Physico-chemical aspects of Thai fermented fish viscera, Tai-Pla, curry powder processed by hot air drying and hybrid microwave-infrared drying.

The objective of this research was to comparatively investigate the effect of hot air drying (HA) and hybrid microwave-infrared drying (MI) on physico-chemical characteristics of Thai fermented fish viscera, Tai-Pla, curry powder (TCP). HA was carried out at 60°C, 70°C, and 80°C and MI was carried out at a microwave power of 740, 780, and 810 W with a constant infrared heating power (500 W) for different drying times to obtain the final moisture content ≤ 12.0% and the water activity (aw) ≤ 0.6. The quality characteristics of TCP were governed by HA temperature and MI output power. TCP dried using HA and MI at all conditions had similar contents of protein, lipid, ash, fiber, and carbohydrate (p>0.05). The fastest drying rate was detected when MI at 810 W for 40 min was applied (p<0.05). In this condition, TCP had the lowest browning index (A294 and A420) and the highest lightness (L* value) (p<0.05). TCP dried with MI at all powers had higher phenolic content and lower TBARS compared to HA (p<0.05). However, no significant differences in DPPH• scavenging activity were observed among TPC made by HA and MI (p>0.05). Similar Fourier transform infrared (FTIR) spectra with different peak intensities were observed in all samples, indicating the same functional groups with different contents were found. The bulk density of all TCP ranged from 0.51 g/mL to 0.61 g/mL and the wettability ranged from 24.02% to 26.70%. MI at 810 W for 40 min effectively reduced the drying time (5-fold faster) and lowered the specific energy consumption (18-fold lower) compared to the HA at 60°C for 210 min. Therefore, MI is a promising drying technique to reduce the drying time and improve the overall quality of TCP.

Introduction

Thailand has long been recognized as “the kitchen of the world” with varieties of tastes and flavors. Strong aromatic components and a spicy edge of Thai cuisine are caused by the formulation of spices, seasoning, and curry paste. Typically, the specific ingredients particularly spices and curry paste with medicinal benefits have long been used for creating the unique flavor in Thai food [1]. A curry of Southern Thai cuisine is one of the most favorite Thai food having a strong flavor and aroma, depending on the type of curry paste used [1,2]. Hot and spicy Tai-Pla curry is well recognized as a unique Southern Thai curry. Its name is derived from the key ingredient, Tai-Pla, which is a salty sauce made from fermented fish viscera [3]. This curry is usually served with fresh vegetables in a separate plate and eaten along with Thai rice noodle. Tai-Pla curry paste can be prepared by mixing herbal ingredients (e.g., dried chilli pepper, garlic, shallot, lemongrass, and galangal) with Tai-Pla sauce. Like other curry pastes, the Tai-Pla curry paste is typically perishable because of high moisture content [4]. In order to commercially distribute this unique local culinary treat over Thailand and export markets, the production of shelf-stable instant Tai-Pla curry powder (TCP) should be developed using an appropriate drying technology.

Drying is one of the most practical preservative methods used in the food sectors to reduce the amount of water from the raw materials and consequently improve the storage stability of food products [5]. The perishable fresh produces and high moisture minimal-processed foods are frequently subjected to drying to minimize the chemical and microbiological deteriorations [6]. There are various commercial drying techniques depending on the requirement of users and the target characteristics of the final product. Hot air convective drying (HA) is one of the most popular and cost-effective methods for agro-food products [5]. However, the major drawbacks of the HA are high drying temperature and long period of drying time, which can substantially result in the deterioration of product quality, such as color, flavor, texture, and nutrition [5,7]. Therefore, a promising drying technology has been developed such as microwave drying (MW) to improve the final product qualities and reduce the drying time. MW is based on dielectric heating by electromagnetic waves and it has several advantages [5]. Demiray et al. [8] indicated that the lower drying temperature, higher drying rate, homogeneous energy diffusion through the material, better space utilization, formation of suitable final product characteristics, and giving better process control are the advantages of MW. However, the main obstacle of MW is the discontinuous heat generation in the food product, which is highly affected by the condition of MW [9]. Also, the other well-known problem of MW is moisture accumulation at the food surface [10]. To encounter these problems, another heating source particularly infrared radiation (IR) is equipped with microwave drier to distribute more uniform heat. Hybrid microwave-infrared drying (MI) combined the time saving advantage of MW with surface moisture removal advantage of IR [10]. IR is the part of the electromagnetic spectrum that is predominantly responsible for the heating effect of the sun. IR energy is absorbed by the food surface and converted to heat [6]. Due to the short time and effective thermal processing, MI can be used to prevent the quality degradation of dried food products [6,11-13].

This present study demonstrated the potential applicability of an alternative MI to produce TCP compared to the traditional HA. The drying rate was monitored to clearly define the drying efficiency. The final product qualities in terms of physico-chemical characteristics were comprehensively determined. These results can be practically presented a promising drying method to prepare TCP with superior quality for further commercial application.

Materials and methods

Tai-Pla curry paste preparation

The main ingredients for Tai-Pla curry paste, including dried whole red chili pepper (5.6%), dried whole garlic (7.5%), fresh lemongrass (11.3%), fresh galangal (3.8%), fresh turmeric (3.8%), fresh shallot (3.8%), dried black pepper (5.6%), dried seedless tamarind (7.5%), Thai fermented shrimp paste (1.9%), fresh kaffir lime leaves (1.9%), and Tai-Pla sauce (47.3%) were purchased from Nap Anuson Food Market, a local market in Thasala, Nakhon Si Thammarat, Thailand (8.6650° N, 99.9225° E). All the ingredients were produced locally in Nakhon Si Thammarat, Thailand. The fresh plant raw materials were hand-trimmed, washed with tap water, and drained with a nylon screen for 30 min at room temperature (27–30°C). Thereafter, all ingredients with the specified proportion were mixed and coarsely ground for 10 min using a grinder (MK 5087M Panasonic Food Processor, Selangor Darul Ehsan, Malaysia) to obtain fresh curry paste. The fresh curry paste (1,000 g) was then pasteurized at 90°C for 10 min [14] in a controlled temperature Hanabishi HGP160S electric pan (Hanabishi Electric Co., Ltd., Bangkok Thailand) under continuous stirring. After cooling down to room temperature, the pasteurized curry paste with the moisture content of 64.7%, referred to as “fresh sample” was subjected to the drying experiment.

Drying experiment

Two drying techniques including HA and MI were used to prepare TCP. The fresh samples (1,000 g) were uniformly spread in the tray with the thickness of 0.5 cm and subjected to dry using a traditional HA drier or an MI drier. The HA drier was operated in a DT 20S tray drier (4,500 W, Owner Foods Machinery Co., Ltd., Bangkok, Thailand) at 60°C, 70°C, and 80°C with the circulation speed of 1 m/s. It has been reported that the maximum temperature used for drying the whole stink bean (Parkia speciosa) seed was at 70°C using the same HA (tray drier) [5]. So, in this study, the drying temperatures were varied on the basis of 70±10°C which were 60°C, 70°C, and 80°C for TCP. The MI drier used in this study was developed by the Center of Excellence in Plasma Science and Electromagnetic Waves, Walailak University (Fig 1). The microwave output powers were adjusted to 740 W, 780 W, and 810 W with a fixed power of infrared heating (500 W) in order to meet the drying temperature of 60°C, 70°C, and 80°C as done by a traditional HA drier. In Thailand, the water activity (aw) of <0.65 and the moisture content of <13% (w/w) are the standards set for seasoning powder [15]. To comply with this standard and to ensure the food safety, the drying was proceeded until the final moisture content was reached ≤12% with the aw of ≤0.6. The drying curves between moisture content and drying time were plotted. The specific energy consumptions of HA and MI were calculated on the basis of the ratio of energy consumed to the initial mass of the samples and expressed as kJ/g [16]. To obtain the TCP, the dried samples were ground using a grinder for 2 min and passed through 35 mesh-sieve. The obtained TCP was packed in an aluminum foil laminated bag (10 cm × 15 cm) to prevent moisture adsorption and kept in an auto desiccator at room temperature for 24 h. Thereafter, the quality characteristics of the samples were determined.

Fig 1

Hybrid microwave-infrared drier.

Determination of chemical characteristics

Proximate composition and aw

The standard AOAC’s methods [17] were used for proximate composition analysis including moisture (AOAC method number 950.46), crude protein (AOAC method number 928.08), fat (AOAC method number 963.15), ash (AOAC method number 920.153), fiber (AOAC method number 962.09), and carbohydrate. Carbohydrate content was estimated by difference (1).

The aw was determined at room temperature using an Aqualab Series 3TE aw meter (Decagon, Pullman, WA, USA).

Total phenolic content (TPC) and DPPH radical scavenging activity

The samples (10 g) were extracted with 80% ethanol (10 mL) at 45°C in an incubator for 2 h with constant stirring. After filtration (Whatman No.1), the filtrates were analyzed for TPC using the Folin-Ciocalteu method [18]. The extracts (0.5 mL) were mixed thoroughly with 9.5 mL of distilled water, 1.5 mL of Folin-Ciocalteu reagent, and 1.2 mL of 7.5% sodium carbonate solution. After standing for 30 min at room temperature, the absorbance was determined at 765 nm (UV-Vis spectrophotometer, Shimadzu Scientific Instruments Inc., Columbia, MD, USA). A standard curve was prepared using gallic acid in the range of 0–100 ppm. The data were expressed as mg gallic acid equivalent (GE)/100 g.

DPPH radical scavenging activity was determined according to the method of Yen and Hsieh [19]. The ethanolic extracts (0.3 mL) was mixed with 1.5 mL of 0.10 mM DPPH in methanol solution and incubated at room temperature for 30 min. The absorbance was then read at 517 nm. The blank was prepared as the same manner, except that distilled water was used instead of the sample extract. A standard curve was prepared using Trolox in the range of 0–0.2 mM. The DPPH radical scavenging activity was expressed as mmoLTrolox equivalent (TE)/g sample.

