Traditionally grapes are air or sun-dried to produce raisins but drying technologies now cater to the massive volume of the fruit that is required to meet demand. The technique of drying fruit was likely discovered by accident. It is conceivable that our ancestors came upon fallen fruit, which had dried in the sun, and discovered its sweetness after tasting it.
The process of drying fruit in the sun is accredited to the Persians with early written references to drying black grapes for raisins dating back to over 2,000 years ago.
Since sun drying is a natural, slightly uncontrolled process, the color of the final raisins can change. Faster, hotter drying tends to give darker brown raisins whereas; slower, cooler drying gives a lighter colored raisin.
Raisins are produced commercially by drying harvested grape berries. 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.
The three steps to commercial raisin production include pre-treatment, drying, and post-drying processes.
Pre-treatment is a necessary step in raisin production 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 historical method of completing this process was developed in the Mediterranean and Asia Minor areas by using a dry emulsion cold dip made of potassium carbonate and ethyl esters of fatty acids. This dip was shown to increase the rate of water loss by two- to three-fold.
Raisins may play a part in preventing anemia. They contain good amounts of iron, copper, and vitamins that are essential for making red blood cells and carrying oxygen throughout the body.
Recently, new methods have been developed such as exposing the grapes to oil emulsions or dilute alkaline solutions. These methods can encourage water transfer to the outer surface of grapes which helps to increase the efficiency of the drying process.
The two types of drying methods are sun drying and mechanical drying.
Sun drying is an inexpensive process; however, environmental contamination, insect infections, and microbial deterioration can occur and the resulting raisins are often of low quality. Additionally, sun drying is a very slow process and may not produce the most desirable raisins
Instead of using the sun to dry the grapes into raisins, manufacturers may also use mechanical drying processes. Once they’ve been pre-treated, the raisins can be dried in tunnels by blowing air past the raisins. Instead of a few weeks, these raisins will be sufficiently dry in less than two days.
After the drying process is complete, raisins send to processing plants, where they are cleaned with water to remove any foreign objects that may have become embedded during the drying process. Stems and off-grade raisins are also removed. The washing process may cause rehydration, so another drying step is completed after washing to ensure that the added moisture has been removed.
All steps in the production of raisins are very important in determining the quality of raisins. Sometimes, sulfur dioxide is applied to raisins after the pretreatment step and before drying to decrease the rate of browning caused by the reaction between polyphenol oxidase and phenolic compounds. Sulfur dioxide also helps to preserve flavor and prevent the loss of certain vitamins during the drying process.
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