Southern-style Pad Thai sauce: From traditional culinary treat to convenience food in retortable pouches.

Pad Thai, a Thai dish of stir-fried rice noodles and other ingredients, is one of the culinary heritages of Thailand. In the southern region of Thailand, Pad Thai has different characteristics from other areas because coconut milk and curry paste are used to produce a thick sauce prior to cooking. To commercially distribute this uniquely local culinary treat in a convenient form all over Thailand and other export markets, a shelf-stable sauce using heat sterilization should be developed. Retort processing technology with retort pouches can be used for this purpose. However, phase separation and lipid oxidation can occur and subsequently reduce the overall quality of retorted southern-style Pad Thai sauce. The application of an appropriate stabilizer and antioxidant can be used to prevent such problems. Thus, the objective of this study was to investigate the effects of stabilizers and antioxidants on the stability of retorted southern-style Pad Thai sauce. Southern-style Pad Thai sauce was prepared according to the traditional recipe in the presence of different stabilizers (2.3% potato starch (PS) + 0.1% xanthan gum (XG), 0.5% soy lecithin (LT), and 4% whey protein isolate (WPI)) and antioxidants (500 mg/kg ascorbyl palmitate + 500 mg/kg α-tocopherol (As + Toc), 100 mg/kg ethylenediaminetetraacetic acid (EDTA), 200 mg/kg butylated hydroxytoluene (BHT), and As+Toc+EDTA+BHT (mixed antioxidants)). Samples were packed in retort pouches and processed in a retort at 121°C with an F0 value of 3.57 min. Results showed that the retorted southern-style Pad Thai sauce can be stabilized by 2.3% PS plus 0.1% XG in combination with mixed antioxidants. PS and XG helped stabilize the sauce with a desirable viscosity, water holding capacity, and color without any creaming layer and negative effect on sensory properties. Mixed antioxidants improved the oxidative stability of the retorted sauce by retarding the changes in the peroxide value and color during retorting. Consequently, the processing of southern-style Pad Thai sauce in retortable pouches could increase the market demand for this traditional product due to its convenience and ready-to-use features.

Introduction

Traditional foods represent the cultural heritage of a particular region. The ingredients and preparation methods of traditional foods can vary considerably depending on the cultural richness of a region [1, 2]. Pad Thai is a popular Thai dish of stir-fried rice noodles and other seasonings [3]. The style of Pad Thai in Southern Thailand differs from other areas because coconut milk and curry paste are used to produce a thick sauce prior to adding rice noodles and other condiments. Especially in the Pak Phanang District of Nakhon Si Thammarat Province in Thailand, coconut milk, nipa palm sugar, salt, shrimp paste, and chili paste are used to produce an emulsion-based sauce leading to the special flavor of this southern-style Pad Thai, so-called Pak Phanang Mee Pad (spicy stir-fried rice noodles with coconut milk) (Fig 1A). This product symbolizes the Pak Phanang River Basin of Nakhon Si Thammarat, and it is a specialty dish in Southern Thailand which can attract the gastronomic tourism of this area. Accordingly, the Pak Phanang Mee Pad fair is organized annually by the Tourism Authority of Thailand.

Fig 1

Southern-style Pad Thai, so-called Pak Phanang Mee Pad (spicy stir-fried rice noodles with coconut milk) (a), preparation of southern-style Pad Thai sauce (b), retortable pouch used to fill the product (c), pilot-scale horizontal stationary retort system (d), monitoring the temperatures in the retort and pouch center with thermocouples (e), and appearance of the final retorted sauce (f).

In order to commercially distribute this uniquely local culinary treat all over Thailand and other export markets, the production of shelf-stable sauce using heat sterilization should be developed. Retort processing technology, one of the thermal processing methods, can be used to achieve this goal. Nowadays, retorted foods in pouches are acceptable worldwide due to their lighter, more appealing, and more convenient end use and longer shelf-life [4]. However, based on the emulsion-based system that produces other emulsified sauces [5, 6, 7, 8, 9], the retort process of southern-style Pad Thai sauce may encounter phase separation and lipid oxidation during retorting. It was reported that the retorting of emulsion at high temperatures (121°C for 16 min) resulted in lipid oxidation and an increase in coalescence of oil droplets [5]. Lipid oxidation is a critical issue in regard to emulsified foods because it is considered as an interfacial phenomenon [10, 11]. The large interfacial area of emulsion provides numerous sites for lipid oxidation to occur [10, 11]. Also, the separation of the solid and liquid phase is one of the main problems associated with the quality of sauces [6]. Polysaccharides and protein hydrocolloids are commonly used as stabilizers of food due to their thickening and gelling properties, which can capture and hold moisture, inhibit its evaporation from foodstuffs, stabilize emulsions, and control rheology [7]. The type and concentration of the applied hydrocolloids and their combinations, the use of the final product, and consumer demands influence the efficiency of hydrocolloids in food products [7]. Whey proteins can play important roles as an emulsifier and stabilizer in heat-treated emulsion [8]. Polysaccharides (e.g. xanthan gum and modified starch) were found to significantly improve emulsion stability against creaming during heat treatment [5, 9, 12]. In addition, lipid-based stabilizers (e.g. commercial lecithins) increased the heat stability of emulsions [13]. Generally, oxidation in emulsions can occur via a free radical route, and it can be triggered by permeation of free radicals generated at the emulsion interface [14]. The addition of free radical scavenging antioxidants is one of the practical strategies to prevent lipid oxidation in food emulsions [15]. Butylated hydroxytoluene (BHT) and ethylenediaminetetraacetic acid (EDTA) were authorized as synthetic antioxidants for use in food [16]. Antioxidant synergism between tocopherols and ascorbyl palmitate in food products has been reported by Bruun-Jensena et al. [17] and Cort [18]. Thus, free radical scavengers and other antioxidants can be used to retard oxidation in southern-style Pad Thai sauce. Therefore, the objective of this study was to determine the effects of stabilizers and antioxidants on the overall quality of retorted southern-style Pad Thai sauce.

