Study On The Effect Of Streptococcus Thermophilus On The Quality Of Fermented Milk From Lactobacillus Fermentum Grx08

There are many kinds of fermented milk,but the fermented milk products with probiotics as the main strain in the market are relatively rare.A strain of Lactobacillus fermentum grx08 screened in the laboratory has the probiotic function of reducing blood lipid.There are some shortcomings in the fermentation of single strain grx08,such as slow acid production rate,poor water holding capacity and insufficient flavor.Streptococcus thermophilus as an industrial starter has the following main advantages:its fast acid production rate can shorten the coagulation time of fermented milk;it can increase the viscosity of fennented products and improve the texture.In addition,S.thermophilus is the main flavor-producing bacteria.The purpose of this study is to explore the influence of S.thermophilus on fermented milk by grx08 and select two strains with excellent ferment S.thermophilus was mixed with grx08 to prepare a fermented milk of good quality.The main results are as follows(1)The fermentation characteristics of S.thermophilus were detennined.The 11 strains were divided into fast acid-producing strains(fennentation time is less than 7.5 hours,average acid-producing rate is more than 10°T/h),medium-fast acid-producing strains(fermentation time is 7.5 hours,average acid-producing rate is 8-10°T/h)and low-speed acid-producing strains(fermentation time is more than 10.5 hours,average acid-producing rate is less than 8°T/h).According to the viscosity of fermented milk,11 strains were divided into high-yield strains(above 10000 cP),medium-yield strains(5000-10000 cP)and low-yield strains(below 5000 cP).According to the lactoacetaldehyde content of fermented milk,11 strains were divided into high-producing strains(more than 20 mg/L),medium-producing strains(15-20 mg/L)and low-producing strains(less than 15 mg/L).According to the diacetyl content of fermented milk,11 strains were divided into high diacetyl producing strains(greater than 6 mg/L)and low diacetyl producing strains(less than 6 mg/L).(2)Determine the titratable acidity,water holding capacity,viscosity,gel strength,acetaldehyde and diacetyl content of 1 d,5 d,10 d and 15 d of different fermented milk.By comparing the indexes of mixed fermented milk and grx08 fermented milk,we can know that the acidity of mixed fermented milk is significantly higher than that of grx08 fermented milk,and the acidification degree of mixed fermented milk produced by high speed acid producing streptococcus is weaker than that of mixed fermented milk.The viscosity of the mixed fermented milk with high viscosity is higher than that of grx08 fermented milk.The gel strength of the mixed fermented milk produced by S.thermophilus 394-1 and XPL-1-4 is stronger than that of grx08 fermented milk.The acetaldehyde content of the mixed fermented milk at 1 d and 15 d is significantly higher than that of the grx08 fermented milk,and that of the mixed fennented milk of S.thermophilus 393-2 is higher than that of the fennented milk.Aldehyde content remained above 18 mg/L.The diacetyl content of 393-2,R704-2 and 937 mixed fermented milk was significantly higher than that of grx08 fermented milk.The diacetyl content of high-yielding S.thermophilus mixed fermented milk was higher than that of low-yielding S.thermophilus mixed fermented milk at 1 and 15 days storage.Correlation analysis of different indicators shows that there is a negative correlation between water holding capacity,viscosity and acidity trend.The change of diacetyl and gel strength is positively correlated with acidity change.Water holding capacity is positively correlated with viscosity change trend,gel strength,diacetyl content and viscosity change trend are negatively correlated;diacetyl content change is positively correlated with acetaldehyde content change;gel strength change.It was positively correlated with the change of diacetyl content and negatively correlated with the change of water holding capacity.(3)In order to explore the effects of two S.thermophilus mixed on grx08 fermented milk,The results showed that the titrative acidity of the mixed fermented milk of two strains of S.thermophilus and grx08 was above 90°T for 15 days,the highest was 130°T;the water holding capacity was above 50%during storage,and the water holding capacity of some mixed fermented milk was above 60%;the acetaldehyde content was above 15 mg/L during storage;the titrative acidity of the mixed fermented milk of two strains of S.thermophilus and grx08 was above 130°T;The diacetyl content of fermented milk of S.thermophilus mucoacetyl was almost above 8 mg/L on the 10d and 15d day.These indexes were higher than those of single strain S.thermophilus and grx08 mixed fermented milk,but the gel strength was less than that of a single strain of S.thermophilus and grx08 mixed fermented milk.Except for single strain of high-yield mixed fermented milk of S.thermophilus 937 and grx08,the viscosity of the other two strains of mixed fermented milk of S.thermophilus and grx08 was higher than that of single strain of mixed fermented milk of S.thermophilus and grx08.(4)Single factor and orthogonal experiments were used to optimize the fermented milk preparation conditions.The results showed that fermentation temperature,sucrose addition,inoculation amount and fermentation time had different effects on sensory evaluation of fermented milk.Orthogonal experiments with sensory evaluation as an index showed that fermentation temperature had the greatest impact on fermented milk,followed by sucrose addition,and fermentation time.Finally,the amount of bacteria received.The preparation conditions of fermented milk were determined as follows:fermentation temperature 38?,sucrose addition 8%,inoculation 2%,fermentation time 4 h.The quality of fermented milk was determined by optimizing conditions.The change of acidity was increased from 77.06°T to 96.63°T,while the water holding capacity decreased by 4.56%during storage,but maintained at over 60%.Viscosity decreased from 13578 cP to 10953 cP;during storage,the gel strength of fermented milk was not significantly different from 5 d,10 d,15 d;acetaldehyde content and diacetyl content increased with storage time.The sensory score of fermented milk reached 92.33 at 5 days of storage.