Study On The Improvement Of Lactobacillus Rhamnosus Fermented Milk Quality By Streptococcus Thermophilus

Lactobacillus rhamnosus is one of the most widely studied probiotics,which has strong stability in fermented milk.Bt at present,the main fermented dairy products sold in the domestic market are fermented by Streptococcus thermophilus and Lactobacillus delbrueckii subsp.bulgaricus.Products by using L.rhamnosus as the main strain appear much less.In this thesis,L.rhamnosus LV108 that isolated from the longevous person of Guangxi Province was researched.Because L.rhamnosus LV108 has the characteristics of long clotting time and slow acid production,by researching the fermentation of L.rhamnosus LV108 mixed with other strain,we selected the best group to improve the clotting time,speed of acid production and product texture,and by the meantime had the least effect on the viable counts of L.rhamnosus.The purpose of this research is to provide a reference for the development of fermented milk that have the advantage of high number of viable bacteria and a better quality during the shelf life,so as to meet the growing consumer demand.The main research contents are as follows.1.The inhibition of L.rhamnosus LV108 and S.thermophilus grx02,393,XPL,937,90-57 on each other was determined by double-layer plate method.According to the size of bacteriostatic circle,it was found that none of the S.thermophilus strains showed bacteriostatic circle to L.rhamnosus LV108,which indicated that none strain had inhibitory effect on L.rhamnosus LV108.So they were suitable for the production of fermented milk that making L.rhamnosus LV108 as the core strain.On the other hand,L.rhamnosus LV108 can inhibit S.thermophilus to a certain extent.The inhibition of L.rhamnosus LV108 on S.thermophilus grx02 was the weakest,so S.thermophilus grx02 was used as the auxiliary fermentation strain.2.The results showed that L.rhamnosus LV108 and S.thermophilus grx02 could form colonies on the improved solid medium of HA 1-MRS and HA2-MRS,which indicated that high concentration of acetic acid(7.5g/1 and 10g/L)did not affect the growth of S.thermophilus grx02.No colony of S.thermophilus grx02 was formed on the LpH1-MRS solid medium(pH 5.0),and L.rhamnosus LV108 could not grow well in this medium either.Therefore,LpH1-MRS solid medium can not be used to identify them.The solid medium of Gly-MRS lacks the carbon source needed by S.thermophilus grx02,but it provides the need for the growth of L.rhamnosus LV108.Therefore,Gly-MRS solid medium can be used as the identification medium for L.rhamnosus LV108 and S.thermophilus grx02.The growth rate of L.rhamnosus LV108 on LM17 solid medium was slower than that of S.thermophilus grx02.When growing 24 hours,grx02 grew well on LM17 solid medium,while L.rhamnosus LV108 did not grow on LM17 solid medium.Therefore,by controlling the culture time of the strain on LM17,grx02 in the mixed strain can be calculated effectively.3.Control group and skim milk with initial pH adjusted to 5.0,5.5,6.0 with lactic acid were introduced into L.rhamnosus LV108,and cultured at 37? for 8 hours.The results showed that the growth of L.rhamnosus LV108 increased first and then decreased with the decrease of pH.The viable cells was the highest when pH was 6.0.It was found that the viable counts of L.rhamnosus LV108 increased with the proportion of tryptone added.The results showed that the growth of L rhamnosus LV108 was promoted by the acid and oligopeptide produced by S.thermophilus grx02.Through the mixed fermentation at 30? and 37?,we found that the effective way to increase the viable number of L.rhmnosus LV108 was to adjust the proportion of S.thermophilus grx02 and the culture temperature.4.By comparing various indexes of fermented milk at different temperatures,two fermented milk groups in which the amount of S.thermophilus grx02 was 0.01%at 37? and 33? were selected as the research objects.The results showed that the viable counts of L rhamnosus LV108 in the fermented milk did not show a decreasing trend with the prolongation of storage time,kept a high range.The viable counts of L.rhamnosus LV108 fermented at 33?was higher than that fermented at 37?,which was 6.4×108-1.1×109 CFU mL-1.The water holding effect of 33? mixed fermentation milk was also better than that fermented at 37?(55.1-71.1%).The indexes of texture also showed that 33? mixed fermented milk was relatively stabler.