Viability of bacterial strain of stored fermented soymilk
Lactobacillus rhamnosus (
LR),
Lactobacillus acidophilus (
LA) and
Wessilla confusa (
WC) were significantly remained constant in fermented soymilk after 28th days of storage at 4°C. The viability of probiotic strains after 28th days of storage of fermented soymilk at 4°C was shown in Fig 2. The colony forming unit (CFU) of
L. rhamnosus fermented food grade soymilk (FR),
L. acidophilus fermented food grade soymilk (FA) and
W. confusa fermented food grade soymilk (FW) was 6.1, 6.5 and 5.6 log cfu/ml respectively at 28th days of storage. In the case of low KTI soybean milk fermented with same bacterial strains, the CFU was reported 5.8 log cfu/ml for LR, 6.1 log cfu/ml for LA and 5.2 log cfu/ml for LW. The cfu count in
L. rhamnosus, L. acidophilus and W. confusa fermented black soymilk as BR, BA and BW were reported 9.23 log cfu/ml, 9.44 log cfu/ml and 8.56 log cfu/ml respectively. Among all three probiotic strains
L. acidophilus showed better cell viability after storage period at 4°C in comparison to
L. rhamnosus and
Weissella confusa. Similarly, among the three soybean varieties, black soybean milk showed better viability of bacterial strains than food grade soybean and low KTI soybean variety. The higher viability of bacterial strains even after 28th days storage at 4°C indicates that black soymilk harbour plant bioactive compounds as anthocyanin or other phenolic compounds which act as source of hydrocarbon and energy for the growth of bacterial strain during storage
(Krishnan et al., 2018; Janpaeng et al., 2018; Kumar et al., 2019; Krishnan et al., 2020; Bhartiya et al., 2020).
Changes in pH and titratable acidity (%TA) of stored fermented soymilk
During the storage pH of the fermented soymilk decreased while titratable acidity (%TA) increased (Table 1). The pH of the unfermented soymilk from food grade soybean (FC), low KTI soybean (LC) and black soybean (BS) reported as 5.1, 5.3 and 4.8 after storage, while the pH of fermented soymilk samples was reported low in compare to unfermented soymilk. The pH of food-grade soybean milk decreased from 5.1 (FC) to 2.8 (FR), 2.5 (FA) and 2.7 (FW) after 28th days of storage at 4°C. In low KTI soybean milk pH declined during the storage from 5.3 (LC), 3.12 (LR), 2.87 (LA) and 2.91 (LW). The reduction in pH was low in black soybean milk in compare to food grade soybean and low KTI soybean milk. The pH of black soymilk declined from 4.8 (BS) to 3.7 (BR), 3.31 (BA) and 3.62 (BW) after storage.
In reverse to pH, the %TA of the samples enhanced after storage. %TA of unfermented food grade soybean milk (FC) observed 1.41, which increased during storage in fermented soymilk 1.59 (FR), 1.75 (FA) and 1.65 (FW). In low KTI soybean milk % TA observed 0.97 (LC), which increased in fermented sample during storage 1.32 (LR), 1.5 (LA) and 1.41 (LW). %TA of fermented samples of black soymilk raised from 1.94 (BS) to 2.34 (BR), 3.98 (BA) and 3.37 (BW).
The reduction of pH in fermented samples indicates that bacterial strains producing some short chain fatty acids by metabolizing the large hydrocarbon molecules as RFOs (raffinose family oligosaccharides) which led to pH reduction
(Nazhand et al., 2020) (Sasi et al., 2022). Among the three mentioned bacterial strains
L. acidophilus (
LA), fermented soymilk samples showed lower value of pH and higher value of %TA in comparisons to remaining probiotic strains.