Thiobarbituric acid reactive substances (TBARS)

TBARS analysis is the most widely used method to determine the secondary lipid oxidation products [20]. Due to the presence of some lipid content in the Tai-Pla curry paste, the lipid oxidation can be taken placed during drying. Herein, the lipid oxidation was monitored using the TBARS. Samples (10 g) were mixed with 50 mL of distilled water and 2.5 mL of 4 M HCl and diluted to 100 mL with distilled water. The samples were distilled at 100°C following the procedure described by Tarladgis et al. [21]. Then, 2 mL of distillate was mixed with 3 mL of a mixed TBA solution containing 0.375% thiobarbituric acid, 15% trichloroacetic acid, and 0.25 M HCl. A reagent blank was prepared by adding 2 mL of distilled water and 3 mL of mixed TBA solution. The mixture was heated in boiling water (100°C) for 10 min, followed by cooling with the running tap water. The absorbance was measured at 532 nm. A standard curve was prepared using malondialdehyde in the range of 0–10 ppm. The results were expressed as mg molondialdehyde equivalent/kg sample.

Fourier transform infrared (FTIR) spectroscopy

The FTIR spectroscopy is a vibrational spectroscopic technique that can be used to characterize the substances by identifying their functional groups presented [22]. FTIR spectra (400–4000 cm-1 with the resolution of 4 cm-1 at the average of 16 scans) of the TCP were obtained using a horizontal Attenuated Total Reflectance (ATR) Trough plate crystal cell (45° ZnSe; 80 mm long, 10 mm wide and 4 mm thick) (Pike Technology, Inc., Madison, WI, USA) equipped with a Bruker Model Vector 33 FTIR spectrometer (Bruker Co., Ettlingen, Germany) at room temperature. Analysis of spectral data was carried out using the OPUS 3.0 data collection software program.

Determination of physical characteristics

Color

Colorimetric values of the samples were measured using a Hunterlab colorimeter (Hunter Assoc. Laboratory; VA, USA) with 10 standard observers and illuminant D65. The instrument was calibrated to a white and black standard. The L* (lightness), a* (redness/greenness), and b* (yellowness/blueness) values were recorded.

Browning intensity

The browning intensity can be used to monitor the formations of the intermediate and final products of the Maillard reaction in the TCP. The presence of the Maillard reaction products (MRPs) affected both color and antioxidant activity of the food products [23]. The UV absorbance at 294 nm (A294) was often used to indicate the intermediate MRPs while the final MRPs was monitored by the absorbance at 420 nm (A420). An aqueous extraction and appropriate dilution were made prior to analysis. The mixture (1 g of TCP and 10 mL of distilled water) was homogenized at 13,000 rpm for 1 min at room temperature (IKA® Model T25 digital Ultra-Turrax®, Staufen, Germany). The centrifugation was applied at 3,000 ×g for 20 min at room temperature using an RC-5B plus centrifuge (Sorvall, Norwalk, CT, USA). Then, the absorbance of the supernatant was read measured at 294 nm and 420 nm using a Shimadzu UV-2100 spectrophotometer (Shimadzu Scientific Instruments Inc., Columbia, MD, USA) [23,24].

Bulk density

The method of Jinapong et al. [25] was used for bulk density determination. The sample (1 g) was gently loaded into a 10 mL graduated cylinder and tapped for 10 times with the same strength. The volume was read directly from the cylinder and then used to calculate the bulk density according to the ratio of mass (g) to volume (mL).

Wettability

To ensure the proper rehydration of the dried power, the wettability of all powders were determined [26,27]. Specifically, 40 mL of distilled water was transferred into a 250 mL beaker. Four grams of samples were added into the beaker and then stirred at 800 rpm for 5 min at room temperature. The mixture was centrifuged at 4,500 ×g for 5 min to separate the insoluble substances. Then, 20 mL of supernatant was transferred into a plate and dried at 100°C for 24 h. The wettability (%) was calculated as the weight of dry matters in the supernatants versus the weight of dry matters in the powders.

Statistical analysis

A completely randomized design was used in this study. The data were expressed as means ± standard deviations (SD) of three replications (n = 3) for all analyses. Data analysis was carried out using one-way ANOVA. The comparison of means was performed by Duncan’s multiple-range test to identify significant differences (p<0.05) among samples [28], using the SPSS program (SPSS Inc., Chicago, IL, USA).

Results and discussion

Drying curves and specific energy consumption

The drying curves for TCP obtained from HA and MI are depicted in Fig 2A and 2B, respectively. The drying time was dependent on the drying technique and drying condition. HA at 60°C showed the longest drying duration (210 min, k ~ 0.2311) (Fig 2A), whereas MI at 810 W showed the fastest drying rate (40 min, Fig 2B). Using MI, the faster moisture transfer from the interior of the samples to its surface was facilitated by MW [29] and the surface moisture removal was enhanced by IR [10]. TCP dried with HA at 70°C and 80°C had the drying time of 120 and 100 min, respectively (Fig 2A), which was shorter than that did at 60°C for 1.75 and 2.10 folds, respectively. This was probably due to an acceleration of the moisture migration at high temperature. Similar result was attained for vacuum hot air dried Kumquat [9], in which a higher temperature can shorten a drying time. However, HA at 70°C and 80°C showed a negligible different in drying rate (k ~ 0.3436 vs 0.3907) (Fig 2A). There were some differences in drying rate at the first 60 min (constant rate period) between 70°C and 80°C, then a slower water removal period was taken place (falling rate period). For MI, the higher MI output power rendered the shorter drying time (740 W for 70 min, 780 W for 45 min, and 810 W for 40 min), with the drying rate (k) of 0.7073, 1.0358, and 1.1446 for 740 W, 780 W, and 810 W, respectively. The higher the MI output, the faster the water removal [30]. Michalska et al. [31] indicated that microwave vacuum drying showed the lowest drying times for plum powder compared to freeze drying, vacuum drying, and convective hot air drying. In addition, Koç [32] reported that drying at high microwave output power resulted in an increase in the driving force of mass transfer and accelerating the rate of water vapor diffusion.

Fig 2

Drying curves of Tai-Pla curry powder (TCP) dried using hot air (HA) (a) and hybrid microwave-infrared drying (MI) (b) under different drying conditions.

The specific energy consumptions of HA and MI are shown in Fig 2C. The HA consumed more energy than MI did. The highest specific energy consumption (57 kJ/g) was found in HA at the lowest temperature (60°C) whereas the lowest specific energy consumption (3 kJ/g) was found in MI at the highest microwave power (810 W). From the results, the specific energy consumption was reduced as hot air temperature and microwave power increased in HA and MI, respectively. Chua et al. [16] reported that high drying intensities corresponded to a short drying duration, thereby reducing the specific energy consumption. In the MI, the MW heating helps to reach the temperature of the product quickly and the IR directly transfers the heat from IR emitters to the product surface without the need for any physical environment. Thus, the production of high-quality food with minimal energy consumption can be obtained [7].

Chemical characteristics

The moisture, protein, fat, ash, fiber, and carbohydrate contents of TCP dried using different drying methods with various drying conditions are presented in Table 1. The significant decrease in moisture content was observed in TCP (~10%) compared to the original fresh sample (~65%). This implied the shelf-stable of TCP by limiting the microbial growth during storage. No significant differences in moisture content among TCP prepared using different drying conditions were found (p>0.05), which was due to the clearly defined final moisture content. As expected, other proximate compositions, including protein, fat, ash, fiber, and carbohydrate of all TCP were gradually increased after removing significant amount of moisture compared to the original fresh paste (p<0.05). Morris and Barnett [33] suggested that the removal of water from the food matrix can increase the nutrient concentration in dried food samples. Generally, no significant differences in protein, fat, ash, fiber, and carbohydrate were found among TCP dried using HA and MI (p>0.05). The crude protein contents of TCP ranged from 18.3% to 19.2% (p>0.05). These values were greater than those reported for three commercial Nigerian seasoning powders (9.2–12.9%) by Lillian et al. [34]. This clearly caused by the addition of Tai-Pla (fermented fish viscera) sauce and fermented shrimp paste, as sources of protein, in the formulation of Tai-Pla curry paste. The crude fat contents of TCP ranged from 8.6% to 9.9% (p>0.05), which were greater than the contents found in dried spices (e.g. Tetrapleura tetraptera [35] and Scorodophleus zenkeri [36]) and commercial Nigerian seasoning powders [34]. The presence of fat in the product may cause the oxidative instability during storage. The ash, crude fiber, and carbohydrate contents of all TCP were in the ranges of 18.3–19.7%, 13.6–14.9%, and 27.2–29.8%, respectively, indicating high levels of minerals, dietary fiber, and carbohydrate. These components originated from the ingredients used for preparation of fresh Tai-Pla curry paste. From the results, TCP can be classified as a nutritive flavoring agent. Summarily, the drying method and drying condition showed a negligible impact on proximate composition of the final TCP. This finding was in line with the study of Ojo et al. [37] who found no significant difference in protein content among sun-dried and solar-dried fufu flours. Moreover, an increasing in some food constituents after drying, particularly ash content, may contribute to the low volatility of certain substances e.g. minerals, which are slightly thermally degraded.

Table 1

Proximate composition and water activity (aw) of Tai-Pla curry paste (fresh sample) and Tai-Pla curry powder (TCP) dried using hot air (HA) and hybrid microwave-infrared drying (MI) under different drying conditions.

