Characterization of a new isolate of Lactobacillus fermentum IFO 3956 from Egyptian Ras cheese with proteolytic activity

More than 200 isolates were obtained from 15 Egyptian traditional dairy products (Domiatti cheese, Ras cheese and Rayeb milk) collected from local markets of Alexandria, Tanta and Kafr El-Sheikh. Examination with optical microscope of these dairy samples allowed to classify 92 bacilli, 64 of which were identified as lactobacilli. The proteolytic activity of lactobacilli isolates was tested on skim milk agar. Eight isolates showing a high proteolytic activity were further tested on UHT skim milk. The strain showing the highest proteolytic activity was purified and identified as Lactobacillus fermentum IFO 3656. The specific proteolytic activity of this strain and the factors affecting it (pH, temperature and presence of inhibitors) were studied. The proteolysis targeted mainly caseins (73% of whole casein), especially beta-casein (85%). Smaller portions of whey proteins were proteolyzed (20%) essentially beta-lactoglobulin. The proteolysis process gave rise to medium-sized peptide populations. The optimum conditions for the proteolysis activity of the studied strains were pH 6.5 and 37 A degrees C. Proteolytic activities were very slightly affected by the increase of the temperature to 42 A degrees C or the pH to 8.2. The protease system of Lactobacillus fermentum IFO 3956 is most probably composed from a high amount of metalloproteases and small amount of cysteine and serine proteases.