Material and methods

Food additives and chemicals

Potato starch (PS) was purchased from Continental Food Co., Ltd. (Bangkok, Thailand). Xanthan gum (XG), soy lecithin (LT), whey protein isolate (WPI), EDTA, BHT, α-tocopherol and ascorbyl palmitate were purchased from Chemipan Corporation Co., Ltd. (Bangkok, Thailand). Other chemicals used for analyses e.g. potassium iodide, sodium thiosulfate, chloroform and acetic acid were obtained from Sigma Aldrich Co. (St. Louis, MO, USA).

Production of southern-style Pad Thai sauce

Southern-style Pad Thai sauce was prepared using a traditional Thai process. The ingredients for the sauce consisted of 34.72% water, 17.36% coconut milk, 16.67% sugar, 13.89% nipa palm sugar, 10.42% red onion, 5.03% chili paste, 1.56% shrimp paste and 0.35% salt. All ingredients used in the preparation of the sauce were obtained from a local market in Thasala, Nakhon Si Thammarat. To prepare the sauce, all ingredients were blended and stir-fried in an open pan (90-100°C) for approximately 12 min (Fig 1B).

Effects of stabilizers on retorted southern-style Pad Thai sauce

To study the effects of stabilizers on retorted southern-style Pad Thai sauce, different stabilizers including 2.3% PS plus 0.1% XG, 0.5% LT, and 4% WPI were separately added at the stir-frying step during the preparation of the sauce to obtain four different sauces–namely, the control (without stabilizer), sauce with PS + XG, sauce with LT, and sauce with WPI. The concentrations of PS and XG were previously optimized using response surface methodology (RSM), and the optimal ratio (2.3% PS + 0.1% XG) was used in this study in comparison with a reported lipid-based stabilizer (0.5% LT, [19]) and protein-based stabilizer (4% WPI, [8]).

For the retort process for sterilization, pre-fabricated multilayer laminated retortable pouches (12 μm polyester/12 μm aluminium foil/75 μm cast polypropylene/15.0 μm biaxially-oriented nylon) with a dimension of 14 cm × 18 cm × 4 cm were used to fill the product (Fig 1C). Each pouch was filled manually with 200 grams of the product, and then air from the head space was squeezed out so that the pouches can be sealed using a Henkovac type H 1502 packaging machine ('s-Hertogenbosch, the Netherlands). The filled and sealed pouches were kept on perforated trays and subsequently transferred into a retort vessel. A pilot-scale horizontal stationary retort system (Owner Foods Machinery Co., Ltd., Bangkok, Thailand) was used (Fig 1D). For thermal processing, the retort temperature was kept constant at 121°C and the processing time was 54 min with an F0 value of 3.57 min calculated using Ball’s method [20]. Pressure was maintained at 1.83 kg/cm2 throughout the process using a hot water-air mixture. Temperatures in the retort and pouch center were continuously monitored with thermocouples (Fig 1E), and the data was recorded using a data logging device (EVal Flex System, ELLAB, Denmark). The temperature input data was converted into corresponding process lethality values, which were expressed as F0 values [20].

After processing the pouches to the required F0 value, the pouches were cooled rapidly by recirculating water to 45°C. The processed retort pouches were stored in a dry place at room temperature (28-30°C), and the samples were subjected to analyses including chemical (moisture, aw, pH, and peroxide value (PV)), physical (creaming layer, viscosity, and color), and sensory (color, viscosity, odor and overall liking) properties. An optimum stabilizer was selected for further study in combination with antioxidants.

Effects of antioxidants on retorted southern-style Pad Thai sauce

In this study, retorted southern-style Pad Thai sauce was prepared in the presence of 2.3% PS + 0.1% XG as previously described. Different antioxidants including 500 mg/kg ascorbyl palmitate plus 500 mg/kg α-tocopherol (As + Toc), 100 mg/kg EDTA, 200 mg/kg BHT and 500 mg/kg ascorbyl palmitate + 500 mg/kg α-tocopherol + 100 mg/kg EDTA + 200 mg/kg BHT (mixed antioxidants) were incorporated in order to determine their effects on the oxidative and color stabilities of the product. The final concentration of all antioxidants in the mixed antioxidants was the same for each one used separately. The resulting sauces were subjected to analyses for PV and color. The appearance of the resulting sauces is shown in Fig 1F.

Chemical analysis

Determination of moisture content, aw and pH

The AOAC method number 950.46 was used for moisture content analysis in the retorted southern-style Pad Thai sauce [21]. The aw was determined at 25°C using an Aqualab Series 3TE aw meter (Decagon, Pullman, WA, USA). The pH was measured using a calibrated pH meter (Cyberscan 500, Singapore).