Drying methods	Moisture (%)	Protein (%)	Fat (%)	Ash (%)	Fiber (%)	Carbohydrate (%)	aw
Fresh sample	64.7±2.4b	3.1±0.2a	1.5±0.1a	3.3±0.1a	2.3±0.5a	25.2±1.4a	0.80±0.02b
HA
60°C/210 min	10.7±0.1a	18.7±0.1b	9.2±1.0b	19.7±0.0b	13.6±1.1b	28.1±1.0b	0.44±0.01a
70°C/120 min	10.2±0.1a	18.3±0.8b	9.2±0.1b	18.4±1.0b	14.2±0.4b	29.8±0.3b	0.45±0.02a
80°C/100 min	10.3±0.3a	19.1±1.2b	8.6±1.6b	19.6±0.4b	14.8±0.1b	27.8±2.0b	0.44±0.01a
MI
740 W/70 min	10.5±0.1a	18.6±0.1b	9.9±0.7b	18.8±1.1b	14.2±0.0b	27.9±0.7b	0.49±0.30a
780 W/45 min	10.5±0.6a	18.4±0.2b	9.2±0.1b	18.3±1.0b	14.9±0.1b	28.7±0.1b	0.53±0.10a
810 W/40 min	10.1±0.4a	19.2±1.0b	9.7±0.5b	19.7±0.1b	14.1±0.0b	27.2±0.2b	0.47±0.00a

*Values are given as mean ± SD from triplicate determinations.

**Different letters in the same column indicate significant differences (p<0.05).

The aw of the fresh sample was 0.80 whereas the aw of the TCP was 0.44–0.53 (Table 1). The aw of all TCP was not significantly different (p>0.05). Dried foods should have an aw < 0.60 [38]. An aw of <0.65 and a moisture content of <13% (w/w) are the standards set for seasoning powder in Thailand [15]. It has been reported that the volumetric heating plus surface heating of MI can be effective for the microbial inactivation and restriction of moisture deteriorative physicochemical reactions [6]. So, the TCP can be considered as a shelf-stable product due to low moisture content and aw.

TPC. A slight increment in TPC was observed in TCP compared to the fresh sample (Fig 3A). This was probably due to the evaporation of moisture, leading to the increased TPC concentration in the dried samples. Szychowski et al. [39] suggested that freeze-, convective hot air-, and vacuum- microwave driers increased the phenolic compounds in dried quince fruits compared to the fresh counterpart. Moreover, higher phenolic compounds in dried guava powders compared to fresh sample could be attributed to the concentration of the phenolic compounds after water evaporation [40]. The increment of phenolic compounds after drying could involve in the cell wall breakdown and disruption by heating effects and consequently eases the release and extractability of bound phenolic compounds [39]. Generally, plant phenolic compounds can be classified into two major forms, free and bound [41]. There were no significant difference in TPC among sample did by HA at 60°C and 70°C compared to the fresh sample (p>0.05). The thermal degradation of phenolic compounds may relate to this result. However, the greater TPC was observed in HA dried sample at 80°C (p<0.05). The availability of precursors of phenolic molecules by non-enzymatic inter-conversion between phenolic molecules may explain the formation of phenolic compounds at high temperature [42]. This was in agreement with the finding of Vega-Gálvez et al. [43] who reported a slight increase in TPC of red pepper after hot air drying at high temperature, in particular at 90°C. It can be seen that drying temperature has an important effect on phenolic remaining in dried product. For MI, drying at 740 W rendered the TCP with the highest TPC (p<0.05). The reduction of TPC at higher MI power intensity could be due to the heat-induced degradation. The opposite trend was reported by Hayat et al. [44] who found an increasing in TPC of citrus mandarin pomace with increasing microwave power (125–500 W/15 min). The different results may be due to the different raw material, microwave power, and drying time. Thus, MI power seemed to play an important role in the TPC retention during drying. Comparatively, sample dried with MI had higher TPC than that did with HA (p<0.05). This was due to a shorter drying time of MI, leading to a higher phenolic retained in dried sample. The result was in line with Özcan et al. [45] who found a higher TPC in microwave dried kiwi and pepino fruits than hot air counterpart. The presence of the TPC in the product may have helped to improve the oxidative stability during storage. The major phenolic compounds composed in the raw materials for production of TCP have been reported intensively, for instance, galangin in galangal [46], capsaicin in red chili pepper [47], allicin in garlic [48], isoorientin in lemongrass [49], piperine in black pepper [50], β-pinene and limonene in kaffir lime leaves [51], curcumin in turmeric [52] and quercetin in shallot [53].

Fig 3

Total phenolic content (TPC) (a), DPPH radical scavenging activity (b), and TBARS (c) of Tai-Pla curry paste (fresh sample; pattern fill) and Tai-Pla curry powder (TCP) dried using hot air (HA) (□) and hybrid microwave-infrared drying (MI) (■) under different drying conditions. Different letters on the bars indicate significant differences (p<0.05).

Free radical scavenging activity

The DPPH radical scavenging activity of TCP made by HA and MI is depicted in Fig 3B. Dehydration temperatures and MI powers had no impact on radical scavenging activity (p>0.05). There were also non-significant difference among radical scavenging activity of all TCP and fresh sample (p>0.05). Although, antioxidative phenolic compounds varied depending on drying conditions (Fig 3A), similar radical scavenging activities of all sample were observed (Fig 3B). This behavior could be related to the degradation of certain original antioxidative compounds and simultaneous formation of new antioxidants (e.g. Maillard reaction products (see browning index in Table 2) which may decrease the antioxidative capacity [54] or promote such activity [55]. A correlation between antioxidant activity and TPC has been reported during food dehydration [56]. Therefore, the net oxidative status of TCP was governed by the rate of degradation and formation of antioxidants during drying. However, numerous factors, such as drying method, type of extraction solvent, antioxidant assays, and interactions of several antioxidant reactions have been reported to affect the TPC and antioxidant activity of dried foods, which somehow resulted in a conflict data [42].

Table 2

Color (L*, a* and b* values) and browning index (A294 and A420) of Tai-Pla curry paste (fresh sample) and Tai-Pla curry powder dried using hot air (HA) and hybrid microwave-infrared drying (MI) under different drying conditions.

Drying methods	L*	a*	b*	A294	A420
Fresh sample	27.65±0.05a	8.52±0.24a	28.96±0.83a	45.24±0.01a	10.03±0.00a
Hot-air
60°C/210 min	43.82±0.08e	11.17±0.10b	43.31±0.17e	79.98±0.02c	18.96±0.00c
70°C/120 min	39.12±0.07b	10.90±0.18b	37.58±0.15b	84.12±0.01d	20.87±0.00d
80°C/100 min	39.97±0.20c	11.17±0.20b	38.59±0.34c	86.18±0.02e	20.83±0.00d
MI
740 W/70 min	45.29±0.12f	11.24±0.20b	40.59±0.34c	77.74±0.04b	16.83±0.00b
780 W/45 min	43.26±0.12d	11.04±0.04b	39.65±0.48c	76.02±0.00b	16.71±0.00b
810 W/40 min	48.27±0.10g	11.33±0.10b	41.54±0.99c	75.09±0.01b	16.64±0.00b

*Values are given as mean ± SD from triplicate determinations.

**Different letters in the same column indicate significant differences (p<0.05).

TBARS

The effect of drying methods on the evolution of TBARS of TCP is shown in Fig 3C. TBARS increased with increasing HA temperature (p<0.05). This was attributed to the fact that unsaturated lipids are easily oxidized at higher temperature. Song et al. [57] reported that the drying temperature had progressively impact on lipid oxidation of lotus pollen. TBARS of pollen rapidly increased when drying at 50, 60, and 70°C in the first two hours of drying. For MI, TBARS value tended to increase with increasing MI power from 740 W to 780 W, then decreased afterward (Fig 3C). The changes in TBARS may be dependent on the level of heat generated by MI. The formation and accumulation of aldehydic lipid oxidation products could occur progressively with increasing MI power from 740 W to 780 W. However, at 810 W, a lower TBARS value was possibly due to the vaporization of volatile aldehydes and the interaction of reactive aldehydes with amines to form Maillard based products. De Pilli et al. [58] suggested that lipid oxidation of pasta increased with increasing microwave power during drying. From the results, the heat induced lipid oxidation and the newly formed antioxidants may involve in the net TBARS of TCP. Comparatively, the TBARS of MI dried powders were lower than HA dried samples (p<0.05). Although the same drying temperature between HA and MI were set, the different TBARS may result from the different drying time. The major changes in the powder structure during HA and MI may facilitate the accessible of oxygen to lipids, resulting in a greater lipid oxidation [59]. Interestingly, the fresh sample exhibited a greater TBARS than dried samples (p<0.05). This result might cause by the oxidation of lipid during curry paste preparation. The lipid oxidation of the fresh sample could be hastened during pasteurization at 90°C for 10 min.

FTIR spectra

Similar FTIR spectra with different peak intensities were observed in all TCP (Fig 4), suggesting the identical compositions with different contents were found in all samples. From the results, the FTIR spectra can be grouped into two main regions. The first region, showing complex spectra, was located at 400–1,800 cm-1 and the second region was located at 1,800–4,000 cm-1. The complexity of the FTIR spectra especially in the bands at 400–1,800 cm-1 was due to the presence of different ingredients used for production of TCP such as Tai-Pla sauce, spices, herbs, and fermented shrimp paste. Herein, the peak at 3,412 cm-1 was typically found in all samples. Sanyal et al. [60] suggested that the peak region at 3,200–3,400 cm-1 represented O-H stretching and this may be due to the combined effect of O-H groups of water and carbohydrates. The absorbance peak around 2,925 cm-1 was related to typical vibration modes of the lipids fatty acids [61]. In addition, the absorbance band at 3,200–3,600 cm-1 can be used to monitor the formation of hydroperoxide, a primary lipid oxidation product, [62] in which the highest bands were observed in TCP produced by HA at 70–80°C. This was somehow related to higher contents of TBARS, a secondary lipid oxidation product, of those TCP (Fig 3C). The peak region at 2,800–3,000 cm-1 assigned to C-N stretching [60]. The peaks around 1000–1600 cm-1 were also found in all samples, which were attributed to the presence of the amide of the characteristic protein bands [63]. The band at 1,033 cm-1 region was assigned to C-N amines stretching [60]. Specifically, the detection of protein secondary structure is based on the amide I region composed of C = O stretching vibrations in the region of 1611–1690 cm-1 [22]. Results indicated that proteins in TCP were in the denatured forms due to the effect of drying. The band at 1,410 cm-1 region was assigned to O-H phenols bending [64], confirming the occurrence of the phenolic compounds in the TCP. Overall, the results indicated the presence of protein, lipid and its oxidation products, moisture, carbohydrate, and phenolic compounds in TCP.

