Effect of transglutaminase enzyme treatment on the physicochemical and microbial properties of synbiotic soy yogurt

The word “probiotic” comes from the Greek word “pro bios” meaning “for life”, which is the opposed of the term “antibiotics” meaning “against life.” Probiotics are alive bacteria and the majority of probiotic microorganisms are belonging to the genera of Lactobacillus and Bifidobacterium. Regular consumption of probiotics in the adequate amounts have many beneficial health such as prevention of diarrhea, constipation, intestinal diseases and help to digest lactose and strengthen the immune system. On the other hand, recently the use of soy products has been progressively increased due to consumer consciousness about soy benefits. Soy probiotic yogurt is of special importance due to the presence of functional compounds such as probiotic microorganisms and natural prebiotics (raffinose, stachyose), the combination which is considered and dubbed as synbiotic. Dairy products, especially probiotic yogurt, are the most common foods that are used as probiotic products. On the other hand, yogurt production and similar products have always been accompanied with problems such as defects in texture, structure and syneresis. The functionality and structure of proteins can be modified with physical, chemical and enzymatic procedures. Enzymatic modification has been recommended as a useful technique owing to high specificity of enzymatic reactions and therefore a little risk of formation of toxic products. Transglutaminase enzyme can produce a gel with a desirable structure by forming covalent bonds between glutamine and lysine in protein systems. Therefore, the effect of application of transglutaminase enzyme in protein food systems (such as milk, yogurt, cheese, ice cream, bread, fish, meat and other food products) have been extensively studied by many researchers. So, the aim of this study was to evaluate the physicochemical and microbial properties of symbiotic soy yogurt as affected by TG-enzymatic treatment at different inulin concentrations.

The effect of different amounts of TG-enzyme (0, 0.015, 0.03 and 0.045%) and inulin (0, 1 and 2%) on physicochemical (pH, acidity, syneresis) and microbial characteristics (probiotic count) of synbiotic soy yogurt during 21 days of cold storage (1, 7, 14 and 21 days) was investigated. To prepare the synbiotic soy yoghurt samples, inulin as a prebiotic and Lactobacillus acidophilus and Bifidobacterium lactis as probiotic bacteria were used and added to the soy milk. After addition of inulin, soy milk was heated (95 °C for 5 min) and cooled to inoculation temperature (45 °C). After the enzyme addition, the probiotic starter culture was added and fermentation was done at 42 °C until the pH was reached to 4.6. Sample without TG-enzyme and inulin was considered as control yogurt. For performing analysis, data were analyzed by a completely randomized factorial design using SPSS software, version 24. The mean of treatments was compared with Duncan test at 95% confidence level.

The results showed that with increasing the enzyme concentration, amount of acidity and syneresis decreased while pH increased significantly (p < 0.001). This was probably due to formation of internal and interstitial bonds in protein network which causes reduction of bioavailability of organic compounds for microorganisms (reduce acidity) and increases water holding capacity (reduce syneresis). However, enzymatic treatment had no significant effect on total solids of the samples. Increasing the amount of inulin also caused a significant increase in acidity and total solids while expressively reduced pH and syneresis of yoghurt samples (p < 0.001). The reduction of syneresis as a consequence of inulin addition was probably due to water absorption and water holding capacity of this compound. The highest total solids was recorded for yogurt sample containing 2 percent inulin and the lowest was recorded for control (0 percent inulin). By passing the time of storage, pH and syneresis significantly decreased and acidity until the fourteenth day increased and thereafter decreased considerably (p < 0.001). However, there was no significant effect on the amount total solids. The results of microbial analysis showed that with increasing enzyme concentration, the survival amount of Bifidobacterium lactis decreased significantly (p < 0/01). Although their number in the sample containing the highest level of enzyme was higher than standard. The formation of covalent bonds due to the addition of the TG enzyme can remove lower the molecular weight of peptides from the available microorganisms and thus reduce their growth. On the other hand, by increasing the enzyme concentration, unlike Bifidobacterium, the survival rate of Lactobacillus acidophilus increased significantly compared to the control sample, but eventually the amount was lower than the standard. By increasing the inulin concentration at level 2% the growth rate of Bifidobacterium lactis was significantly increased (p < 0/01). The effect of storage period on the survival rate of Bifidobacterium showed that during storage, their number decreased significantly (p < 0/05). However, the number of probiotic bacteria in all soy synbiotic yogurts at the end of 21-day maintenance were over 107 logcfu/g. Our results also showed significant interactions between three tested variables on the physicochemical characteristics and the probiotic counts.

In general, the results of this study showed that the addition of enzyme and inulin during storage period did not have an adverse effect on pH and acidity and also reduced the syneresis. The sample containing 0.045% enzyme and 2% inulin at the end of the storage period showed the least syneresis compared to the control sample. In this study, amongst two probiotic bacteria, Bifidobacterium lactis, in comparison with Lactobacillus acidophilus, was able to maintain its survival rate above the standard level. Among the different samples, the sample containing 2% inulin on the first day of storage period had the highest probiotic content. According to the results, the best synbiotic soy yogurt with acceptable probiotic count and physicochemical properties could be produced using 2% inulin and 0.015% TG-enzyme concentration.