Determination of PV

PV was determined according to the method of Low and Ng [22]. The sample (1 g) was treated with 25 mL of a chloroform:acetic acid mixture at 2:3 ratio. The mixture was shaken vigorously, followed by the addition of 1 mL of saturated potassium iodide. The mixture was kept in the dark for 5 min, and 75 mL of distilled water was added. A 0.5 mL starch solution (1%, w/v) was added as an indicator. The PV was determined by titrating the iodine liberated from potassium iodide with a standardized 0.01 N sodium thiosulfate solution. The PV was expressed as milliequivalents of free iodine/kg sample.

Physical analysis

Creaming layer

A creaming layer was determined using a modified method of Agboola et al. [5], where 15 mL of freshly retorted southern-style Pad Thai sauce was poured into a glass cylinder. After 24 h of storage at 25°C, the separated cream layer was read and the creaming layer was calculated according to Eq (1).

Viscosity measurement

Viscosity of the retorted southern-style Pad Thai sauce was determined at 25°C using a Brookfield Viscometer Model RV-DV II Pro+ (Brookfield Engineering Inc, Middleborough, MA, USA) at 200 rpm. A UL cylindrical spindle (ULA-15E) was used along with a Brookfield Ametek UL adapter.

Determination of color

Colorimetric values of the retorted southern-style Pad Thai sauce were obtained in triplicate using a portable Hunterlab Miniscan/EX instrument (10° standard observers, illuminant D65, Hunter Assoc. Laboratory, VA, USA). The instrument was calibrated to a white and black standard. The tristimulus L* (lightness), a* (redness/greenness), and b* (yellowness/blueness) measurement mode was used as it relates to the human eye response to color. Total difference in color (ΔE*) was then calculated according to Eq (2) [23] where ΔL*, Δa*, and Δb* are the differences between the corresponding color parameter of the sample and that of the original control sauce without retorting (L* = 31.98, a* = 19.41, and b* = 38.22).

Sensory analysis

Sensory properties of the retorted southern-style Pad Thai sauce including color, viscosity, odor and, overall liking were evaluated by 30 untrained panelists using the 9-point hedonic scale (a score of 1 = not like very much, 5 = neither like nor dislike, and 9 = like extremely) [24]. The samples were labelled with random three-digit codes. The panelists were instructed to rinse their mouth with water after each sample evaluation, and the order of presentation of the samples was randomized [25].

Statistical analysis

Three different lots for each type of samples were produced (n = 3). Data was presented as means ± standard deviation from triplicate determinations. Statistical comparisons were performed using the One-Way Analysis of Variance (ANOVA) statistics method and by Duncan’s new multiple range test (DMRT) by SPSS software. P-value < 0.05 was considered as significantly different [26].

Results and discussion

Chemical property of retorted southern-style Pad Thai sauce added with different stabilizers

Chemical property of the retorted southern-style Pad Thai sauce added with different stabilizers is shown in Table 1. The control (without added stabilizer) had the highest moisture content and aw (p<0.05). The addition of stabilizers led to a reduction of moisture content and aw. The lowest moisture content was found in the sauce stabilized by WPI, followed by LT and PS mixed with XG. Protein-based stabilizers may undergo gelation upon retorting and the moisture could be entrapped in the gel network. Carbohydrate hydrocolloids (PS and XG) could form a composite gel and hold water as well, whereas lipid-based stabilizers (e.g. lecithin) could bind water with their polar part. Both chemical binding and physical entrapping resulted in the increased water holding capacity of the sauce, which was difficult to evaporate upon the determination of moisture and aw. The pH values of the sauces were altered by the stabilizers. When compared to the control, PS mixed with XG led to a negligible decrease in pH, whereas LT caused a gradual decrease. However, WPI showed a different trend in which it caused an increase in pH of the sauce (p<0.05). The highest pH of the sauce stabilized by WPI would result in the highest water holding capacity of the sauce. The PV of the control sauce was similar to the stabilized sauces with PS mixed with XG and WPI (p>0.05), whereas the sauce stabilized by LT had about 50% reduction in PV. This was probably due to the antioxidant activity of LT. It has been reported that LT can function as an antioxidant in emulsion system [14], vegetable oil [27], and margarine [28]. Pan et al. [14] reported that the antioxidant activity of LT reduced permeation of free radicals across the emulsion interface. XG has been reported to show some antioxidant activity in emulsion [29], but it could not be seen in this study. This was probably due to the different emulsion systems and different concentrations applied.

Table 1

Chemical property of retorted southern-style Pad Thai sauce added with different stabilizers.

Sample	Moisture (%)	aw	pH	PV (meq/kg sample)
Control	52.28±1.35c	0.97±0.00b	4.97±0.01c	19.60±0.05b
PS + XG	45.97±0.28b	0.95±0.00a	4.95±0.00b	23.03±5.78b
LT	45.03±1.77b	0.95±0.00a	4.38±0.01a	9.77±0.04a
WPI	40.17±0.15a	0.95±0.00a	5.01±0.00d	19.65±0.33b

PS + XG = 2.3% potato starch + 0.1% xanthan gum, LT = 0.5% lecithin and WPI = 4% whey protein isolate.

Values are given as mean ± standard deviation from triplicate determinations.

