PHYSICOCHEMICAL AND BACTERIAL-MORPHOLOGICAL CHARACTERISATION OF DAIRY PRODUCTS

ABSTRACT. Lactic acid bacteria possess the ability to efficiently convert lactose into lactic acid. This leads to an increase in acidity, coagulation of proteins, and prevents the growth of unwanted microorganisms. The most consumed fermented milk product worldwide is yoghurt, followed by ayran, kefir, koumiss, and others. Unlike Bulgarian yoghurt, which is obtained by adding starter cultures of Streptococcus thermophilus and Lactobacillus delbrueckii subspecies bulgaricus, kefir and koumiss are obtained by a mixed starter culture of lactic acid bacteria and yeast. To determine the physicochemical and morphological differences between fermented dairy products (homemade yoghurt, purchased yoghurt with 2 % fat as per the Bulgarian State Standard, ayran with 1.8 % fat, and kefir with 3 % fat), active (pH) and titratable acidity (⁰T), lactic acid (%), and dry matter (%) were determined. The morphology of the microorganisms was established through the Loeffler method and observation of the microscopic preparations with an immersion objective (x100). The results were close and similar for both types of yoghurt: 13.4-13.5 % dry matter, 1.5-1.7 % lactic acid, and active acidity around 4.00. In the case of the drinking products, large differences were found related to the technological process of their production and the starter cultures used.