The reduction of water activity in dried grapes is of greatest importance, because the products become more stable and with storage under appropriate conditions the optimum microbiological, nutritional, and sensory properties of the finished product are achieved.
Pre-treatment is a necessary step in raisin production in order to ensure the increased rate of water removal during the drying process. A faster water removal rate decreases the rate of browning and helps to produce more desirable raisins.
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.
Pre-treating helps keep light-colored fruits from darkening during drying and storage and it speeds the drying of fruits with tough and waxy skins.
One way of removing this waxy layer is to apply pretreatments before drying grapes so that water gets migrated easily, thus improving the process. This not only decreases the energy consumption but also promotes quality preservation of the final product, such as retention of antioxidant during drying.
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.
Therefore, chemical dip treatments (both hot and cold) have been used to increase the drying rate of grapes which not only reduce the drying time but also improve quality of the raisins. The hot dip treatment with dipping solution causes cracking and perforation in the waxy cuticle and thus increases the drying rate over the cold dip treatment.
The effects of pretreatment solutions on seedless grapes during drying were reported in literature.
Study shows dipping grapes in to 0.5% NaOH solution at 90 °C for 2-3 sec repeated for two to three times and then dipped in to Lye solution 0.5% + Ethyl oleate 1.0% + Potassium metabisulfite 1.0% will improve the physical and chemical properties of raisins and also reduce the time required for drying.
Pre-treatment of fresh grape