Fig 4

FTIR spectra of Tai-Pla curry paste (fresh sample) and Tai-Pla curry powder (TCP) dried using hot air (HA) and hybrid microwave-infrared drying (MI) under different drying conditions.

Physical characteristics

Color and browning intensity

Color is a crucial quality attribute of food, which influences the consumer’s acceptance. The appearances of the fresh sample and final TCP prepared from the different drying conditions are shown in Fig 5.

Fig 5

Appearances of Tai-Pla curry paste (fresh sample) and Tai-Pla curry powder (TCP) dried using hot air (HA) and hybrid microwave-infrared drying (MI) under different drying conditions.

Generally, the natural color of fresh Tai-Pla paste is roughly dark, caused by the dark color of Tai-Pla sauce, the key ingredient of this curry paste. Color parameters of TCP are displayed in Table 2. After drying, the dried powders had higher L*, a*, and b* values than that the original fresh sample (Table 2). The increased L* value was probably be due to the alteration of light reflection plane caused by water removal. A reduction of water content let the particles to freely flow and allowed more light to pass through the sample. The increased a* and b* values were mainly due to the evolution of browning reactions, e.g. Maillard reaction, caramelization, and polyphenol oxidation. Sharifian et al. [65] reported that the changes in color of dried food might contribute to the complex chemical reactions such as pigment oxidation, chemical degradation, and browning reaction.

For HA, a lower drying temperature (60°C) rendered the sample with greater L* and b* values (p<0.05). With increasing temperature in HA, L* and b* values decreased but a* value remained constant (Table 2). No significant differences in a* values were detected among HA dried samples (p>0.05). For MI, no significant differences in a* and b* values were observed among samples (p>0.05) whereas the L* values were slightly different among samples. This finding was in accordance with Inchuen et al. [66] for microwave-dried red curry, Pereira et al. [67] for combined microwave/hot-air-dried banana, Soysal [68] for microwave-dried parsley, and Maskan [69] for microwave-dried banana.

Browning effects are mainly contributed to the quality deterioration of dried food products caused by thermally induced browning reactions [70]. The absorption peak at a wavelength of 294 nm (A294) indicates the presence of intermediate compounds whereas the intensity at 420 nm (A420) indicates the formation of brown pigments of Maillard reaction or caramelization. The browning index of TCP are presented in Table 2. The TCP showed the greater A294 and A420 than the fresh paste, demonstrating the evolution of browning reaction during drying. An increase in HA temperature caused a significant increase in the formation of Maillard intermediate products (p<0.05). Non-enzymatic browning reactions particularly Maillard reaction and caramelization are temperature dependent. The higher the temperature, the greater the browning intensity [65]. Vega-Gálvez et al. [43] reported that the amounts of reducing sugars and amino acids in the raw materials played an important role in the degree of Maillard reaction during the drying process. On the flip side, the MI power had no impact on the accumulation of both intermediate products and brown pigment (p>0.05). Comparatively, the browning intermediates and brown pigments in traditional HA dried samples were greater than those in MI dried samples, suggesting a better color protective effect of the MI. Since the similar drying temperatures were employed between HA and MI, heat generated during MW by molecular friction caused the greater heat load inside the sample. In addition, MI quickly produced heat and effectively removed water at surface. This can shorten the drying time and hence reduce the degree of browning reactions. TCP produced by HA had a higher browning index (Table 2) whereas TCP produced by MI tended to have a higher TPC (Fig 3A). Both browning intermediate/final products and phenolic compounds can function as antioxidant. So, the net free radical scavenging activity of TCP was similar among MI and HA (Fig 3B).

The bulk density of food powder can typically present its textural characteristics. The bulk density of fresh sample was 1.26 g/mL (Table 3). A lower bulk density was observed in all TCP, ranging between 0.51 g/mL to 0.61 g/mL (Table 3). The larger particle sizes with higher void spaces could originally lower the bulk density of dried sample [71]. Among HA dried powders, TCP dried at 80°C showed the highest bulk density, followed by samples dried at 60°C and 70°C, respectively (Table 3). No significant difference in bulk density was noticed among MI dried samples (p>0.05), suggesting a comparable textural characteristic. This was in accordance with the finding of Koç and Çabuk [72] who found a constant bulk density of egg white powder dried with different microwave powers (120–350 W). TCP dried with MI method showed higher bulk density than those HA dried samples (p<0.05). A higher bulk density may contribute to a decrease in the inter-particle void volume of dried powder with larger contact surface area per unit volume. Changes in bulk density and porosity of powder were influenced by powder characteristics such as particle size [73].

Table 3

Bulk density and wettability of Tai-Pla curry paste (fresh sample) and Tai-Pla curry powder dried using hot air (HA) and hybrid microwave-infrared drying (MI) under different drying conditions.

Drying methods	Bulk density (g/mL)	Wettability (%)
Fresh sample	1.26±0.01d	8.95±0.72a
HA
60°C/210 min	0.54±0.01ab	24.39±0.35b
70°C/120 min	0.51±0.02a	24.02±0.55b
80°C/100 min	0.57±0.02b	24.10±0.80b
MI
740 W/70 min	0.61±0.02c	25.39±0.15c
780 W/45 min	0.57±0.12bc	26.70±0.06c
810 W/40 min	0.62±0.02c	24.87±0.14b

*Values are given as mean ± SD from triplicate determinations.

**Different letters in the same column indicate significant differences (p<0.05).

Wettability presents the reliable criterion of powder behavior in aqueous solution. The sinkability, dispersability, and wettability of powder are underwent after the dissolution steps [74]. The wettability of the TCP made by different drying methods is shown in Table 3. Generally, all TCP had greater water wettability than the fresh sample (p<0.05). This result could contribute to the higher degree of macromolecular disorganization of the dried particles as affected by drying process. There were no significant difference in the wettability among HA dried TCP (p>0.05) (Table 3). The MI dried TCP had higher wettability than HA dried TCP (p<0.05). This may be due to the lower structure disruption of MI dried sample caused by the shorter drying time. It should be noted that drying using MI output power of 810 W resulted in a slight decrease in the wettability of TCP. Koç and Çabuk [72] reported that the wettability of egg white powder increased with increasing microwave power from 120 W to 460 W and subsequently decreased with further increasing microwave power from 460 W to 600 W. From the results, the wettability of TCP was governed by drying condition.

Conclusion

The effects of two different drying methods (HA and MI) on quality of TCP were evaluated. Results suggested that the color, bulk density, wettability, TPC, radical scavenging activity, and lipid oxidation of final TCP were dependent on drying conditions. The optimal MI condition for TCP preparation was observed at 810 W for 40 min, which was faster than the traditional HA for 5 times. At this condition, TCP had the superior physico-chemical characteristics (TPC, oxidative stability, color, and bulk density) to the traditional HA without a negative effect on the proximate compositions. Therefore, MI is a promising method for TCP production after considering the overall quality characteristics. Further studies are recommended to determine the sensory quality, flavor profile, and storage stability of the TCP.

10 May 2021

PONE-D-21-10233

Physico-chemical aspects of Thai fermented fish viscera, Tai-Pla, curry powder processed by hot air drying and hybrid microwave-infrared drying

PLOS ONE

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"This research was financially supported by Research and Researchers for Industries (RRI) program and Shaw Processing Food Co. Ltd. [Grant No. MSD61I0053] and the new strategic research project (P2P), Walailak University, Thailand. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript."

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Reviewer #1: The manuscript entitled Physico-chemical aspects of Thai fermented fish viscera, Tai-Pla, curry powder processed by hot air drying and hybrid microwave-infrared drying finds to be interesting. Further some major revision is required before final acceptance.

General Comments:

1. In this work, author used hybrid MI drying method for drying of Thai fermented fish viscera, Tai-Pla, curry powder (TCP). But throughout the manuscript only microwave power level varies and nothing is mentioned about Infra-red drying. In Fig 1 also there is no provision of Infra-red heating, then how this method become hybrid drying.

2. There is no comparison of energy consumption of HA and MI in the manuscript.

3. The applied Drying time in MI method is higher range (above 40 min). Normally drying in microwave upto 10 min maximum.

Specific Comments:

1. In Introduction give more recent references related with MI drying method.

2. Line no. 120-121, On what basis final moisture content 12% and water activity 0.6 was chosen. Whether its safe moisture content of storage of TCP. If yes, then give reference for it.

3. What is the sample size dry in both the method?

4. In this research work, TCP dried at three different temperatures 60, 70 and 800C. On what basis this range was selected. Please mentioned in the manuscript.

5. Line 152-153; please specify why TBRS is important to study in this research work.

6. Line 162-163, why author want to use FTIR in this study. Please mention? On what basis the FTIR range was selected?

7. Line 170-171, How Browning Intensity influence the product quality. Please specify

Reviewer #2: The paper ”Physico-chemical aspects of Thai fermented fish viscera, Tai-Pla, curry powder processed by hot air drying and hybrid microwave-infrared drying” aimed to comparatively investigate the effect of hot air drying (HA) and hybrid microwave-infrared drying (MI) on physico-chemical characteristics of Thai fermented fish viscera, Tai-Pla, curry powder (TCP). The subject is of great interest for the industrials willing to reinterpret the traditional technology into convenient innovative alternatives.

There are few aspects that need attention from the authors.

Indicate how many replicates were taken in this study.

When a product is designed for further industrial scale production, the readers are willing to see the sensory analysis results. Did you perform sensory data or are you able to perform it and include the results in this manuscript?

For better point the advantage of MI treatment, authors should perform also some stability tests and specify the shelf life of the obtained powders. Is it possible to perform also the microbiological analysis? At least the common ones ? You can also insert a discussion related to the influence of fat content to product stability.

Please argument the necessity of antioxidant activity (AA) and correlate the your AA results to total phenolic content, not just simply say they correlate based on literature analysis, because it is not always true. So please check this with your results. Why did authors decide not to include also the individual phenolic compound analysis?

