Drying Kinetics And Anti-browning Mechanism Of Thompson Seedless Grapes

Grape is one of the most popular fruit crops all over the word and China is the world’s largest producer. Drying grapes into raisins is a major processing method in almost all grape-growing countries. Long time drying process and browning seriously are two big issues of drying grapes. High-humidity hot air impingement blanching (HHAIB) technology was applied to the pre-treatment of grape drying. The effect of HHAIB pre-treatment on the Polyphenol oxidase (PPO) residual activity, the drying kinetics of grape and the color of the dried products were investigated. The grapes were dried by the pulse vacuum dryer with no pre-treatment. The process of heat and mass transfer mechanism and browning kinetics were studied during the drying. Weibull distribution model was applied to describe the drying process. The influencing factors and physical meaning of parameters in the Weibull distribution model were also studied.Blanching temperature and the humidity of hot air had significant impact on the PPO activity of grape samples during the blanching. PPO residual activity in the grape samples will be about10%when the blanching time is90s and blanching temperature is110℃. The zero-order model was most suitable model for describing the PPO inactivation kinetics. Activation energy (Ea) of PPO inactivation was17.34kJ/molby Arrhenius equation.Blanching temperature and blanching time had significant impact on the drying time, but the effect of HHAIB time on drying rate was more significant than the effect of HHAIB temperature. Grapes samples did not show a constant rate period of drying under the experimental conditions and showed only a falling rate period. Moisture effective diffusivity (Deff) values of grape samples range from1.08×10-10to1.65×10-10m2/s, when the samples were treated by HHAIB at different temperatures for different time and then dried at65℃. The activation energy of Thompson seedless grapes pretreated by HHAIB at110℃for90s was56.39kJ/mol. In terms of the PPO residual activity, drying behavior and the color quality of the dried product, HHAIB grape pretreated at110℃for90s then dried with55-60℃was the most favorable condition for gape drying result in desirable green-yellow to green raisins.Grape drying characteristics of pulse vacuum was investigated. It was found that the atmospheric pressure duration and vacuum pressure duration had significant effects on the drying time. The better drying parameter was the atmospheric pressure duration for4min and vacuum pressure duration for15min. The drying rate initially increased and then decreased gradually with the drying time, and a constant rate period would show at some drying parameters. High and low temperature of grapes internal temperature occurred alternately with the pressure change of the drying chamber. According to the internal temperature of grapes, the process of heat and mass transfer during the drying was studied.The browning index increased during the drying. The zero-order, first-order and first order fraction kinetics model were tested for their applicability to the browning index data for grape samples. The kinetics model of browning was established. Activation energy (Ea) of grape browning was66.40kJ/mol by Arrhenius equation. Weibull distribution model could well describe the drying curves under different pre-treatments and drying methods. It was found that the scale parameter (a) was depended on the drying temperature, the drying method and the material tissue structure. The scale parameter (a) decreased with the increase of drying temperature. The shape parameter (β) was depending on the drying method and materials tissue structure. For the same drying method, drying temperature had little impact on the shape parameter, the calculated moisture diffusion coefficient (Dcal) of the grape samples were calculated, ranging from0.2982×10-9m2/s to2.7700×10-9m2/s. The activation energy for moisture diffusion of grape samples was72.87and61.43kJ/mol by hot air drying and pulse vacuum drying, respectively.