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The History of Potatoes
The potato is a member of the nightshade family (Solanaceae) and originated in the Peruvian and Bolivian Andes of South America. Related plants in the nightshade family include peppers, eggplants and tomatoes, and weeds like hairy nightshade and black nightshade. The same potato species as the one we cultivate today (Solanum tuberosum) and related species were gathered and cultivated for thousands of years before the European explorations. Potatoes were taken first to Spain around 1570 and about 20 years later to England. They first reached North America from England in the early 1600s. Potato production spread from these two early European introductions and they are now grown in nearly every part of the world.
During the early period of cultivation in Europe, North America and elsewhere, potatoes were grown decade after decade from original selections and no new cultivars were introduced. Following the Irish potato blight in the 1840s, crossing was increased in North America and many new cultivars were selected in the last half of that century. From this early breeding work the forerunners of modern potato varieties were developed.
Common cultivars of 25 to 100 years ago included: Early Rose, Green Mountain, Irish Cobbler, White Rose, Early Ohio, Burbank and Red McLure. Russet Burbank was selected over 100 years ago and still remains one of the most widely grown varieties in North America. Records from Manitoba show that Irish Cobbler, Gold Coin, Netted Gem (Russet Burbank) and Early Ohio were recommended varieties in 1926.
Potato Production in Alberta
Records of potato production in the early history of Alberta are scarce. Small-scale potato production in home and market gardens presumably spread with settlement. On October 3, 1810, Daniel Harman, the agent for the North West Trading Company post in Dunvegan, wrote, "... we harvested the potatoes that were planted on May 10 and found the nine bushels planted produced 150 bushels ..."
The best account of early potato production in Alberta is recorded in Development of Horticulture on the Canadian Prairies which was compiled by the Western Canadian Society for Horticulture in 1956.
By the 1930s commercial potato production was well established and records of the Western Canadian Society for Horticulture show that 6,000 ac (2,430 ha) of irrigated potatoes were grown in the Lethbridge area in 1937. It was a record year and the crop averaged 6.3 ton/ac (14.2 t/ha).
During the 1930s and 1940s certified seed production and grading regulations were implemented and table stock quality improved. In the early part of the 1940s the industry was set back by a severe outbreak of bacterial ring rot which damaged established export markets. By the mid 1950s, quality, markets and credibility had been re-established and Alberta potatoes received good publicity with their performance at the Toronto Royal Agricultural Fair.
During the past two decades major changes have been made in management techniques, mechanization, storages, packaging facilities and in processing. From 1985 to 1990 Alberta's potato growers produced between 220,000 and 300,000 tonnes of potatoes annually on 24,000 to 28,000 acres (9700 to 10400 ha).
The Potato Plant and Tubers
Potato tubers are not roots, but enlarged portions of underground stems called a rhizomes or stolons (figure 1). Eyes are the buds from which next season's growth will sprout. Eyes are concentrated near the apical end of the tuber and are fewer near the stolon or basal end.
The tuber skin is composed of two layers of cells: a layer of single cells called the epidermis, and several layers of corky cells called the periderm. The cells in the periderm layer may contain a red pigment that produces red-colored potatoes. Next to the periderm is the cortex followed by the vascular ring which contains the cells that transport food products to the tuber from the above-ground stems. The internal part of the tuber is called the medulla, which is the part of the tuber that contributes to the bulking of potatoes. Excess food produced by the potato plant is transported to the medulla via the vascular ring. Cells in the medulla increase in number and size, as they are supplied with food, and the tuber increases in size.
Figure 1. Diagram of potato plant
Note: For simplicity one main stem is shown. Productive palnts have two or more main stems.
Potato Plant Growth
Growth and quality of potatoes are influenced by environmental factors such as temperature, moisture, light, soil type and nutrients. Many factors that influence potato growth are uncontrollable: length of growing season, air and soil temperatures, light intensity and duration, humidity and wind. Other factors that affect growth can be controlled by the grower: variety, size of mother seed tubers, seed-piece cutting, seed-piece types, seed size, planter operation, plant stand, stem population, moisture, nutrition, pest management, planting date and harvest date. Only when all factors are at optimum levels can profitable yields of quality potatoes be attained.
