Potato (Solanum tuberosum), English potato, Irish potato, White potato.
Botanical name
Solanum tuberosum L.
Family
Solanaceae.
Other names
Aardappel (Nether.); Alu (Ind.); Batata (Port.); Jaga-imo (Japan); Kartoffel (Ger.); Papas (Lat. Am.); Patatas (Sp.); Pomme de terre (Fr.); Viazi (E. Afr.); Watalu (Pak.); Yang shu (China); Yeomilan (Cy.).
Botany
The potato is a herbaceous, freely branching dicotyledonous perennial, usually between 30 and 100 cm tall, with alternate, pinnately compound leaves, made up of three or four pairs of oval leaflets and a terminal leaflet. Cymose inflorescences, arising opposite to leaves (not axillary) near the ends of branches, bear gamopetalous flowers with 5-lobed corollas, which are white, yellow, purple, blue or striped, and about 3 cm across.
The fruit is a spherical berry about 1.5-2 cm in diameter, green or purplish, containing a large number of small seeds. The roots are numerous, fine, fibrous and adventitious. Short stolons with hooked tips are produced from the axils of the lower leaves and become thickened to form stem tubers which have buds (eyes) mainly towards the distal end.
When the aerial part of the plant dies back following the normal maturity cycle in adverse climatic conditions the tubers remain in the ground and sprout to form new plants when the dormancy of the tuber breaks and climatic conditions are favourable.
Authorities differ as to the number of distinct species of cultivated potato, some recognise as many as 20 species, but the classification of Dodds is widely accepted in which one cultivated species S. tuberosum is recognised, plus five horticultural groups and two hybrid cultivars S. x juzepczukii and S. x curtilobum. There are a great many cultivars, most of which have been bred for temperate climates. Until very recently all potatoes tested in the tropics were selected from such material (India, for example, has tested over 600 temperate cultivars). Only a minute proportion could even be considered for commercial culture under the changed day-length and temperature conditions. However, substantial breeding programmes are underway in several tropical (and temperate) countries towards the improvement of potatoes for the lowland tropics, often using tropical varieties and species in the breeding lines - varieties that are both physiologically adapted to tropical conditions, and resistant to the major tropical diseases; particular attention is being paid to developing resistance against Pseudomonas solanocearum. True potato seed (see Planting procedure - Material) from appropriate crosses (hybrid seed) may have an important part to play in such improvements.
Origin and distribution
The potato is believed to have originated in the altiplano around Lake Titicaca, at a height of about 3 000 m in the Bolivian Andes, and the main centre of diversity is in the Andes between 10°N and 20°S at altitudes above 2 000 m. This area is still an important source of germplasm for the breeding of new cultivars. Wild species (varieties) are distributed from Central America to southern Chile. The plant has been cultivated in the Andes since Inca times, but its spread throughout the world has been relatively recent, following its introduction into Spain from Colombia by the Conquistadores in 1570, and its independent introduction into England in 1586. The first potatoes to reach North America came from Europe in about 1621. The potato did not become popular in Europe until about 1663, when it became a staple food in Ireland. During the 16th to 18th centuries it was introduced into the Philippines and other Pacific countries, reaching India in the 17th century. It was little used in England until the 19th century, and about the same time became widely grown in Europe. It is now grown all over the world, especially in temperate climates, though production is increasing rapidly in the tropics and it is now the root crop with by far the greatest production and the leading position in world trade.
Cultivation conditions
Level land is ideal, especially where operations are mechanised. In tropical highlands, plateaux are often used; in tropical lowlands, valleys are likely to be more suitable than extensive plains as they funnel cool air during the night from neighbouring highlands.
