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.

Food Dehydration Recent Advances and Approaches

“…Grapes, prunes, and figs, among other dried fruits, have been preserved using this method to an extensive degree in recent years [8]. Some have been dried using hot-air dehydrators rather than sun drying in recent years because the fruit dries faster with the former, and problems caused by inclement weather are avoided with the latter [84]. Because there is a good chance that future energy limits will be reached, the availability of fossil fuels and natural gas used in dehydration may be severely restricted.…”

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

“…The Aegean Region of the Türkiye occupies about 28% of the total vineyard area in Türkiye. Anthocyanin from grapes and raisins provides many health benefits such as antioxidant, anticancer, and anti‐inflammatory properties (Adiletta et al, 2016; Colombo et al, 2019; Nasser et al, 2020; Olejar et al, 2019; Patidar et al, 2021). Compounds like protocatechuic and oxidized cinnamic acids are more prominent in raisins compared to grapes (Williamson & Carughi, 2010).…”

“…Therefore, removal or weakening of the waxy layer is the main target of different pretreatments used in grape drying (Adiletta et al, 2016; Bai et al, 2013; Pawar, Giri, Sharma, & Kotwaliwale, 2021). Chemical pretreatment is most common and commercially used by all raisin producers (Patidar et al, 2021). Alkali (sodium hydroxide, potassium carbonate) and oil emulsion (ethyl oleate, olive oil) are the commonly used chemical in this pretreatment process (Doymaz & Pala, 2002; Patidar et al, 2021).…”

“…Chemical pretreatment is most common and commercially used by all raisin producers (Patidar et al, 2021). Alkali (sodium hydroxide, potassium carbonate) and oil emulsion (ethyl oleate, olive oil) are the commonly used chemical in this pretreatment process (Doymaz & Pala, 2002; Patidar et al, 2021).…”

“…The use of sulfur for color enhancement is also a practice in some areas of raisin production. However, use of sulfur is being discouraged because of its allergic reaction in sensitive individuals (Deng et al, 2019; Kamiloglu et al, 2016; Patidar et al, 2021). The chemical pretreatment process also generates wastewater which harms the soil and environment (Adiletta et al, 2016; Bai et al, 2013; Deng et al, 2019; Wang et al, 2017; Wang et al., 2007).…”