Starter culture design to overcome phage infection during yogurt fermentation

Abstract

Streptococcus thermophilus phages present hazards for yogurt processing. In order to avoid adverse effects, different component yogurt starters were designed. For 2-component yogurt starters consisting of an S. thermophilus sensitive and a Lactobacillus delbrueckii ssp. bulgaricus strain, phage infection was destructive. Phages resulted in increased fermentation time, unbalanced cocci to rod ratios, decreased viscosity values, and varied sensory properties. For 3-component yogurt starters consisting of S. thermophilus sensitive, S. thermophilus insensitive, and L. bulgaricus strains, phage invasion did not result in the undesirable effects of slow acidification, decreased viscosity values, and unbalanced cocci to rod ratios. The 3-component yogurt starter stabilized fermentation and improved the practical fermentation performance in the presence of phages. Combinations of different S. thermophilus strains in the design and use of yogurt starters should be considered.