High-Density Cultivation Of Strptococcus Thermophilus And Development Of Direct-Vat-Set Culture

With the development of our national living, more and more dairy products appear in our daily life, and one of them is yoghurt which is popular in most parts of the world. To make the yoghurt, streptococcus thermophilus is considered as one of the most important bacteria for a long history. Currently manufacturers would be more likely to choose the direct-vat-set cultures, but Chinese markets of direct-vat-set cultures are monopolized by oversea corporations, while our national researches and developments of direct-vat-set cultures having the independent intellectual property rights, which seriously conflicted with the development of national dairy industry and the sudden growth in people’s need. Therefore it is high time developed superior direct-vat-set cultures having the independent intellectual property rights.After several superior strains of streptococcus thermophilus were selected, this study determined the growth property of them, and used skim milk culture for the high density cultivation, collected the bacteria through centrifugal ization, and prepared direct-vat-set cultures after the freeze drying. With two commercial direct-vat-set cultures as control groups, the self-made culture was then used to produce yoghurt by imitating the industrialized method. A large quantity of data about fermentations and youghurts was got to evaluate the quality of the self-made culture. All of the data would be used to develop direct-vat-set cultures for reference.This study consisted of three main parts:selection of the superior strains; high density cultivations of the superior strains of streptococcus thermophilus; development of direct-vat-set cultures. At first,8strains of streptococcus thermophilus and3strains of Lactobacillus bulgaricus conserved in laboratory were used to ferment milk in ways of single strain and associative strains respectively. Two strains of streptococcus thermophilus(KLDS3.1012and KLDS3.1014) and1strain of Lactobacillus bulgaricus(KLDS1.0209) were selected out, and the two streptococcus thermophilus were chosen to be the experimental strains for the next high-density cultivation.The two stains of streptococcus thermophilus were cultivated for high density, and this part contained selection of the culture, determination of the growth property, and optimization of the fermentation tank. The results showed that the two stains made a good performance on the growth and acid production in the culture of skim milk (11%skim milk,1%whey power,1%yeast power). After2hours of lag phase, strains entered the exponential phase, and maintained the stationary phases at about4hour, additionally strains had the biggest biomass in the culture of skim milk. Strains performed differently in fermentation of milk after cultivated in three different cultures. A little disparity existed in the speed of acid production, and skim milk group took0.5hours less than the other groups; while the character of viscosity showed big difference, and the skim milk group held the best capacity.The high density cultivation was operated in a3.7L Bio-auto fermentation tank which was made in Switzerland. The rotation speed was100r/min, and the optimal initial pH of the culture was6.5-6.6, while the temperature was37℃.The rates of inoculation were2%and3%for KLDS3.1012and KLDS3.1014respectively. The optimal neutralizer was20%(W/W) Ca(OH)2, and the optimal pH for the two strains (KLDS3.1012and KLDS3.1014) were6.1-6.2and6.2-6.3respectively. Under the optimal condition the cell density reached7.03×109CFU/mL and4.06x109CFU/mL respectively. The optimal harvest time was5hour among the the late log phase or early stationary phase (4.5-6h), when the cell density reached7.10×109CFU/mL and4.12×109CFU/mL respectively.After the high density cultivation the cells were collected by centrifugation and then freeze-drying. Then the power of streptococcus thermophilus and Lactobacillus bulgaricus were mixed in the ratio of1:1to make the direct vat culture. Yoghurt was produced with the self-made culture by imitating the industrialized mode to comprehensively evaluate the fermentation performances of the starter culture. The optimal centrifugal force and time were30000xg and20min, and the harvest rate were97.24%and93.91%(KLDS3.1012and KLDS3.1014) respectively. After lyophilization, the cell density of KLDS3.1012powder was1.60×1010CFU/g with the livability of42.05%which was comparatively low; For the mistake during the lyophilization, the cell density of KLDS3.1014powder was5.97×109CFU/g with the livability of22.32%only. By comparing the coagulation time before and after lyophilization, it could be seen that the cell vitality became lower after lyophilization.During the process of producing yoghurt, the starter showed a good recovery of activity. The bacteria enter the exponential phase after1h, and after5hours the pH of self-made starter culture and commercial starter culture were4.32and4.34, while the titratable acidity were91.9and86°T, respectively. During the storage phase, a weak post acidification was observed. Compared with the other two control groups, yoghurt made from self-made starter had a better quality, because of the results of organoleptic evaluation which showed no significant difference (P>0.05).As a result, the strain of Streptococcus themophilus KLDS3.1012was proved to be superior for its performances, such as the strong capacity of acid production, quick speed of growth, high cell density, high viscosity of the yoghurt and so on, and it was proper to be used to prepare the DVS culture.