Rainfall - a rainfall of 50-75 cm, evenly distributed throughout the growing period, is considered essential (approximately 2.5-3 cm per week is required). In the tropics up to 15 cm per month can be tolerated. Drought, even for short periods, can have a serious effect upon yields and the quality of the crop, especially when it is accompanied by high temperatures or when it occurs during the last 9 weeks of growth. Inadequate or irregular water supply not only results in poor yields, but the tubers are usually malformed, often having very thick skins and a knobbly appearance. It is therefore recommended that natural water supplies be supplemented by irrigation when the available soil moisture is only 50-60 per cent of the requirement, particularly during the emergence period. Under arid conditions irrigation, either spray or flood, should be carried out at frequent intervals until the tubers are formed, then the interval is gradually decreased to reduce the possibility of the potatoes developing pre-harvest rots. On light soils, combined with moderately high temperatures irrigation at 3-4 day intervals has been recommended, while on heavier soils 5-7 day intervals should be adequate. When grown under humid conditions, control of late blight (Phytophthora infestans) is often difficult and this is particularly serious in many tropical areas.
Soil - potatoes can be grown on all soil types, except heavy water-logged clays, but for optimum yields need a well-drained loam or sandy loam, relatively free from stones. Well-drained peat soils are particularly suitable and where the growing season is short, light, well-aerated soils are necessary. The pH may range from 4.8 to 6 (optimum 5.5-6); above pH 6 potatoes are liable to suffer from scab.
The potato is a heavy feeder and responds well to fertilisers, though requirements vary greatly according to cultivar, soil type and climatic conditions. Adequate fertilising is particularly important in the tropics because of the shorter growing period. In the UK 1.25 t/ha of a 13:13:20 complete (NPK) fertiliser is common; for India, nitrogen 120-180 kg/ha, phosphorus 80-100 kg/ha and potassium 80-100 kg/ha or FYM at 12-15 t/ha have been recommended.
Altitude - the importance of altitude is its effect on temperature. Many indigenous South American cultivars can be grown in the tropics at levels of about 2 000 m, but at lower altitudes yields are generally poor. Main crop or late-maturing cultivars from temperate climates can usually be grown with moderate success in the tropics at altitudes between about 400 and 2 000 m, and even down to sea level if there is a marked cool season.
Day-length, light intensity and temperature all interact in their effects on the productivity of potato crops. In general, indigenous South American cultivars will only produce reasonable yields with a day-length of 12-13 hours, but the early-maturing cultivars of temperate regions require a daylength of 15-16 hours; however, the main or late-crop temperate climate cultivars are successful under long- or short-day conditions.
Certain generalisations can be stated. Yields are dependent upon both tuberisation and the subsequent development of the tubers. In most cultivars the optimum temperature for tuber formation is 15-20°C (up to 22°C in a few cultivars), but short day-length permits tuber formation at higher temperatures. Also, in general, the higher the light intensity during the growing season, the higher the maximum temperature allowing tuberisation. Subsequent development depends upon the deposition of assimilates in the tubers. Net assimilation is at a maximum at about 25°C: above this temperature the rate of respiration increases substantially so that fewer assimilates are retained by the plant. In addition, at high temperatures the partitioning of assimilates between tubers and vegetative parts favours the latter, particularly for stem and branch growth, this itself reducing the photosynthetic capacity of the plant as leaf area does not increase proportionately.
The best balance of these various factors has been found in temperate regions and is reflected in the high yields obtained in most European countries, the USA and Canada (Table 1). At the other extreme are the lowlands tropics where, until very recently, commercial production was virtually impossible. In tropical uplands the lower (though still high) temperatures, combined with short day-lengths, allow 'reasonable' yields to be obtained. Current trials with heat tolerant clones are showing much improved yields.
Planting procedure
Material - the cultivar to be used is critical. There are many suitable for temperate conditions, both early-maturing and late-maturing. Early-maturing cultivars are not suitable for the tropics, as they require a long day-length, but a few late-maturing temperate cultivars have given moderate success in parts of the tropics.
