Mechanical drying of raisins

Grapes intended for dehydration are hand harvested and transported to the drying facility in pallet bins. In artificial drying, grapes are first dipped n 0.25 – 1 % lye (sodium hydroxide) solution at 200 – 212 degree F for 2 – 5 sec to remove a natural wax that impedes drying and to produce tiny cracks in the skin of the fruit in order to speed drying.

The fruit is then placed on trays and exposed to gaseous sulfur dioxide. This is to prevent enzymatic and nonenzymatic browning during drying. Hence instead of having the dark brown color of raisins that normally be expected, the raisins will be of a light yellow color when dried. The purpose of the pretreatment is to increase the permeability of the grape skin to moisture.

There are three main methods that are used for fruit drying: sun drying, shade drying and mechanical drying. Mechanical drying which is safe, rapid, and controllable is attractive to dry fruit production, especially when high throughput is needed.

Raisins are dried at temperatures not exceeding 165 degree F and at a low relative humidity. After the moisture content has been lowered to about 16 – 18%, the raisins are packaged in containers of various sizes to be sold at retail or for use by the food industry.
Mechanical drying of raisins