Raisins are produced commercially by drying harvested grape berries. In order for a grape berry to dry, water inside the grape must be removed completely from the interior of the cells onto the surface of the grape where the water droplets can evaporate. However, this diffusion process is very difficult because the grape skin contains wax in its cuticle, which prevents the water from passing through. In addition to this, the physical and chemical mechanisms located on the outer layers of the grape are adapted to prevent water loss.
Generally, there are 4 steps to commercial raisins production in Turkey, and that includes farming and harvesting, drying of grapes, post-drying processes and packaging of raisins.
Source Link: http://fooddryingoven.com/drying_ideas/Turkey-raisins-production-by-grapes-dryin.html