The primary raw material for making raisins is grapes. To make 1 lb (453.59 g) of raisins, over 4 lb (1,814.36 g) of fresh grapes are required. These grapes must have certain qualities in order to produce quality raisins. For example, they must ripen early and be easy to dry. Additionally, they must have a soft texture, not stick together when stored, have no seeds, and have a pleasing flavor. The most important grapes for raisin production include Thompson Seedless, Black Corinth, Fiesta, Muscats, and Sultans.
By far, the most widely grown raisin grape is the Thompson Seedless variety. They are used in the production of over half the world's raisins. Ninety percent of these come from California. The Thompson was first developed in 1872 by William Thompson, who created it by taking cuttings from an English seedless grape and grafting them with a Muscat grape vine. The resulting plant produced the first Thompson seedless grapes. It is believed that all of the subsequent Thompson seedless vines came from this original grafting.
The Thompson seedless is a white, thin skinned grape, which produces the best raisins available today. Its small berries are oval and elongated. It does not contain seeds and has high sugar content. From a raisin production standpoint, Thompson grapes are ideal because they ripen fairly early in the season and do not stick to each other during shipping.