Figure 2. cross-section of potato tuber.
In Alberta, the upper limits of yield are likely about 30 ton/ac (70 t/ha) given optimum inputs and ideal environmental conditions. Growers occasionally, but not consistently, have achieved yields as high as 24 ton/ac (55 t/ha).
Temperature
Potatoes are a cool-season crop and they grow well in Alberta. Yields are highest when average daytime temperatures are about 21°C. Cool night temperatures are important because they affect the accumulation of carbohydrates and dry matter in the tubers. At lower night temperatures, the respiration process is slowed, less dry matter is burned up and is stored in the tubers as starch. The optimum soil temperature for initiating tubers is 16-19°C. Tuber development declines as soil temperatures rise above 20°C and tuber growth practically stops at soil temperatures above 30°C.
Development of sprouts from seed pieces is more rapid at higher temperatures. Very little sprout elongation occurs at 6°C, elongation is slow at 9°C and maximizes at about 18°C. The number of tubers per plant is greater at lower temperatures than at higher temperatures, whereas higher temperatures favor development of large tubers. Second growth of tubers, associated with irregular moisture levels, can also develop under high temperatures. Second growth may occur in Russet Burbank fields in Alberta because of high temperatures in late July.
These uniform tubers are well
on their way to producing a
profitable crop.
Moisture
Potatoes require a continuous supply of soil water and good soil aeration. Yields are greatest when soil moisture is maintained uniformly above 60 to 70 per cent of total available capacity.
Tuber set is affected by moisture levels at the time of tuber initiation and there are generally fewer tubers set when moisture at tuber initiation is less than 70 per cent of total available capacity. The amount of water needed by potatoes varies with the amount of fertilizer used, soil type, temperature, air movement, plant and stem populations and cultural practices. During the growing period too little moisture, and fluctuating moisture, generally cause more problems than excessive moisture. Experience in Alberta indicates that water shortages are more common than excesses in moisture. Low or fluctuating moisture levels can contribute to scab, hollow heart, knobby tubers, low dry matter, low tuber set and low yield. Excess moisture can result in poor aeration and water logging, which will reduce root growth and yields. An excess of moisture may also lead to enlarged lenticels or openings of the skin on tubers which detract from their appearance. Water-soaked soil at harvest can cause bitterness of Russet Burbank tubers.
Light
Intensity and duration of light affects potatoes. When light intensity is high, tuber initiation and development start earlier, maximum stem length is reached earlier, yields are higher and tubers contain more dry matter than at lower light intensities. However, at very high light intensities the plant may die earlier and tuber weight may be limited by this early senescence. When the duration of light is long, vegetative top growth, particularly stem elongation is enhanced; but when light periods are shorter, stolons are shorter and the rate of tuber set increases.
Tuber Initiation and Growth
Tuber initiation or tuberization starts when tubers begin to swell at the ends of stolons. This occurs between early June and mid-July in Alberta, depending on location, planting date, climate, soil type, and cultivar. Tubers form when the plant produces more carbohydrates than are required for vine growth. Varying climate and moisture conditions cause uneven tube set and growth.
The number of tubers formed per plant is called tuber set. Early in the year, 20 to 30 small tubers may form; however, only 5 to 15 mature tubers are normally found on each plant at harvest. Some of the small tubers that are initially set are used by the growing plant and the number that are finally retained and grow is directly related to available moisture and nutrition. Optimum moisture and nutrient levels are therefore critical to the maintenance and development of tubers very early in the growing season. The effect of uniform high levels of moisture has been well documented in Alberta. (See Irrigation Section). Maintaining high levels of moisture (above 70 per cent of available moisture capacity) has been shown to produce high tuber set compared to lower moisture levels.
Uneven growth of the potato plant, and therefore uneven growth of tubers, can result in tuber abnormalities. Fluctuating temperatures, moisture, and nitrogen, especially in the formative stages, can cause malformed tubers. The uneven growth will be made worse by poor plant stands and by variations in seed-piece types.
