An Index of Proteolysis Degree in Ewes' Milk and Ewes' Milk Yoghurt, by Single Strains and Combinations of Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus, Isolated from Traditional Greek Yoghurt

Abstract

The proteolysis degree in ewes' milk by 5 Streptococcus thermophilus and 4 Lactobacillus delbrueckii subsp. bulgaricus strains, as well as by their 20 combinations in ewes' milk yoghurt, was determined. Samples for analysis were taken after 24 h and 5 days' storage at 4°C. Determination of proteolysis degree in the 2% and 12% TCA soluble fractions was performed by the photometric ninhydrin method. The proteolysis degree varied among the single strains and their combinations. The proteolytic activity of Lb. bulgaricus strains was greater than or at the same level as the activity determined for Str. thermophilus strains. In yoghurt, the proteolysis determined was higher than that of the single strains. Changes in the proteolysis degree were also observed between the 24-h and 5-day samples. The highest total amount of free amino acids/peptides soluble in the 2% and 12% TCA fraction, in the 24-h as well as in the 5-day samples, was produced by Str. thermophilus strain CNRZ 404, Lb. bulgaricus strain CNRZ 1159 and the combinations CNRZ 1198/CNRZ 1159, ACA-DC 116/CNRZ 1159.