Effect of thermosonication on quality attributes of hog plum (Spondias mombin L.) juice.

The use of thermosonication (TS) technique to preserve the qualities of fruit juice as an alternative to conventional pasteurization has attracted research interest in recent times. In the present study, freshly prepared hog plum juice (control), and the juice samples subjected to pasteurization (90 °C for 60 s) and thermosonication (40 kHz, 400 W at 40, 50 and 60 °C each for 5, 10, 20 and 30 min) were each analyzed for physicochemical, bioactive, microbial and sensory properties. After treatment, no significant changes in pH, total soluble solids and titratable acidity were observed. Notably, TS at 40 and 50 °C significantly (p < 0.05) improved color parameters, cloudiness and browning index. Furthermore, thermosonication increased ascorbic acid (11.40-18.55%), total phenolic content (17.98-18.35%), carotenoids (2.19-4.30%), flavonoids (10-16%) and antioxidant activity (32.52-48.5%) relative to the control. Both treatments significantly reduced the microbial count to non-detectable level after processing, while sensory attributes slightly improved. However, TS treatment at 60 °C decreased most of the quality parameters. Results showed that TS can improve quality, safety and economic potential of hog plum juice as a feasible alternative to pasteurization.

Introduction

Fruit juices constitute an integral component of the human diet. They are regarded as excellent sources of micronutrients such as vitamins, minerals and some phytochemicals which offer nutritional and health benefits [1], [2]. Meanwhile, fruit juices are often subject to quality deterioration due to poor processing and storage conditions, thus constituting quality and safety challenges [3]. Traditionally, pasteurization (at <100 °C for few seconds/minutes to achieve a 5-log reduction) has been applied to attain microbial safety and quality preservation of fruit juices [4], [5].

Hog plum (Spondias mombin L.) also known as “yellow mombin” is a wild, but exotic fruit from a small deciduous tree belonging to the Anacardiaceae family [6]. The fruit which is indigenous to tropical areas of America, Asia and Africa has a unique blend of sweet-sour taste and is recently gaining research attention in Nigeria [7]. Nutritionally, hog plum juice is a rich source of vitamins A and C, minerals such as potassium and phosphorus, and certain health-promoting phytochemicals [8], [9]. Pasteurization of this juice for quality retention has been investigated, but the authors reported some quality losses and deleterious alterations [10]. As a result of this development, the need for novel and alternative processing method to meet consumers’ demand becomes imminent. The potential use of ultrasound technology has gained veritable attention in recent times to meet the requirements of the U.S. Food and Drug Administration (FDA) in the beverage industry [11]. Furthermore, the combination of controlled temperature (thermo) with ultrasound (sonication) called thermosonication (TS) has been reported to improve quality and safety characteristics of fruit juice [12], [13], [14]. This technique which operates at a frequency of 20–100 kHz causes cavitation in cellular structures, leading to membrane disruption, pore formation and eventual breakage or fragmentation [15], [16], [17]. Thermosonication treatment has reportedly achieved significant improvement in the color, bioactive compounds and microbial safety of several fruit juices such as mango [18], pineapple [19], star fruit [20], apple [21], soursop [22], pitaya [23] and black mulberry [24]. Hence, preserving the quality of juice from this exotic but seasonal fruit using TS will be another way to enhance its nutritional and economic value to its various consumers. To the best of authors’ knowledge, there has been a paucity of information on thermosonicated hog plum juice. Therefore, the objective of this study was to investigate the quality attributes of juice from hog plum using thermosonication technique.

Materials and method

Fruit preparation.

Matured and ripe hog plum fruits (Spondias mombin L.) were harvested from the University of Ibadan’s horticultural garden into plastic crates and brought to the Food Technology research laboratory. The fruits were properly rinsed with distilled water (pH 7.0 ± 0.2) while defective ones were removed. Rinsed fruits were air-dried at room temperature for 5 min in a Class II biosafety cabinet (BSC-1500II B2-X Labotech. Midrand 1685, South Africa) before juice extraction. All chemicals used in the present study were of analytical grade from Sigma Aldrich (Buchs Switzerland).

