Drying Characteristics And Discoloration Mechanism Of The Thompson Seedless Grape

In this paper, Xinjiang thompson seedless grape used as the material, the air-drying characteristics were firstly studied, the grape drying model was established and the effective diffusion coefficient was calculated, the effect of temperature on the shrinkage of grape was also analyzed; then the characteristics of grape color change was studied, the chemical mechanism of the typical color composition change were studied to Clarify the mechanism of color change of the grape during the drying process; based on the study of the water diffusion and the color change of the grape during the air-drying process, the suitable drying temperature range were selected to study the heat pump drying, the effect on drying characteristics and dried product’s quality of different temperature and different pre-treatment during the heat pump drying were studied, according to the improvement of the drying rate and the quality of the product, the best pre-treatment and drying condition were determined. The main results were outlined as following:1. Logarithmic model can well discribe the drying process. The effective diffusion coefficient of the Thompson seedless is smaller than Thompson seedless.cv. Effective diffusion coefficient increases with temperature rise, but at a lower temperature effective diffusion coefficient affected by temperature changes is small. Grape shrinkage occurred during drying resists the internal moisture diffusion, the rate control of drying belongs to the internal conditions. Shrinkage occurrs during drying, Shrinkage resists the water diffusion and the drying rate decreases with the shrinkage,Temprature affects the effect between the shrinkage and the rate.2. Based on Lab color system, the quantitative analysis about the color of the Thompson seedless grape was carried out, the color dynamics model was established. And the color parameters of L*, a*, b* and A E reaction activation energy were 40.19,24.29,35.43, and 39.46 kJ mol-1, respectively. L* value of activation energy was the largest and was mostly effected by the temperature. Browning reaction occurred in the middle of the drying process, as the moisture content was between 50% and 60%. L* and b* values dropped, a* and △ E value increased, browning degree was related to the temperature, the higher the temperature, the more serious the browning. The color of raisins was relatively good at 30 ℃, and the color of raisins was worst at 40 ℃.3. Temperature and oxygen is the important factors to change color in the process of drying, the existence of oxygen will promote structural changes of grape color composition and promotes to change the grape color.4. Free radical reaction and the formation of quinone is the important reason for the color changes of grape in the process of drying.(1) The gallic acid, quercetin and Yang mei, using quantum chemical calculation of density functional method found that:Gallic acid phenol hydroxyl 013-H17 longest bond length and bond order minimum, dissociation to lowest, the spin density distribution is stable, relative to other phenolic hydroxyl groups, the hydroxyl free radicals due to hydrogen reaction and form quinone structure. Quercetin molecule hydroxyl in B ring is A ring hydroxyl prone to seize the hydrogen reaction and form quinone.Quercetin in B ring of phenol hydroxy 4 ’phenolic hydroxyl (027-H32) bond length is relatively long, bond order is low, free radicals high reactivity, prone to seize the hydrogen reaction and form quinone structure. The free radicals reaction activity sites of Myricetin also in B ring ’4, followed by its ortho hydroxyl H33.Free radical reaction and form quinone material is the important reason for the grape changes color in the process of drying.5. Based on the above study, the suitable drying temperature range were selected to study the heat pump drying, showing that the best way of heat pump drying for Thompson seedless grapes was that uses dry-promoter to pre-treat grapes and the the drying temperature was set 35 ℃ , which not only can effectively shorten the drying time and also can inhibit fruit browning.