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

Physical property of retorted southern-style Pad Thai sauce added with different stabilizers

Stability, viscosity, and color generally contribute to the acceptance of a sauce [6]. From the results, creaming layer can be observed in the control sauce after retorting (Table 2), whereas no creaming layer was detected in the sauces stabilized by all the stabilizers used (Table 2). With the stabilizers, emulsion should be more stabilized especially by increasing the apparent viscosity of the system (Table 2). The apparent viscosity of the sauce markedly increased when the stabilizers were used. Mixed carbohydrate hydrocolloids (PS + XG) rendered the sauce with the highest viscosity, followed by WPI and LT. Polysaccharides are mainly used as thickening agents for modifying the texture of the food system and enhancing the viscosity of the aqueous phase, which increase emulsion stability, particularly towards creaming [7]. Polysaccharides play important roles as thickening, stabilizing, and gelling agents in many foods. Also, for emulsion systems, polysaccharides are very often used to improve emulsion stability and textural properties [30]. However, the influence of polysaccharides on the stability of emulsions depends on the concentration of polysaccharides and the characteristics of the polysaccharides and emulsion system [9]. XG has relatively high water binding capacity and high viscosity of aqueous solutions [31]. Krystyjan et al. [7] reported the positive effect of combining XG with PS on the texture and acceptability of caramel sauce. Whey proteins can play a dual role in heat-treated emulsion by acting as an emulsifier and stabilizer [8] due to their hydrophobic and hydrophilic segments within the polypeptide chain. LTs are important ingredients in the commercial manufacturing of emulsions. Commercial LT has increased the heat stability of recombined milk systems [13]. The addition of 2.5% (w/w) LT slightly improved the stability of the retorted emulsions [5]. Many studies have been carried out on LTs reporting on their surface active properties, their competition with proteins at oil/water interfaces and their interactions with proteins [5].

Table 2

Physical property of retorted southern-style Pad Thai sauce added with different stabilizers.

Sample	Creaming layer (%)	Apparent viscosity (mPa.s)	Color
			L*	a*	b*	ΔE*
Control	10±1	25±1a	29.52±0.35a	22.22±0.20a	38.32±0.85a	3.88±0.82a
PS + XG	ND#	14167±205d	31.48±0.07c	23.14±0.21b	41.52±0.93b	5.09±0.80b
LT	ND	2738±18b	30.60±0.12b	23.37±0.35b	39.60±0.48a	4.48±0.14ab
WPI	ND	5697±35c	29.33±0.20a	22.23±0.28a	38.21±0.99a	3.91±0.11a

PS + XG = 2.3% potato starch + 0.1% xanthan gum, LT = 0.5% lecithin and WPI = 4% whey protein isolate.

Values are given as mean ± standard deviation from triplicate determinations.

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

ΔE* was calculated relative to the color of the original control sauce without retorting. L*, a*, and b* of the original control sauce without retorting were 31.98±0.06, 19.41±0.20, and 38.22±0.54, respectively.

#ND, not detected.

For the color of the retorted sauces, changes in L* and a* values were found when stabilizers were added (p<0.05). The sauces stabilized by PS + XG and by LT were lighter and redder than the control and the WPI-stabilized sauce. The PS + XG-stabilized sauce had a higher b* value than other sauces (p<0.05). The total difference in color (ΔE*) was calculated relative to the color of the original control sauce without retorting. Only the PS + XG-stabilized sauce had a higher ΔE* than the control. This was due to an increase in L*, a*, and b* of that sauce after retorting. However, the ΔE* of all sauces was quite low, suggesting a similar color of the sauces with and without stabilizers, which was related to the sensory analysis (Table 3). Carbohydrate hydrocolloids may undergo thermal degradation and the monomers released can further react with amine groups in the sauce to form brown pigments via Maillard reaction as indicated by an increase in the a* and b* values. XG was found to be less thermally stable than starch [32]. XG is a heteropolysaccharide made up of the building blocks of D-glucose, D-mannose, and D-glucuronic acid residues [12].

Table 3

Sensory property of retorted southern-style Pad Thai sauce added with different stabilizers.

Sample	Color	Viscosity	Odor	Overall liking
Control	6.87±1.22a	4.09±0.64a	6.47±1.66a	6.47±1.66a
PS + XG	7.03±0.98a	6.60±1.07b	6.77±0.97a	6.80±1.00a
LT	7.00±0.83a	6.83±1.18b	7.00±1.12a	7.13±1.11a
WPI	7.15±1.03a	6.70±1.06b	6.97±1.19a	7.25±1.08a

PS + XG = 2.3% potato starch + 0.1% xanthan gum, LT = 0.5% lecithin and WPI = 4% whey protein isolate.

Values are given as mean ± standard deviation from 30 determinations.

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

Sensory property of retorted southern-style Pad Thai sauce added with different stabilizers

Sensory property of the sterilized southern-style Pad Thai sauce added with different stabilizers is shown in Table 3. Panelists rated all sauces with the same range of scores for color, odor, and overall liking (p<0.05). All samples showed acceptability with the same score for likeness being equal to 7 (moderately like) for all attributes tested except for the viscosity of the control (score ~4). The viscosity scores of the stabilizer-added sauces were higher than the control. The instrumental color values were different among the samples, but the panelists scored the color similarly. Overall, the stabilizers used did not affect the odor and overall liking of the resulting sauces. Thus, the sauce stabilized by PS + XG was selected for further study due to its highest apparent viscosity.

