## 2 Production and Processing Of Raisins: Effect on Final Quality
Natural open sun drying is a low-cost method, performed by placing grapes in thin layers on the ground or hanging on the vines, directly exposed to sunlight, for a period of up to 3 weeks [18; 24]. Regardless the simplicity of these methods, the reliance on weather conditions might trigger microbial spoilage and insect infections. Application of solar drying is relying on the utilization of solar energy, performed via direct, indirect, and mixed types of driers [32; 33]. [PERSON] et al. [32] stressed out that wide acceptance of solar driers is hindered by the high initial capital cost, specifically for small and medium scale producers, regardless the short payback time that renders the process economically feasible and environmentally benign. Shade drying constitutes another natural way to remove water from raisins, also referred to as natural rack dryers, commonly used in China, Australia, and India [18]. Grapes can be placed either on trays, on the ground or hanging on the vine and subsequently covered so that ambient air circulates to dry the fruit. A reduction in drying time of raisins (equal to 43%) has been reported using shade drying compared to open sun drying [34]; however, the effect on the final quality and the antioxidant properties is indistinctly affected by sanitary conditions [18; 23]. Mechanical or conventional drying was developed to coincide with the commercialization of raisins and the need to meet the consumers' demands. Grapes are placed in dehydration tunnels for at least 24 h, using air circulation at controlled temperature [17]. This method confers low labor costs and can be easily controlled, and results in products of superior quality however it requires high energy generation [26; 34]. Thus, new technologies and methods have been developed to mitigate the drawbacks of conventional  
Figure 1: Grapes processing steps for currants and raisins production [18; 19; 24]. air drying in chambers and accelerate the moisture diffusion rate, including microwave drying, vacuum pulsed drying, infrared drying, etc. [18; 19]. For instance, microwave drying exhibits reduced energy consumption, enhanced dehydration rate thus less time to obtain the final product [35]. Likewise, combination of methods (microwave, infra-red, and conventional drying) has been also studied aiming to improve the final quality of the products. Indeed, as [PERSON] et al. [17] noted the combination of advanced and alternative processes entails a reduction on the impediments observed on the nutritional and functional characteristics of the final product impaired by drying. Drying of the grapes does not constitute the final stage of processing. Post-drying treatments include washing of the raisins, stem, and other materials removal, packaging and storage [36]. The conditions of packaging and storing of raisins may also affect the final quality of the product, with entomological and microbiological infections also being the dominant concerns. All processing steps of grapes to formulate raisins, along with storage and packaging, constitute crucial factors that affect the final quality of the product. On top of that, a dynamic interaction is established among these parameters and the variety of grape and culture conditions before harvesting (e.g., sun exposure during maturation). Color has been employed as an indicator of the chemical pre-treatment effects on raisins in previous studies, whereby it has been widely acknowledged that sulfite treatments entail raisins with lighter color. This could be attributed to the inhibition of PPO activity thereby the prevention of non-enzymatic browning. Carbonic maceration using CO\({}_{2}\)-rich atmosphere on red globe grapes led to a more accepted color for raisins [37]. Raisins demonstrating lighter color, thus, highest quality, were also obtained via dipping in alkaline solution at 40 \({}^{\circ}\)C [38]. Likewise, [PERSON] and [PERSON] [39] tested a potassium carbonate and olive oil solution, and compared the raisins with untreated samples, observing lighter color in the treated ones.