The Application Of NMR In Storage And Quality Evaluation Of Yogurt

Yogurt has become an important part of the daily diet, and the probiotics and the various other nutrients in yogurt have played a huge role in improving human health. However, as the rising of living standards,people have more requirements for yogurt, they want yogurt to have more health functions and at the same time to taste better. Recent researches on dietary fiber made people realize that this kind of material was also important for human health, so dietary fiber was defined as the seventh major nutrients.These days people eat more meat than vegetables, the intake of dietary fiber is seriously inadequate. Resistant starch not only has similar heslth function features as dietary fiber, but also has more other functions, such as it has smaller particle size and smooth texture, so the addition of resistant starch in food will not have sandy taste. The addition of resistant fiber will increase the health functions of yogurt.In this thesis, we combined nondestructive and noninvasive feature of nuclear magnetic resonance with the physical and chemical experiments to improve the production process of resistant starch, at the same time we studied the impact of resistant starch on the water holding capacity of yogurt and developed a relationship between NMR relaxation parameters and the yogurt physical and chemical properties. The main conclusions are as follows:(1) Through single factor and orthogonal experiment designs and tests, we obtained the optimum processing conditons for the yogurt with high level resistant starch:4%resistant starch,7.5%white sugar,5min homogenization time,44℃fermentation for5h.With the developed production process,the obtained yogurt not only had good yogurt flavor,but also was more uniform and delicate in structure,and no coarse and sandy texture.(2) During the fermentation process, the addition of resistant starch could significantly increase the number of Streptococcus thermophilus, but not the number of Lactobacillus bulgaricus significantly. During the storage process, the network structure formed by resistant starch played an important role in preventing water migration and loss which help maintain the yogurt structure and water content。 The addition of resistant starch ensured the water content at a proper level for lactic acid bacteria growth in the yogurt during storage, ensuring the yogurt still maintained a high content of lactic acid bacteria even after a long storage time. (3) The addition of resistant starch in yogurt not only improved the functional properties, but also provided physiological functions of the resistant starch. Through the analysis of nuclear magnetic resonance properties, it was observed that during the storage of yogurt, the state of water changed tremendously,and the addition of resistant starch clearly influenced the state of water changes,so NMR technique could play a significant role in the non-destructive and noninvasive water analysis in yogurt.(4) Through the analysis of pH changes and lactic acid bacteria changes during the fermentation process, it could be observed that the growth and division of lactic acid bacteria promoted acid production, but the acid would also inhibit the metabolic activity of the lactic acid bacteria. During the fermentation process of resistant starch added yogurt, the proton density corresponding to T21showed an upward trend as the pH declined. When the pH reached to4.50, the proton density would not increase anymore. During the fermentation process,it could be observed that there is a good linear relationship between pH and the NMR proton density corresponding to the monolayer water.Therefore NMR relaxation techniques could be used to predict the fermentation extent of yogurt non-destructively and non-invasively.