Juice extraction and treatment procedure

The fruit peels were carefully removed and pulp scraped from the seed using a sterilized stainless steel knife. The pulp was thereafter fed into a motorized juice extractor (Model: TMG 410025; Mumbai, India) for about 10 sec to obtain the juice. The juice was filtered with a sterile muslin cloth for homogenous consistency. Extracted juice was treated as raw (RJ), pasteurized (PJ) and thermosonicated (TS).

Pasteurization and thermosonication treatment

In the pasteurized sample, hog plum juice was pasteurized in a laboratory-scale pasteurizer (JBN 26. Grant Inst., Ltd. Cambridge SG86GB, UK) at 90 °C for 60 s to achieve a 5 log microbial reduction following a modified method of Santhirasegaram, Razali [25]. The juice sample was thereafter cooled in an ice bath and stored under refrigerated condition (4 °C) before analysis. Thermosonication treatment of hog plum juice sample was carried out according to a modified method reported by Aadil, Zeng [11] in a digital ultrasonic bath (Grant Inst. XUB 25 Cambridge, UK) at a frequency of 40 kHz. The digital ultrasonic bath (with a maximum tank capacity of 28 L) has a rectangular dimension of 365 × 385 × 546 mm. The 40 kHz transducer underneath transmits 400 W ultrasonic wave power into the bath vessel at acoustic energy density (AED) of 0.348 W/cm3. Hog plum juice sample in 500 ml beaker was placed carefully in the middle of the ultrasonic bath at the same water level in the bath. This process was varied at 40, 50 and 60 °C each for 5, 10, 20 and 30 min at the stated frequency, power and AED (Table 1). All the treatments were done in the dark during sonication to avoid any possible interference of light. After processing, juice samples were cooled in an ice bath, poured carefully into plastic bottles and kept under refrigerated storage (4 °C) before further analysis.

Table 1

Experimental treatment for raw, pasteurized and thermosonicated hog plum juice.

Treatment	Temperature (oC)	Time (min)	AED (W/cm3)	Ultrasonic power (W)	Frequency (kHz)
Raw juice	–	–	–	–	–
Pasteurized juice	90	1	–	–	–
TS-A	40	5	0.348	400	40
TS-B	40	10	0.348	400	40
TS-C	40	20	0.348	400	40
TS-D	40	30	0.348	400	40
TS-E	50	5	0.348	400	40
TS-F	50	10	0.348	400	40
TS-G	50	20	0.348	400	40
TS-H	50	30	0.348	400	40
TS-I	60	5	0.348	400	40
TS-J	60	10	0.348	400	40
TS-K	60	20	0.348	400	40
TS-L	60	30	0.348	400	40

*AED: Acoustic energy density, TS; Thermosonicated samples

Physicochemical analysis-pH, TSS, TA and color parameters

The pH values of raw and treated juices were obtained using pH meter (Model Basic 2°; Crison Instrument, Barcelona, Spain) at 20 °C. Total soluble solids (TSS, o Brix) was determined by measurement of refractive index at 25 ± 1 °C using a digital refractometer (Atago Co. Ltd., Tokyo, Japan). Titratable acidity (TA) was determined by diluting 10 ml of each juice sample with 10 ml of distilled water in a 100 ml volumetric flask. Diluted samples were titrated against 0.1 N NaOH up to 8.1 pH value using phenolphthalein as an indicator. The titration was done in triplicate and calculated as percentage citric acid.

Color parameters L* (lightness), a* (redness to greenness), b* (yellowness to blueness) of the juice was measured using a colorimeter (Chroma meter CR-400, Konika Minolta. USA). The color intensity was calculated using the formula

All determinations were carried out three times per treatment.

Cloudiness and browning indices

Cloudiness and browning indices were determined following a modified method reported by Cervantes-Elizarrarás, Piloni-Martini [26]. About 5 ml of hog plum juice was centrifuged (5810R; Eppendorf Hamburg, Germany) at 3500×g for 20 min. The supernatant obtained was used for calculation of both indices. For cloudiness index, absorbance value was obtained at 660 nm using a spectrophotometer (T70 UV–VIS spectrophotometer, PG instruments, Alma Park, UK), while for the browning index, 5 ml of the supernatant together with ethanol was centrifuged at 3500×g for 20 min and the absorbance value of supernatant obtained again was read at 420 nm in the spectrophotometer.

