The effects of different drying methods on the sugar, organic acid, volatile composition, and textural properties of black 'Isabel' grape

In this study, the black 'Isabel' grapes were dried by hot-air (HAD), vacuum (VD), ultrasound-assisted vacuum (US-VD), and freeze-drying (FD) methods, and the sugar, organic acid, volatile composition, and textural properties of raisins and fresh grapes were both evaluated. Depending on the method of drying, the fructose and glucose content of grapes were decreased with less reduction by the FD. Similarly, a decrease in tartaric and malic acids was observed by drying. A total of 29 different volatile compounds were identified and PCA analysis revealed that esters were more associated with fresh grapes whereas HAD and VD raisins became more related to alcohols. The aldehydes probably derived from the auto-oxidation of fatty acids were increased by drying and their presence became more associated with HAD and VD methods. The hardness, gumminess, and chewiness values of grapes were elevated by drying, and the HAD raisins provided the highest hardness values. The whiteness (L*) and redness (a*) values of the grapes were increased by FD, whereas they were not significantly changed with other drying methods. While HAD raisins were preferred by the panelists, the US-VD raisins had the lowest score in each sensory property.