(i) Tubers, either whole or cut, are normally used for planting. They should be from virus-free stock and these are normally specially grown in areas where the aphid vectors are absent or strictly controlled, often at a considerable distance from the actual production area for the crop. This necessity for virus-free 'seed' tubers has often caused problems in the tropics, requiring the costly importation of planting material from other countries. In some tropical countries it has been possible to grow virus-free seed potatoes in hill areas for planting in the lowlands. Recent work in India has considerably reduced the cost of seed potatoes, by what is termed the 'Seed Plot Technique'. At certain seasons the aphid (vector) population in the plains is low, and when healthy seed potatoes from the hills are grown during these periods the incidence of virus disease is also low. In the seed plots further precautions are taken by applying systemic insecticides, rogueing infected plants, and removing haulms before the aphid population reaches a certain critical level. Infection is minimal and such seed plots can be maintained in a healthy condition for several generations, supplying seed potatoes to surrounding farms for a number of years before a new injection of hill-grown seed potatoes is required.
Potato tubers enter into a period of dormancy after harvesting; the period of dormancy depends very much upon genetic and environmental factors, but is usually at least 8 weeks and can be a problem when planting material is required for continuous cropping. However, dormancy can be broken by holding the tubers at 20-30°C for 30-45 days (which are common ambient conditions in the tropics, ie dormancy is broken quickly in such climates), or by treating them with chemicals such as chlorhydrin, potassium or sodium thiocyanate, or gibberellic acid. Naturally-broken dormancy, however, is preferred as it gives a more uniform rate of germination and better growth. Once the period of natural dormancy has ended potatoes will begin to sprout provided that the temperature is above 5°C; the higher the temperature the quicker the rate of sprouting and many growers, particularly those producing early crops in temperate regions, expose the seed potatoes in a 'chitting house' to diffuse light and a temperature of approximately 10°C to accelerate the process. Often the tubers are treated with fungicide before planting to prevent disease, and when pieces are used these are frequently cut by hand and planted as soon as possible after cutting. Cut tubers can be stored for up to 30 days if they have been cured at 15-21°C and 85 per cent RH for 7-10 days.
(ii) True potato seed (TPS), formerly used only in breeding work, is now becoming a commercial technique. It is easy and cheap to distribute over long distances, which makes it unnecessary to have a source of virus-free tubers relatively close to the main potato growing area. The production of TPS is a specialised but low-cost operation requiring healthy parent plants. Open pollinated seed is collected from commercial varieties. F-l hybrid seed which has greater disease resistance or higher yield potential is being developed. TPS planted directly into the field is often not satisfactory, but planted in trays or a nursery will give seedlings suitable for later transplanting, or planted in a suitable nursery will give seed tubers for subsequent planting in the usual way. TPS may well prove to be especially useful in the tropics, avoiding the problems associated with obtaining virus-free tubers for planting in these regions.
Method - potatoes may be planted by hand or with mechanical planters, which can be fully mechanical or hand-fed semi-automatic machines. Before planting the field should be ploughed to a depth of 25-30 cm and harrowed so as to produce a fine deep filth. The seed tubers are usually planted in ridges at a depth of 5-15 cm and kept free from weeds; preemergence herbicides can be used for this purpose. In Europe the crop is often repeatedly hoed, up to five times during the season, to control weeds and earthed-up to avoid greening of the tubers.
Tubers grown in a nursery from TPS are planted in a similar manner, or seedlings grown from TPS may be transplanted by hand or mechanically.
In the tropics, planting on ridges would normally be recommended, but whether ridges or level soil be used depends upon local conditions. A flat soil heats up less during the day than ridged soil, as the exposed surface is less: conversely ridged soil is cooler at night, and this may be especially important in areas where nights are warm. Ridging is advantageous in the rainy season or in generally wet locations because it gives better drainage and minimises erosion; flat soil is recommended for relatively dry locations or in the dry season, where daytime temperatures are high and soil moisture is to be conserved. Hilling or ridging after planting helps to control weeds, covers emerging stolons and thus prevents tuber greening, but conversely it loosens the soil and encourages moisture loss. In general, however, some degree of earthing-up appears to be desirable.
