Production of yoghurt with mild taste by a Lactobacillus delbrueckii subsp. bulgaricus mutant with altered proteolytic properties

In this communication, we describe the isolation of a Lactobacillus delbrueckii subsp. bulgaricus 92063 mutant strain named pH‐P11, which differed from the parent strain by low proteolytic activity and altered regulation of expression of lacZ in the presence of glucose or lactose. In the presence of lactose, β‐galactosidase activity was approximately twice as high in pH‐P11 than in the wild type. pH‐P11 exhibited protosymbiosis together with Streptococcus thermophilus. Yoghurt produced with pH‐P11 was characterized by low acidity and little post‐acidification during storage. The organoleptic properties (absence of bitterness and other off‐flavors, weak sourness, and clear yoghurt taste) were those of a typical “yoghurt mild”. This mild flavor was achieved at rather high cell counts of lactobacilli even at the end of shelf‐life. High cell counts in conjunction with high β‐galactosidase activity make pH‐P11 an interesting strain for application in yoghurt especially designed for consumers with lactose malabsorption. In contrast to “yoghurt mild”, which is predominantly produced with Lactobacillus acidophilus together with Streptococcus thermophilus , the product obtained by fermentation with pH‐P11 and Streptococcus thermophilus concurs with international standards for yoghurt. During frequent sub‐culturing, strain pH‐P11, which is supposed to differ from the wild type by one or a few so‐far‐not‐characterized mutations, showed sufficient stability for application in industrial production.