【作者】 陈洁；

【导师】 陈卫；

【作者基本信息】 江南大学 ， 食品科学， 2008， 硕士

【摘要】 酸乳的发酵温度会影响酸乳的整体品质。高温发酵能更快生产酸乳,对乳制品企业来说更经济。而中温发酵能提供给微生物更多的时间产生风味物质,甚至产生更多的风味物质。首先对收集得到的样品,通过分离和初筛,得到16株球杆菌。菌株经35℃凝乳实验,综合考虑酸度、凝乳时间、粘度和感官各项指标,选出5株球杆菌。将优选出的球菌和杆菌以1:1(v/v)混合发酵脱脂乳,根据酸度、凝乳时间、粘度和感官各项指标选出发酵性能最好的球菌和杆菌组合为S28-L24。采用正交设计试验研究了球杆菌比例、全脂乳浓度、发酵终点以及砂糖含量对中温发酵酸乳的风味和游离氨基酸,游离脂肪酸含量和挥发性风味物质等的影响。分别测定酸乳的游离氨基酸,游离脂肪酸含量,并采用描述分析感官评价法进行感官评价,固相微萃取/气质联用(SPME-GC-MS)进行风味成分的测定。主成分分析法(PCA)和偏最小二乘回归分析法(PLSR1)研究了感官评价结果和仪器测定结果的相关性。结果表明:球杆菌比例和全脂乳浓度是影响酸乳品质的主要因素。偏最小二乘回归分析表明酸乳气味与游离脂肪酸含量相关性较强,而风味和余味与游离氨基酸含量相关性强,并得到奶香味、典型酸奶香味、尖酸气味、涩味风味、苦味、持久性余味等与挥发性成分,乳酸和柠檬酸量的相关性。对中温发酵酸乳和高温发酵酸乳(按42℃发酵4小时左右制作的酸乳)的后熟结束的乳酸量、柠檬酸量、游离氨基酸量、游离脂肪酸含量、挥发性风味物质含量和感官品质等指标进行了对比研究。结果表明:经感官评定,中温发酵酸乳的凝乳状态粘稠、细腻,香气浓郁纯正,酸甜适口,其典型酸乳挥发性物质和游离氨基酸量更多,表观粘度更大,而STS更小。酸奶的品质很大程度上取决于乳酸菌发酵剂。瑞士乳杆菌6024发酵乳偏酸,但粘度大,凝乳粘稠。S28发酵乳的游离脂肪酸含量和酸乳典型挥发性物质含量多,表观粘度大,STS小,因此其风味和质构最好。L24蛋白水解力强,其发酵乳游离氨基酸最多。而BD-Ⅱ干酪乳杆菌和嗜酸乳杆菌GM发酵乳挥发性物质较简单,表观粘度小,STS大,酸乳风味和质构都有所缺陷。更多还原

【Abstract】 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.更多还原