Ascorbic acid content

The ascorbic acid (AA) content was determined according to a method of Rahman, Khan [27] with slight modification. The method is based on the oxidizing strength of bromine water on ascorbic acid to dehydroascorbic acid using acetic acid. Here, the mixture containing hog plum juice was treated with 1 ml of 2, 4-dinitrophenyl hydrazine at 37 °C for 3 h, and the resulting solution was further treated with H2SO4 acid (1.0 mol⋅dm−3) to produce red color complex. Absorbance was measured at 521 nm using a spectrophotometer (T70 UV–VIS spectrophotometer, PG instruments, Alma Park, UK). AA content was calculated and expressed as milligram per 100 ml of juice (mg/100 ml) using a calibration curve of standard ascorbic acid.

Determination of total phenolic, total carotenoid, total flavonoid and antioxidant capacity.

Total phenolic content (TPC) was determined using the Folin-Ciocalteu assay method reported by Matkowski and Piotrowska [28]. Absorbance was measured at 760 nm using a spectrophotometer (T70 UV–VIS spectrophotometer, PG instruments, Alma Park, UK). Total phenol content was expressed as mg/g of juice extract using Gallic acid standard curve.

Total carotenoid (TC) determination was carried out following the modified method of Lee and Castle [29]. Juice sample (2 ml) was added to 50 ml of water-saturated n-butanol and mixed thoroughly by handshaking. The mixture was kept in the dark for 16–18 h for proper extraction of carotenoid and centrifuged (5810R; Eppendorf Hamburg, Germany) at 10,000×g for 10 min. The supernatant of the solvent containing the carotenoid was recovered and the absorbance measured at 440 nm on a spectrophotometer (T70 UV–VIS spectrophotometer, PG instruments, Alma Park, UK) where β-carotene was used to prepare the standard curve.

Total flavonoid (TF) determination was carried out via the colorimetric aluminium chloride method of Nayak, Rayaguru [30] with slight modification. Total flavonoid contents were calculated as milligram of catechin equivalents per millilitre of the sample. Antioxidant capacity of the juice was determined by the stable 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical method [31]. One (1) ml from each tested juice sample was added to 4 ml of a methanol solution of DPPH. Absorbance was recorded at 517 nm after 1 h. Inhibition of free radical by DPPH was calculated using the formulawhere: Ac – absorbance of the control mixture (containing all reagents except the test compound); As – absorbance of the prepared sample or standard

Microbial analysis

Microbial enumeration of total plate counts, yeasts, and mold counts was carried out according to the modified method of Nayak, Rayaguru [30]. Total plate count was evaluated using the plate count agar (PCA; Oxoid, Basingstoke, Hampshire, UK) and incubating at 37 °C for 48 h. The enumeration of yeast and mold was also done using potato dextrose agar (PDA; Oxoid, Basingstoke, Hampshire, UK) via spread plate technique. Adjustment of media was done to pH 3.5 during preparation using 100 g/L tartaric acid and incubated at 25 °C for 120 h. and colonies counted and expressed as log CFU/ml.

Sensory analysis

All sensory evaluation process was performed at the food laboratory, the University of Ibadan using an untrained panel consisting of 50 students (24 males and 26 females) with 26 years mean age who were familiar with the fruit. Panelists were instructed to rinse their mouth with tap water between sample evaluations. The juice samples were assessed for taste, color, flavour, mouth feel, and overall acceptability according to the hedonic scale of nine points (9 = like extremely to 1 = dislike extremely) as reported by Basu, Shivhare [32].

Statistical analysis

All experiments were performed in triplicates (n = 3). Data from each treatment were subjected to analysis of variance (ANOVA) using SPSS software (IBM 24 SPSS Inc., Chicago, IL, USA) and means separated using Duncan multiple range tests (p < 0.05).

Results and discussion

Physicochemical properties (pH, TSS, TA)

In the present study, the pH, TSS and TA of the raw juice was not significantly different from those of both pasteurized and thermosonicated (TSA-L) samples (Table 1). Hence the pH (2.32–2.36), TSS (5.05–5.58° Brix) and TA (0.46–0.49%) of the treated juices are within acceptable limit for fresh hog plum juice [10]. Acidity is a very important factor of juice stability and it is dependent on the nature of the fruit. A similar observation of non-significance of TS treatment on physicochemical attributes of juice from apple, star and blood fruits have been reported [20], [21], [33].

