Conclusions
The effects of flour protein content and freezing conditions (freezing air temperature and freezing air speed) on dough quality and steamed bread quality were examined. The freezing process caused a damage to gluten network as well as a decrease in yeast activity, which resulted in lower Hmax and total gas production in Rheofermentometer analysis, and lower specific volume and form ratio and increased hardness of steamed bread.
Protein contents affected the extent of freezing damage to dough quality; the low-protein dough samples were the most damaged by freezing, as shown by the greatest decrease in average Hmax and form ratio after freezing, as well as the most disrupted gluten network observed in SEM analysis and lowest steamed bread specific volume. However, few quality parameters were found to have a clear dependence on protein content for frozen dough; dough extensibility and Rmax increased with protein content, while R/E ratio and Hmax of dough and specific volume, form ratio, and hardness of steamed bread did not show clear trends as protein content varied. Therefore, protein content alone could not explain the differences in dough quality parameters.
Freezing rate of the dough also affected the extent of damage to the gluten network and yeast activity; moderate freezing rates resulted in dough with higher extensibility and a less damaged dough microstructure with more continuous gluten matrices as observed in SEM analysis. Fast freezing rates, however, led to a decrease in total gas production as the yeast activity was compromised. These resulted in a lower specific volume and higher hardness for steamed bread made from the dough that was frozen at fast freezing rates, especially for low-protein samples. Generally, the extreme freezing conditions 2A (−20 °C, 0 m/s) and 4C (−40 °C, 6 m/s) had the poorest dough and steamed bread quality. Freezing air temperature and air speed had significant effects on several dough and steamed bread quality parameters. Samples frozen at −20 °C had poorer form ratio of steamed bread than those frozen at −30 and −40 °C, while those frozen at −40 °C, 6 m/s had the lowest specific volume compared to lower air speeds (0 and 3 m/s). The volume of total gas produced and Hmax were higher at a moderate air speed of 3 m/s, possibly due to the resultant moderate freezing rate that preserved a better balance between both yeast activity and gluten network. Lastly, there are practical insights that are relevant to the industry for making steamed bread from frozen dough. Steamed bread is typically made from flours with protein contents of 7.5–11 %, but frozen dough made with higher protein content (9.5–11 %) had better resistance to freezing damage, resulting in a final product of closer specific volume, form ratio and texture to those of bread made from fresh dough. Low freezing air temperatures are costly to maintain for practical applications in the industry. Therefore, it is recommended that to achieve a particular freezing rate, a combination of relatively higher freezing air temperature and moderate air speed should be employed.
Acknowledgments The authors are grateful to the Singapore Ministry of Education for funding the project through the Academic Research Fund Tier 1 research grant R143-000-404-112 and the National University of Singapore (Suzhou) Research Institute under the grant number NUSRI2011-007 and Jiangsu Province under the Scientific Research Platform scheme. The first author also likes to thank the Agency for Science, Technology and Research (A*STAR) and the National University of Singapore (NUS) for financial support.