Comparative studies on the effect of probiotic additions on the physicochemical and microbiological properties of yoghurt made from soymilk and cow's milk during refrigeration storage (R2)

Soymilk yoghurt is an option to consumers who are allergic to cow's milk yoghurt. Probiotics can provide gut health. The objective of this study was to investigate the physicochemical characteristics, including pH, colour, firmness and syneresis of yoghurt made from soymilk and cow's milk as affected by the addition of probiotics during storage at 4 degrees C for 28 days at 7-day intervals. Microstructure and bacterial populations were also determined. Two types of yoghurt were prepared. The first type was prepared with the additions of commercially available yogurt starter containing Lactobacillus delbrueckii ssp. bulgaricus (LB) and Streptococcus thermophilus (ST). The second type was prepared with the same yogurt starters plus three probiotics, including Bifidobacterium animalis subsp. lactis BB-12 (BB), L. acidophilus La-5 (LA), and L. rhamnosus (LGG). Probiotics reduced syneresis and increased compact network of soymilk yoghurt significantly (p < 0.05) and shortened the time needed to reach pH 4.5 in both cow's milk and soymilk yogurts. During storage, the pH of cow's milk yoghurts slightly decreased (4.37-4.26 with probiotics and 4.54 to 4.33 without probiotics), however, the pH of soymilk yoghurts remained relatively constant. The firmness of yoghurt made from cow's milk was significantly decreased with the addition of probiotics, however, it remained stable in soy milk yoghurt during storage. Soymilk yoghurt (34-50 g force during compression) was generally firmer than cow's milk yoghurts (9.8-12.5 g force). The cow's milk yogurt had slightly higher counts of ST, LB and LA than soymilk yogurt, but the count of BB in soymilk yogurt is significantly higher than that in cow's milk yogurt.