Studies On The Screening And Fermentation Conditions Of Fine LAB Strains From Natural Fermented Milk

The main purpose of this paper is to find out fine starter culture strains from 109 lacticacid bacteria (LAB) strains, which were isolated from the western Sichuan Plateau andthen identified with biochemical and physiological tests(lactobacillus 59 strains andlactococcus50 strains). After looking for the optimum fermentation condition, optimumcombination, the result are the following:The first step: First screeningBy testing the genetical stability, primely, 14 and 15 lactococcus, with stable acidproducing rate and having good test, were selected, totally 29 stains, Subsequently, thebest temperature for the growth of these 29 strains is 42℃, at the same time, 3-7,4-1,4-7,6-2,B-2,E-1,E-5,J-2could be identified as good yoghurt producers, respectively,2 lactococcus and 6 lactobacillus.The second step: Further screeningBy inoculating, these 8 strains in the liquid medium and in milk, the growth graphwas figured out. The log phase ended for lactococcus by 10 hours, for lactobacillus by 14hours; strongly suggesting, the optimum time for starter culture was for lactococcus by 10hours and for lactobacillus between 12～14 hours. By fermentation at 42℃, thefermentation and storage performances of 8 LAB strains were studied, among them, 4had good fermentation performance, lactococcus 3-7, 6-2 and lactobacillus E-1, E-5had the characteristic of weak post acidification and had stabile quality duringstorage. Then, the resistance to the extreme conditions of 4 LAB was tested. The growthof 2 lactococcus was completely inhibited at pH3.5, in contrast, the lactobacillus couldgrow at pH2.5. The growth of 6-2 was basically inhibited in 0.1% bile salt, while, theother 3 strains could grow; All of the 4 strains could grow in 6%Nacl, notably, 3-7 couldeven grow in 8%Nacl. 2 lactococcus were found to be having no inhabitation to Staphylococcus aureus, Bacillus subtilis and Escherichia coli, while the other 2lactobacillus could; All of the 4 LAB strains are very sensitive to penicillin.The third step: Combined fermentationThe optimum combination was confirmed, that is D4: (3-7:E-1: E-5=2:1:1). Thebest fermentation technological parameters of the combination swains were gained by theorthogonal experiment: the cultivation temperature is 42℃, and the inoculum amount is3%, and the concentration of sugar in milk is 7%. In comparison with the single strainfermentation, the co-inoculation had shorter fermentation time, better postacidification, higher viscosity, more acetaldehyde, and had alive strains of more than10¹⁰ cfu/ml.