The discussion on FTIR results is poor. Please improve this part otherwise is not useful for the readers.

Reviewer #3: The manuscript deals with physico-chemical aspects of Thai fermented fish viscera, Tai-Pla, curry powder processed by hot air drying and hybrid microwave-infrared drying.

The English language must be revised.

Please separate values from units, e.g. “60 ºC” not “60ºC”.

Please number all sections.

Abstract

This section is vague. Please present your main results.

Introduction

The topics must be better linked.

Materials and methods

Line 107- “Thereafter, all ingredients with the specified proportion were mixed and coarsely ground for 10 min using a grinder (MK 5087M Panasonic Food Processor, Selangor Darul Ehsan, Malaysia) to obtain fresh curry paste.”??amounts used of each ingredient??

Line 109- “The fresh curry paste was then pasteurized at 90ºC for 10 min.”??how was the sample pasteurized?amount used??

Line 114- “Two drying techniques including HA and MI were used to prepare Tai-Pla curry powder. The fresh samples were uniformly spread in the tray with the thickness of 0.5 cm and subjected to dry using a traditional HA drier (tray drier) or an MI drier. The MI drier used in this study was developed by the Center of Excellence in Plasma Science and Electromagnetic Waves, Walailak University (Fig. 1). The MI output powers were adjusted to 740, 780, and 810 W in order to meet the drying temperature of 60ºC, 70ºC, and 80ºC as done by a traditional HA drier.”??used conditions??tray dryer air speed??

Line 115- “The fresh samples were uniformly spread in the tray with the thickness of 0.5 cm and subjected to dry using a traditional HA drier (tray drier) or an MI drier.”??amount used??

Line 123- “The obtained TCP was packed in an aluminum foil laminated bag to prevent moisture adsorption and kept in an auto desiccator at room temperature for 24 h.”??packaging dimensions??

Line 167- “Colorimetric values of the samples were measured using a Hunterlab colorimeter (Hunter Assoc. Laboratory; VA, USA). The L*, a*, and b* values were recorded.”???illuminant used??ºobserver??calibration??

Line 184- “Wettability”??or solubility in water??

Results and discussion

This section has lack of depth and must be improved.

Line 251- “From the results, TCP contributed not only for taste and flavor enhancers, but also played a part in an extra-source of nutrients.”?? Aroma???flavor??measured??

Particle size??

Figure 1- Please define each component.

Figure 4- Please add wavenumber in each peak.

Conclusion

Line 451- “Results suggested that TCP prepared by MI method showed the superior physico-chemical characteristics to the traditional HA.”???superior??in which results??

References

37 references have more than 5 years. Please update your list of references.

Reviewer #4: In the present study titled “Physico-chemical aspects of Thai fermented fish viscera, Tai-Pla, curry powder processed by hot air drying and hybrid microwave-infrared drying” is presented in detailed a well elaborated research which evaluated the properties of Thai fermented fish viscera, Tai-Pla, curry powder (TCP) dryed on different approaches: hot air drying (HA) and hybrid microwave-infrared drying (MI). The properties of both fresh and dried powders were evaluated by proximate composition, aw, total phenolic content, DPPH radical scavenging activity, TBARS, FT-IR spectroscopy and physical characteristics (colour, browning intensity, bulk density, wetability).

The research led to the identification of the best drying procedure of the fresh pasta with the desired properties, namely the hybrid microwave-infrared drying (MI).

In general the data are strong, and convincingly shows that the hybrid microwave-infrared drying (MI) approach could be used as a drying procedure to obtain good quality food products. The manuscript is well written, concise and the appropriate analyses are performed.

Overall, this is a well performed study that I consider that is important and represent a new strategy to conveniently obtain Thai fermented fish viscera, Tai-Pla, curry powder (TCP) with good quality characteristics.

The authors need to address the below comments to strengthen the quality of the manuscript:

1. Please insert the characterization methods used in the present study in the Abstract (e.g. FT-IR, DPPH assay).

2. Please replace the phrases from line 37 and 455: „Therefore, MI was a promising drying

3. technique to reduce the drying time and improve the overall quality of TCP.” By „Therefore, MI is a promising drying technique to reduce the drying time and improve the overall quality of TCP.”

4. In the preparation method of the pasta (in Materials and methods) please include the mass percentage of the main ingredients used to obtain the product.

Reviewer #5: In this manuscript, the authors compared the effect of hot air drying (HA) and hybrid microwave-infrared drying (MI) on physico-chemical characteristics of Thai fermented fish viscera, Tai-Pla, curry powder (TCP). The data in this article is not solid and well analyzed. Besides, the article doesn't present the application superiority of MI thoroughly, in other words doesn't fit with the average originality found of PLOS ONE. I recommend to not publish the article.

Major comments

The data succeed to show the different effect of HA and MI, but the experiments are too superficial, as well the test of antioxidant activities. According to the introduction, the application of TCP mainly gives the Thai food a special flavor and aroma, which should be concerned in this article because the HA and MI processing would change the flavor of TCP. In the overall the text is well written but is superficial and is out of the journal standards.

Detailed comments

1. The authors developed a hybrid microwave-infrared drier but the schematic diagram (Fig. 1) didn’t show how the hybrid microwave-infrared drier works.

2. SEM is recommended to show the influence of these physical processes on tissues of TCP during drying.

3. Line 36: better change “between … to …” to “from … to …”.

4. Line 61: “a” should be “its” or deleted.

5. Line 76-77: Please check this sentence and revise.

6. Line 163: FTIR spectra of fresh samples can’t be found in Figure 4.

7. Line 251-252: Please check this sentence and revise.

8. Line 271: Please check this sentence and revise.

9. Line 380-381: Please check this sentence and revise.

10. Line 411-412: Please check this sentence and revise.

11. Line 421: “P<0.05” is wrong.

12. Please replace “governed” with another word in line 30, line 307, and line 441.

13. Line 51: “cuisin” should be “cuisine”.

14. Please add a comma before the second subject in line 215-217, line 217-218, and line 402-403.

15. Please mark “a, b, c” in the Figure 3 according to “Fig. 3a, Fig. 3b, Fig. 3c” in line 302-313.

Reviewer #6: In this manuscript, Choopan et al. compared the hot air (HA) and hybrid microwave-infrared (MI) drying ways on the Physico-chemical characteristics of TCP. They have demonstrated that MI drying with a condition of 810 W for 40 min effectively reduced the drying time by five-fold. They also showed that MI-dried TCP had the lowest browning index, the highest lightness, higher phenolic content, and lower TBARS, indicating improved overall quality. The manuscript has convincing data to support their conclusion.

The concern I have is whether MI drying affects the flavor of TCP. Is it possible to evaluate the flavor?

Reviewer #7: The paper presents an application of Physico-chemical aspects for Thai fermented fish viscera, Tai-Pla, curry powder processed. It is a topic of interest to the researchers in the related areas but this paper needs improvement before acceptance for publication. My detailed comments are as follows:

1. The sample source and size are both important for this paper, and please provide more details about the sample information in the section Tai-Pla curry paste preparation.

2. Instrument model and manufacturer used for the drying process are not provided in the section Drying experiment. Please provide more details about these information.

3. In the sections “Total phenolic content (TPC) and DPPH radical scavenging activity”, “Thiobarbituric acid reactive substances (TBARS)”, and “Fourier transform infrared (FTIR) spectroscopy”, only 20 g (i.e., 10g for Total phenolic content (TPC) and DPPH radical scavenging activity and 10 g for Thiobarbituric acid reactive substances (TBARS)) is not enough for us to consider the robustness of your chemical measurements. You must provide more samples to ensure it.

4. Please provide more details in the section Fourier transform infrared (FTIR) spectroscopy and Color.

5. In the section “Statistical analysis”, please provide the sample size and the sample category for the data analysis.

6. The increase of TPC from the fresh sample to the drying sample is not reasonable in the Figure 3. Please show us more reason or information about it .

Reviewer #8: The manuscript entitled “Physico-chemical aspects of Thai fermented fish viscera, Tai-Pla, curry powder processed by hot air drying and hybrid microwave-infrared drying” investigated the effect of hot air drying (HA) and hybrid microwave-infrared drying (MI) on physico-chemical characteristics of Thai fermented fish viscera, Tai-Pla, curry powder (TCP). The present manuscript requires major revision before considering for the acceptance.

1. Add references in the line 54-64

2. Mention the quantity of each ingredients taken for the preparation of Tai-Pla curry paste (Line 102-104)

3. The authors mentioned that the fresh curry paste was pasteurized at 90oC for 10 min. There is any standard pre-optimized protocol/references available?

4. In case of MI how output power level calculation was done? And what basis the power level of 740, 780, and 810 W was chosen?

5. Elaborate the standard methodology (AOAC) followed to determine moisture, protein, fat, ash, and carbohydrate

6. In line 168, add the description for color value L*, a* and b*

7. In line 207 author have mentioned drying time 70oC and 80 oC had the similar drying time of 120 min. Why so? If there is difference in 10 oC could achieved at 120 min?

8. What would be the final temperature achieved at 740W/70 min, 780W/45 min and 810W/40 min?

9. Rewrite the whole section in results part -Free radical scavenging activity, TBARS- precisely with the obtained values and compare with other studies

10. Explain the effect of drying on functional group in FTIR-Rewrite the paragraph

11. Revise the conclusion part as per the obtained results

12. Need to add statistical design or experimental design

13. Fig 1. label the each parts

14. Fig 4 & 5 labelling is inappropriate

**********

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Reviewer #1: No

Reviewer #2: No

Reviewer #3: No

Reviewer #4: No

Reviewer #5: No

Reviewer #6: No

Reviewer #7: No

Reviewer #8: Yes: VENKATACHALAPATHY N

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21 May 2021

Response to Reviewers

All points raised by the reviewers were carefully addressed and answered point-by-point. A revision was made in highlighted red fonts. The revised manuscript was carefully prepared to meet PLOS ONE's style requirements.

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Ans: The revised manuscript was carefully prepared to meet PLOS ONE's style requirements.

