الفهرس 
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Abstract
Quality, safety and acceptability of yoghurt significantly improved through addition of natural antimicrobial substances as bacteriocins. These substances produced by lactic acid bacteria and offer a potential application in food industry. During this study, bacteriocin was extracted from Lactobacillus acidophilus and Lactobacillus plantarum then determined their inhibitory effect against Bacillus subtilis and Staphylococcus aureus (Gram-positive bacteria) and Escherichia coli (Gram-negative bacteria). 5 groups of yoghurt was manufactured from fresh raw mixed milk of cows and buffalos (1:1) and inoculated with active cultures as following:2% yoghurt starter culture (Lb. delbrueckii subspp bulgaricus and Streptococcus salivarius subspp thermophilus), 1% yoghurt starter culture + 1% Lb. acidophilus DSMZ 20079, 1% yoghurt starter culture + 1% Lb. plantarum subspp plantarum DSMZ 20174, 1% yoghurt starter culture + bacteriocin of Lb. acidophilus DSMZ 20079 and 1% yoghurt starter culture + bacteriocin of Lb plantarum subspp plantarum DSMZ
20174 for C, G1, G2, G3 and G4, respectively.
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 obtained with Lb. plantarum
bacteriocin against E.coli. Addition of bacteriocins in G3 and G4 increased
the coagulation time compared with control yoghurt, G1 and G2. The
organoleptic evaluation showed highest scores in G3 followed by C, G4,
G1 and G2, respectively.