Field spacing - a spacing of 20-30 cm in rows 75-120 cm apart is widely used in temperate conditions, but optimum spacing depends very much upon cultivar and environmental conditions. Wider spacing between the rows normally increases the number of tubers produced; closer spacing increases yield but may decrease the proportion of tubers of marketable size. However, owing to the more rapid maturing of potato plants in the tropics, it is important for maximum light interception to be achieved as early as possible in the life cycle. This may be achieved by closer plant spacing (brought about by smaller distances between rows), but when carried to excess, a high proportion of small tubers is produced, though this also depends very much on the cultivar. It is not possible to give ideal spacings as so many factors are involved; however, spacings as low as 30 cm between plants in rows 40 cm apart (80 000 plants/ha) have given good results with certain cultivars in Peru, and 60 x 20 cm has been recommended for India. Trials under warm conditions have given maximum tuber yields when clones have reached maximum cover before tuber initiation.
Seed rate - depends upon the spacing and whether whole tubers or pieces are used, but usually ranges from 1.2 to 2.5 t of whole tubers per hectare.
Mulching - is not normally practiced in temperate regions, but for the tropics it is recommended, both for minimising evaporation and for keeping the soil cool. Light-coloured (highly reflective) material should be used, eg rice straw, rice hulls, mature maize leaves, etc are suitable. Mulch should be applied immediately after planting as the improvement in conditions that it creates is especially important in hastening emergence of the shoots. When the leaves begin to form ground cover the importance of the mulch is reduced.
Shading - in temperate climates this is not normally practiced but recent work in Peru has indicated that artificial shading in the early stages of growth has a ground cooling effect and is beneficial, but reduces yield if long continued. Intercropping can be used to provide shading, but the intercrop should provide only a minimum of competition, eg maize (planted 1-2 months before the potato and widely spaced), palm or citrus groves.
Pests and diseases
Over 100 insect pests, about 100 bacteria and fungi, about 30 viruses and some 40 or more disorders of unknown cause have been recorded for this crop. Proper control measures are of the utmost importance, together with the breeding of resistant cultivars and the multiplication and distribution of healthy planting material. Some of the common pests and diseases are listed here; many are of world wide distribution though a few are limited to tropical or subtropical areas.
Pests - Aphids are widespread and of considerable economic importance, not only on account of the damage they inflict on the crop but because they are responsible for spreading virus diseases such as leaf roll and mosaic. Several different species attack the crop, including Macrosiphum euphorbiae, Myzus persicae, Aphis gossypii and A. nasturtii. The potato tuber moth Phthorimaea operculella can be very destructive, especially in the tropics, and attacks both the young plants and the stored tubers. Root eating ants, Dorylus orientalis, have been reported as troublesome in parts of the tropics. Various species of cutworms cause considerable damage to the crop, particularly Agrotis spp., of which the black cutworm, A. ipsilon, is of economic importance in many parts of the world. Diabrotica speciosa (pin worm), leaf miners (Liriomyza spp.) and other leaf-eating insects may cause serious damage in the tropics, especially in the wet season; probably the best method of control is to avoid growing potatoes at this time of the year. The flea beetles, Epitrix cacumeris and E. tuberis, are widespread. The larvae and the adult form of the potato stalk borer, Trichobaris trinotata can cause considerable losses in certain areas, eg the USA. Ladybirds, Epilachna spp., are another widespread pest particularly in dry conditions, and unless controlled can completely defoliate the plants. Wireworms, especially Agriotes spp., are of considerable economic importance in the USA and the UK. The Colorado beetle, Leptinotarsa decemlineata, is widespread in the USA and many parts of Europe. Both the adults and larvae feed on the potato plants and can quickly defoliate an entire field; in addition, the adult beetles are known to transmit several potato diseases. Root eelworm, Heterodera rostochiensis, is a serious pest, particularly in the UK, where losses as high as 50-60 per cent have been reported for early potatoes grown on infected land during a dry spring and early summer. Other eelworms attacking the crop are Ditylenchus dipsaci and D. destructor; attacks of the latter cause the tuber to dry out, shrink and sometimes crack. The root-knot eelworms, Meloidogyne spp., cause wart-like growths on the tubers, adversely affecting yields, and are serious pests in both temperate and tropical areas. In addition, potatoes may be attacked by slugs and snails; of these the grey field slug Agriolimax agrestis and the banded grey slug Arion circumscriptus are the most important.
