ACCELERATION OF YOGURT FERMENTATION BY BACTERIAL MEMBRANE FRACTION BIOCATALYSTS

Bacterial membrane fraction biocatalysts obtained from Escherichia coli (E-8), Gluconobacter oxydans (Gox) and Acetobacter xylinum (Acx), as well as the commercial oxygen scavenger Oxyrase^TM, at concentrations of 0.1–2.0 U/ml effectively stimulated cell growth of Lactobacillus bulgaricus and Sreptococcus thermophilus, and lactic acid production during yogurt fermentation. The membrane fractions scavenged oxygen in the fermented milk to an optimal oxygen tension for growth of yogurt cultures. The yogurt culture populations with membrane fraction(s) increased faster than those without. Total counts in the presence of yogurts with Oxyrase, E-8, Gox, and Acx were 0.5–1, 0.5, 1.5, or 1.2 log cycles, respectively, greater than counts of the control after 3 h of fermentation. The commercial membrane fraction Oxyrase reduced the fermentation time by 1 h needed to reach pH 4.5 compared to the controls, while E-8, Gox, and Acx reduced time by about 0.5 h, 1.5 h and 1–1.5 h, respectively, depending on the membrane concentrations. The titratable acidity was corresponded well with the reduction in pH. Ratios of rods to cocci among the samples with and without membrane fraction supplementation were not different (P > .05). Each membrane fraction biocatalyst enhanced the depletion rate of dissolved oxygen in a yogurt mix differently.