Influence of dipping pretreatments in drying of ‘Pedro Ximenez’ grapes in chamber at controlled temperature

Abstract

The influence of dipping pretreatments on drying of ‘Pedro Ximenez’ grapes, destined to the elaboration of sweet wines, in chamber at controlled temperature, has been studied. Changes in color and phenolic composition of raisins produced were observed during the process of raisining. Dipping pretreatments can improve the quality of the raisins produced in terms of lightness, sweet flavor, nutritional quality and better hygiene, in addition shortening drying time. Dipping pretreatments used in this study were solutions of potassium carbonate and olive oil (D1), and alkaline emulsion of ethyl oleate (D2). CIELab space showed that musts of raisins and their fractions of high molecular weight (HMW), obtained by dialysis, increased a^* and b^* components, while lightness (L^*) decreased. In relation to phenolic compounds, the drying processes of grapes could increase their concentrations as a result of dehydration, although they could also decrease by participating to non- enzymatic browning, auto-oxidation or enzymatic oxidation reactions. All fractions of phenolic compounds increased during drying of the grapes, with and without dipping pretreatments, and particularly the flavonol fraction.