Control of pests is normally by chemicals, eg carbofuran or aldicarb in the furrows before planting for soil pests, and foliar sprays of methomyl or carbaryl for insects attacking foliage; pirimicarb is used against aphids.
Diseases - Late blight caused by the fungus Phytophthora infestans is one of the most widespread and serious of all potato diseases and was responsible for the great potato famine in Ireland during the 1840s. All parts of the potato plant are affected by the disease and infected tubers develop dry or wet rots either before or after harvest, depending upon the degree of infection, environmental conditions and the presence or absence of secondary organisms. There is no cultivar completely resistant to late blight, although some have a high degree of resistance for several years.
Early blight, caused by Alternaria solani, is another widespread fungal disease of considerable economic importance, but is easier to control than late blight. Scab, due to Streptomyces scabies, often affects potatoes grown in the tropics and in more temperate regions in soils of pH above 6, causing raised greyish white to brown corky areas on the tubers. Black scurf, due to Rhizoctonia solani, is a serious fungal disease, also known as stem canker and 'rhizoc'. It attacks the stems and the tubers, both at and below the soil level resulting in a decrease in the size and the set of the tubers. The fungus has a wide host range and can survive as a saprophyte in the soil, which makes it difficult to control. Verticillium wilt, Verticillium albo-atrum, has become of increasing importance in recent years in both temperate and tropical regions. Several types of tuber rots, including black rot, powdery dry rot and seed piece decay, are caused by certain species of Fusarium, while other species cause wilting of the plants and stem-end discoloration very similar to Verticillium wilt. In cool temperate regions, skin spot, Oospora pustulans, can sometimes result in the tubers being rejected for human consumption, while in the more northerly areas of the USA powdery scab, Spongospora subterranea, is sometimes a problem.
Of the bacterial diseases, the foul smelling wet rot in the tubers, caused by Erwinia carotovora, is of economic importance. Black leg is another form of soft rot which affects the stems in addition to seed pieces and tubers. Both these diseases are widespread and can only be effectively controlled by the planting of disease-free tubers, careful sanitation and the prevention of tuber injuries. Brown rot or southern bacterial wilt, Pseudomonas solanacearum, is common in subtropical and tropical regions. Mild attacks cause wilting and death of the plants and if it invades the tubers these break down with an offensive odour. A similar disease which is very contagious and easily spread, particularly when cut pieces of tubers are used for seed is ring rot, Corynebacterium sepedonicum.
Several virus diseases cause serious losses to potato crops. Rugose mosaic, known also as severe mosaic, leaf drop mosaic, potato virus Y and potato vein banding virus, is one of the most serious and widespread, sometimes causing premature death of the plants. It is transmitted by the peach aphid, Myzus persicae; no commercial cultivars have yet been developed which are completely immune to it, though some do possess a moderately high degree of resistance. Leaf roll is another of the more serious virus diseases, and is also transmitted by the peach aphid and other insects. It is widespread and plants grown from infected tubers are often dwarfed and pale green in colour and may show a characteristic rolling of the upper leaves. The number and size of tubers is greatly reduced and these frequently develop phloem necrosis. Mild mosaic is characterised by a chlorotic mottling of the foliage usually accompanied by a slight crinkling, while other virus diseases include mottle or potato virus X, spindle tuber, yellow dwarf and paracrinkle virus.
The use of healthy planting material is extremely important in disease control. Where Rhizoctonia is a problem the seed potatoes may be treated with benomyl before planting, and shallow planting allows rapid emergence of sprouts, reducing the chance of infection. Verticillium is at least partially avoided by rotation of crops and there are resistant cultivars. Alternaria is minimised by keeping plants healthy with proper nutrition and water; maneb or zineb are used for control. Phytophthora is treated with copper-based sprays or maneb or zineb. In some countries farmers are advised in advance about the development of weather conditions likely to favour Phytophthora, and preventive spraying is carried out. Viruses are particularly difficult to control; methods include (in addition to the use of virus-free planting material) rogueing and destroying infected plants, control of aphids by insecticides, and, when potatoes are being grown for seed tubers, early destruction of the haulm.