Effects of antioxidants on lipid oxidation and color of retorted southern-style Pad Thai sauce

Effects of the antioxidants on the PV of the retorted southern-style Pad Thai sauce is shown in Fig 2. The control sauce had the highest PV, followed by the sauces with As + Tc, EDTA, BHT, and mixed antioxidants (p<0.05). Generally, lipid oxidation in the emulsion is triggered by permeation of free radicals generated at the emulsion interface [10, 11, 14]. Thus, the polarity of free radicals may influence their movement to the oil or water phase. As + Tc and BHT are lipid-solubilizing antioxidants that may function as free radical scavengers mainly in the oil phase, but their activities may be different to some degree. Ascorbyl palmitate is a potent antioxidant in protecting lipids from peroxidation and is a free radical scavenger [33]. BHT is fairly heat stable and is used in heat-processed foods [34]. Thus, BHT may show a better carry-through property than As + Tc in the retorted southern-style Pad Thai sauce, which can pronounce superior antioxidant activity as indicated by a lower PV compared to As + Tc. EDTA may chelate the metal ion presented in the water phase due to its water solubility and chelating ability. Mixed antioxidants showed the best activity in retarding the PV, which was due to the presence of antioxidants in both the oil and aqueous phases with different modes of action. Synergistic mechanism among antioxidants may lead to the lowest PV.

Fig 2

PV of retorted southern-style Pad Thai sauce added with different antioxidants.

Different letters on the bars indicate significant differences (p<0.05). As + Toc = 500 mg/kg ascorbyl palmitate + 500 mg/kg α-tocopherol, EDTA = 100 mg/kg EDTA, BHT = 200 mg/kg BHT, and Mix = 500 mg/kg ascorbyl palmitate + 500 mg/kg α-tocopherol + 100 mg/kg EDTA + 200 mg/kg BHT.

Effects of antioxidants on the color of the sterilized southern-style Pad Thai sauce is shown in Table 4. Generally, the L*, a*, and b* values changed considerably when the antioxidants were added. The L* value of the sauce with EDTA was higher than the control, whereas other treatments were lower (p<0.05). The lowest L* value was noticeable in the sauce with the mixed antioxidants (p<0.05). For the a* value, the sauces with the antioxidants except for the mixed antioxidants had a higher a* value than the control (p<0.05). The sauce with the mixed antioxidants showed the most similar a* value to the control. For the b* value, the control sauce had a similar b* value to the sauces with As + Toc and EDTA (p>0.05), whereas the b* values slightly decreased in the presence of BHT (p<0.05). The b* value of the sauce markedly decreased when the mixed antioxidants were used (p<0.05). For the color difference (ΔE*) which was calculated against the original control sauce without retorting, it was found that the control sauce had the lowest ΔE* value. Among the samples with the antioxidants, the mixed antioxidants and BHT-added sauces had lower ΔE* values than the As + Toc and EDTA-treated ones. Thus, based on antioxidant activity and color stability, the mixed antioxidants were selected as the optimum antioxidant for the sterilized southern-style Pad Thai sauce.

Table 4

Color of retorted southern-style Pad Thai sauce added with different antioxidants.

Sample	L*	a*	b*	ΔE*
Control	29.95 ± 0.15d	24.66 ± 0.18b	43.37 ± 0.66c	7.28±0.22a
As + Toc	28.45 ± 0.31b	29.28 ± 0.33d	44.13 ± 0.94c	11.20±0.24c
EDTA	30.17 ± 0.08e	30.03 ± 0.95e	43.92 ± 0.60c	11.29±0.37c
BHT	29.31 ± 0.27c	27.82 ± 0.32c	42.02 ± 1.68b	8.63±0.62b
Mix	23.67 ± 0.03a	23.67 ± 0.03a	35.57 ± 0.13a	8.51±0.19b

As + Toc = 500 mg/kg ascorbyl palmitate + 500 mg/kg α-tocopherol, EDTA = 100 mg/kg EDTA, BHT = 200 mg/kg BHT, and Mix = 500 mg/kg ascorbyl palmitate + 500 mg/kg α-tocopherol + 100 mg/kg EDTA 200 mg/kg BHT.

ΔE* was calculated relative to the color of the original control sauce without retorting. L*, a*, and b* of the original control sauce without retorting were 31.67±0.04, 21.01±0.14, and 37.31±0.76, respectively.

Values are given as mean ± standard deviation from triplicate determinations.

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

Conclusion

Retorted southern-style Pad Thai sauce was successfully produced, and it can be stabilized by 2.3% PS + 0.1% XG in combination with mixed antioxidants during retorting. This sauce stabilized by 2.3% PS + 0.1% XG showed a desirable viscosity, water holding capacity, and color without any creaming layer or detrimental effects on sensory properties. The mixed antioxidants showed the best activity in retarding the PV and ΔE* values, which was due to the presence of antioxidants in both the oil and aqueous phases. Thus, the use of an appropriate stabilizer and antioxidant can be a strategy in improving textural property and preventing lipid oxidation of retorted southern-style Pad Thai sauce. However, storage test should be done in the future to ensure the stability of the product.

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14 Apr 2020

PONE-D-19-34660

Southern style Pad Thai sauce: from traditional culinary treat to convenience food in retortable pouches

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Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: The study is very interesting because it relates culture (gastronomy) to food science and technology. It focuses on the availability of an important sauce in Thailand to other cities and countries with good sensory quality. Some improvements are necessary to better comprehension of data and conclusions. English revision is necessary and I will point out some mistakes, but other need to be fixed.

