The Character Change During Chinese Steamed Bread In Storage

In this thesis, on the basis of examining and analyzing the quality of steamed bread on the market, the author mainly expounds the physiochemical and microbiological changes of steamed bread during storage. First of all, the thesis clarifies how some vital factors such as moisture, temprature and humidity affect the characters of steamed bread during storage. Then it deals with the qualitative changes of steamed bread during storage. After that, the author discusses and confirms the concept of spoilage of steamed bread. Meanwhile, the best storage way to prevent steamed bread from decaying has been approached and pursued. Moreover, the recommendatory standard for safety index of the microbe in steamed bread has also been put forward, and some major microbes, which may lead to the decay of steamed bread, have been isolated and identified. The main findings of this program are as follows:1.The data of the investigation on the steamed bread on the market: their pH value tends to be medium, which is between 6.1and 7.2; the water content is high, exactly to say, between 36% and 45%; the activity coefficient of water is between 0.945 and 0.986; the number of bacteria in steamed bread is higher, that is, the bacteria quantity of about 85 percent of those samples is between 800~5000/g; the coliform of 80% of those samples is under the national standard (<30/100g), which is optimistic; however, the content of sulphite of 70% of steamed bread is much higher than the food maximal safety dosage (0.25g/kg), which is very serious; meanwhile, the investigation also indicates that the samples whose sulphite content is higher will hold higher white degree and lesser bacteria.2.The role of ferment of tough dough by way of staying over in low-temperature: to slow the sclerosis speed and restrain the growth of bacteria. On the one hand, the sclerosis speed of steamed bread will be slow and the steamed bread will be kept soft for longer time when water content retains such high degree as between 35% and 40%; on the other hand, there will be some peculiar smell and the cuticle of steamed bread will be pasty when the quantity of bacteria reaches 106/g while there will be obviously spoiled smell and sticky phenomenon within the steamed bread when the quantity of bacteria reaches 107/g. Moreover, lower temperature and lower humidity can inhibit the proliferation of microbes. The prime microorganisms causing the spoilage of steamed bread stored in higher temperature are bacteria while the prime microorganisms causing the spoilage of steamed bread stored in the higher humidity are moulds.3.The course of qualitative change during the storage of steamed bread: the moisture of the coat, the pulp and the core tends to decrease gradually. In the first 12h, the downtrend is fastest. At the end, the moisture content of pulp is a little higher than those of the coat and the core. As a matter fact, the water activity and vitamin B6 decrease at first and then increase later. The reducing carbohydrate shows the tendency of decreasing while the enzyme activity presents the tendency of increasing. Therefore, the lower temperature will make the loss of nutritive elements (vitamin B6 and reducing carbohydrate) lesser and make the uptrend of the enzyme activity slower. Meanwhile the pH value is decreasing in fluctuation.4. The changes of steamed bread in macrostructure: it is mainly bacteria that result in the sticky phenomenon. By isolating and identifing these bacteria, we can find out that most of them are gemma bacillus and capsule bacteria. It is moulds that make steamed bread coming out yellow, gray, cyan spots on the surface. By isolating and identifing these moulds, we can find out that most of them are aspergillus and few of them are penicillium.5. The change of steamed bread in microstructure: there is no clear separation between solid gluten and mushy starch in fresh steamed bread because of their cohesion. However, the starch in the pulp of aging steamed bread is clearly visible. The division among starch particles is more legible as the steamed bread is aging. Meanwhile, the volume of starch particles in pulp is becoming smaller in the aging of steamed bread. However, the gluten in core cell remains unchangeable.6. The optimum re-heating time: the re-heating time for 100g cut steamed bread should be 10min by steaming while it should be 1min by using microwave.