Novel high-humidity hot air impingement blanching (HHAIB) pretreatment enhances drying kinetics and color attributes of seedless grapes

Seedless grapes blanched by high-humidity hot air impingement blanching (HHAIB) at different temperatures (90, 100, 110, and 120 °C) and several durations (30, 60, 90, and 120 s) were air-dried at temperatures ranging from 55 to 70 °C. The PPO activity, drying kinetics, and the product color parameters were investigated to evaluate the effect of HHAIB on drying kinetics and color of seedless grapes. The results clearly show that HHAIB not only extensively decreases the drying time but also effectively inhibits enzymatic browning and results in desirable green–yellow or green raisins. In view of the PPO residual activity, drying kinetics and color attributes, HHAIB at 110 °C for 90 s followed by air drying at 60 °C are proposed as the most favorable conditions for drying grapes. These findings indicate a new pretreatment method to try to enhance both the drying kinetics and quality of seedless grapes. Drying grapes into raisins is a major processing method in almost all grape-growing countries. Drying grapes is more difficult than some other biological materials, since a thin-layer of wax covers on its surface peel. Currently, chemical pre-treatment methods are used frequently to dissolve the wax layer and accelerate dry rate. However, the chemical additive residue in the raisins may cause food safety problems and how to deal with larger quantities of corrosive chemicals is a serious problem. HHAIB is a new and effective thermal treatment technology with advantages such as minimum solids loss, uniform, rapid and energy-efficient blanching process. The current work indicates that HHAIB may be a useful non-chemical pretreatment technology for seedless grape drying, which can not only accelerate drying kinetics but also improve color parameters of seedless grape.