Storage and Processing
There are more than 16,000 producers, packers, shippers, and processors of potatoes in the United States. Packers and shippers are responsible for storing the potatoes so that their quality remains as high as possible. Both retail customers and processors want a top-quality potato that meets their needs.
Potato processing contributes a huge amount of revenue to the economy. In the northwestern region of the U.S, almost $2 billion is contributed per year from value-added frozen potato products. Washington, Oregon and Idaho lead the nation in the frozen potato market, producing 80% of the country's frozen French fries and hash brown potatoes. Potato processing is the Northwest's primary vegetable processing industry, employing more than 15,000 people on a full-time, year-round basis and many more seasonally. The food processing industry as a whole is the largest industrial employer in Idaho, and is second behind aerospace in Washington.
How well potatoes store and how they process when removed from storage depends largely upon the condition of the tubers going into storage. Growth, harvest conditions and tuber maturity are as influential as the actual storage process. Bruising is probably the single most important factor that reduces the financial returns of the potato industry.
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Potatoes grown under stress have uneven tuber growth. Stressful conditions such as temperature extremes, non-uniform moisture, defoliation, nutrient imbalance, or inadequate or excess fertilizer result in tubers more prone to problems. Early stress, when plants are young and tubers are forming, is more serious than stress later on in the growing season. Manifestations of stress are, most commonly, malformed tubers, accumulation of sugars both in the field and in storage, development of sugar and jelly-end tubers, and premature physiological aging and sprouting of tubers in storage.
Achieving tuber maturity is more complicated. As tubers grow, develop and mature, a peak in dry matter occurs. A minimum amount of sugars is achieved shortly thereafter. This is important because high dry matter and low sugar content are important for processing. This stage is considered physiological maturity and is an indicator of when to start vine-kill and harvest.
When the skin has been set properly, which occurs along with physiologic maturity under non-stress conditions, the potatoes will store well. But when stress conditions occur, physiologic maturity may occur without adequate skin set. Stress can also change physiologic maturity by continuing to mature tubers on plants that have already died, changing reducing sugar content, weight, etc. Storage of tubers that are immature and "skinned" results in a variety of problems. The maturation process will continue in storage; skins will continue to thicken and set.
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Several criteria are considered in preparing to harvest and store potatoes. Vine kill must be carefully controlled. Rapid vine kill can cause stem-end discoloration. Excess late fertilization interferes with vine and tuber maturation. Too much soil moisture can increase blackspot susceptibility; too little soil moisture can hinder rapidity and degree of skin set.
Growers try to harvest during optimum soil conditions. Soil should be lightly moist and soil and pulp temperatures should be between 45 and 60° F. Otherwise tuber bruising can result. Care is taken to avoid transporting soil into storage; excess soil in storage prevents good air circulation and increases susceptibility to rot.
When operating the harvester and piler, care is taken to reduce bouncing and rolling the tubers or bruising can result. When transporting the tubers from harvest to storage, care is taken to avoid wind and sun.
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Optimally, growers like to store only sound, unbruised tubers that have not been stressed during growth. Heat from the field must be removed quickly, preferably within three days. Removal of field heat is important as rapid wound healing is necessary. In addition, tuber maturation will occur for a month or more but only at temperatures of 50° F. with relative humidity above 95%.
After the wound healing period, the temperature is slowly lowered one-half degree per day to the long-term holding temperature. The growers goal is to prevent sprouting and maintain weight. Long-term storage temperatures vary with the potato's future use, the variety, and the amount of bruising and rot that is present. Humidity levels are maintained at 95%.
Daily storage management now becomes extremely important. Proper ventilation and temperatures must be maintained and humidity and condensation monitored. But no machine can take the place of the human monitor. Smelling for rot and looking for condensation and wet tubers is still a necessary daily task.
Seed potatoes store best at 38-40° F. Physiologic aging and sprouting can be kept to a minimum at this temperature. Tubers stored at higher temperatures will loose weight.
Processing potatoes are stored at 45-47° F. If temperatures are lower, the tubers will accumulate sugars which results in dark fry color and poorer textures. Again tubers stored at higher temperatures will lose weight. Fresh market potatoes are stored at 43-45° F. to minimize weight loss and sprouting. Chip potatoes are stored at 50° F. to prevent starch to sugar conversion though some varieties can be stored at slightly lower temperatures.
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The biggest potential for tuber loss while in storage is storage rot. Most rot organisms are present in the field and only require bruises to enter the tuber. Tubers which have been subject to growth stress are much more susceptible.
Rot development in storage is normally very noticeable. Putrid odors, wet and slimy tubers, disintegration (from dry rot), and surface blemishes are all characteristics of rot. Soft rot is the most common storage rot.
Soft rot (Erwinia spp.) spreads rapidly in the presence of free water on the surface of tubers. Infected areas are initially cream-colored and later may become brown, producing slimy areas with a foul odor.