Producing Profitable Yields of Quality Potatoes
Marketing | Site selection | Cultivar and seed selection | Planting | Fertility and irrigation | Pest management | Harvest management | Storage management
Potato growers want and need to produce profitable yields of quality potatoes. Yield is important because the crop must generate enough income to cover costs and return a profit. Quality is important because the potatoes must be marketable with minimal grading or processing cost. Production inputs that produce high quality potatoes also tend to produce the maximum economic yield. The following is a brief summary of the major factors the producer can influence to produce profitable yields of quality potatoes. More detailed discussion of these practices is presented in subsequent sections.
Healthy uniform crops result in high quality potatoes.
Marketing
A crop is only profitable when it is sold at a reasonable price; therefore, growers must target production to suit a projected market, not market to suit production. Market projections must consider volume and quality requirements. Over one-half of the potatoes produced in Alberta are sold to processors for French frying or chipping, and all of these are grown under contract. Many fresh-market growers produce potatoes for specific packers, although they may not sign a contract they discuss market agreements early in the crop year. Prospective seed growers need to be aware that credibility and reputation as a supplier of quality potato seed may take several years to establish. Provisions for low-waste harvesting, handling, storage and marketing must be made in advance of production.
New growers who wish more information about processors, fresh packers and marketing in general are invited to contact the Potato Growers of Alberta All potato growers producing more than 5 acres(2 ha) of potatoes are required by regulations under the Agricultural Products Marketing Act, to be licensed by the Potato Growers of Alberta (PGA). New growers should contact the PGA office for a licence.
Site Selection
In choosing a site for a potato farm a prospective potato grower has to be aware of two general factors: climate and soil. Potato production should be confined to those areas shown on the climate map with 100 or more frost-free days. Certain micro-climates outside of this zone, such as in river valleys, may be suitable; and conversely, some small areas within the 100-day-or-more zone may be subject to abnormal early or late frosts. Areas with longer frost-free periods have the potential to produce the highest yields, however, other factors such as soil type, availability of irrigation water and nearness to markets have to be considered. A long frost-free season allows the grower a wider choice of cultivars, a longer harvest period and generally contributes to the production of high yields of mature potatoes. Deep, well-drained, coarse-textured soils such as loamy sands, sandy loams and loams are the best soils for potato production. Coarse-textured soils tend to warm up sooner in the spring, are easily worked and have good drainage. However, very coarse soils (sands) have a low water-holding capacity and require irrigation in most parts of Alberta to produce consistent yields of quality potatoes. Irrigated sands will also require the addition of nitrogen through the irrigation system during the growing season. Finer-textured soils with high amounts of clay can be difficult to work into a clod-free seed bed and they become sticky when wet or lumpy when dry. Clay soils adhere to tubers and they may require considerable washing. Poorly drained soils can result in delayed tillage or harvest operations and potatoes may be more subject to disease.
Cultivar and Seed Selection
The most commonly grown cultivars are described in detail in the next section. New producers need to become familiar with varieties, so they can select the best ones for their market and climate. Varieties are commonly described by their maturity (early, mid, late), skin color (white, russet, buff, red), skin texture (smooth, netted), flesh color (white, cream, yellow), flesh texture (dry and mealy or, wet), and tuber shape (round, oval, oblong, long). There are about 25 cultivars grown in Alberta; however, less than 10 varieties account for over 90 per cent of the potatoes produced in recent years.
The importance of using certified seed cannot be overemphasized. Potatoes are grown from fleshy tubers and bacterial, fungal and viral diseases can be carried from generation to generation. The primary reason for the potato seed-certification regulations is to maintain disease-free stocks of potato seed and therefore keep diseases to a minimum in commercial fields. If these diseases were left unchecked, production of potatoes would soon decline. Many diseases merely reduce plant vigor and yield, but others will kill the crop under the right conditions. The objective of certified seed regulations and of using certified seed is to keep diseases at very low levels.
Planting
Three factors are of paramount importance to a potato crop: stand, stem population and vigor. Establishing a stand of uniformly vigorous seedlings with an adequate number of stems is demanding and no amount of other cultural inputs such as fertilization, irrigation and pest control will compensate for a poorly planted crop. Potato plants adjacent to a "miss" will have an increased yield, but the increase does not make up for the zero yield of the missing plant. Usually the tubers produced by plants next to a miss are oversized, misshapen and have a higher incidence of hollow heart in susceptible cultivars. Potato producers achieve good stands by maintaining and operating their planters properly and by cutting uniformly sized seed pieces. Seed pieces that are uniform in shape and size can only be cut from a mother seed lot with a narrow range in size with few tubers over 8 ounces (225 g).
