Traditional and recent development of pretreatment and drying process of grapes during raisin production: A review of novel pretreatment and drying methods of grapes

Abstract

Raisins are highly produced value-added products that are extracted from grape processing. The most important unit operations in raisin production are pretreatment, followed by drying. Due to the presence of waxy layer, the drying cannot be used alone; however, the layer can be removed by pretreating the grapes with chemicals or physical abrasion. The food safety and environmental issue with the chemicals have forced to find alternative methods. The physical method, such as surface abrasion, is explored but showed negative effect of color. The nonthermal methods such as microwave (MW) heating, pulsed electric field, ultrasound, and ohmic heating are observed, and their outcomes are discussed in this review. Carbonic maceration, high-humidity hot air impingement blanching, and MW hydrodiffusion and gravity are the novel methods that showed promising results. In the latter part, the traditional drying process of grapes is discussed, where shade drying is found to be used remarkably in raisin production. However, the time of drying was long, and the process required large land area. Alternatively, novel methods such as hot air, MW, vacuum, and infrared drying provide higher drying rate and good-quality raisins. However, the combined drying methods such as MW-assisted air drying, pulsed vacuum drying, and MW vacuum drying have shown better results than single drying. Pretreatment and drying cause changes in physical and chemical properties of grapes, such as changes in color, size, nutrient content, and polyphenol content. Overall, the review covered the crux output with the advancement in raisin development, which could be useful for novel research on grape processing.