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The Influence of Abrasive Pretreatment on Hot Air Drying of Grape

Fresh grapes are very sensitive to microbial spoilage during storage, even under refrigerated conditions, having relatively high moisture contents. Therefore, within a few weeks following harvest, they must either be consumed or processed into various products. Drying is the most common method for grape processing. It can process grapes into raisins for longer shelf-life by reducing the moisture content to a low level.

The peel of the grape plays a critical role in controlling the drying process. Grape peel consists of a wax layer as a protective barrier against fungal pathogens. However, this coating reduces permeability and therefore hinders the process of water transfer. Due to this, prior to drying, several chemical and physical pretreatments are used to enhance permeability and increase the drying rate, while preserving the physical, chemical, nutritional and sensory qualities of the final product.

In this paper, an abrasive pretreatment for enhancing the drying rate and preserving the grape samples was proposed. Grapes selected from Regina (white cultivar) and Red Globe (red cultivar) were used for the experiments. The drying kinetics of untreated and treated samples were studied using a convective oven at 50°C with an air velocity of 2.3 m/s until similar water content for both grapes was achieved. Samples were obtained during the drying experiments by taking six fruit berries for each cultivar. 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. Experiments were performed in triplicate. Data reported are the mean and the standard deviation calculated from three replicates.   

The results revealed that the pretreated grapes showed reduced drying times and rehydration time, due to faster transport of water, and lower shrinkage with respect to untreated ones. 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.

In conclusion, abrasive pretreatment contributed to reduce drying time and rehydration time. Moreover, pretreated grapes presented lower reduction in sugar content and in tartaric acid, but similar malic and citric acids and texture properties to untreated ones.

Article by Giuseppina Adiletta, et al, from Italy and Australia.

Full access: http://t.cn/E51rzXM
Image by Speleolog, from Flickr-cc.

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