Effect of freezing methods on quality of frozen raw baozi

The freezing treatments had adversely affected the viable cell counts and the structure of the gluten network in the baozi dough, thereby deteriorating the baozi quality. To investigate the optimal freezing method for raw dough baozi, the freezing rates of various methods including conventional refrigerator freezing (RF_{-18 ℃}), traditional quick-freezing cabinet (TF_{-40 ℃}), liquid nitrogen quick-freezing (LNF_{-30 ℃}, LNF_{-40 ℃}, LNF_{-50 ℃}, LNF_{-60 ℃}, LNF_{-90 ℃}), and liquid nitrogen immersion freezing (IF_{-196 ℃}) were determined. The effects of these freezing methods on the rheological properties, moisture distribution, the content of free sulfhydryl groups and freezable water, and microstructure of the frozen dough were also assessed. Additionally, the freezing rates, specific volume, texture properties, and sensory evaluation of the frozen raw baozi were determined to analyze the impact of freezing methods on the quality of the bao wrapper. The results showed that the freezing rates were in the order of IF_{-196 ℃} (50.38 ℃/min)>LNF_{-90 ℃} (7.01 ℃/min)>LNF_{-60 ℃} (4.48 ℃/min)>LNF_{-50 ℃} (3.66 ℃/min)>LNF_{-40 ℃} (2.69 ℃/min)>LNF_{-30 ℃} (1.49 ℃/min)>TF_{-40 ℃} (1.30 ℃/min)>RF_{-18 ℃} (0.38 ℃/min). Faster freezing rates resulted in the freezable water content, water distribution, and dynamic rheological properties of the raw dough more closely resembling those of fresh raw dough. However, the fermentation rheological properties, viable count of yeast, the maximum dough fermentation height, the maximum height of the gas emission curve, and the total gas volume were quite different from fresh dough. Raw baozi when frozen under IF_{-196 ℃}、LNF_{-90 ℃}、LNF_{-60 ℃}, and LNF_{-50 ℃} were significantly higher, the elasticity was significantly lower than other freezing methods, and the quality deteriorated in terms of flavor and color. The viable count of yeast, the maximum dough fermentation height, the maximum height of the gas emission curve and the total gas volume of dough, the specific volume, the texture and the sensory scores of baozi through LNF_{-40 ℃} were significantly higher than other freezing methods. The quality of raw baozi through LNF_{-40 ℃} was closer to fresh baozi. Therefore, LNF_{-40 ℃} is a suitable freezing method for the processing of frozen raw baozi.