| Instant controlled pressure drop drying(DIC)had been gradually applied to the fruit and vegetable processing industry.However,because the drying mechanism was unclear,the quality of DIC products was still unstable.Therefore,in this study,apple cubes were used as the sample material to explore the drying mechanism of DIC and the factors affecting the product quality.The results of this study can give advice to accurately controlling the quality of DIC products.(1)The internal water migration of apple cube during the DIC drying process was studied.In the hot air pre-drying process,smaller apple cube had a lower effective moisture diffusion coefficient,and the drying time was decreased due to a reduced moisture diffusion path.The higher temperature increased the effective moisture diffusion coefficient of apple cube,and the drying time was decreased due to the accelerated water transfer rate.After pre-drying to a moisture content of 2.14 g/g d.b.,the apple cubes had obvious deformation.The shrinkage of apple cube was stable in the later period of hot air drying.When the apple cubes were dried to the water content of 0.60 g / g d.b.,the thermal characteristics changed significantly.Comparing the changes of the water status of apple cube before and after the MEP,the results showed that 4 ℃ and relative humidity of 40%-45% were the most proper conditions for EMP.After EMP,the internal moisture of apple cube self-diffusion,the range of T2 of water became narrow,and the water status tended to be stable.The results showed that the MEP could induce a more homogenous spatial distribution of water in the semi-dried apple cube after pre-drying.(2)The state of apple cube in the DIC process was studied and the expansion mechanism was investigated in detail.The results showed that the energy generated by the water instantaneous vaporization at the moment of decompression caused the expansion of the internal pores of the apple cube,which increased the volume(maximum volume expansion ratio reached 2.627)and absolute porosity(maximum absolute porosity ratio reached 0.627)of the apple cube.Meanwhile,the instantaneous vaporization energy of water was significantly positively correlated with the transition moisture content of apple cube(p < 0.01).In addition,the apple cube with the transition moisture content range of 0.134-0.248 g/g(d.b.)had homogeneous large pores after DIC treatment,whereas the pore walls of apple cube with the transition moisture content of 1.068 g/g(d.b.)were collapsed after DIC.Furthermore,the transition moisture content and the moisture vaporization energy significantly affected the volume expansion ratio and hardness of the DIC apple cube(p < 0.01).Moreover,DIC dried apple cube products had the optimal volume expansion ratio(2.317 ± 0.310),hardness(139.44 ± 16.52 N)and crispness(90 ± 17)with the transition moisture content of 0.152±0.018 g/g(d.b.).(3)Hydrolysis by pectinase and pectin osmosis were applied to apple cubes before instant controlled pressure drop(DIC)drying.The water migration in the apple cubes during the pretreatment and drying process were studied by low-field NMR.The results showed that the galacturonic acid(Gal A)content of the apple cubes hydrolyzed by pectinase decreased(the maximum decrease was 49%)whereas the pectin osmosis for 1 h could increase the Gal A content of the apple cubes(the maximum increase was 14%).The osmosis treatment hindered the moisture migration inside the apple cubes during drying,whereas,the short-time pectinase hydrolysis could accelerate the moisture transport.On the other hand,the hardness and crispness of apple cubes which were osmosed by pectin was higher(the maximum value was 106 ± 13 N,and 53 ± 8 respectively)than that hydrolyzed by pectinase(the maximum value was 38 ± 15 N,and 2 ± 1 respectively).Simultaneously,the enzymolysis and osmosis of pectin caused the non-uniform pore distribution.In addition,the enzymolysis and osmosis treatments of pectin had no significant correlation with the expansion ratio of the apple cubes(p > 0.05),whereas the hardness and crispness of DIC dried apple cubes were related to the peak area of cytoplasm water(A22).(4)Effects of instant controlled pressure drop drying(DIC)on the modification effect of water-soluble pectin(WSP),chelator-soluble pectin(CSP)and sodium-carbonate-soluble pectin(NSP)of apple were studied to explore the effect of DIC on the texture formation of apple cube from the aspect of material characteristics.The results showed that there were obvious differences in hygroscopicity of the three pectin fractions after DIC.The increase in the bond water ratio of CSP fraction was the most significant after DIC,which was 21.62% and 35.91%,estimated from DSC and NMR data respectively.The Gal A content of WSP increased significantly,whereas the Gal A content of NSP decreased significantly after DIC,which was 117.35% and 36.60%,respectively.Meanwhile,the degree of methylesterification(DE)of CSP was decreased by 43.04%.Although the FT-IR results showed that the change of group structures was slightly,and the state of the pectin changed significantly after DIC.The results of the pectin properties analysis have shown that there were strong correlations among the water binding properties of pectin and the Gal A content,DE,state of pectin after drying by DIC.(5)By changing the external conditions,the cause of the texture formation of DIC dried apple cube was further revealed.The results showed that the water status of apple cube which were dried by hot air and/or vacuum freeze pre-drying combined with DIC had different trends.Free water changed drastically in hot air pre-drying,while water in vacuum freeze pre-drying was mainly the immobile water.Vacuum freezing-DIC dried apple cube had poor texture.The hardness and crispness of apple cube during the hot air drying-DIC were significantly related to the strong binding water;while the hardness and crispness of apple cube during the vacuum freeze drying-DIC were related to the immobile water.The MEP treated and DIC dried apple cubes showed better quality in terms of the appearance and texture properties including high porosity(71.77%),large pores(maximum pore size of 1.55 mm),and thin pore walls(pore wall thickness of 0.079 mm).Besides,osmotic pretreatment and DIC combined with HAD could significantly increase crispness(57-64)and reduce hardness(138-145 N)of apple cubes. |
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