title: NADH Oxidase of Streptococcus thermophilus 1131 is Required for the Effective Yogurt Fermentation with Lactobacillus delbrueckii subsp. bulgaricus 2038.
abstract: We previously reported that dissolved oxygen (DO) suppresses yogurt fermentation with an industrial starter culture composed of Lactobacillus delbrueckii subsp. bulgaricus (L. bulgaricus) 2038 and Streptococcus thermophilus 1131, and also found that reducing the DO in the medium prior to fermentation (deoxygenated fermentation) shortens the fermentation time. In this study, we found that deoxygenated fermentation primarily increased the cell number of S. thermophilus 1131 rather than that of L. bulgaricus 2038, resulting in earlier l-lactate and formate accumulation. Measurement of the DO concentration and hydrogen peroxide generation in the milk medium suggested that DO is mainly removed by S. thermophilus 1131. The results using an H2O-forming NADH oxidase (Nox)-defective mutant of S. thermophilus 1131 revealed that Nox is the major oxygen-consuming enzyme of the bacterium. Yogurt fermentation with the S. thermophilus Δnox mutant and L. bulgaricus 2038 was significantly slower than with S. thermophilus 1131 and L. bulgaricus 2038, and the DO concentrations of the mixed culture did not decrease to less than 2 mg/kg within 3 hr. These observations suggest that Nox of S. thermophilus 1131 contributes greatly to yogurt fermentation, presumably by removing the DO in milk.
authors.affiliation: School of Agriculture, Meiji University, 1-1-1 Higashimita, Tama-ku, Kawasaki, Kanagawa 214-8571, Japan.
authors.forename: Yasuko
authors.initials: Y
authors.lastname: Sasaki
authors.affiliation: Food Science Institute, Meiji Co., Ltd., 540 Naruda, Odawara, Kanagawa 250-0862, Japan.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Horiuchi
authors.affiliation: Food Science Institute, Meiji Co., Ltd., 540 Naruda, Odawara, Kanagawa 250-0862, Japan.
authors.forename: Hiroko
authors.initials: H
authors.lastname: Kawashima
authors.affiliation: School of Veterinary Medicine, Kitasato University, 35-1 Higashi 23, Towada, Aomori, 034-8628, Japan.
authors.forename: Takao
authors.initials: T
authors.lastname: Mukai
authors.affiliation: School of Veterinary Medicine, Kitasato University, 35-1 Higashi 23, Towada, Aomori, 034-8628, Japan.
authors.forename: Yuji
authors.initials: Y
authors.lastname: Yamamoto
keywords: Lactobacillus delbrueckii subsp. bulgaricus; NADH oxidase; Streptococcus thermophilus; yogurt fermentation