Color, cloudiness and browning indices.

The color parameters (CIE L* a* b*) of raw (49.48, −8.58 and 23.11) and pasteurized (52.81, −10.34 and 24.39) samples, respectively are as shown in Table 3. Notably, TS increased L* (lightness) and b* (yellowness) value with treatment period (TSA-H), while treatment at elevated temperature of 60 °C (TSI-L) resulted in decreased values. Increase in color parameters with TS at 40 and 50 °C could be associated with partial precipitation of unstable suspended particles during cavitation which induced more colored compounds. The decrease in color parameters (L* and b* values) at elevated temperature and longer exposure time has been associated with the development of Maillard reaction which agrees with the previous report on thermosonicated tomatoes and red grape juice [34], [35]. Color is a vital parameter for microbial safety and sensory quality of juice during processing, storage and consumption [17].

Table 3

Effect of thermosonication on color parameters (L* a* b* and ΔE), cloudiness and non-enzymatic browning index on hog plum juice.

Treatment	L*	a*	b*	ΔE	Cloudiness	Browning index
RJ	49.48 ± 0.05b	−8.58 ± 0.01c	23.11 ± 0.04b	–	23.50 ± 0.13a	0.09 ± 0.15b
PJ	52.81 ± 0.07a	−10.34 ± 0.01e	24.39 ± 1.15a	3.98 ± 0.15a	8.40 ± 0.22c	0.15 ± 0.25a
TS-A	51.23 ± 0.56a	−7.76 ± 0.96bc	24.20 ± 0.97b	3.03 ± 0.22a	24.10 ± 0.21a	0.09 ± 0.15b
TS-B	50.06 ± 0.01a	−6.22 ± 0.06b	26.71 ± 0.23ab	3.16 ± 0.12a	24.30 ± 0.11a	0.10 ± 0.12ab
TS-C	52.01 ± 0.04a	−8.83 ± 0.03c	27.30 ± 0.02ab	3.31 ± 0.11a	24.60 ± 0.16a	0.10 ± 0.14ab
TS-D	53.04 ± 0.06a	−9.22 ± 0.04d	28.14 ± 0.03a	1.38 ± 0.07b	24.80 ± 0.15a	0.11 ± 0.20b
TS-E	51.66 ± 0.18a	− 9.25 ± 0.03d	28.16 ± 0.11a	1.38 ± 0.18b	27.40 ± 0.11b	0.09 ± 0.10b
TS-F	50.63 ± 0.08a	− 7.80 ± 0.04bc	29.94 ± 0.07a	2.51 ± 0.19a	27.60 ± 0.19b	0.10 ± 0.11ab
TS-G	51.36 ± 0.01a	− 8.82 ± 0.01c	28.03 ± 0.05a	2.28 ± 0.18a	27.80 ± 0.21b	0.11 ± 0.17b
TS-H	52.22 ± 0.03a	−8.79 ± 0.02c	29.98 ± 0.09a	2.13 ± 0.09b	28.20 ± 0.22b	0.11 ± 0.13b
TS-I	52.17 ± 0.18a	8.57 ± 0.03a	27.16 ± 0.11a	2.82 ± 0.25a	28.10 ± 0.25b	0.11 ± 0.22b
TS-J	51.19 ± 0.08a	8.64 ± 0.04a	26.94 ± 0.07a	1.01 ± 0.13b	28.30 ± 0.21b	0.12 ± 0.11ab
TS-K	49.86 ± 0.01b	8.72 ± 0.08a	25.03 ± 0.05ab	1.61 ± 0.07b	28.80 ± 0.18b	0.12 ± 0.17ab
TS-L	47.34 ± 0.03b	8.82 ± 0.04a	22.98 ± 0.09b	3.25 ± 0.05a	29.30 ± 0.19a	0.13 ± 0.18a

Values are means ± standard deviation of three replicates experiments. Mean values in the same column with the same superscripts are not significantly different (p < 0.05). ΔE: change in chromacity

