Research On The Safety Of Lactic Acid Bacteria And Selection Of Starters For Yogurt

Lactic Acid Bacteria (LAB) produce lactic acid as the major metabolic end-product of carbohydrate fermentation. The major genera that comprise the LAB are Lactobacillus (such as Lactobacillus acidophilus, Lactobacillus casei, Lactobacillus rhamnosus), Bifidobacterium (such as Bifidobacterium longum, Bifidobacterium breve, Bifidobacterium bifidum, Bifidobacterium adolesentis), Streptococcus (such as Streptococcus thermophilus) and Lactococcus (such as Lactococcus lactis). Researchs show they are beneficial to human body as probiotics.The evaluation of antibiotic susceptibility of LAB is a major part of the safety of probiotic foods. However, there is no international standard method of the susceptibility testing of LAB till now. We investigated 6 culture media and selected a proper commercial one from them. We optimized the agar disc diffusion method and assayed 9 strains of LAB encompassing the genera Lactobacillus, Bifidobacterium, Streptococcus and Lactococcus for susceptibility to 51 antibiotics on RCA agar. The optimized method proved to be an applicable technique for antibiotic susceptibility testing of LAB. This contributes to the establishment of a standardized procedure for antibiotic susceptibility testing of LAB.Yoghurt is a dairy product produced by bacterial fermentation of milk. It is inseparable with LAB which make yoghurt rich in nutrition and do good to gastrointestinal tract of human being. Stirred yoghurt is faced with problems such as post-acidification and texture deficiency. Quality of starter culture determines the quality of yoghurt during the shelf life. We selected 4 starter formulas through layers of screening. The quality of yoghurt used with these 4 starter formulas could be assured. The acidification capability was weak and texture deficiency was improved without additional stabilizers during the shelf life of 28 d. In addition, we chose the strains of weak acidification capability by Accelerated Shelf Life Testing. We changed the yoghurt storage temperature to 43 "C (the same as the fermentation temperature) and observed the sample pH till the pH reached stable. This method proved to be effective.