Drying of 'Perlette' grape under different physical treatment for raisin making

Experiments were conducted to evaluate the suitability of grape (Vitis vinifera L) cv. 'Perlette' for raisin preparation with different physical treatments. The treatments consisted of whole berries, half cut berries and berries with superficial abrasion of peel/waxy cuticle. The effectiveness of these treatments on drying characteristics and quality of the prepared raisin were studied. The half cut berries took less time to dry as compared to scratchy surfaced berries and whole berries. Drying of berries was carried out at 60 °C in an air circulatory tray dryer with loading capacity of 6.0 kg/m(2). Initial moisture content of berries was 67% (wb) with varying total soluble solids of 15-18% which was dried to moisture content of around 15% (wb). Empirical/semi-theoretical/mathematical models, commonly describing thin-layer drying of various biological materials have been investigated to illustrate the drying characteristics of the physically treated berries. A non-linear regression analysis using a standard statistical program was used to evaluate the constants of mathematical models to describe appropriately the drying behaviour. The results as indicated that the abrasion method is effective for faster drying as against the chemical method; however this treatment gave comparatively darker raisin which was less attractive.