2. We understand that you purchased ingredients from local markets for this study. In your Methods section, please provide additional regarding the source of this material. Please provide the geographic coordinates and names of the purchase locations (e.g., stores, markets), if available, as well as any further details about the purchased items (e.g., lot number, source origin, description of appearance) to ensure reproducibility of the analyses.

Ans: The geographic coordinate and name of the purchase location were given. The details about the purchased items were also given.

3. Thank you for stating the following in the Financial Disclosure section:

"This research was financially supported by Research and Researchers for Industries (RRI) program and Shaw Processing Food Co. Ltd. [Grant No. MSD61I0053] and the new strategic research project (P2P), Walailak University, Thailand. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript."

We note that you received funding from a commercial source: Shaw Processing Food Co. Ltd.

Please provide an amended Competing Interests Statement that explicitly states this commercial funder, along with any other relevant declarations relating to employment, consultancy, patents, products in development, marketed products, etc.

Within this Competing Interests Statement, please confirm that this does not alter your adherence to all PLOS ONE policies on sharing data and materials by including the following statement: "This does not alter our adherence to PLOS ONE policies on sharing data and materials.” (as detailed online in our guide for authors http://journals.plos.org/plosone/s/competing-interests). If there are restrictions on sharing of data and/or materials, please state these. Please note that we cannot proceed with consideration of your article until this information has been declared.

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Ans: We got the co-funding grant from Research and Researchers for Industries (RRI) program and Shaw Processing Food Co. Ltd. [Grant No. MSD61I0053]. The main funding came from the RRI program and a partial funding from a commercial source: Shaw Processing Food Co. Ltd. This grant aims to encourage the graduate student to work with industry and the results can be published as signed in the grant contract. Therefore, we confirmed that this does not alter our adherence to all PLOS ONE policies on sharing data and materials. The Competing Interests Statement "This does not alter our adherence to PLOS ONE policies on sharing data and materials.” was added in the cover letter.

Reviewer #1: The manuscript entitled Physico-chemical aspects of Thai fermented fish viscera, Tai-Pla, curry powder processed by hot air drying and hybrid microwave-infrared drying finds to be interesting. Further some major revision is required before final acceptance.

General Comments:

1. In this work, author used hybrid MI drying method for drying of Thai fermented fish viscera, Tai-Pla, curry powder (TCP). But throughout the manuscript only microwave power level varies and nothing is mentioned about Infra-red drying. In Fig 1 also there is no provision of Infra-red heating, then how this method become hybrid drying.

Ans: In this study, the infrared heater is fixed at 500 watts. The Fig. 1 was revised to indicate the presence of infrared heater in the oven. So, the use of an adjustable power microwave heating (740 W, 780 W, and 810 W) coupled with a fixed power infrared heating (500 W) can be recognized as a hybrid drying. The information regarding this issue was added in the Abstract and Materials and methods.

2. There is no comparison of energy consumption of HA and MI in the manuscript.

Ans: The specific energy consumption of HA and MI was given in Fig. 2c. The Method and the Discussion regarding the energy consumption were also given.

3. The applied Drying time in MI method is higher range (above 40 min). Normally drying in microwave upto 10 min maximum.

Ans: In this study, the power of microwave was set at 740-810 W and the target moisture content was around 12%. In comparison with previous report using the same MI machine, the drying time for stink bean seed (600 W/target moisture content of 15%) was about 6-10 h (Nisoa M, Wattanasit K, Tamman A, Sirisathitkul Y, Sirisathitkul C. Microwave drying for production of rehydrated foods: A case study of stink bean (Parkia speciosa) seed. Appl Sci. 2021;11:2918.). So, the drying time using MI was dependent on the applied power, target moisture content, and the type and nature of raw material.

Specific Comments:

1. In Introduction give more recent references related with MI drying method.

Ans: The recent references related with MI drying method were added.

2. Line no. 120-121, On what basis final moisture content 12% and water activity 0.6 was chosen. Whether its safe moisture content of storage of TCP. If yes, then give reference for it.

Ans: In Thailand, the water activity (aw) of <0.65 and the moisture content of <13% (w/w) are the standards set for seasoning powder [15]. To comply with this standard and to ensure the food safety, the drying was proceeded until the final moisture content was reached �12% with the aw of �0.6. This was given in the “Drying experiment”.

3. What is the sample size dry in both the method?

Ans: The sample size was given for both methods. “The fresh samples (1000 g) were uniformly spread in the tray with the thickness of 0.5 cm and subjected to dry using a traditional HA drier (tray drier) or an MI drier.”

4. In this research work, TCP dried at three different temperatures 60, 70 and 800C. On what basis this range was selected. Please mentioned in the manuscript.

Ans: From the report of our colleges using the same HA (tray drier), they suggested the maximum temperature for drying whole stink bean at 70�C. They stated that if the temperature is higher than 70�C, the dehydration rate may be too high for that product. So, in this study we tried to vary the drying temperatures using the basis of 70�10�C (60, 70, and 80�C) for TCP. This was mentioned in the manuscript “It has been reported that the maximum temperature used for drying the whole stink bean (Parkia speciosa) seed was at 70�C using the same HA (tray drier) [5]. So, in this study, the drying temperatures were varied on the basis of 70�10 �C which were 60 �C, 70 �C, and 80 �C for TCP.”

5. Line 152-153; please specify why TBRS is important to study in this research work.

Ans: We stated in the Materials and method that “TBARS analysis is the most widely used method to determine the secondary lipid oxidation products [20]. Due to the presence of some lipid content in the Tai-Pla curry paste, the lipid oxidation can be taken placed during drying. Herein, the lipid oxidation was monitored using the TBARS.”

6. Line 162-163, why author want to use FTIR in this study. Please mention? On what basis the FTIR range was selected?

Ans: The detail about the FTIR was given in the Materials and methods. “The FTIR spectroscopy is a vibrational spectroscopic technique that can be used to characterize the substances by identifying their functional groups presented [22]. FTIR spectra (400-4000 cm-1 with the resolution of 4 cm-1 at the average of 16 scans) of the TCP were obtained using a horizontal Attenuated Total Reflectance (ATR) Trough plate crystal cell (45° ZnSe; 80 mm long, 10 mm wide and 4 mm thick) (Pike Technology, Inc., Madison, WI, USA) equipped with a Bruker Model Vector 33 FTIR spectrometer (Bruker Co., Ettlingen, Germany) at room temperature. Analysis of spectral data was carried out using the OPUS 3.0 data collection software program.”

7. Line 170-171, How Browning Intensity influence the product quality. Please specify

Ans: The effect of browning intensity on the product quality was given and this section was revised accordingly. “The browning intensity can be used to monitor the formations of the intermediate and final products of the Maillard reaction in the TCP. The presence of the Maillard reaction products (MRPs) affected both color and antioxidant activity of the food products [23]. The UV absorbance at 294 nm (A294) was often used to indicate the intermediate MRPs while the final MRPs was monitored by the absorbance at 420 nm (A420)…..”

Reviewer #2: The paper ”Physico-chemical aspects of Thai fermented fish viscera, Tai-Pla, curry powder processed by hot air drying and hybrid microwave-infrared drying” aimed to comparatively investigate the effect of hot air drying (HA) and hybrid microwave-infrared drying (MI) on physico-chemical characteristics of Thai fermented fish viscera, Tai-Pla, curry powder (TCP). The subject is of great interest for the industrials willing to reinterpret the traditional technology into convenient innovative alternatives.

There are few aspects that need attention from the authors.

Indicate how many replicates were taken in this study.

Ans: Three replications were taken in this study as stated in the Statistical analysis section. “A completely randomized design was used in this study. The data were expressed as means ± standard deviations (SD) of three replications (n =3) for all analyses.”

When a product is designed for further industrial scale production, the readers are willing to see the sensory analysis results. Did you perform sensory data or are you able to perform it and include the results in this manuscript?

Ans: Thank you for your invaluable suggestion. This study aimed to explore the physico-chemical aspects of the TCP. We will do the sensory analysis in the future. However, from our own observation, the sensory aspects (odor, flavor, taste) of the TCP were remained unchanged. The recommendation regarding this issue was added in the Conclusion. “Further studies are recommended to determine the sensory quality, flavor profile, and storage stability of the TCP.”

For better point the advantage of MI treatment, authors should perform also some stability tests and specify the shelf life of the obtained powders. Is it possible to perform also the microbiological analysis? At least the common ones ?

Ans: Thank you for your invaluable suggestion. We will do the stability test including the microbiological analysis in the future. However, the recommendation regarding this issue was added in the Conclusion. “Further studies are recommended to determine the sensory quality, flavor profile, and storage stability of the TCP.”

You can also insert a discussion related to the influence of fat content to product stability.

Ans: The discussion regarding the fat content to product stability, especially oxidative stability, was included.

“TBARS analysis is the most widely used method to determine the secondary lipid oxidation products [20]. Due to the presence of some lipid content in the Tai-Pla curry paste, the lipid oxidation can be taken placed during drying. Herein, the lipid oxidation was monitored using the TBARS.”

“The presence of fat in the product may cause the oxidative instability during storage.”

“The effect of drying methods on the evolution of TBARS of TCP is shown in Fig. 3c. TBARS increased with increasing HA temperature (p<0.05). This was attributed to the fact that unsaturated lipids are easily oxidized at higher temperature.”

Please argument the necessity of antioxidant activity (AA) and correlate the your AA results to total phenolic content, not just simply say they correlate based on literature analysis, because it is not always true. So please check this with your results.

Ans: A revision was made. “Although, antioxidative phenolic compounds varied depending on drying conditions (Fig. 3a), similar radical scavenging activities of all sample were observed (Fig. 3b). This behavior could be related to the degradation of certain original antioxidative compounds and simultaneous formation of new antioxidants (e.g. Maillard reaction products (see browning index in Table 2) which may decrease the antioxidative capacity [54] or promote such activity [55]. A correlation between antioxidant activity and TPC has been reported during food dehydration [56]. Therefore, the net oxidative status of TCP was governed by the rate of degradation and formation of antioxidants during drying. However, numerous factors, such as drying method, type of extraction solvent, antioxidant assays, and interactions of several antioxidant reactions have been reported to affect the TPC and antioxidant activity of dried foods, which somehow resulted in a conflict data [42].”