Cloudiness which indicates the level of turbidity and suspended particles in the juice significantly (p < 0.05) reduced from 23.50 to 8.4 after pasteurization (Table 2). Thermosonication increased level of cloudiness from 24.10 (TS-A) to 29.30 (TS-L). Increase in cloudiness could be ascribed to proteins, lipids, pectins, cellulose and hemicellulose present as suspended particles in the juice which are broken down (sonolysis) during cavitation of bubbles [11]. This observation supported the previous report on thermosonicated black mulberry and grape juice [11], [24]. Pasteurization significantly (p < 0.05) increased browning index (0.15) compared to the raw juice (0.09) in Table 2. However, samples showed a slight increase in browning index with the treatment period. Increase in browning index with TS could be linked to the development of Maillard reaction [36]. Previous authors’ report on the inefficacy of TS to cause Maillard effect at ≤50 °C could be responsible for its insignificant effect at TSA-H [22], [37]. Conversely, TS at elevated temperature could trigger Maillard reaction and thus correlates with the decrease in lightness (L*) and yellowness (b*) of chroma values at 60 °C (TSI-L) [35]

Table 2

Effect of thermosonication on physicochemical properties of hog plum juice.

Treatment	pH	Total soluble solids (oBrix)	Titratable acidity (%)
Raw Juice	2.35 ± 0.15a	5.58 ± 0.14a	0.46 ± 0.16a
Pasteurized Juice	2.36 ± 0.02a	5.58 ± 0.14a	0.49 ± 0.13a
TS-A	2.34 ± 0.02a	5.57 ± 0.12a	0.48 ± 0.20
TS-B	2.33 ± 0.02a	5.58 ± 0.12a	0.49 ± 0.12a
TS-C	2.32 ± 0.02a	5.58 ± 0.14a	0.49 ± 0.18a
TS-D	2.32 ± 0.02a	5.58 ± 0.13a	0.49 ± 0.21a
TS-E	2.33 ± 0.01a	5.07 ± 0.13a	0.47 ± 0.20a
TS-F	2.33 ± 0.01a	5.13 ± 0.12a	0.49 ± 0.12a
TS-G	2.34 ± 0.01a	5.13 ± 0.11a	0.49 ± 0.18a
TS-H	2.34 ± 0.01a	5.13 ± 0.13a	0.49 ± 0.21a
TS-I	2.33 ± 0.02a	5.05 ± 0.09a	0.47 ± 0.20a
TS-J	2.34 ± 0.01a	5.07 ± 0.12a	0.49 ± 0.12a
TS-K	2.34 ± 0.01a	5.13 ± 0.12a	0.48 ± 0.18a
TS-L	2.34 ± 0.01a	5.13 ± 0.13a	0.48 ± 0.21a

Values are means ± standard deviation of three replicates experiments. Mean values in the same column with the same superscripts are not significantly different (p < 0.05).

Ascorbic acid content (AA)

The ascorbic acid (AA) contents of raw and pasteurized juice obtained were 24.10 and 11.35 mg/100 ml respectively (Table 3). However, TS significantly (p < 0.05) increased the level of ascorbic acid along the treatment period at 40 °C (TSA-D) and 50 °C (TSE-H) than TS treatment at 60 °C (TSI-L). This increase could be associated with effective elimination of dissolved oxygen (which promotes AA stability during processing) in the juice medium during cavitation [11], [38], while elevated temperature which degrades AA could be responsible for lower AA level of pasteurized and (TSI-L) samples. A similar report of AA increase in thermosonicated apple and carrot juice has been reported [21], [39]. Higher loss of AA with TS treatment at 60 °C (TSI-L) could also result from severe sonolytic activity occurring during the collapse of bubbles formed via cavitation [40]. A similar observation of AA loss has been reported in thermosonicated strawberry juice [37].

