Novel alternative pretreatment approaches for production of quality raisins from grapes: Opportunities and future prospects

Abstract

Raisins are high-value products obtained by drying grape berries. Natural drying of grapes is difficult because a hydrophobic waxy cuticle is present on the berry surface. Pretreatment is an important operation in raisin production in which the waxy layer is dissolved or removed for enhancing the drying rate of grapes. Chemical pretreatment consisting of dipping grape bunches in a solution of alkali and oil emulsion is commonly followed for raisin production. However, the threat of chemicals to human health, the environment, and longer processing time lead to the development of an alternative to chemical pretreatment. Various alternative methods have been explored including the abrasive pretreatment and some novel thermal and nonthermal techniques. The potential of these alternative pretreatments has been discussed in this review along with a critical analysis of literature on grape drying. The fundamentals of grape drying and unit operations of the chemical and abrasive process of raisin production have also been narrated. An abrasive process as the best alternative to chemicals in raisin production has been described in detail along with future prospects of different physical pretreatments. This review provides comprehensive information on raisin production with a focus on chemical-free processing of grape for raisin production. Practical applications: Raisins are dried grapes rich in nutritional and functional ingredients that are beneficial for human health. A predrying treatment to remove or to dissolve the waxy layer from surface of grapes is an important operation in raisin production. The chemical method of pretreatment is prevalent among raisin processors. However, a chemical method has some limitations like longer processing time, intense use of chemicals, food safety issues, and so forth. Many novel pretreatments like superficial abrasion, thermal, and nonthermal techniques have been explored by researchers and it has the potential to produce safe raisins without use of any chemicals. These alternative techniques can be adopted for industrial-scale processing of raisins. A lot of literature is available on the grape drying process, modeling, and quality aspects of raisins. All this information has been summarized in this comprehensive review with a comparison of unit operations, analysis of findings, and future aspects for the production of safe raisins. This review will be very useful for industrial-scale processing of raisins adopting novel processes and it will also provide in-depth knowledge to the academician and research scholars.