Why did authors decide not to include also the individual phenolic compound analysis?

Ans: Thank you very much for you invaluable suggestion. We will try to detect the phenolic profile of our product in the future. At present, we have no equipment to do it. However, major phenolic compounds found in the raw materials for production of TCP were added using the previous reports from the literatures. “The presence of the TPC in the product may have helped to improve the oxidative stability during storage. The major phenolic compounds composed in the raw materials for production of TCP have been reported intensively, for instance, galangin in galangal [46], capsaicin in red chili pepper [47], allicin in garlic [48], isoorientin in lemongrass [49], piperine in black pepper [50], β-pinene and limonene in kaffir lime leaves [51], curcumin in turmeric [52] and quercetin in shallot [53].”

The discussion on FTIR results is poor. Please improve this part otherwise is not useful for the readers.

Ans: The discussion on FTIR was intensively revised (See FTIR spectra).

Reviewer #3: The manuscript deals with physico-chemical aspects of Thai fermented fish viscera, Tai-Pla, curry powder processed by hot air drying and hybrid microwave-infrared drying.

The English language must be revised.

Ans: English language was rechecked and polished.

Please separate values from units, e.g. “60 ºC” not “60ºC”.

Ans: Done throughout the text.

Please number all sections.

Ans: We prepared the manuscript following the PLOS ONE style templates.

Abstract

This section is vague. Please present your main results.

Ans: Abstract was revised intensively.

Introduction

The topics must be better linked.

Ans: The Introduction was revised and all topics in the Introduction were linked.

Materials and methods

Line 107- “Thereafter, all ingredients with the specified proportion were mixed and coarsely ground for 10 min using a grinder (MK 5087M Panasonic Food Processor, Selangor Darul Ehsan, Malaysia) to obtain fresh curry paste.”??amounts used of each ingredient??

Ans: The mass percentage of the main ingredients used for production of Tai-Pla curry was given.

Line 109- “The fresh curry paste was then pasteurized at 90ºC for 10 min.”??how was the sample pasteurized?amount used??

Ans: The information was added. “The fresh curry paste (1,000 g) was then pasteurized at 90 �C for 10 min [14] in a controlled temperature Hanabishi HGP160S electric pan (Hanabishi Electric Co., Ltd., Bangkok Thailand) under continuous stirring.”

Line 114- “Two drying techniques including HA and MI were used to prepare Tai-Pla curry powder. The fresh samples were uniformly spread in the tray with the thickness of 0.5 cm and subjected to dry using a traditional HA drier (tray drier) or an MI drier. The MI drier used in this study was developed by the Center of Excellence in Plasma Science and Electromagnetic Waves, Walailak University (Fig. 1). The MI output powers were adjusted to 740, 780, and 810 W in order to meet the drying temperature of 60ºC, 70ºC, and 80ºC as done by a traditional HA drier.”??used conditions??tray dryer air speed??

Ans: It was changed to “Two drying techniques including HA and MI were used to prepare TCP. The fresh samples (1,000 g) were uniformly spread in the tray with the thickness of 0.5 cm and subjected to dry using a traditional HA drier or an MI drier. The HA drier was operated in a DT 20S tray drier (4,500 W, Owner Foods Machinery Co., Ltd., Bangkok, Thailand) at 60 �C, 70 �C, and 80 �C with the circulation speed of 1 m/s. It has been reported that the maximum temperature used for drying the whole stink bean (Parkia speciosa) seed was at 70�C using the same HA (tray drier) [5]. So, in this study, the drying temperatures were varied on the basis of 70�10 �C which were 60 �C, 70 �C, and 80 �C for TCP. The MI drier used in this study was developed by the Center of Excellence in Plasma Science and Electromagnetic Waves, Walailak University (Fig. 1). The microwave output powers were adjusted to 740 W, 780 W, and 810 W with a fixed power of infrared heating (500 W) in order to meet the drying temperature of 60 �C, 70 �C, and 80 �C as done by a traditional HA drier.”

Line 115- “The fresh samples were uniformly spread in the tray with the thickness of 0.5 cm and subjected to dry using a traditional HA drier (tray drier) or an MI drier.”??amount used??

Ans: The fresh samples (1,000 g) were uniformly spread in the tray with the thickness of 0.5 cm and subjected to dry using a traditional HA drier or an MI drier.

Line 123- “The obtained TCP was packed in an aluminum foil laminated bag to prevent moisture adsorption and kept in an auto desiccator at room temperature for 24 h.”??packaging dimensions??

Ans: It was “10 cm � 15 cm”. It was added in the text already. “The obtained TCP was packed in an aluminum foil laminated bag (10 cm � 15 cm) to prevent moisture adsorption and kept in an auto desiccator at room temperature for 24 h.”

Line 167- “Colorimetric values of the samples were measured using a Hunterlab colorimeter (Hunter Assoc. Laboratory; VA, USA). The L*, a*, and b* values were recorded.”???illuminant used??ºobserver??calibration??

Ans: It was detailed “Colorimetric values of the samples were measured using a Hunterlab colorimeter (Hunter Assoc. Laboratory; VA, USA) with 10 standard observers and illuminant D65. The instrument was calibrated to a white and black standard. The L* (lightness), a* (redness/greenness), and b* (yellowness/blueness) values were recorded.”

Line 184- “Wettability”??or solubility in water??

Ans: It was a wettability, according to the references (no. 26 and 27).

Results and discussion

This section has lack of depth and must be improved.

Ans: This section was intensively revised.

Line 251- “From the results, TCP contributed not only for taste and flavor enhancers, but also played a part in an extra-source of nutrients.”?? Aroma???flavor??measured??

Particle size??

Ans: The aroma, flavor, and particle size were not determined in this study. Based on the nutrient compositions in the proximate analysis, it was changed to “From the results, TCP can be classified as a nutritive flavoring agent.”

Figure 1- Please define each component.

Ans: Fig 1 was revised and labelled.

Figure 4- Please add wavenumber in each peak.

Ans: Wavenumbers were added in each peak.

Conclusion

Line 451- “Results suggested that TCP prepared by MI method showed the superior physico-chemical characteristics to the traditional HA.”???superior??in which results??

Ans: Conclusion was revised intensively. (see Conclusion).

References

37 references have more than 5 years. Please update your list of references.

Ans: The references were updated.

Reviewer #4: In the present study titled “Physico-chemical aspects of Thai fermented fish viscera, Tai-Pla, curry powder processed by hot air drying and hybrid microwave-infrared drying” is presented in detailed a well elaborated research which evaluated the properties of Thai fermented fish viscera, Tai-Pla, curry powder (TCP) dryed on different approaches: hot air drying (HA) and hybrid microwave-infrared drying (MI). The properties of both fresh and dried powders were evaluated by proximate composition, aw, total phenolic content, DPPH radical scavenging activity, TBARS, FT-IR spectroscopy and physical characteristics (colour, browning intensity, bulk density, wetability).

The research led to the identification of the best drying procedure of the fresh pasta with the desired properties, namely the hybrid microwave-infrared drying (MI).

In general the data are strong, and convincingly shows that the hybrid microwave-infrared drying (MI) approach could be used as a drying procedure to obtain good quality food products. The manuscript is well written, concise and the appropriate analyses are performed.

Overall, this is a well performed study that I consider that is important and represent a new strategy to conveniently obtain Thai fermented fish viscera, Tai-Pla, curry powder (TCP) with good quality characteristics.

The authors need to address the below comments to strengthen the quality of the manuscript:

1. Please insert the characterization methods used in the present study in the Abstract (e.g. FT-IR, DPPH assay).

Ans: The DPPH and FTIR results were added in the Abstract.

2. Please replace the phrases from line 37 and 455: „Therefore, MI was a promising drying technique to reduce the drying time and improve the overall quality of TCP.” By „Therefore, MI is a promising drying technique to reduce the drying time and improve the overall quality of TCP.”

Ans: Done.

3. In the preparation method of the pasta (in Materials and methods) please include the mass percentage of the main ingredients used to obtain the product.

Ans: The mass percentage of the main ingredients used for production of Tai-Pla curry was given.

Reviewer #5: In this manuscript, the authors compared the effect of hot air drying (HA) and hybrid microwave-infrared drying (MI) on physico-chemical characteristics of Thai fermented fish viscera, Tai-Pla, curry powder (TCP). The data in this article is not solid and well analyzed. Besides, the article doesn't present the application superiority of MI thoroughly, in other words doesn't fit with the average originality found of PLOS ONE. I recommend to not publish the article.

Ans: All data obtained scientifically from the experiments. The research justification, background, experimental design, and methodology were optimally stated. This research was the first report on the production of TCP using MI in comparison with HA which can guarantee the originality of the work.

Major comments

The data succeed to show the different effect of HA and MI, but the experiments are too superficial, as well the test of antioxidant activities. According to the introduction, the application of TCP mainly gives the Thai food a special flavor and aroma, which should be concerned in this article because the HA and MI processing would change the flavor of TCP. In the overall the text is well written but is superficial and is out of the journal standards.

Ans: Thank you very much for your invaluable suggestion. Regarding the flavor and aroma, we will do the sensory analysis in the future. However, from our own observation, the sensory aspects (odor, flavor, taste) of the TCP were remained unchanged. The recommendation regarding this issue was added in the Conclusion.

Detailed comments

1. The authors developed a hybrid microwave-infrared drier but the schematic diagram (Fig. 1) didn’t show how the hybrid microwave-infrared drier works.

Ans: Fig 1 was revised and labelled.

2. SEM is recommended to show the influence of these physical processes on tissues of TCP during drying.

Ans: Thank you for your invaluable suggestion. Unfortunately, the SEM was not measured in this study. We will do it in the future work. Basically, without SEM, the methods used in this study can characterize the physico-chemical properties of the TCP.