Growth period
Potatoes show varying maturity periods; in temperate climates early cultivars will mature in 3-3 1/2 months, medium in about 4-6 months and late cultivars may take up to 7 months, depending upon the environmental conditions. Short day-lengths (12-13 hours) lead to earlier cropping and main crop potatoes may mature in as short a time as 4 months, but with reduced tuber yield.
Harvesting and handling
Main crop potatoes should not be harvested until they are fully mature, when the skins have set and there is less likelihood of damage during lifting. In temperate climates the crop is usually harvested 2 or 3 weeks after the foliage has died down either naturally or been killed by mechanical or chemical methods, or a combination of these, or sometimes by frost. In most areas where potatoes are grown on a commercial scale, the crop is harvested mechanically and there is a wide range of methods and machinery used, depending upon the area to be harvested, the type of soil, labour costs, etc. The more common types of equipment used include diggers of various types, spinners, ploughs and complete harvesters, which lift the tubers, clean them of soil and other extraneous matter, and deliver them into trailers or bags. Frequently the tubers are left on top of the soil for 15-60 minutes, depending upon the temperature, in order for the skin to dry and toughen. Early potatoes are frequently harvested by hand, as the immature skins are easily damaged or rubbed off.
During storage the tubers may be affected by a number of pests, diseases and disorders, and storage procedures are designed to minimise such problems. However, virtually all pests and diseases that cause damage to potatoes in storage also attack the growing crop; many have already been mentioned. Tubers that develop these conditions during storage in most cases are already infected when put into storage or become infected by the spread of the pest or disease from other previously-infected tubers. In general, low storage temperatures and good aeration markedly reduce the spread both of pests and of diseases.
Physiological disorders are common. Physical damage during harvesting may not only permit easy entry of pathogens, but can lead to internal bruising with discoloration of the flesh, even if the skin is not broken. High or low extremes of temperature cause damage: high temperatures give rise to black heart, resulting from asphyxiation of the tissues at the centre of the potato where the accelerated respiration rate produces shortage of oxygen; chilling (below 2°C) causes collapse of the tissues. Warm temperatures accelerate sprouting, and the associated changes of starch to sugar give sweetness and softening of the tissues, but storage below 6°C also causes rapid changes from starch to sugars which give an uncharacteristic sweetness and can also cause darkening during certain types of cooking and processing, eg frying or dehydration. Exposure to sunlight leads to greening (and production of toxin). It is therefore essential to put only healthy and dry potatoes into storage, and then to seek a compromise involving as low a temperature as practicable to minimise the development of sprouting and spread of disease, but high enough to avoid low temperature sweetening: in most temperate countries potatoes for eating are stored at about 5-7°C, along with the use of chemicals to delay sprouting: 6-8 months storage life is expected. Among the chemicals used are MENA (methyl ester of naphthalene acetic acid), TCNB (tetrachloronitrobenzene), propham, chlorpropham and nonanol (trimethyl-hexanol). The solids may be dusted onto the tubers as they are placed in store or mixed with the tubers in granular formulations or the chemical may be vaporised and blown through the stacks of tubers.
Losses can be considerably reduced if the tubers are cured at 12-18°C and 85 per cent RH or higher to encourage cell suberisation, and periderm formation to heal wounds. However, the treatments noted above, except for TCNB and the late application of a vapour, prevent wound healing.
Potato stores range from clamp storage to specially designed buildings. In clamp storage, until recently common in temperate climates, the potatoes are stored in heaps in the field and covered with straw and soil, which gives them protection from light and fluctuations in ambient temperatures, but no humidity control. The lack of aeration often results in serious losses. Storage in slatted boxes or in sacks, or in dark sheds or other buildings in which cold air is circulated, may be used, though again air circulation around the tubers may be inadequate and may permit overheating in the centre of the bag or box. Purpose-built bulk stores have underfloor ducting through which air can be blown through stacks of potatoes up to about 3.7 m high; vaporised chemicals may be introduced through this ducting. (Storage of potatoes is dealt with in detail by Rastovski and van Es (1981) and Cargill (1976): see Bibliography.)