Plants must be vigorous and strong to produce large yields. Large, uniform seed pieces with few cut surfaces produce strong plants. A seed lot should have an average seed-piece weight of 2 ounces(60 g) with no pieces less than 1.5 ounces (45 g) or greater than 3 ounces (90 g). Because seed-piece sizing is so important, potato producers should purchase uniform seed tubers ranging from 2 to 8 ounces (60-250 g) in weight. Large mother seed tubers (greater than 10 oz) result in many seed pieces with a few or no eyes. These pieces reduce plant stand, have fewer stems, and result in much size variability.
Potato growers who want high yields treat the cut seed lot with a registered fungicidal treatment to reduce seed-piece decay and improve stand and plant vigor. Cut seed must be kept out of the sun and drying wind to prevent moisture loss and is usually planted within a day of cutting. Cut seed that must be stored is suberized in warm humid conditions to promote rapid healing of the cut surface. Cut seed that is to be suberized and stored is not treated until planting time.
Fertility and Irrigation
The coarse-textured, well-drained soils used for potatoes will require adequate amounts of nutrients to produce profitable yields and growers should soil test annually. Typically, nitrogen and phosphorus are the two nutrients applied to potatoes, although potassium is often required, especially in southern Alberta. Potatoes forage for nutrients and water up to a depth of about 2.5 ft (75 cm) and soil sampling to at least 2 ft (60 cm) is recommended to get an accurate assessment of available nitrogen. Tissue sampling and analysis (especially of irrigated potatoes) helps a grower monitor nitrogen levels and add nitrogen as required to maintain growth and plant health. However, it must be stressed that over application of nitrogen can delay maturity, reduce dry matter yield and increase the incidence of hollow heart.
Irrigation in central and northern Alberta is good insurance in some years, and mandatory every year in southern Alberta to produce high yields. A uniform supply of moisture (above 60 to 70 per cent of available moisture capacity) generally increases tuber set and yield, and reduces the number of deformities.
Pest Management
Diseases, insects and weeds can ravage potatoes if left unchecked. New growers must become familiar with pests if they expect to grow quality potatoes and make a profit. The importance of pest management is best stressed by reviewing the seed potato certification program in Alberta and in Canada. A handful of tubers that are completely disease free are released by Agriculture Canada, propagated by Alberta Agriculture and then go through four to seven years of growing and inspection on seed farms before certified seed is available for commercial growers. This whole process is aimed primarily at disease control.
The best tool available to commercial growers to control disease is to use certified classes of seed. Producers also need to be concerned about seed piece decay, early blight, and storage diseases like soft rot and dry rot. Bacterial ring rot, which has nearly disappeared in Alberta, will always be a major concern because it is highly contagious and destructive if left uncontrolled. All commercial potato fields in Alberta are inspected for ring rot every year, and for this reason all potato growers with 5 acres, or more, are required to be licensed by the Potato Growers of Alberta.
A rotation with a minimum of three other crops between potatoes is required to sustain potato production over a long period. Shorter rotations result in a steady increase in soil-borne diseases.
Insects such as the Colorado potato beetle can completely defoliate a potato crop if left unchecked. Wireworms, which are ever present in newly broken pasture land, cause unsightly tunnels in tubers which can result in losses on the grading line and reduction in grade if damage is severe. Seed growers need to control aphids to prevent the spread of viruses that may cause loss of certification.
Weed control in potatoes is not normally a problem because the combination of cultivation and herbicides is usually effective. However, a few weeds left unchecked in the spring may become an unmanageable problem by fall. Severe weed infestations reduce yields, interfere with harvest and are a bountiful weed seed source for subsequent crops.
