Effects of yam ( Dioscorea opposita Thunb.) juice on fermentation and textural attributes of set-style skimmed yoghurt

Yam ( Dioscorea opposita Thunb.) juice was used to replace water by 10% and 20% (w/w), respectively, while the diluted yam juice was used to disperse skimmed milk powder. Set-style skimmed yoghurt samples were thereby prepared using a commercial direct vat set starter at 42 °C. Assaying results indicated that yam juice had a slight impact on the contents of total proteins and total solids of yoghurt samples but could promote yoghurt fermentation and induce texture changes. Firstly, yam juice shortened fermentation time by about 1 h and caused a higher post-acidification during yoghurt storage at 4 °C. Secondly, yam juice reduced syneresis extent during the preformed yoghurt storage, led to value increases for these textural and rheological indices like hardness, adhesiveness, cohesiveness, viscosity as well as elastic/viscous moduli, and caused a finer and compact microstructure. Furthermore, yam juice also resulted in higher viable numbers for two starter microorganisms Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus in the stored yoghurt samples. However, replacing water with yam juice by 20% other than 10% occasionally showed a negative impact on some textural attributes of yoghurt samples. It is thus concluded that yam juice at suitable level could be used in the production of set-style yoghurt without significant negative effect on yoghurt compositions or its fermentation, when aiming to obtain a firm yoghurt texture and endow yoghurt with enhanced health benefits from the starter strains.