Research On Effect Of Different Drying Methods On Moisture Diffusion Properties In Apple Slices

In this paper, Fuji apple was chosen as raw materials to study the moisture diffusion properties in different drying process from both macro and micro perspectives. Based on drying curves and effective moisture diffusion coefficient, LF-NMR (low-field nuclear magnetic resonance), DSC (differential scanning calorimetry), SEM (scanning electron microscopy) were innovatively applied in the study. Moreover, the effects of different methods on moisture diffusion were carried on in the comparative analysis. This study aimed at providing theoretical guidance for the drying process of apple slices. Through the experiment, main conclusions are as follow:1. Results of the effect of hot air drying on moisture diffusion property showed that:the speed of internal temperature increased with ambient temperature rising, in which higher temperature could promote moisture diffusing and the drying rate ratio of 50℃,70℃ and 90℃ was 1:1.56:2.82. Higher temperature could also accelerate the change of water state, and water gradually shifted from moisture with high freedom to moisture with low freedom. Along with water content decrease, Tg and aw rose and declined, respectively which had important role on food shelf-life prolonging. Immobilized water and free water had extremely significant effect on aw (p<0.01) and immobilized water had significant effect on Tg(p<0.01). In addition, extremely significant correlation was obtained between Tg and aw (p<0.01). Water removal made volume obviously shrunk and showed contraction distortion in its microsture, in which formed cavities and relatively compact structure, so as to influence the quality of apple slices.2. Results of the effect of short- and medium-wave infrared drying (SMW-IFD) on moisture diffusion property showed that:temperature rising could improve water diffusing where the drying rate ratio of 50℃,70℃ and 90℃ was 1:1.6:4.4. Thus, SMW-IFD could effectively increase water diffusion rate. Mutual transform phenomen occurred in water state and represented freedom decrease during drying. Water diffusion caused changes of Tg and aw, in which immobilized water influenced greater. Correlation ship between Tg and aw (|r| >0.85) that could be used to predict water change. Rapid water loss changed internal water gradient and led to irregular shrinkage to center of apple slice because of the volume changing along with water content. Thus, obvious contraction distortion occurred in microstructure.3. Results of the effect of instant controlled decompression drying (ICDD) on moisture diffusion property showed that:internal water instantaneous vaporized with the high temperature and instant pressure change which caused rapid phase transition. High temperature not only enhanced water diffusing, but also provided power for water vaporization, so that it could induce internal structure puffing and avoided shrinkage phenomenon during drying. Flashing times increase was conducive to water diffusion which shortened vacuum drying time. Water state might affect water loss rate, where it was easily removal due to relatively higher mobility of free water and immobilized water before flashing 5 times, and then decreased along with freedom of moisture declining.5 times was appropriate after comprehensive analysis of quality in apple slices.4. Results of the effect of hot air-instant controlled decompression drying (HA-ICDD) on moisture diffusion property showed that:this kind of unit drying method could shorten drying time and enhanced drying efficiency. Pre-drying water content and samming ways had different effects on water diffusion property, where the lower the pre-drying water content, the faster the moisture freedom changed and different samming processes partly increased moisture freedom and free water content. Changes of water content and water state could also influence Tg and aw, in which higher pre-drying water content made aw increase and Tg decrease in apple chips. After samming treatments, awwas relatively higher and Tg was a little lower that weaken crispy taste of apple chips. Pre-drying water content 30% and normal samming way were appropriate after comprehensive analysis of quality in apple slices.