Total phenolic content (TPC)

The TPC content of raw and pasteurized hog plum juice were 8.01 and 7.35 mg GAE/ml respectively (Table 3). During TS, TPC of samples at 40 °C increased from 8.55 to 9.45 mg GAE/ml (TSA-D), while samples at 50 °C increased from 8.39 to 9.48 mg GAE/ml (TSE-H). However, the reduction in TPC values from 7.83 to 7.61 mg GAE/ml was observed in samples treated at 60 °C (TSI-L). Increase in TPC with TS at 40 and 50 °C could be associated with the release of phenolic compounds (secondary metabolites) from the bond to free form during cavitation [39], [41]. Cavitation during TS has been reported to increase mechanical disruption of plant cell wall resulting in the extraction of certain bioactive components [19], [42]. Reduced value of TPC at elevated temperature could be linked with processing condition such as temperature, treatment period and power rating [43], [44]. A similar observation of reduction was reported in thermosonicated apple juice [21].

Total carotenoid content (TC)

Effect of TS treatment on carotenoid content which is a precursor of vitamin A with concomitant health-promoting characteristics is as shown in Table 3. Carotenoid content of raw hog plum juice was 95.15 µg/100 ml while that of pasteurized juice was 75.13 µg/100 ml. Thermosonicated samples treated at 40 °C (TSA-D) increased carotenoid content from 86.05 to 97.23 µg/100 ml along the treatment period, while samples treated at 50 °C (TSE-H) increased from 88.26 to 99.24 µg/100 ml. A decline from 69.06 to 67.56 µg/100 ml was observed in samples treated at 60 °C (TSI-L). Increase in carotenoid content with TS treatment could be associated with disruption and inactivation of certain bioactive enzymes such as lipoxygenase in the cell wall during cavitation [25], [45]. A similar increase in carotenoid via TS in mango, apple and star fruit juices have been reported [20], [21], [46]. However, the shearing effect of TS leading to isomerization during elevated temperature could be responsible for the reduction in the level of carotenoid [18].

Total flavonoid content (TFC)

Flavonoid contents of raw and pasteurized juice samples were 1.50 and 1.39 mg CE/ml respectively. TFC of hog plum juice samples thermosonicated at 40 °C increased from 1.45 to 1.65 mg CE/mL (TSA-D), while samples treated at 50 °C increased from 1.47 to 1.74 mg CE/mL (TSE-H). However, samples treated at 60 °C decreased from 1.48 to 1.41 mg CE/mL (TS1-L). Increase in the level of TFC with TS conditions was similar to the previous report on thermosonicated juices from orange, apple, litchi, watermelon, pineapple, carambola and black janum [21], [46], [47]. However, reduction in TFC at elevated temperature of 60 °C could be linked with excessive cavitation with the corresponding disruption of a cell which has been reported to exert a negative effect on most bioactive compounds [48].

Antioxidant activity

The antioxidant capacities which were measured using the DPPH assay were not significantly (p < 0.05) different from each other (Table 3). The antioxidant activity of raw juice was 38.72% while that of pasteurized juice was 34.96%. However, thermosonication treatment significantly increased the values of antioxidant activity with an increase in the treatment period. TS treatment increased antioxidant activity of juice treated at 40 °C (TSA-D) from 49.19 to 71.24%, and juice treated at 50 °C (TSE-H) increased from 45.29 to 87.22%, while juice treated at 60 °C (TS1-L) decreased from 65.29 to 43.33%. Increase in antioxidant activity at 40 and 50 °C could be associated with the bioavailability of phenolic compounds during cavitation [46], [49]. Similar increase with treatment condition has been reported in carrot and golden berry juice [39], [50]. Other authors associated increase in antioxidant activity with decreased formation of free hydroxyl radical during TS on the juice component [51], [52]. High level of these radicals for a longer period has been reported to have a deleterious impact on the antioxidant activity [18]. This development may be responsible for decreasing antioxidant values at 60 °C.