Harvest Management
Getting a bumper crop of potatoes to market or into storage while maintaining excellent quality involves planning and good management. Vine killing is usually done 10 or more days before harvest to ensure a good skin set and reduce skinning. Harvest equipment must be in good working order and personnel must be trained to operate harvesters so that bruising is minimized. Bruises, cuts and scrapes are typically caused by roll-back, drops and sharp edges on harvesters; all of which can be reduced by machinery modification, chain-speed monitoring and maintenance.
Storage Management
Seed must be stored for eight months, through to the next crop year, without rotting or sprouting. Potatoes for French frying and especially chipping must have low levels of sugars so that they will produce light colored frys or chips throughout the storage season.
Obviously a grower must place a good quality crop into storage to take quality potatoes out. Management throughout the season (cultivar selection, seed cutting, planting, fertilization, irrigation, pest control and harvest management) contributes to the quality of tubers placed in storage. Cold, freezing weather during harvest in late September and early October can severely affect quality. Following adverse harvest weather, heavy culling and curing potatoes with high volumes of air may enable a producer to keep a problem crop for a reasonable amount of time.
Sanitation of storages and equipment helps reduce losses caused by disease. Maintenance of proper temperatures, humidity and airflow prevent heat build up and rots, reduces shrink to a minimum, and maintains processing quality. To do all of this, growers require facilities that have the necessary environmental controls. 
Potato Quality
Nutritional quality | Fresh market quality | Quality of pre-peeled potatoes | Potato chip quality | French fry quality
Nutritional Quality
Potatoes are a non- fattening, nutritious and wholesome food which supply many important nutrients to the diet. Potatoes contain approximately 78 per cent water, 22 per cent dry matter and less than 1 per cent fat. About 82 per cent of the dry matter is carbohydrates which occur mainly as starch, however some sugars are also present in small amounts. Potatoes contain 11 per cent protein by dry weight, which is less than that of soybeans; however, the nutritional quality of potato protein is better than that of soybeans. Potatoes contain at least 12 essential vitamins and minerals. They are an economical source of vitamin C, and contain thiamine, iron, folic acid and some fibre.
Potatoes are graded prior to storage
and prior to shipping to remove diseased
or damaged tubers and dirt.
Fresh-Market Quality
Quality is an important factor influencing consumer preference and saleability of potatoes. Good quality potatoes are clean, uniform in shape and size, and have a unmarked skin, firm flesh and shallow eyes. Consumers don't like the waste caused by trimming potatoes with deep eyes and surface defects. Other tuber defects which may adversely influence quality are greening, second growth, growth cracks, scab, storage rots, internal black spot, skinning, bruising and mechanical damage.
In addition to physical quality and appearance, potatoes must also have good cooking and eating quality. The main culinary quality factors of home-prepared potatoes are texture, color, flavor and odor. The texture of cooked potatoes is directly related to the dry matter content or the specific gravity, and is described as mealy and dry or soggy and wet. Potatoes such as Russet Burbank, which have a high dry matter content, usually have a dry, mealy texture when baked. However, they may slough or break up when boiled which is a problem when preparing boiled potato pieces. They do make excellent mashed potatoes. Potatoes such as Norland, with a lower specific gravity, are inclined to be somewhat soggy and wet when baked, but hold together quite well when boiled.
White-fleshed potatoes, when boiled or baked, are expected to be a creamy white color. Yellow-fleshed varieties should retain their yellow color during cooking as well.
White- or yellow-fleshed potatoes should exhibit minimal after-cooking darkening. This darkening, which is often more prevalent at the stem end of the tuber, is believed to be caused by a dark pigment formed by the reaction between chlorogenic acid and iron, both naturally occurring compounds in potatoes. Research at the Agriculture Canada Research Station, Lethbridge, showed that Russet Burbank potatoes grown in central Alberta had a significantly higher chlorogenic acid content than potatoes grown in southern Alberta. Varieties such as Norland, which naturally contain higher amount of chlorogenic acid, blacken more than varieties containing less chlorogenic acid. In the home, after-cooking darkening may be controlled with lemon juice (which contains citric acid). In the commercial potato processing industry sodium acid pyrophosphate is used.
