Study On Effects Of Freezing And Thawing On Quality Of Tofu

As a traditional soybean food,frozen tofu has a long history with rich nutrition. It is not only more easily stored and distributed than bean curd,but also quite loved by consumers with its unique taste. However, at present, domestic frozen tofu are confined to the production mode with homemade or individual workshop, the texture, nutrition and other quality characteristics cannot be guaranteed. As the result, this study focused on the effects of different freezing and thawing methods on quality of tofu with textural properties, sensory qualities, nutrient components and health statistics as indicators. In addition, the freezing– thawing curve was established and their microstructure were also observed, the analyses were carried on to get high quality frozen tofu and provide the reference for the industrialized production of frozen tofu. The research content and main results were as follows:(1) Freezing methods played an extremely important role on the quality of frozen tofu. Considering about economy and practicability, Refrigerator freezing and blast freezing were taken to freeze tofu, and the single factor and orthogonal design experiments were done to determine the optimal freezing conditions, which were: under –10 ℃, freezing for 3.5 h, and then placed in –18 ℃ for 21 days; under –15 ℃, freezing for 2 h, and then placed in –18 ℃ for 21 days.(2) Frozen tofu was thawed under different microwave power(50 W、150 W、400 W、500 W、675 W) and time(30 s、60 s、90 s、120 s、150 s), and compared with fresh tofu. To explore the effects of microwave thawing on the quality of frozen tofu, the characteristics such as basic composition, texture, pH, color and sensory quality were investigated, the result showed that the optimal defrosting parameters was 150 W、90 s.(3) The frozen tofu under optimal conditions were thawed by air thawing, water bath thawing, cold-storage and microwave thawing to study the change of textural properties and sensory qualities. The results showed that: for freezing processes, blast freezing had a slightly greater effect on hardness and springiness than that with refrigerator freezing; the color and pH of air thawing tofu were poor, and its sensory evaluation score was also low. The texture properties and sensory quality of water bath thawing tofu were also not so good, microwave thawing got better product quality, and sensory evaluation scores were significantly higher than others.(4) Nutrition and safety of tofu with different freezing-thawing combination were analysised, the loss of the nutrition before and after freezing were measured by the methods of quantitative approaches, and the total colony and coliform count were also measured. The results showed that: air and water bath thawing showed greater loss, and the nutrient content were lower than those by other ways, cold-storage and microwave thawing have less nutrient loss and better nutrient retention, and microwave thawing was more close to fresh tofu; the total colony of blast freezing was significantly lower than that of refrigerator freezing, the number of coliform count was not higher than that of refrigerator freezing, the total colony exceed the standard by air thawing only, and the total number of colonies in microwave thawing was the lowest. Blast freezing had less nutrients loss and lower microorganism index because of its rapid freezing process, so its nutrition and security are better.(5) The freezing-thawing curve was established, the microscopic structure were observed, and a further insight in the mechanism of frozen denaturation was given. The results showed that: the heat transfer efficiency had a direct link with the outside temperature and wind speed. Two methods of freezing had large differences on time to pass the zone of maximum crystallization with 60, 138 min respectively. Combined with the scanning electron microscopy(SEM) subsequently, we could see that the hole of tofu by blast freezing was compact and uniform, and that of refrigerator freezing was relatively rough, so its texture properties and taste were worse than the former. Two kinds of freezing thawing curve was consistent, and the time of air thawing, water bath thawing, cold-storage and microwave thawing were: the refrigerator freezing, 120 min, 21 min, 9 h 10 min, 90 s, blast freezing, 150 min, 51 min, 9 h 50 min, 90 s. In practice, blast freezing technology was the first choice to produce frozen Tofu. With SEM, it showed that the structure of water bath thawing was confused, there was no obvious difference in the gel network structure of others; infrared spectroscopy showed that, the characteristic peaks during thawing process did not transfer, so the thawing methods did not change the functional groups, just changed the content of feature structure. Research on the correlation between secondary protein structure and texture properties had been studied, the result showed that the chewiness was positively correlated with the content of α-helix and β-turn, negatively with content of β-sheet.