Abstract:

The objective is missing in the abstract.Also, it is missing information related to methodology. which stabilizers were used? and antioxidants?

For key words, choose or stabilizer or stabilization

Introduction

Line 2 - take out the comma before differs - subject and verb come together

Lines 66 and 67 - Based on what? Is there a study with Pad Thai sauce? Is there a study relating the sauce and lipid oxidation? little is shown in this part for oxidation of sauces.

take out this sentence: Xanthan gum was approved by the FDA for use as a stabilizer, emulsifier, thickener, suspending agent, bodying agent, or foam enhancer in foods [10].

This information is not necessary in the introduction.

Make objective simpler, take out from in order to.

References must be improved with newer articles related to the study

Methods:

the way procedures are described, it is very confusing.

As a suggestion starting writing everything related to the production of the sauce including the retort procedures.

Then present the additions of stabilizers and antioxidants - differences, percentages. all the possibilities. Follow with all the chemical analysis and close with sensory and statistical analysis.

For the sauce preparation: were the coconut milk and chili and shrimp paste industrialized? or were they prepared by researchers? how?

line 101 - take out respectively

It is not clear how stabilizers were added, separately, together, how many samples. only at the results section, we understand the number of samples.

The mix with all the antioxidants was prepared with the same amount of each one used separately?

For the sensory analysis, why 30 panelists? is this number enough? refer to a study.

How many samples each panelist tasted in one session?

For statistical analysis - authors state 3 different lots of retorted southern style Pad Thai sauce. three lots for each type of sample?

Results and discussion

Lines 192, 193 - PV was similar, how with p<0.05?

no discussion fo pH and Aw. include

197 - 198 - rewrite sentence. it is not understandable

It is strange that pH so similar are all statistically different. Can you check this information.

line 221 - no creamy layer was not were

Line 241 - Table shows statistical differences so it is not negligible changes as written.

line 247 - Which differences may not be detected by consumers? Relate with sensory analysis.

line 255 - To investigate the effect of different stabilizers on sensory properties, a nine-point hedonic scale

256 was used to evaluate the likeness of retorted southern style Pad Thai sauce. take out this part. it is methodology

line 262 - rewrite - English problems

Line 265 - Why did you select PS + XG? based on sensory, it could be any of them.

What are the limitations of the study? include

Reviewer #2: The present work on "Southern style Pad Thai sauce: from traditional culinary treat to convenience food in retortable pouches" is interesting. The work has been executed meticulously and the results have been expressed aptly. However, for the benefit of the international readers, it is recommended to share pictures of "Pad Thai sauce" containing without and with stabilizers and antioxidants. Further, since this product is meant for long term storage, results of the effect of the optimized ingredients on the storage stability should have incorporated at least under accelerated storage conditions.

**********

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Reviewer #1: No

Reviewer #2: Yes: Dr. Narender Raju Panjagari

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files to be viewed.]

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22 Apr 2020

Response to Reviewers

All points raised by the academic editor and reviewers were carefully addressed and answered point-by-point. A revision was made in highlighted red fonts. The revised manuscript was carefully proofread for English by a native speaker, Mr. Dave Chang from USA. The revised manuscript was carefully prepared to meet PLOS ONE's style requirements.

Additional Editor Comments (if provided):

Dear author,

Finally, we were able to have the required reviewers comments. Once you supply appropriate point by point answer to their queries I will be glad to revaluate your manuscript.

Journal requirements:

When submitting your revision, we need you to address these additional requirements:

1. Please ensure that your manuscript meets PLOS ONE's style requirements, including those for file naming. The PLOS ONE style templates can be found at http://www.plosone.org/attachments/PLOSOne_formatting_sample_main_body.pdf and http://www.plosone.org/attachments/PLOSOne_formatting_sample_title_authors_affiliations.pdf

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

2. In your Methods, please state the exact source of all ingredients used in the preparation of the Pad Thai sauce.

Ans: The exact sources of all ingredients used in the preparation of the Pad Thai sauce were added “All ingredients used in the preparation of the sauce were obtained from a local market in Thasala, Nakhon Si Thammarat.”

Reviewer #1: The study is very interesting because it relates culture (gastronomy) to food science and technology. It focuses on the availability of an important sauce in Thailand to other cities and countries with good sensory quality. Some improvements are necessary to better comprehension of data and conclusions. English revision is necessary and I will point out some mistakes, but other need to be fixed.

Ans: The revised manuscript was carefully proofread for English by a native speaker, Mr. Dave Chang from USA.

Abstract:

The objective is missing in the abstract. Also, it is missing information related to methodology. which stabilizers were used? and antioxidants?

For key words, choose or stabilizer or stabilization

Ans: The objective was added in the abstract “Thus, the objective of this study was to investigate the effects of stabilizers and antioxidants on the stability of retorted southern-style Pad Thai sauce.” The stabilizers and antioxidants used were also mentioned in the abstract “Southern-style Pad Thai sauce was prepared according to the traditional recipe in the presence of different stabilizers (2.3% potato starch (PS) + 0.1% xanthan gum (XG), 0.5% soy lecithin (LT), and 4% whey protein isolate (WPI)) and antioxidants (500 mg/kg ascorbyl palmitate + 500 mg/kg �-tocopherol (As + Toc), 100 mg/kg ethylenediaminetetraacetic acid (EDTA), 200 mg/kg butylated hydroxytoluene (BHT), and As+Toc+EDTA+BHT (mixed antioxidants))..” For the keywords, “stabilizer” was selected and “stabilization” was deleted.

