Study On Preservation Of Chinese Steamed Bread Stored At Room Temperature

Chinese steamed bread(CSB) is a traditional staple food, which plays an important role in the diet and culture of Chinese people. CSB is attracting more and more people for its nutrition quality, texture and palatability. With the high-speed development of urbanization, it is of great practical significance to realize the industrialization of CSB. Nevertheless, CSB is easy to get rotten during storage at room temperature due to high moisture(40%) and water activity(0.970). Thus, maintaining freshness of the products is the key issue and premise for preventing quality deterioration in steamed bread during transpotation, storage and sales process.Firstly, the changes of the quality parameters of CSB stored at room temperature were evaluated. Results showed that the microorganisms grew fast, and the total plate count(TPC) exceeded the value of 6 log cfu/g only after 36 h; L* value and p H decreased firstly and reduced at last; Water holding capacity and the content of water soluble starch decresed significantly(P<0.05); During the first 48 h of storage, hardness, gumminess and chewiness of CSB increased significantly(P<0.05), and cohensiveness and resilience decerased remarkably(P<0.05) as well, however, the texture parameters of re-steamed bread got close to that of the fresh CSB. Large molecular weight proteins were partly decomposed with the deterioration of CSB, and amylopectin recrystallization took place, and ΔH increased from 0.04 J/g to 2.38 J/g during storage. In addition, water sorption isotherm of CSB decreased during storage, and the original structure in CSB was damaged, water distribution and loss took place. With the correlation and factor analysis of the changes in quality parameters, TPC and staling parameters(textue parameters and water holding capacity) were selected as the important indicators to evaluate the deterioration of CSB. Howerver, in this study, the stored CSB was consumed after re-steaming process. Therefore, staling parameters were not regarded as the important reference index.Secondly, CSB was made using 3 kinds of flour, which were native to Shandong, Henan and Hebe, respectively. The spoilage microorganisms in air-packaged CSB(ultraviolet radiation was used during cooling) were isolated and identified by means of morphological test and biochemistry test. Results showed that the main spoilage microorganism in CSB was Bacillus, regardless that microorganisms were different in different kinds of flour. In this study, 3 kinds of Bacillus(Bacillus licheniformis, Bacillus megatherium and Bacillus subtilis) were isolated from CSB.Thirdly, the effect of active packaging {oxygen absorber(OA), oxygen absorber combined with ethanol emitter [(OA+EE) 1% and 3% v/v] }and modified atmosphere packaging(MAP) on quality and microbial stability of CSB was evaluated. During storage, the oxygen content in the headspace of OA was under 0.1%, which prevented the growth of aerobic bacteria effectively, and the microbial shelf-life of the CSB could be extended to 6 days; There was a synergism between OA and EE, CSB packaged with OA+EE(1% and 3% v/v) had longer shelf-life. However, OA+EE(3% v/v) gave a strong objectionable flavor and taste, which was not fit for CSB preservation. OA+EE(1% v/v) could extend the microbial shelf-life of CSB to 11 days. Moreover, MAP could inhibit the growth of microbes. With increasing concentration of CO2, growth rate was decreased, and it is possible to prolong the microbial shelf-life of CSB to at least 8 days when packaged with 20% N2:80% CO2.Then, the impact of sourdough(10%, 20%, 30%) on quality and microbial shelf-life of CSB was investigated. In addition, the agar diffusion method and the microtiter plate well assay were used to assess the antimicrobial activity of sourdough and organic acid. Results showed that the addition of sourdough(10%, 20% and 30%) decreased the p H of CSB to 4.5, 4.3, 4.0, and extended the microbial shelf-life to 8 days, 30 days and 40 days, respectively. The proper addition amount of sourdough(10% and 20%) could improve the viscoelastic properties of the batters, improve the specific volume and texture of CSB. As p H value decreased and organic acid concentration increased, the bacteristasis effect increased, which indicated that p H decrease and the presence of organic acid were partly responsible for shelf-life extension of acidified CSB. However, the inhibition zone test proved that sourdough had stronger inhibiting effects on Bacillus than organic acid.Finally, the effect of hurdle technology(ultraviolet radiation during cooling, sourdough fermentation and active packaging) on microbial shelf-life of CSB was investigated. Results showed that there were synergy effects between each hurdle factor and longer shelf-life of CSB was achieved. The TPC of CSB(with 20% sourdough addition) packaged with OA was still under 4.5 log cfu/g after 90 days of storage. The p H maintained more or less constant(p H 4.3); Water loss and aging velocity of CSB decreased, the water loss rate was 2.65% after 90 days of storage, and the hardness of CSB was 4567 g on day 30 of storage. Confocal laser scanning micrographs of CSB showed that the protein network structure weakened during storage, and starch retrogradation took place.