Fusarium dry rot (Fusarium spp.) takes several months to develop in storage. The organism enters through cuts and bruises. Infected areas will completely fall apart and disintegrate.
Pythium water rot (Pythium spp.) infects tubers in the field. High storage temperatures and low storage humidity promote its development. It will affect isolated tubers throughout the pile.
Early blight tuber blemish (Alternia solani) enters the tuber through cuts and bruises. Shallow, necrotic sunken blemishes will appear after a period of storage.
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Man has dehydrated potatoes for more than 1000 years. First the Inca Indians of the Andes created chuño , then the Spaniards carried the process back with them to Europe. Dried potatoes were used as staples and to create potato flour. During the Industrial Revolution, drying processes emerged in Germany and England. Production of potato starch and fried potato chips developed in the 1800's. Finally, both World Wars I and II resulted in the development of quick rehydrating dried mashed potatoes.
The era of convenience foods began after World War II. It escalated in the late 1950s and has led to the development of a world-wide market for a wide range of dehydrated and frozen potato products. Thanks to the processing industry, in the U.S. alone, annual consumption of processed potatoes is now at more than 140 pounds per person.
The most important product in the potato processing industry is the French fry. American soldiers were introduced to fried potatoes in Belgium during World War I. U.S. restaurants adopted the product and popularized it. But developing a frozen product for home use wasn't perfected until the 1950s. Then the Mac-fry process revolutionized the fast-food French fry business and French fries became a primary menu item. Today, more than 50% of the potatoes grown in the U.S. are destined for processing.
A processing potato must have a high specific gravity and a low sugar content. A high specific gravity is necessary because it indicates how much water must be evaporated from the potato during the dehydration process. It is actually a measure of the dry matter or "solids" in the potato. High specific gravity potatoes make the best french fries and dehydrated potato products. Russet potatoes are most often used in processing.
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The chief raw ingredient for dehydrated mashed potatoes are small, damaged, and misshapen potatoes sorted from field run potatoes in major producing areas. The Russet Burbank is commonly used because rigorous grading will eliminate about 50% of the yield from being sold as bakers. About 20% will be process grade. Production is a five-step process.
First the potatoes are precooked and cooled. The peels are removed by various dry brushing methods, the tubers are cooked, mashed and the granules are dried. Sometimes the granules are combined with previously dried granules. Granules are used to make mashed potatoes; extruded products like hash-browns; dry mixes for extruded French fries; and fried, baked and pelleted snacks. Potatoes are also processed into flakes to make mashed potatoes for home use and fried potato snacks. Flakes are used as an ingredient in baked goods, ice cream, pet food, breading mixes and dried soups. They are also used as a binder for fish cakes, fishing bait and eel food (in Japan).
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Steam-peeled potatoes are trimmed, diced or sliced, blanched, lightly sulfited, dried, and sorted. Diced products are used for canned food products, potato salad, hash-browns, and dry soup mixes. Slices (usually 3mm thick) are used in retail and institutional casseroles and potato salads. Crushed or ground products are used in extruded snack pellets, as a thickener, in dumpling mixes and potato pancakes, and in dry soup mixes. Elongated strips are used in restaurant hash browns.
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Raw, high-quality, potatoes are steam-peeled, cut lengthwise, sorted, blanched, pre-dryed, and then fried. Customers prefer long French fries so processors use Russet Burbanks from Idaho, Washington, Oregon, Wisconsin, North Dakota, and Maine. Small potatoes and nubbins are used for chopped/molded products, southern-style hashbrowns and fabricated products. New specialty products are being created such as frozen baked potatoes, twice baked potatoes, breaded French fries, waffle chips, curly fries, and mashed potato products.
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A Saratoga, New York chef named George Crum is generally credited with the invention of the potato chip in 1853. They quickly became popular around the country. Potato chips are still made from cut slices in continuous fryers but flakes and granules are also used for chips.
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Flour has been made from ground, dried potatoes for centuries and was used as a food staple when mixed with water. Now it is used in breadmaking, cookies and candies to improve texture and flavor. Potato flour is also used as a thickener or breading agent.
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First produced in Germany in the late 1700s, potato starch plants were numerous in the U.S. by the late 1800s. These plants have disappeared because disposal of waste effluent from potato starch plants is such a problem. There is only one conventional potato starch plant in the U.S. today. Food grade potato starch is imported from Germany and Holland where the industry is government subsidized. However, starch modification plants exist which convert by-product starch from processing for use in the paper industry. Starch recovery reduces the cost of effluent treatment.
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The gas-packed pre-peeled industry has suffered recently concerning the use of sulfite in food products. But this process is still used to produce blanched, parfried French fried strips and precooked shredded potatoes. Canning is still of minor importance. Potato alcohol production is used to make vodka, and also used in the production of gasohol.
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