Introduction

Line 2 - take out the comma before differs - subject and verb come together

Ans: Done as recommended.

Lines 66 and 67 - Based on what? Is there a study with Pad Thai sauce? Is there a study relating the sauce and lipid oxidation? little is shown in this part for oxidation of sauces.

Ans: The assumption was based on the previous reports in other emulsified sauces. We also observed during the preliminary test after retorting the southern style Pad Thai sauce. Here is the detail we provided in the revision “However, based on the emulsion-based system that produces other emulsified sauces [5, 6, 7, 8, 9], the retort process of southern-style Pad Thai sauce may encounter phase separation and lipid oxidation during retorting. It was reported that the retorting of emulsion at high temperatures (121�C for 16 min) resulted in lipid oxidation and an increase in coalescence of oil droplets [5]. Lipid oxidation is a critical issue in regard to emulsified foods because it is considered as an interfacial phenomenon [10, 11]. The large interfacial area of emulsion provides numerous sites for lipid oxidation to occur [10, 11]. Also, the separation of the solid and liquid phase is one of the main problems associated with the quality of sauces [6].”

take out this sentence: Xanthan gum was approved by the FDA for use as a stabilizer, emulsifier, thickener, suspending agent, bodying agent, or foam enhancer in foods [10].

This information is not necessary in the introduction.

Ans: Done as recommended.

Make objective simpler, take out from in order to.

Ans: Done as recommended.

References must be improved with newer articles related to the study

Ans: It is difficult to find the new articles with related references because there is no report on the production of characteristics of southern style Pad Thai sauce, especially the heat-treated one. So, we tried our best to find the related references from other sauces which published from 1992-2018. We have added two more updated references in 2019 and 2020 regarding the lipid oxidation of emulsion in general. “Lipid oxidation is a critical issue in regard to emulsified foods because it is considered as an interfacial phenomenon [10, 11]. The large interfacial area of emulsion provides numerous sites for lipid oxidation to occur [10, 11].”

10. Mirzanajafi-Zanjani M, Yousefi M, Ehsani A. Challenges and approaches for production of a healthy and functional mayonnaise sauce. Food Sci Nutr.2019;7:2471-2484.

11. Noon J, Mills TB, Norton IT. The use of natural antioxidants to combat lipid oxidation in O/W emulsions. J Food Eng.2020:110006.

Methods:

the way procedures are described, it is very confusing.

As a suggestion starting writing everything related to the production of the sauce including the retort procedures.

Then present the additions of stabilizers and antioxidants - differences, percentages. all the possibilities. Follow with all the chemical analysis and close with sensory and statistical analysis.

Ans: The procedures stared with the production of southern-style Pad Thai sauce, the effect of stabilizer on retorted southern-style Pad Thai sauce including the retort procedures, the effect of antioxidants on retorted southern-style Pad Thai sauce, the chemical analysis (moisture content, aw, pH, and PV), the physical analysis (creaming layer, viscosity, and color), sensory analysis, and statistical analysis.

For the sauce preparation: were the coconut milk and chili and shrimp paste industrialized? or were they prepared by researchers? how?

Ans: We added in this section that “All ingredients used in the preparation of the sauce were obtained from a local market in Thasala, Nakhon Si Thammarat.” as recommended by the editor. In Thailand, coconut milk, chili paste and shrimp paste can be purchased in the market.

line 101 - take out respectively

Ans: Done as recommended.

It is not clear how stabilizers were added, separately, together, how many samples. only at the results section, we understand the number of samples.

Ans: We added the detail of samples in the section of Effects of stabilizers on retorted southern-style Pad Thai sauce “To study the effects of stabilizers on retorted southern-style Pad Thai sauce, different stabilizers including 2.3% PS plus 0.1% XG, 0.5% LT, and 4% WPI were separately added at the stir-frying step during the preparation of the sauce to obtain four different sauces – namely, the control (without stabilizer), sauce with PS + XG, sauce with LT, and sauce with WPI.”

The mix with all the antioxidants was prepared with the same amount of each one used separately?

Ans: Yes, the final concentration of all antioxidants in the mixed antioxidants was the same of each one used separately. We also stated this issue in the Material and methods that “Different antioxidants including 500 mg/kg ascorbyl palmitate plus 500 mg/kg �-tocopherol (As + Toc), 100 mg/kg EDTA, 200 mg/kg BHT and 500 mg/kg ascorbyl palmitate + 500 mg/kg �-tocopherol + 100 mg/kg EDTA + 200 mg/kg BHT (mixed antioxidants) were incorporated in order to determine their effects on the oxidative and color stabilities of the product. The final concentration of all antioxidants in the mixed antioxidants was the same for each one used separately.”

For the sensory analysis, why 30 panelists? is this number enough? refer to a study.

Ans: In this study we used 30 panelists. From the textbook reference (Watts, B. M., Ylimaki, G. L., Jeffery, L. E., & Elias, L. G. (1989). Basic sensory methods for food evaluation. IDRC, Ottawa, ON, CA.), the in-house consumer panels (pilot consumer panels) usually consist of 30 to 50 untrained panelists selected from personnel within the organization where the product development or research is being conducted. We also added this reference in the revised version.

How many samples each panelist tasted in one session?

