## 2 Production and Processing Of Raisins: Effect on Final Quality  
Grape constitutes one of the dominant fruit crops worldwide; its global production increased from 75.1 mt in 2016 to 77.8 mt in 2018 [20]. Approximately 57% of the total production is used in wine making, 36% for table grape and a share of 7% is further treated for dried grape, yielding an annual production of 1.2 mt of dried raisins for 2018/19, demonstrating a 5% increase from 2017/18 based on market and trade data obtained from the United States Department of Agriculture (USDA) [21]. The main raisin producers are Turkey, USA, China, and Iran, contributing to approximately 73% of the global raisin production for 2018/19 [22]. Raisins, Sultanas and currants are the three categories of dried grapes. More specifically, 95% of the total production refers to Thompson seedless variety whereas 1.5% corresponds to black Corinthian currants [17], with more than 80% deriving from Greek production, representing a 3% of the global dried vine production [23]. Indigenous Greek dried grapes production is classified on the following dominant varieties: Zante currants obtained from black Corinth grapes (_Vitis vinifera_ L., var. _Ayprena_), the Corinthian currants Vostiraza and Sultana or \"Sultanina\" from the cultivation of _Vitis vinifera_ L., cv. Sultanina [24; 25]. Specifically, Zante currants are cultivated without irrigation and processed to dried product in the island of Zakynthos, using _Vitis corinthica_ grapes [24]. Both Zante and Vostiraza currants are characterized as a protected designation of origin (PDO) product [23]. The process of raisin production from grapes includes three main steps, pre-treatment, drying, and post-drying (Figure 1). The pre-treatment step is optional but is usually employed to remove the waxy layer formed on the skin of the grape during the ripening stage, providing a barrier to permeability and water diffusion [18; 26]. On top of that, pre-treatment steps have been shown to increase the dehydration rate during drying process and enhance the quality of the final product [19; 27]. Pre-treatment methods include physical and chemical methods [18; 19]. Chemical methods entail the application of an alkaline solution (e.g., NaOH, K\({}_{2}\)CO\({}_{3}\), and NaHCO\({}_{3}\)) combined with olive oil, ethyl oleate solution [24; 27; 28]. Gas treatments (CO\({}_{2}\), SO\({}_{2}\), and O\({}_{3}\)) have been also reported as a process of chemical treatment to modify the permeability of cell membranes on fruit [29]. With respect to physical treatments, they are differentiated between thermal and non-thermal applications, including blanching, abrasive processes, ultrasound, pulsed electric fields and hydrostatic pressure [19]. Apart from softening the waxy layer of grapes, physical processes like blanching or microwave irradiation, entail the inactivation of enzymes, such as pectinmethylesterase (PME), peroxidase (POD), and polyphenol oxidase (PPO). Enzymatic activity leads to browning or changes in the phenolic content and antioxidant activity [17]. Blanching is performed by immersion in hot water, acid or alkaline solutions, steaming or microwave for designated time periods. Treatment with sulfite solutions or sulfitation is carried out using K\({}_{2}\)S\({}_{2}\)O\({}_{5}\), Na\({}_{2}\)S\({}_{2}\)O\({}_{5}\), and NaHCO\({}_{3}\), and prevents enzymatic browning via the inactivation of PPO. In the case of _Zante_ currants, pre-treatment methods are not applied as it is a traditional and naturally produced food, although some pre-treatment steps with sulfitation may be employed for the Sultanina in Crete to prevent microbiological spoilage after drying [24; 30]. Drying of grapes is performed until the final moisture content is approximately 13% in the obtained product [31]. Several ways have been reported for dehydration, the dominant ones being natural sun drying, solar drying, shade drying and mechanical (or conventional) drying. Sun and solar drying have been the methods traditionally employed in the production of raisins.