Research On Effect And Mechanism Of Freeze-thaw Treatment On The Quality Of Explosion Puffing Drying Sweet Potato At Variable Temperature And Pressure Difference

In this dissertation, crisp sweet potato was dried by explosion puffing drying at variable temperature and pressure difference. On the basis of the effects of pretreatment methods on the characteristic of sweet potato dried by explosion puffing drying, sweet potato hot air drying as a control, the effects and mechanism of freeze-thaw on the quality and drying characteristics of sweet potato by explosion puffing drying at variable temperature and pressure difference were investigated.With puffing dried product quality characteristics as indicators, the effects of moisture content, boiling time of maltose syrup solution and freeze-thaw treatment on explosion puffing drying sweet potato were analyzed. The appropriate pretreatment condition was moisture content of75%-80%, sugar boiled by40%maltose syrup solution at100℃for2min, freezing at-18℃for12h and then at room temperature20℃natural thawing. The optimum puffing drying condition of sweet potato was puffing temperature100-105℃, vacuum drying temperature75-80℃. At this puffing condition, the dehydration sweet potato was of good color and crispness.The effects of freeze-thaw cycles on the quality of the explosion puffing sweet potato were compared. The results showed that, with the increase of freeze-thaw times, the porosity and rehydration of the dried product gradually increased, and AE gradually became smaller, while the L*value, hardness and conductivity decreased slightly and then increased, but the anthocyanins increased and then decreased. Crude protein content changed less with the increasing of freeze-thaw cycles. It was found that the isothermal adsorption curves of crisp sweet potato were counter-S obviously.The effects of freeze-thaw cycles on drying characteristics of explosion puffing sweet potato under different vacuum drying temperature were compared. The drying kinetics models of freeze-thaw sweet potato were founded. It was found that freeze-thaw pre-treatments could significantly decrease the drying time and increase the drying rate, and the effective moisture diffusivity (Deff) increased with the increasing of times of freeze-thaw. Henderson and Pabis model provided better simulation of drying curves for freeze-thaw sweet potato at different drying conditions.The water distribution and water status of each group in the sweet potato after freeze-thaw were studied using low-field nuclear magnetic resonance (LF-NMR). The gelatinization properties and cell tissue structure of sweet potato were studied using rapid viscosity analyzer (RVA) and scanning electron micrographs (SEM), respectively. The results showed that there were four distinct water populations in the data of multi-exponential fitting of sweet potato. With the increasing of freeze-thaw cycles, the free water content (mT24) and the closely combined water content (mT2i) increased first and then decreased, the intercellular spaces of the sweet potato after freeze-thaw gradually increased, and the cell tissue structure became looser. The correlation analysis showed that porosity had significant relationship with T24and mT23(P＜0.05), and the correlation coefficients were0.995,0.989, respectively. The correlation coefficient between mT22and AE was0.984. The anthocyanin content had significant relationship with conductivity (P<0.05).Freeze-thaw pretreatment process will help to improve the quality and increase the drying rate of sweet potato explosion puffing drying at variable temperature and pressure difference. The quality of fruits and vegetables by explosion puffing drying at variable temperature and pressure difference were predicted using low-field nuclear magnetic resonance. The T24value of freeze-thaw sweet potato on can be a symbol of the porosity of explosion puffing drying sweet potato. Gelatinization properties and cell structure is closely related to the quality of sweet potato dried products. The raw materials cell structure were appropriately changed will help to improve the quality of fruits and vegetables explosion puffing drying at variable temperature and pressure difference.