Cooked potatoes are expected to have "normal" potato flavor and aroma, without off-flavors or off-odors. One group of chemicals in potatoes which contribute to the characteristic potato flavor when present in small amounts, are alkaloid compounds, such as solanine and chaconine. Collectively these compounds are referred to as total glycoalkaloids or TGAs. They are usually present at low levels(2-15 mg/100 g fresh tuber weight); however, when TGA levels increase to 20 mg/100 g the potatoes are bitter. Above 20 mg/100 g, the potatoes are considered unfit for human consumption because the glycoalkaloids have reached toxic levels.
The development of bitterness in potatoes is often signalled by surface greening of the tubers, although the development of bitterness compounds and green color are independent. A green pigment, chlorophyll, forms when potatoes are exposed to light. This same light also triggers chemical reactions in the outer layer of the potato tuber leading to increased TGA levels. Light-green potatoes are harmless if peeled. Moderately green tubers should not be used for consumption without first tasting the peel of several raw tubers.
Bitterness or burning indicates that TGA levels are too high and the potatoes should be discarded. Elevated TGA levels in potatoes may result not only from exposure to light but from harvesting immature potatoes, bruising, skinning and improper storage temperature. Wet conditions before harvest, and chilling or freezing may also cause high TGA levels. Cold potatoes can be slowly warmed up to 15°C for 10 days after harvest; however, growers should seek further advice on the storage of chilled tubers. Unlike other problems associated with potato quality, such as after-cooking darkening, TGA accumulation cannot be counteracted with chemicals or special processing techniques. Glycoalkaloids do not leach from potatoes into blanching or cooking water, nor are they destroyed during the high temperatures associated with boiling, baking or deep frying.
Quality of Pre-peeled Potatoes
Some potatoes (often cull material) are used to produce pre-peeled products for the food-service industry. These potatoes are peeled by chemicals, hot water or abrasion. Following peeling and trimming, the potatoes are packed for shipment to the user or distributor. These pre-peeled potatoes must be refrigerated at all times since they have a limited shelf life. Pre-peeled potatoes are treated with anti-browning agents.
Potato Chip Quality
Four important factors affect chip quality: the yield of chips from fresh potatoes, the color of chips, the oil content of the chips and chip flavor. One of the most important qualities of chipping potatoes is high specific gravity. The yield of potato chips increases as the specific gravity, or dry matter, of the fresh potatoes increases. Potato chipping is a water-removal process, hence the higher the water content of the potato the more water will be lost to evaporation and the lower the yield of chips. Potatoes with more solids (higher specific gravity) will lose less water and the chip yield will be higher. In addition to its direct influence on chip yield, specific gravity has a direct bearing in the amount of oil absorbed by the potato slices during the deep frying process. Slices from low
specific gravity tubers absorb more oil than slices from tubers with high specific gravity. Some oil absorption by the potato slices during deep frying is desirable for flavor development. However, too much oil absorption results in greasy chips with a high oil content. Production costs are therefore increased because more oil is used in the frying process.
Quality potato chips have a light color and little vascular discoloration. Processors have little control over chip color (manipulation of oil temperature, slice thickness and length of frying period) because the color of potato chips is largely determined by the sugar content of the potato tuber. Potatoes with high sugar levels make dark chips. When potato slices are fried, the reducing sugars react with proteins and amino acids to form dark products in a non- enzymatic browning reaction. The concentration of chemical constituents, such as reducing sugars, depends on many factors including variety, growing conditions, maturity and storage conditions.
Potato chips must have a pleasing and desirable flavor. Thus potatoes should not be bitter or have other off-flavors. The flavor of potato chips is more complex than that of boiled, baked or mashed potatoes, since the cooking temperatures are much higher, and the absorbed oil contributes to the overall flavor profile of the product.
French Fry Quality
Good quality French fries have a uniform light cream to golden color. They have a good potato flavor free from rancidity, bitterness, and are free from a scorched taste and off odors. Their texture consists of external surfaces that are moderately crisp, showing no separation from the inner portion. The inside is tender, mealy and free from sogginess. Other undesirable defects include sunburn, carbon spots and light and dark brown areas.
As in the case of potato chips, the color of french fries also depends on the reducing-sugar content of the potatoes; however, French fry processors have more control over reducing-sugar levels because of the blanching process. Mealiness, crispness and lack of oiliness in French fries increases with specific gravity.
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