Production and processing of raisins

Raisins are the second most important product of the grapevine wine, being the first. The quality of raisins depends on the size of the raisins berries, the uniformity and brilliance of the berry color, the condition of the berry surface, the texture of the skin and pulp in the berry, moisture content, chemical position and presence of decay, mold, yeast and foreign matter.

Raisins are produced from grapes by sun during and artificial drying. In the traditional drying method, grape bunches are spread over either the ground or on a platform or on wooden trays in a thin layer between the rows of grapes vines.

The trays are tilted to face the sun. After they are partially dried, the grape bunches are turned and allowed to dry to the point where no juice can be pressed out. The trays are then stacked, and the air drying is continued in the shade until moisture content of about 17% reached. After drying, the raisins are placed in sweat boxes to equilibrate, or to even out, the moisture that is present, and then they are packed in retail size containers or in larger containers to be sold to the bakery trade.

This method of drying is most cost-effective and takes approximately 8-10 days to produce dried products.

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 check or crack the skin of the grape to facilitate drying

They are then washed, placed on trays, and treated with sulfur dioxide 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.

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.

In California, light-colored raisins are produced mostly by the golden bleach process. First, Thompson Seedless grapes are sorted, then they are dipped to produce slight checking cracking of the skins, and they are cooled and washed in a spray of cold water. They are exposed to the fumes of burning sulfur in a sulfur house. After being sulfured, the grapes are dehydrated at a temperature of 140 to 160 degree F.
Production and processing of raisins