Ans: There were 4 samples (control, PS + XG, LT, and WPI) each panelist tasted in one session.

For statistical analysis - authors state 3 different lots of retorted southern style Pad Thai sauce. three lots for each type of sample?

Ans: Yes, it is three lots for each type of sample. We also stated in the statistical analysis “Three different lots for each type of samples were produced (n = 3)."

Results and discussion

Lines 192, 193 - PV was similar, how with p<0.05?

Ans: It was changed to “p>0.05”.

no discussion fo pH and Aw. Include

Ans: The pH was originally discussed “The pH values of the sauces were altered by the stabilizers. When compared to the control, PS mixed with XG led to a negligible decrease in pH, whereas LT caused a gradual decrease. However, WPI showed a different trend in which it caused an increase in pH of the sauce (p<0.05). The highest pH of the sauce stabilized by WPI would result in the highest water holding capacity of the sauce.” For the aw, the discussion was extended. “The control (without added stabilizer) had the highest moisture content and aw (p<0.05). The addition of stabilizers led to a reduction of moisture content and aw. The lowest moisture content was found in the sauce stabilized by WPI, followed by LT and PS mixed with XG. Protein-based stabilizers may undergo gelation upon retorting and the moisture could be entrapped in the gel network. Carbohydrate hydrocolloids (PS and XG) could form a composite gel and hold water as well, whereas lipid-based stabilizers (e.g. lecithin) could bind water with their polar part. Both chemical binding and physical entrapping resulted in the increased water holding capacity of the sauce, which was difficult to evaporate upon the determination of moisture and aw.”

197 - 198 - rewrite sentence. it is not understandable

Ans: It was rewritten “Pan et al. [14] reported that the antioxidant activity of LT reduced permeation of free radicals across the emulsion interface. XG has been reported to show some antioxidant activity in emulsion [29], but it could not be seen in this study.”

It is strange that pH so similar are all statistically different. Can you check this information.

Ans: We rechecked the information regarding the pH and they were all statistically different. The means of the pH were quite similar but the standard deviations were very low. Therefore, they were statistically different.

line 221 - no creamy layer was not were

Ans: Done as recommended.

Line 241 - Table shows statistical differences so it is not negligible changes as written.

Ans: We deleted the term “negligible” and the p value was added. “For the color of the retorted sauces, changes in L* and a* values were found when stabilizers were added (p<0.05).”

line 247 - Which differences may not be detected by consumers? Relate with sensory analysis.

Ans: The sentence was changed to “However, the �E* of all sauces was quite low, suggesting a similar color of the sauces with and without stabilizers, which was related to the sensory analysis (Table 3).”

line 255 - To investigate the effect of different stabilizers on sensory properties, a nine-point hedonic scale was used to evaluate the likeness of retorted southern style Pad Thai sauce. take out this part. it is methodology

Ans: Done as recommended.

line 262 - rewrite - English problems

Ans: The sentence and the issue about the viscosity score were rewritten.

Line 265 - Why did you select PS + XG? based on sensory, it could be any of them.

What are the limitations of the study? Include

Ans: In this study we selected PS+XG because it can improve the apparent viscosity to the highest extent without a negative effect on sensory characteristic. “Thus, the sauce stabilized by PS + XG was selected for further study due to its highest apparent viscosity.”

Reviewer #2: The present work on "Southern style Pad Thai sauce: from traditional culinary treat to convenience food in retortable pouches" is interesting. The work has been executed meticulously and the results have been expressed aptly. However, for the benefit of the international readers, it is recommended to share pictures of "Pad Thai sauce" containing without and with stabilizers and antioxidants. Further, since this product is meant for long term storage, results of the effect of the optimized ingredients on the storage stability should have incorporated at least under accelerated storage conditions.

Ans: The pictures relating to the Pad Thai sauce are shown in Fig. 1 a-f.

“Fig. 1 Southern-style Pad Thai, so-called Pak Phanang Mee Pad (spicy stir-fried rice noodles with coconut milk) (a), preparation of southern-style Pad Thai sauce (b), retortable pouch used to fill the product (c), pilot-scale horizontal stationary retort system (d), monitoring the temperatures in the retort and pouch center with thermocouples (e), and appearance of the final retorted sauce (f).”

For the storage test, unfortunately, it was not done yet in the present study. However, in the conclusion, we recommended to do the storage test for the further research as recommended. “However, storage test should be done in the future to ensure the stability of the product.”

Submitted filename: Response to Reviewers.docx

Click here for additional data file.

5 May 2020

Southern-style Pad Thai sauce: from traditional culinary treat to convenience food in retortable pouches

PONE-D-19-34660R1

Dear Dr. Chaijan,

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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 #2: 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 #2: Yes

**********

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

Reviewer #1: Yes

Reviewer #2: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: Yes

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PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #2: Yes

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6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: All the changes were done and now the article is clear and suitable to be published. Authors reviewed the English language for mistakes and included references that were necessary.

Reviewer #2: (No Response)

**********

7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: Yes: raquel braz assuncao botelho

Reviewer #2: Yes: Narender Raju Panjagari

7 May 2020

PONE-D-19-34660R1

Southern-style Pad Thai sauce: from traditional culinary treat to convenience food in retortable pouches

Dear Dr. Chaijan:

I am pleased to inform you that your manuscript has been deemed suitable for publication in PLOS ONE. Congratulations! Your manuscript is now with our production department.

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