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.
Raisins are composed of important food elements such as sugars, fruit acids, and mineral salts. The sugars provide a good source for carbohydrates. Fruit acids such as folic acid and pantothenic acid, which have been shown to promote growth, are also significant components. Vitamin B6 is found in raisins and is an essential part of human nutrition. Important minerals in raisins include calcium, magnesium, and phosphorus. Additionally, iron, copper, zinc, and other nutrients are found in trace amounts in raisins. Considering the composition of raisins and the fact that they have no fat, it is no wonder that this fruit is considered a healthy snack.
The majority of grapes used for making raisins in the United States are grown in California. This area has an ideal climate for grape growing because it has plenty of sun during the summer and very mild winters. Five other countries, which produce a substantial amount of raisins include Greece, Australia, Turkey, Iran, and Afghanistan. Each of these countries have their own variety of raisin that they consistently grow.
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. Evidence has shown that raisins were produced by the Egyptians as early as 2000 B.C. Raisins specifically have been mentioned in ancient writings and it suggests that they were used for eating, treating illnesses, and even paying taxes.
Throughout the ages, wine making has been the most important use for grapes, however, a small amount of these grapes have always been made into raisins. During the late 1800s, Spanish missionaries from Mexico introduced grapes into the United States. Many of the vineyards established by these missionaries in California are still producing today. These early vineyards were primarily used to make wines, however in 1873 when the vineyards discovered they could make quicker profits by making raisins, the raisin industry was born.
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, thinskinned grape, which produces the best raisins available today. Its small berries are oval and elongated. It does not contain seeds and has a 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.
The Black Corinth is a grape that originated in Greece, which has become an important variety of raisin grape. They are about one fourth the size of the Thompson grapes and have a juicy, tangy/tart flavor. These grapes are quite small, spherical in shape, and reddish-black in color. They are thin skinned and nearly seedless. They make good raisins and are excellent for production because they ripen early and dry easily. Because of their flavor, they are more often used for baking cookies, specialty breads, and fruitcakes than for eating.
Next in line of importance to raisin production is the Muscat grapes. These are large, sweet grapes that contain some seeds. Originally grown in Alexandria, Egypt, these grapes were the primary raisin grape before the advent of the Thompson. They were introduced in the United States in 1851. Muscat grapes are juicy, dull green in color, and have a sweet, muscat flavor. They have moderately tough skins and result in excellent tasting, large, soft-textured raisins. When they are used for raisin making, they are subjected to a mechanical process, which removes the seeds after the grapes are dried. These seeds are a significant drawback to using the muscat, and additionally, they do not ship well.
Two minor varieties of grape that find some use as raisins include the Fiesta and the Sultana. The Fiesta is a white seedless grape with a good flavor. A major problem with these grapes is that their stems are more difficult to remove. The Sultana grape is nearly seedless, but they make inferior raisins because they are less meaty, have a high acid content, and have some small, very hard seeds. Both Fiesta and Sultana raisins are used more often as baking 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.
- 1 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. Since these compounds can have an effect on the overall grape quality, attempts are made to minimize the amounts used
Harvesting and drying
- 2 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.
- 3 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
- 4 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. In the United States, dried raisins are inspected by the United States Department of Agriculture to ensure that all state and federal food laws are followed. 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.
Whether or not some of the fruit will be stored for later processing or moved to the production lines, is determined by the needs of the manufacturer. If the raisins are moved for storage, they are stacked outside the plant in temporary storage enclosures. These enclosures are constructed with polyethylene sheeting fastened to wooden frames. They are made tight enough to hold the fumigation gasses, which are applied periodically to inhibit insect growth. Methyl bromide and phosphine gases are the primary fumigants used.
- 5 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.
- 6 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.
Quality control is an important part of each step in the raisin making process. While the grapes are growing, they are checked for ripeness by squeezing the juice from a grape and using a refractometer. This allows the growers to determine how much sugar is in the grape. They are also tasted and their weight per volume is measured to give a measure of the quality of the fruit. During picking, workers are careful not to place bunches with insects or mold on the trays. They also try not to break berries as the liquid will attract insects. Knives are used to cut down the grape bunches to prevent damage. At the factory, the raisins are thoroughly inspected. They are also subjected to a variety of laboratory analyses to ensure the production of a consistent, high quality product.
Advancements in raisin production will focus on improvements in raisin yield, variety, and processing. Currently, the amount of grapes that can be produced are limited by the amount of land available. To increase yield, researchers are developing improved farming methods and new, genetically modified vine types. Experimentation is also being done on improving grape variety and characteristics through traditional grafting and biochemical means. It is expected that processing equipment will improve to reduce the amount of time required and improve the quality of the finished product.
Where to Learn More
Densley, Barbara. Food Preservation Pack: Fun With Fruit Preservation, ABC's of Home Food Dehydration, New Concepts in Dehydrated Food Cookery. Horizon Publishing Co., 1994.
Macrae, R., et al., ed. Encyclopedia of Food Science, Food Technology and Nutrition. San Diego: Academic Press, 1993.
Mullins, Michael, Alain Bouquet, and Larry E. Williams. "Biology of the Grapevine." In Biology of Horticultural Crops. Cambridge University Press, 1992.
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