Structure development and acidification kinetics in fermented milk containing oat β-glucan, a yogurt culture and a probiotic strain

The impact of oat β-glucan fortification (1.4%) on the acidification and gelation kinetics of fermented skimmed milk (12% total solids) as well as the effects of inclusion of a probiotic strain (Lactobacillus paracasei subsp. paracasei B117) into the yogurt starter culture (Lactobacillus delbrueckii subsp. bulgaricus Y 6.15 and Streptococcus thermophilus Y 4.10) and fermentation temperature (36, 38 and 40 °C) have been explored by dynamic rheometry, pH measurements and microbial enumeration. The viability/growth of the mixed culture bacteria was also evaluated upon storage. Incorporation of β-glucan to milk resulted in phase separation between proteins and the added polysaccharide, altering the entire gelation process as manifested by the development of a transient gel structure at the early stages, followed by significant (P < 0.05) retardation of both protein aggregation and acidification kinetics, and finally the formation of significantly weaker gels compared to control formulations. Furthermore, fortification of milk with β-glucan led to a liquid-like structure (tan δ > 1) at the end of fermentation (pH = 4.6) when performed at 36 °C. A higher fermentation temperature accelerated the acidification kinetics, yielding a shorter gelation time, but the storage modulus values of milk gels decreased. Inclusion of the probiotic strain into the yogurt starter culture seemed to increase the gelation rate without affecting the gelation time and gel strength. Viability of all three culture bacteria was enhanced by decreasing fermentation temperature and during storage. Moreover, the L. paracasei showed good compatibility with the yogurt starter culture and the addition of β-glucan enhanced the viability of the probiotic strain in the fermented products throughout cold storage (4 °C).