Abstract
The drying of grapes is a more complex process compared to the dehydration of other agricultural materials due to the necessity of a pretreatment operation prior to drying. Grape drying to produce raisins is a very slow process, due to the peculiar structure of grape peel, that is covered by a waxy layer. Its removal has been so far carried out by using several chemical pre-treatments. However, they cause heterogeneity in the waxes removal and create microscopic cracks. In this paper an abrasive pretreatment for enhancing the drying rate and preserving the grape samples is proposed. Two cultivars of grape were investigated: Regina white grape and Red Globe red grape. The drying kinetics of untreated and treated samples were studied using a convective oven at 50°C. Fruit quality parameters such as sugar and organic acid contents, shrinkage, texture, peel damage (i.e. by SEM analysis) and rehydration capacity were studied to evaluate the effectiveness of abrasive pretreatment on raisins. Abrasive pretreatment contributed to reduce drying time and rehydration time. The treated and untreated dried grapes were significantly different (p < 0.05) in sugar and in tartaric acid content. On the contrary, no significant differences (p < 0.05) in malic and citric acids and in texture properties between untreated and treated samples were observed.
Highlights
Fresh grapes are very sensitive to microbial spoilage during storage, even under refrigerated conditions, having relatively high moisture contents (80% - 85% wet basis)
In a previous study [15], we found that treated white and red grapes showed reduced drying times due to faster release of moisture and lower color changes
To compare the effect of pretreatment on the drying kinetics of Regina and Red Globe grapes, the curves of Moisture ratio (MR) versus drying time are shown in Figure 1 and Figure 2, respectively
Summary
Fresh grapes are very sensitive to microbial spoilage during storage, even under refrigerated conditions, having relatively high moisture contents (80% - 85% wet basis). Raisins are obtained by sun drying of the fruit for eight to ten days, which substantially reduces water content This drying method is cheap, but there is a risk of damage due to dust and insect infection [2] and the sensory quality can be seriously damaged during exposure to sunlight [3]. Convective drying is one of the oldest and alternative dehydration methods in which hot air removes the water from the fruit surface This creates a diffusion gradient in the food that moves the water from the interior to the outer surface [4]. This process decreases the quality of the final product [5]. The drying of grape should be carried out quickly (less exposure time to oxygen) and at relatively low drying temperature [7]
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