The first part of this study: Growth and survival of some probiotics under gastrointestinal tract (GIT) and manufacturing conditions of yoghurt and Ras cheese. The obtained results showed that all tested strains could survive either under gastrointestinal tract environment or manufacturing conditions of yoghurt and Ras cheese. The final viable cell counts of these strains were within the levels needed (106-107 cfu/ml) for achieving the probiotic bacterial delivery system (enhance their health benefits). The second part: Use of some probiotic bacteria strains in making functional yoghurt. The results concluded that yoghurt treated with some probiotic bacteria increased coagulation time and thereby delaying acid production. Also, probiotic yoghurt can be successfully produced using Bifi. angulatum, Bifi. longum, Lb. rhamnosus, Lb. plantarum and Lb. gasseri as a mixed culture with yoghurt starter (1:1) with good organoleptic characteristics, and prolonging shelf life of such products during cold storage up to 28 days. The third part: Making of probiotic Ras cheese. The results showed that probiotic Ras cheese can be successfully produced using different probiotic strains i.e. (Bifi. angulatum, Lb. casei, Lb. plantarum, Lb. rhamnosus and Lb. gasseri) as a mixed culture with starter culture (1:1) with a good sensory, biochemical and rheological properties after ripening for 2 to 3 months.

The first part of this study: Isolation, classification and identification of lactic acid bacteria (LAB) from 50 samples of raw milk. Fifty five strains of (LAB) were selected from 140 isolates. The second part: Ten isolates were selected and employed to produce bacteriocins in different media (milk permeate supplemented, MPS; M17; MRS and TY. The preferable medium for production of bacteriocin extracts was a MPS. All bacteriocin extracts were active against indicator bacteria. Four extracts (from different 4 strains) were selected from the previous 10 extracts for MPS medium which gave the highest inhibition zone against indicator bacteria to study the characteristics of bacteriocin extracts. The third part: Use of bacteriocins produced by some LAB as a natural preservative in yoghurt compared with control yoghurt and yoghurt with nisin.Use of nisin and bacteriocins produced and/or exteracted with a level of ~100 AU/ml. Yoghurt treated with nisin or bacteriocins produced and/or extracted by some LAB increased coagulation time and retarded growth of lactic acid bacteria, thereby delaying acid production. The rheological properties, sensory evaluation and shelf-life of yoghurt were improved, when nisin and bacteriocins were used. The quality and shelf-life of yoghurt containing extracted bacteriocins or nisin were still acceptable after 28 days of storage. It was concluded that the addition of bacteriocins extracted especially from Lactococcus lactis subsp. lactis isolate and/or nisin to milk could be produced yoghurt with good organoleptic properties and prolonged the shelf-life.