Raisins are made primarily by sun drying several different types of grapes. They are small and sweetly flavored with a wrinkled texture. The technique for making raisins has been known since ancient times and evidence of their production has been found in the writings of ancient Egyptians. Currently, over 500 million lb (227 million kg) of raisins are sold each year in the United States, and that number is expected to increase because raisins are recognized as a healthy snack.
Most raisins are small, dark, and wrinkled. They have a flavor similar to the grapes from which they are made, but the drying process which creates them concentrates the amount of sugar making them taste much sweeter. They are a naturally stable food and resist spoilage due to their low moisture and low pH.
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.
The manufacturing process
There are four primary methods for producing raisins including the natural, dehydration, continuous tray, and dried-on-the-vine methods. The most popular of these is the natural method which will be explained in some detail. The basic steps in natural raisin manufacturing include harvesting, processing, and packaging. While a small portion of raisins are made by mechanically dehydrating grapes, the majority of them are produced by sun drying.
The first step to producing good raisins is growing quality grapes in the vineyards. Grape farming is a year-round commitment and includes the practices of pruning, irrigation, fertilization, and pest control. Most of the work done in these vineyards is still done by hand.
– Pruning involves the removal of parts of the vine to control its growth pattern. This has the benefits of equalizing the quality of grape throughout the vineyard, making other farming tasks easier and reducing costs. It is typically done when the vines are dormant between December and March.
– Irrigation is done during the summer while the vines are growing to keep a continuous supply of water in the vineyard soil.
– While fertilizers are not needed in all vineyards, some vines respond well to the use of nitrogen and zinc based fertilizers. Fertilization is typically done during the summer growing season.
– Vineyards are susceptible to various diseases and insect attacks, so it is important for these factors to be controlled. Chemical and biological agents are used to control mites and other insects. Sulfur dusting is used to prevent the growth of mildew and other fungi.
Harvesting and drying
– Starting in late August and continuing through September, the grapes are harvested. At this point in the year they are at their optimum sweetness. Bunches of grapes are handpicked by field workers and placed on paper trays, which are laid out on the ground between the vine rows. To provide a good surface for the trays, the soil between the rows is leveled.
– Depending on the weather, the grapes are allowed to dry on the trays for two to four weeks. During this time, the moisture content of the grape is reduced from 75% to under 15% and the color of the fruit changes to a brownish purple. At night, the trays are rolled to minimize the accumulation of sand and protect against raisin moth infestation. The paper trays are embedded with a compound, which kills insects that can damage the grapes as they dry. After the fruit is dried, the paper trays are rolled up around the raisins to form a package. The rolls are gathered and stored in boxes or bins before being transported by truck to a processing plant.
Inspection and storage
When the rolls of fruit arrive at the manufacturing plant, they are emptied out onto wire screens and shaken to remove dirt and other unwanted debris. They are also inspected to ensure that they meet previously determined specifications. Factors such as moisture content, color, and taste are all used to evaluate the shipment. Based on their quality, the raisins are graded as either standard or substandard. Only the standard graded raisins can be immediately used.
– The dried grapes are moved from the storage bins to the processing plant. Here they are emptied out onto a conveyor line and mechanically modified. The residual sand and other debris are first removed by running the raisins on a fine mesh screen while air is blown on them. Immature fruit is removed by suction devices. Next, the raisins are separated from the bunch stem by shaking. The cap stems on each raisin are removed by being passed through two rotating conical surfaces. If there are seeds in the raisins, they are mechanically removed. When all these processing steps are completed, the raisins are run through a series of mesh screens to sort them according to size.
– At this point the raisins can be put into a variety of packaging. These range in size from small half ounce cardboard containers for individual consumption to 1,100 lb (499.4 kg) containers for industrial use. Each package is run through metal detectors, in order to detect any unwanted metal particles, and then checked for the appropriate weight. They are packed onto trucks and shipped to customers. The whole process of receiving the raisins at the factory, processing them and putting them into packaging takes about 10 minutes.
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