3. Line 36: better change “between … to …” to “from … to …”.

Ans: Done.

4. Line 61: “a” should be “its” or deleted.

Ans: Deleted.

5. Line 76-77: Please check this sentence and revise.

Ans: It was changed to “MW is based on dielectric heating by electromagnetic waves and it has several advantages [5].”

6. Line 163: FTIR spectra of fresh samples can’t be found in Figure 4.

Ans: It was changed to “FTIR spectra (400-4000 cm-1 with the resolution of 4 cm-1 at the average of 16 scans) of the TCP were obtained using…….”

7. Line 251-252: Please check this sentence and revise.

Ans: It was changed to “From the results, TCP can be classified as a nutritive flavoring agent.”

8. Line 271: Please check this sentence and revise.

Ans: It was changed to “This was probably due to the evaporation of moisture, leading to the increased TPC concentration in the dried samples.”

9. Line 380-381: Please check this sentence and revise.

Ans: It was changed to “With increasing temperature in HA, L* and b* values decreased but a* value remained constant (Table 2).”

10. Line 411-412: Please check this sentence and revise.

Ans: It was changed to “In addition, MI quickly produced heat and effectively removed water at surface. This can shorten the drying time and hence reduce the degree of browning reactions.”

11. Line 421: “P<0.05” is wrong.

Ans: It was changed to “p>0.05”.

12. Please replace “governed” with another word in line 30, line 307, and line 441.

Ans: It was changed to “influenced”, “affected”, and “influenced”, respectively.

13. Line 51: “cuisin” should be “cuisine”.

Ans: Done.

14. Please add a comma before the second subject in line 215-217, line 217-218, and line 402-403.

Ans: Done.

15. Please mark “a, b, c” in the Figure 3 according to “Fig. 3a, Fig. 3b, Fig. 3c” in line 302-313.

Ans: Done.

Reviewer #6: In this manuscript, Choopan et al. compared the hot air (HA) and hybrid microwave-infrared (MI) drying ways on the Physico-chemical characteristics of TCP. They have demonstrated that MI drying with a condition of 810 W for 40 min effectively reduced the drying time by five-fold. They also showed that MI-dried TCP had the lowest browning index, the highest lightness, higher phenolic content, and lower TBARS, indicating improved overall quality. The manuscript has convincing data to support their conclusion.

The concern I have is whether MI drying affects the flavor of TCP. Is it possible to evaluate the flavor?

Ans: Thank you for your invaluable suggestion. This study aimed to explore the physico-chemical aspects of the TCP. We will do the sensory analysis in the future. However, from our own observation, the sensory aspects (odor, flavor, taste) of the TCP were remained unchanged. The recommendation regarding this issue was added in the Conclusion.

Reviewer #7: The paper presents an application of Physico-chemical aspects for Thai fermented fish viscera, Tai-Pla, curry powder processed. It is a topic of interest to the researchers in the related areas but this paper needs improvement before acceptance for publication. My detailed comments are as follows:

1. The sample source and size are both important for this paper, and please provide more details about the sample information in the section Tai-Pla curry paste preparation.

Ans: The mass percentage of the ingredients used for production of Tai-Pla curry and the sample information were given.

2. Instrument model and manufacturer used for the drying process are not provided in the section Drying experiment. Please provide more details about these information.

Ans: Instrument model and manufacturers were given.

3. In the sections “Total phenolic content (TPC) and DPPH radical scavenging activity”, “Thiobarbituric acid reactive substances (TBARS)”, and “Fourier transform infrared (FTIR) spectroscopy”, only 20 g (i.e., 10g for Total phenolic content (TPC) and DPPH radical scavenging activity and 10 g for Thiobarbituric acid reactive substances (TBARS)) is not enough for us to consider the robustness of your chemical measurements. You must provide more samples to ensure it.

Ans: The sample sizes were used following the published standard methods from references. Also, the analyses were run in triplicate. So, measurements were reliable and repeatable.

4. Please provide more details in the section Fourier transform infrared (FTIR) spectroscopy and Color.

Ans: Done.

5. In the section “Statistical analysis”, please provide the sample size and the sample category for the data analysis.

Ans: Done. (See Statistical analysis section).

6. The increase of TPC from the fresh sample to the drying sample is not reasonable in the Figure 3. Please show us more reason or information about it .

Ans: The possible reasons were given.

“A slight increment in TPC was observed in TCP compared to the fresh sample (Fig. 3a). This was probably due to the evaporation of moisture, leading to the increased TPC concentration in the dried samples.”

“The increment of phenolic compounds after drying could involve in the cell wall breakdown and disruption by heating effects and consequently eases the release and extractability of bound phenolic compounds [39]. Generally, plant phenolic compounds can be classified into two major forms, free and bound [41].”

Reviewer #8: The manuscript entitled “Physico-chemical aspects of Thai fermented fish viscera, Tai-Pla, curry powder processed by hot air drying and hybrid microwave-infrared drying” investigated the effect of hot air drying (HA) and hybrid microwave-infrared drying (MI) on physico-chemical characteristics of Thai fermented fish viscera, Tai-Pla, curry powder (TCP). The present manuscript requires major revision before considering for the acceptance.

1. Add references in the line 54-64

Ans: References were added.

2. Mention the quantity of each ingredients taken for the preparation of Tai-Pla curry paste (Line 102-104)

Ans: Quantity of each ingredients were mentioned.

3. The authors mentioned that the fresh curry paste was pasteurized at 90oC for 10 min. There is any standard pre-optimized protocol/references available?

Ans: Reference was added.

4. In case of MI how output power level calculation was done? And what basis the power level of 740, 780, and 810 W was chosen?

Ans: The microwave output powers were adjusted to 740 W, 780 W, and 810 W with a fixed power of infrared heating (500 W) in order to meet the drying temperature of 60 �C, 70 �C, and 80 �C as done by a traditional HA drier.

5. Elaborate the standard methodology (AOAC) followed to determine moisture, protein, fat, ash, and carbohydrate

Ans: The standard AOAC’s methods [17] were used for proximate composition analysis including moisture (AOAC method number 950.46), crude protein (AOAC method number 928.08), fat (AOAC method number 963.15), ash (AOAC method number 920.153), fiber (AOAC method number 962.09), and carbohydrate. Carbohydrate content was estimated by difference (1).

Carbohydrate content (%)=100-[moisture+protein+fat+ash+fiber] (1)

6. In line 168, add the description for color value L*, a* and b*

Ans: It was changed to “The L* (lightness), a* (redness/greenness), and b* (yellowness/blueness) values were recorded.”

7. In line 207 author have mentioned drying time 70oC and 80 oC had the similar drying time of 120 min. Why so? If there is difference in 10 oC could achieved at 120 min?

Ans: We apologized for the mistake. From the drying curve, the moisture content of sample dried using HA at 80 �C reached the target moisture content at 100 min. So, the drying time of 80 �C was corrected to 100 min and the Results and discussion was revised accordingly.

“TCP dried with HA at 70 �C and 80 �C had the drying time of 120 and 100 min, respectively (Fig. 2a), which was shorter than that did at 60°C for 1.75 and 2.10 folds, respectively.”

“However, HA at 70 �C and 80 �C showed a negligible different in drying rate (k � 0.3436 vs 0.3907) (Fig.2a).”

8. What would be the final temperature achieved at 740W/70 min, 780W/45 min and 810W/40 min?

Ans: We stated earlier that the final temperature achieved at 740W/70 min, 780W/45 min and 810W/40 min was 60, 70, and 80�C. “The microwave output powers were adjusted to 740 W, 780 W, and 810 W with a fixed power of infrared heating (500 W) in order to meet the drying temperature of 60 �C, 70 �C, and 80 �C as done by a traditional HA drier.”

9. Rewrite the whole section in results part -Free radical scavenging activity, TBARS- precisely with the obtained values and compare with other studies

Ans: These parts were revised.

10. Explain the effect of drying on functional group in FTIR-Rewrite the paragraph

Ans: The FTIR section was rewritten.

11. Revise the conclusion part as per the obtained results

Ans: The Conclusion was revised intensively.

12. Need to add statistical design or experimental design

Ans: Done. (see Statistical analysis section)

13. Fig 1. label the each parts

Ans: Fig 1 was revised and labelled.

14. Fig 4 & 5 labelling is inappropriate

Ans: Fig 4 and 5 were revised.

Submitted filename: Response to Reviewers.docx

Click here for additional data file.

15 Jun 2021

Physico-chemical aspects of Thai fermented fish viscera, Tai-Pla, curry powder processed by hot air drying and hybrid microwave-infrared drying

PONE-D-21-10233R1

Dear Dr. Panpipat,

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #1: All comments have been addressed

Reviewer #3: All comments have been addressed

Reviewer #4: All comments have been addressed

Reviewer #6: All comments have been addressed

Reviewer #7: All comments have been addressed

Reviewer #8: All comments have been addressed

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

Reviewer #3: Yes

Reviewer #4: Yes

Reviewer #6: Yes

Reviewer #7: Yes

Reviewer #8: Yes

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3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #3: Yes

Reviewer #4: Yes

Reviewer #6: Yes

Reviewer #7: Yes

Reviewer #8: Yes

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Reviewer #1: Yes

Reviewer #3: Yes

Reviewer #4: Yes

Reviewer #6: Yes

Reviewer #7: Yes

Reviewer #8: Yes

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Reviewer #1: Yes

Reviewer #3: Yes

Reviewer #4: Yes

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Reviewer #7: Yes

Reviewer #8: Yes

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Reviewer #6: In the revised manuscript, the authors addressed all my questions. I have no more concerns and suggest accepting the paper for publication.

Reviewer #7: (No Response)

Reviewer #8: (No Response)

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Reviewer #8: Yes: VENKATACHALAPATHY NATARAJAN

18 Jun 2021

PONE-D-21-10233R1

Physico-chemical aspects of Thai fermented fish viscera, Tai-Pla, curry powder processed by hot air drying and hybrid microwave-infrared drying

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