Screening Of Streptococcus Thermophilus With High Aroma Production Capacity And Its Application In Sour Milk

Fermented milk is characterized by high nutritional value and unique flavor and taste,which is popular with modern consumers.With the increasing awareness of consumers' health,the market's demand for the quality of fermented milk products is also increasing as well as the safety of food additives.It is required to use high-quality starters as much as possible in the production process of fermented milk to improve the flavor of the product.While the main factor that determines the flavor of fermented milk is the type of starters,therefore,the screening of strains with high aroma production ability to apply to fermented milk is the key to the development of dairy industry.In this research experiment,the Streptococcus thermophilus was isolated and identified from the traditional fermented milk,and its aroma production ability and fermentation characteristics were studied;the ideal strain was determined,and the effects of different addition ratios between the ideal strain and Lactobacillus bulgaricus on aroma production and fermentation were studied;the ideal addition ratio was determined,the aroma production process was further optimized through response surface optimization,and a fermented milk with rich flavor and good quality was developed;moreover,the quality of this milk product is compared with that of fermented milk at home and abroad.The specific research results were as follows:(1)In this research experiment,commercial fermented milk was used as the experimental object,a total of 15 strains of lactic acid bacteria were isolated,and 8 strains of lactic acid bacteria were identified as Streptococcus thermophilus by 16 S r DNA.The aroma production capacity and fermentation characteristics of different Streptococcus thermophilus were studied,and the results showed that: There are significant differences in aroma production capacity and fermentation characteristics of different strains;XN-04 S and XN-10 S had faster curd,shorter time to reach the end of fermentation,and faster acid production in the early stage of fermentation;XN-01 S and XN-10 S produced diacetyl faster in the early stage;at the end of fermentation,XN-10 S fermented milk had the highest water retention rate of 58.51%,and a high content of diacetyl of 14.80 ?g/m L.Therefore,XN-10 S was selected as the best aroma production strain.(2)Taking Streptococcus thermophilus and Lactobacillus bulgaricus as experimental objects,the effects of different strain addition ratio on the aroma and fermentation of fermented milk were studied.Experimental studies have shown that different strain addition ratio has a significant effect on aroma production and fermentation.In the early stage of fermentation,the acid production rate was faster and the ability to produce diacetyl was higher when the coccobacillus ratio is 1:10,1:20 and 1:50;at the end of fermentation,when the coccobacillus ratio is 1:20,the fermented milk had a viscosity of 5865 m Pa.s,a water retention rate of 54.50%,a logarithmic viable count of 7.36 lg CFU/m L,and a diacetyl content of 19.16 ?g/m L.Therefore,1:20 was chosen as the optimal coccobacillus ratio.(3)The aroma production formula and process of fermented milk were studied.The effects of different carbon sources including sucrose,glucose,glucose syrup,fructose syrup,fructose,maltose,lactose,etc.on the aroma production and fermentation of fermented milk were studied.On this basis,the effects of milk solid content,inoculum amount,carbon source addition amount,and fermentation temperature on fermented milk were studied;p H,viable bacterial count,and diacetyl content were as response indicators;single factor experiments and response surface optimization experiments were used to determine the best fermentation process parameters.The results showed that the best carbon source was fructose syrup;on this basis,the best fermented milk fermented milk process conditions were: milk solid content of 14%,fructose syrup addition amount of 10%,and fermentation temperature 42?.(4)Taking p H,titration acidity,water holding capacity,viable bacteria count,viscosity,diacetyl content and sensory evaluation as experimental indicators,the variance analysis and principal component analysis were used to study the quality difference of fermented milk with different starters,diacetyl content,and diacetyl effect on the senses.The results showed that: There were significant differences between fermented milk with different starters at home and abroad;fermented milk prepared with starters from Chinese brands Angel,Jiajiale and Chuanxiu had a good texture but a poor aroma;foreign Danisco YF-L904 starter had strong acid-producing ability and fermented milk had a high viscosity of 9076 m Pa.s,the water retention rate of fermented milk by Danisco YO-MIX883 starter was up to 68.07%,the fermented milk prepared by Danisco YO-MIX883 and Y250A starters had a unique aroma.After optimization of the experiment,fermented milk had a high diacetyl content,the sensory aroma index mainly focuses on the unique aroma of fermented milk,the water retention rate and the number of viable bacteria of fermented milk are similar to those of Danisco YF-L904 starter.Therefore,the fermented milk optimized through the experiment achieved the ability of fermented milk produced with foreign starters in terms of sensory,quality and aroma;and the fermented milk optimized had the characteristics of high viable bacterial count.