Microbial inactivation

Microbial count in Table 4 showed that total plate count, mold and yeast count of raw hog plum juice were 5.23 and 4.51 log CFU/ml respectively. Pasteurized samples showed no detectable microbial growth. After thermosonication treatment at 40 °C (TSA-D), total bacterial count reduced from 3.75 to 1.01 log CFU/ml, while mold and yeast count reduced from 4.17 to 3.64 log CFU/ml. Further TS treatment at 50 and 60 °C showed no detectable growth in the bacterial count, while TS reduced mold and yeast count at 50 °C from 3.36 to 1.93 log CFU/ml, with no detectable growth at 60 °C. Complete inactivation by pasteurization treatment could be associated with rupturing of cell wall/membrane and nuclear components leading to cell death [52]. Microbial inactivation via thermosonication is majorly associated with acoustic cavitation on the cellular structure which could either be transient (producing implosion of bubbles in the liquid) or stable (producing free radicals with antimicrobial properties) in nature [12]. In the present study, the high acidity level of juice, induced osmotic pressure together with other reactions could also enhance the cavitation impact on the microbial structure during TS process leading to release of intracellular components such as protein lipids and nuclear compounds [53], [54]. Similar findings on increased microbial inactivation of thermosonicated juices from carrot, apple, cranberry, pineapple and grapefruit have been reported [39], [55]. However, incomplete inactivation of yeast and mold at 40 and partly 50 °C could be associated with spore formation which could resist such treatment condition [25], [55], [56]. However, with increase in treatment condition, complete inactivation which could meet established regulatory requirement was achieved.

Table 4

Effect of thermosonication on some bioactive compounds and microbial inactivation of hog plum juice.

Treatment	AA (mg/100 ml)	TPC (mgGAE/ml)	TCC (µg/100 ml)	AC (%)	Total plate count (log CFU/ml)	Yeast and mold count (log CFU/ml)
Raw Juice	24.10 ± 0.15e	8.01 ± 0.03b	95.15 ± 0.05a	38.72 ± 0.55f	5.32 ± 0.03a	4.51 ± 0.04a
Pasteurized Juice	11.35 ± 0.08 h	7.35 ± 0.06c	75.13 ± 0.03b	34.96 ± 0.23f	ND	ND
TS-A	22.40 ± 0.03b	8.55 ± 0.04b	86.05 ± 0.06ab	49.19 ± 0.25e	3.75 ± 0.03b	4.17 ± 0.14a
TS-B	23.95 ± 0.05f	8.94 ± 0.34b	88.24 ± 0.08ab	54.54 ± 0.20c	2.23 ± 0.12c	3.97 ± 0.08b
TS-C	24.15 ± 0.16e	9.04 ± 0.01a	92.19 ± 0.03a	61.17 ± 0.24c	1.01 ± 0.15d	3.64 ± 0.19b
TS-D	26.85 ± 0.13c	9.45 ± 0.23a	97.23 ± 0.08a	71.24 ± 0.15b	ND	2.36 ± 0.18c
TS-E	23.18 ± 0.13f	8.39 ± 0.04b	88.26 ± 0.18ab	45.29 ± 0.15b	ND	2.13 ± 0.24c
TS-F	25.85 ± 0.08d	8.86 ± 0.24b	89.06 ± 0.22ab	58.54 ± 0.10d	ND	2.03 ± 0.17c
TS-G	27.36 ± 0.26b	9.18 ± 0.01a	95.13 ± 0.17a	72.11 ± 0.14b	ND	ND
TS-H	28.57 ± 0.17a	9.48 ± 0.22a	99.24 ± 0.15a	87.22 ± 0.25a	ND	ND
TS-I	22.86 ± 0.13 g	7.83 ± 0.04c	89.06 ± 0.18ab	65.29 ± 0.15c	ND	ND
TS-J	22.75 ± 0.08 g	7.78 ± 0.24c	88.78 ± 0.22ab	58.54 ± 0.10d	ND	ND
TS-K	22.67 ± 0.26 g	7.67 ± 0.01c	80.42 ± 0.17b	52.11 ± 0.14d	ND	ND
TS-L	22.66 ± 0.17 g	7.61 ± 0.22c	77.56 ± 0.15c	43.33 ± 0.25e	5.32 ± 0.03a	4.51 ± 0.04a

AA; Ascorbic acid, TPC; Total phenol content, TCC; Total carotenoid content, AC; Antioxidant capacity, ND: Not detected. Values are means ± standard deviation of three replicates experiments. Mean values in the same column with the same superscripts are not significantly different (p < 0.05).

