Study On Fermentation Techniques And Quality Of Hong Qu Yogurt

Hong Qu yogurt is a dairy product that fermented by lactic acid bacteria with addition of extraction of Hong Qu rice. It has a unique flavor and high nutritional value. In this study, the processes of Hong Qu yogurt were optimized with the response surface optimization design method, the changes of lactic acid bacteria during the optimal processes were investigated with the method of fluorescence in situ hybridization (FISH), and the texture features as well as flavor components of Hong Qu yogurt were also measured. The main study contents and results of this thesis are listed as follows:1. The nutrients of Hong Qu rice were extracted with ultrasonic assisted extraction method. Hong Qu rice extracts as a fermentation makings, added 12%(w/v) whole milk powder and 0.1%(w/v) compound stabilizer, the sensory score as index, optimization of process parameters using single factor combined with response surface method to determine the optimum fermentation process as:extraction of Hong Qu rice additive concentration 4.8%(w/v), fermentation time 5.5h, strains inoculation dose 4.2%(v/v), saccharose additive amount 8%(w/v). Under the optimization processes, the sensory score of Hong Qu yogurt was 95.3 which consistent with theoretical prediction. The protein content was 3.6%, fat content was 3.7%, non-fat milk solid content was 12.7%, pH value was 4.2, and no citrinin was detected, accorded with national standard.2. Traced and checked the change of lactic acid bacteria in Hong Qu yogurt and ordinary yogurt using FISH method. The results indicated that the amount of lactic acid bacteria in Hong Qu yogurt is higher than ordinary yogurt. When fermented for 2 hours, the cell number of Lactobacillus bulgaricus and Streptococcus thermophilus in Hong Qu yogurt were 2.63 and 2.24 times respectively as in ordinary yogurt. When fermented for 5 hours, which is close to the end of fermentation, the cell number of Lactobacillus bulgaricus and Streptococcus thermophilus in Hong Qu yogurt were 2.29 and 2.75 times respectively as in ordinary yogurt. These results illustrated that Hong Qu extracts could promote the growth of lactic acid bacteria in the fermentation system.3. Studied the influence of different fermentation conditions on the texture features (hardness, viscosity, cohesiveness and water-retaining property) of Hong Qu yogurt with texture analyzer, the results indicated:the influence of fermentation time and inoculation quantity of strains on the texture characteristics of Hong Qu yogurt are rising before fall; The Hong Qu rice additive amount has little influence on the texture features of Hong Qu yogurt; In a certain range, sucrose additive amount and indexes of texture characteristic of Hong Qu yogurt presented positive correlation relationship.4. Determined the flavor components in yogurt with headspace solid phase microextraction (SPME) with GC-MS technology. The main flavor components detected in Hong Qu yogurt and ordinary yogurt were ketones, aldehydes, acids, alcohols, esters, alkane, olefin etc. However, there were significant differences in quality. The highest flavor substances content of Hong Qu yogurt was ketone, which accounted for 57.26% of total flavor components while the ketones in ordinary yogurt accounted for 28.95%; the highest flavor substances content of ordinary yogurt was acids, which accounted for 38.61% of the ordinary yogurt flavor components while the acids in Hong Qu yogurt accounted for 16.35%. These indicated that Hong Qu extracts has a significant impact on the formation of Hong Qu yogurt flavor, thus leading to the differences in flavor components between these two types of yogurt.