الفهرس 
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Abstract
Preservation of foods by natural and microbiological methods may be a satisfactory approach solving economic losses due to microbial spoilage of raw materials and food products, nowadays bacteriocin from lactic acid bacteria offer a potential application in food industry as natural preservatives. During this study, bacteriocin was extracted from Lactobacillus acidophilus DSMZ 20079 then determined their antibacterial activity against Bacillus subtilis and Staphylococcus aureus (Gram-positive bacteria) and Escherichia coli (Gram-negative bacteria). The stability of crude bacteriocin extracted from Lactobacillus acidophilus was determined at different pH values, heat treatments, proteinase K and during storage at 37, 4 and -20°C. The crude bacteriocin of Lb. acidophilus was purified to apparent homogeneity using a three step purification protocol which ammonium sulphate precipitation, gel filtration on a Sephadex G-75 column and Reversed Phase High Performance Liquid chromatography. Then characterization of the purified bacteriocin at different pH values, heat treatments, proteinase K. The molecular weight of purified bacteriocin was estimated on SDS PAGE and obtained single protein band at 52 K.Da. Application of crude and purified bacteriocin in pasteurized milk three groups of raw milk samples were subjected to low temperature short time pasteurization at 65°C for 30 min then rapid cooling at 7 °C. The pasteurized milk was inoculated as the following: C, control without bacteriocin, T1: milk with 200IU/ml of crude bacteriocin T2: it was inoculated with the purified bacteriocin at the concentration of 200IU/ml T3: it was inoculated with 200IU/ml of nisin. All the above milk samples were kept at refrigerator and the samples were examined for titratable acidity and microbiologically at zero time, then at three days intervals till the signs of spoilage were detected. Each treatment was triplicate. The results revealed that: The inhibitory effect of bacteriocins was better on Gram-positive bacteria than Gram-negative bacteria. As well as, the largest inhibition zone was obtained by Lb. acidophilus bacteriocin against Bacillus subtilis while the smallest inhibition zone b against E.coli. The antimicrobial activity of crude bacteriocin was stable after heating at 100 °C/ 30 min and declined thereafter. Stability of antimicrobial activity was observed at pH ranged from 2.0 to 8.0. Its active principle was proteinaceous in nature since the bacteriocin was inactivated by proteinase K enzyme. Also, the current study concluded that freezing temperature at -20 °C was the most appropriate technique for preservation of bacteriocin. The purification of bacteriocin resulted in protein fraction with high antibacterial activity, heat stability and act as natural preservative of pasteurized milk with improving its keeping quality. Titratable acidity was high in C than the other group from production till spoilage occurs. There were highly significant in T2 and T3, T5 as the count of aerobic spore former was sharply decreased from 6th day of storage and not detected at 12th and 15th days of storage. bacteriocin treated pasteurized milk samples of T2 and T3showed the best role in controlling the psychrotrophic count extended to be not detected at 12thday of storage The coliform counts were failed to be detected in all pasteurized milk samples, while yeast and mould counts increased in C group than T1, T2 and T3. The current study concluded that the purified bacteriocin of Lb. acidophilus DSMZ20079 should have much attention as it could be another dietary adjunct to be approved by the international communities as its commercial application is still limited. It plays an important role in controlling the total bacterial count, aerobic-spore formers and psychrotrophic bacterial counts in pasteurized milk. Moreover, it can extend shelf-life of pasteurized milk up to 15 days of storage.