Changes in chemical composition of stored fermented soymilk
Chemical composition as protein, total fat, carbohydrate, moisture, ash and energy content of non-fermented and fermented soymilk determined by the AOAC methods (AOAC 2000) and the obtained results are shown in Table 1 and Fig 3 (A-D). The protein content in non-fermented soymilk were reported as 1.94 g/100g (FC), 2.26 g/100g (LC) and 2.05 g/100g (BS) which are low in comparison to fermented soymilk (Fig 3A). The higher amount of protein content 6.85 g/100g was found in
L. acidophilus fermented black soymilk (BA). High bacterial fermentation may cause the enhancement of protein content
(Hong et al., 2012). The total fat content in non-fermented soymilk were reported as 1.21 g/100 g (FC), 1.87 g/100 g (LC) and 2.53 g/100 g (BS) which are high in comparison to fermented soymilk (Fig 3B). The low amount of total fat 0.7 g/100g was found in
L. acidophilus fermented low KTI soymilk (LA). The
L. acidophilus fermented soymilk may be responsible for the substantial decrease in total fat
(Lee et al., 2015). The higher amount of total carbohydrate 5.41 g/100g was found in
L. acidophilus fermented food grade soymilk (FA) (Fig 3C) while the high energy value 49.75 Kcal was reported in
W. confusa fermented black soymilk (BW) (Fig 3D). During the fermentation moisture content in the sample declined while the ash content raised. Hence high amount of moisture content was observed in non-fermented low KTI soymilk sample (LC) was 93.67 g/100 g and low amount of moisture content 88.87 g/100 g was reported in
L. acidophilus fermented black soymilk sample (BA). Similarly, the high ash content 0.79 g/100 g was reported in
L. acidophilus fermented black soymilk (BA), while low ash content 0.21 g/100g was observed in non-fermented low KTI soymilk (LC).
Sensory profiling of stored fermented soymilk
Along with the nutritive and health-promoting qualities, fermented products must also have sensory qualities that appeal to consumers. The sensory profiles of stored soymilk samples are shown in Fig 4. The lowest colour value (Fig 4A) was found in non-fermented black soymilk (BS- 3 unit), while the highest colour value was found in non-fermented food grade soymilk (FC-5.8 unit),
W. confusa fermented food grade soymilk (FW-6.0 unit), non-fermented low KTI soymilk (LC- 5.9 unit),
L. rhamnosus fermented low KTI soymilk (LR- 6.1 unit) and
L. acidophilus fermented low KTI soymilk (LA- 6.23 unit). The high colour value of low KTI soybean milk over food grade soymilk and black soymilk might be due to the presence of lower amount of fat in these samples which responsible for the off-white colour of the product.
L. rhamnosus fermented food grade soymilk (FR) showed the lowest value (1.1 unit) of mouthfeel (Fig 4B), while the highest value of mouthfeel observed in
L. acidophilus fermented black soymilk (BA- 5.2 unit). The higher value of mouthfeel might be due to the presence of excess protein content in the BA sample which precipitate in acidic pH during storage.
L. acidophilus fermented low KTI soymilk (LA) showed the higher value (5.3 unit) of consistency (Fig 4C), while for the taste point of view
W. confusa fermented low KTI soymilk (LW) and
L. acidophilus fermented black soymilk (BA) showed higher value of taste 5.1-unit, 4.9 unit, respectively (Fig 4D).
L. rhamnosus fermented low KTI soymilk (LR) showed the high flavour value (6.3 unit) (Fig 4E). Higher value of overall acceptance observed in
L. rhamnosus fermented low KTI soymilk (LR- 6.25 unit) followed by
L. acidophilus fermented food grade soymilk (FA-6.19 unit) (Fig 4F).
L. rhamnosus secretes exopolysaccharides (EPS), which enhance the texture of fermented products, hence
L. rhamnosus fermented low KTI soymilk (LR) demonstrated the best consumer approval
(Li et al., 2014). Additionally, the fragrance of the
L. rhamnosus-fermented soymilk was pleasant, which encouraged more people to choose the
L. rhamnosus fermented low KTI soymilk (LR) sample.