CHARACTERIZATION OF Streptococcus thermophilus ASSOCIATED WITH EGYPTIAN DAIRY PRODUCTS

Abstract

Streptococcus thermophilus is an important industrial species that is extensively used in the preparation of fermented dairy products. The present study was designed to characterize S. thermophilus strains isolated from traditional Egyptian dairy products. Ninety-five samples of Kariesh cheese, Zabady, Laban Rayeb and Ras cheese were randomly collected, and examined for the presence of S. thermophilus. A total of 255 of lactic acid bacteria cultures could be isolated from these products and subjected to morphological and physiological examinations. Out of these isolates, 20 cultures isolated from Kariesh cheese and Laban Rayeb could be identified as S. thermophilus using a PCR analysis targeting the serB gene. These isolates were characterized on the basis of biochemical traits related to their potential use in dairy industries. This involved examining their ability to develop acidity, utilize galactose, produce exopolysaccharides and express the urease enzyme. S. thermophilus isolates were diverse in their ability to acidify milk, with the most of isolates developing moderate levels of acidity of 0.4%-< 0.6% after 6 h of incubation at 37°C. Isolates showed diversity in utilizing galactose and urease activity, yet the majority of them was unable to ferment galactose, or expressed urease. Only 1 isolate could produce exopolysaccharides. These results show that Kariesh cheese and Laban Rayeb could serve as a local source of S. thermophilus isolates of diverse technological traits that could be exploited in dairy processing.