Sensory properties

The sensory score which represents consumer acceptability of the raw and processed juice showed that raw juice had slightly higher scores in all the sensory parameters and overall acceptability than pasteurized juice (Table 5). In raw juice, sensory scores of taste, color, flavor, mouth feel and overall acceptability were 6.11, 6.19, 6.21, 6.39 and 6.45, respectively. While the sensory scores for pasteurized juice in the same order of parameter were 6.05, 6.11, 6.12, 6.22 and 6.38 respectively. However, increase in sensory scores of taste (6.31–6.48), color (6.12–6.39), flavor (6.31–6.45), mouth feel (6.22–6.49) and overall acceptability (6.50–6.59) were obtained for thermosonicated samples. The slight increase in sensory scores shows a positive impact with TS treatment. However, extended processing time at elevated temperature could influence decreasing sensory values as similarly reported in blood fruit [33].

Table 5

Sensory qualities of raw and treated (pasteurized and thermosonicated) hog plum juice.

Treatment	Taste	Color	Flavor	Mouth feel	Overall acceptability
Raw juice	6.11 ± 0.12a	6.19 ± 0.17a	6.21 ± 0.15a	6.39 ± 0.18a	6.45 ± 0.13a
Pasteurized juice	6.05 ± 0.21a	6.11 ± 0.22a	6.12 ± 0.14a	6.22 ± 0.23a	6.38 ± 0.26a
Ts-A	6.47 ± 0.07a	6.36 ± 0.23a	6.40 ± 0.24a	6.47 ± 0.19a	6.50 ± 0.25a
Ts-B	6.45 ± 0.11a	6.31 ± 0.16a	6.38 ± 0.13a	6.43 ± 0.23a	6.58 ± 0.22a
Ts-C	6.31 ± 0.05a	6.35 ± 0.17a	6.31 ± 0.27a	6.43 ± 0.15a	6.57 ± 0.19a
Ts-D	6.42 ± 0.10a	6.31 ± 0.11a	6.39 ± 0.11a	6.47 ± 0.11a	6.55 ± 0.21a
Ts-E	6.40 ± 0.04a	6.30 ± 0.11a	6.33 ± 0.15a	6.45 ± 0.12a	6.51 ± 0.01a
Ts-F	6.45 ± 0.03a	6.12 ± 0.25a	6.38 ± 0.24a	6.43 ± 0.11a	6.58 ± 0.11a
Ts-G	6.48 ± 0.11a	6.39 ± 0.18a	6.40 ± 0.29a	6.41 ± 0.21a	6.59 ± 0.13a
Ts-H	6.46 ± 0.14a	6.31 ± 0.22a	6.36 ± 0.21a	6.42 ± 0.19a	6.55 ± 0.14a
Ts-I	6.48 ± 0.17a	6.39 ± 0.19a	6.45 ± 0.07a	6.49 ± 0.18a	6.59 ± 0.13a
Ts-J	6.47 ± 0.22a	6.37 ± 0.24a	6.42 ± 0.10a	6.46 ± 0.23a	6.57 ± 0.23a
Ts-K	6.45 ± 0.31a	6.32 ± 0.25a	6.35 ± 0.15a	6.45 ± 0.19a	6.55 ± 0.25a
Ts-L	6.41 ± 0.18a	6.30 ± 0.26a	6.31 ± 0.25a	6.42 ± 0.17a	6.52 ± 0.21a

Values are means ± standard deviation of three replicates experiments. Mean values in the same column with the same superscripts are not significantly different (p < 0.05).

Conclusion

In the present study, hog plum juice was subjected to thermosonication as a way of enhancing the quality and consequently its economic value over conventional pasteurization treatment. Thermosonication significantly improved color parameters, cloudiness and browning index. It also improved bioactive components such as ascorbic acid, phenolic, carotenoid, flavonoid and antioxidant activity. Significant microbial inactivation was obtained, and sensory parameters were also enhanced. However, elevated temperature (60 °C) of TS treatment did not favor most of these quality attributes which may necessitate further optimization of the treatment parameters for maximum quality retention. Furthermore, storage or shelf stability of the treated juice is open to further investigation for its commercial potentials using this method.

CRediT authorship contribution statement

Adebola O. Oladunjoye: Conceptualization, Methodology, Software, Validation, Writing - review & editing. Folasade O. Adeboyejo: Visualization, Supervision, Project administration. Titilola A. Okekunbi: Formal analysis, Investigation, Resources, Data curation, Writing - original draft. Olaide R. Aderibigbe: Supervision, Project administration, Funding acquisition.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.