Study On The Quality Of Middle-temperature Fermented Yogurt

Incubation temperature affects overall yogurt quality. There are high temperature and low temperature incubation methods for yogurt making. Generally, the high temperature incubation method is used in yogurt production since it allows faster production of yogurt, and thus is more economical for dairy plants. The middle temperature incubation method is applied to give microorganisms more time to produce aroma substance, even more flavour substance.The starter cultures from different sources were isolated in special medium and selected primary, 16 of cocci and bacilli were obtained. The strains had been cultivated for continuous six generations using skim milk after activation, the acidity, time of curd, viscosity and sensory value were comprehensively studied, then 5 strains of cocci and bacilli were selected. The selected cocci and bacilli were mixed in the ratio of 1:1(v/v) to ferment the skim milk. According to the acidity, time of curd, viscosity and sensory value, the combination of optimal fermentation characteristics were screened, and they were S24-L24.The effect of different factors (including the ratio of cocci and bacilli, concentrations of whole milk powder, final acidity and addition of sucrose) on the lactic acid and citric acid concentrations, amount of free fatty acid and free amino acid, sensory and volatile compounds profiles of middle-temperature fermented yoghurt was investigated according to the orthogonal statistical design test. The organoleptic quality was evaluated by descriptive sensory analysis. The flavour profile was analyzed by a SPME-GC-MS method. Relationships between flavour attributes and the volatile composition, lactic acid, citric acid amount, fatty acid composition and free amino acid composition of middle-temperature fermented yogurt were determined by principle component analysis(PCA)and partial least square regression analysis (PLSR1). Results showed that the ratio of cocci and bacilli and concentrations of whole milk powder were major factors which influenced characteristics of yogurt. Using PLSR1, positive relationships were found between yogurt aroma and fatty acids composition, also between flavour, aftertaste and free amino acids. Additionally, relationships between milky ordor, yogurt ordor, pungent ordor, astringent ordor, bitter flavour, persistent aftertaste and volatile compounds,lactic acid,citric acid constituents were found.Acidity, lactic acid, citric acid, free amino acid, free fatty acid, volatile compound contents and sensory score of two yogurt (middle temperature and high temperature fermented yougrt) were checked and measured after storing at 4℃for 24 h. The result indicated that middle-temperature fermented yoghurt had good viscous and smooth texture which possessed desirable flavour, more volatile compounds and free amino acid, higher viscosity and lower STS.The quality of yogurt are greatly determined by the starter. Lactobacillus helveticus 6024 fermented milk had much sourer flavour, but was more viscous and thicker. S28 fermented milk had most fatty acid and typical yogurt volatile compounds, with higher viscosity and lower STS. It possessed desirable flavour and texture. With high proteolysis ability, L24 fermented milk had more free amino acids. However Lactobacillus acidophilus GM and Lactobailluc casei BD-Ⅱfermented milk had defect in flavour and texture because of less volatile compound, lower apparent viscosity and higher STS.