Primary product
Tubers - which are characteristic of the cultivar in size, shape and colour: they can be round, oval or cylindrical, with smooth or somewhat roughened skin which may be white, yellowish or red.
Yield
In temperate regions yields may be up to about 40 t/ha, but in the tropics yields are much lower (Table 1): although experimentally 40 t/ha has been realised, commercial yields under lowland conditions may be as little as 4-6 t/ha (even in Bolivia, in the Andean highlands, the original home of the potato, yields are only 6-7 t/ha though this is probably associated more with unimproved cultivars than with climate).
Main use
Potatoes are the leading starchy root crop of northern temperate countries and are one of the eight leading staple food crops of the world. They are eaten boiled, roasted, baked or fried and are processed into a very wide range of products, such as canned whole potatoes, frozen french fries or chips, crisps, dehydrated flakes, powder or granules, potato salad, etc.
Subsidiary uses
Stockleed - since the end of the 18th century potatoes, particularly culls, have been used for stockfeed in European countries. The tubers are fed fresh to cattle or sheep or are stored as silage or dried and used in the form of a meal.
Starch - in certain countries, notably the Netherlands, Denmark and the USA, considerable quantities of potatoes are used for the preparation of a large-grained starch, which is used by the food, paper and textile industries, in the manufacture of adhesives, in the preparation of modified starch products such as amylopectin, and for the preparation of glucose and dextrins, etc.
Table 1: Potato - Average yields for selected countries (t/ha)
Flour - potato flour is the oldest commercially-processed potato product and is utilised by the baking industry, particularly in the USA, where it is used in the preparation of certain types of bread, pastries, cakes, biscuits, etc.
Alcohol - potatoes may be pulped and fermented to produce alcohol; potato spirits have been used extensively in some European countries for many years.
Secondary and waste products
Potato pulp - which is obtained as a by-product in the manufacture of starch, can be fed to livestock, either wet or as a dried pulp. The dried pulp has the following average composition: water 12.3 per cent; protein 8.4 per cent; nitrogen-free extract 69.4 per cent; fat 0.4 per cent; fibre 5.3 per cent; ash 4.2 per cent.
For every 10 t of potato starch produced, 54 t of pulp of 96 per cent moisture content are obtained, or 1.9 t of dried pulp of 10 per cent moisture content.
Potato processing water effluent - increasing quantities of potatoes are being processed and the effluent water can be utilised as a source of high grade starch or for the production of butane or acetone.
Protein - potato juices obtained in the production of potato starch are being used in the Netherlands and Japan as a source of high grade protein.
Peels - potato peels are sometimes used for pig feed.
Potato sprouts - are used as a vegetable in certain parts of the world.
Citric acid - has been extracted as a by-product in the manufacture of starch or by the hydrolysis of potato starch and fermentation of the resultant sugars.
Special features
The composition of potatoes varies greatly according to the cultivar, environment, cultural practices, etc, but typical figures for the edible portion are: energy 318 kJ/100 g; water 79.8 per cent; protein 2.1 per cent; fat 0.1 per cent; carbohydrate 17.1 per cent; fibre 0.5 per cent; ash 0.9 per cent; calcium 7 mg/100 g; phosphorus 53 mg/100 mg; iron 0.6 mg/100 g; potassium 407 mg/100 g; thiamine 0.1 mg/100 g; riboflavin 0.04 mg/100 g; niacin 1.5 mg/100 g; ascorbic acid 20 mg/100 g.
In addition to being a valuable source of carbohydrate, potatoes are also an important source of protein, iron, riboflavin and ascorbic acid. The ascorbic acid value is the approximate value at harvest: this falls to about 15, 10 and 6 mg after 3, 6 and 9 months respectively. The principal amino acids present are leucine, glutamic and aspartic acids and serine.
Starch makes up some 65-80 per cent of the dry weight of the tuber, while sugars can vary from trace amounts to as much as 10 per cent. Sugar content is largely dependent upon cultivar, maturity, and the temperature and length of time the tubers have been stored. Potatoes high in sugar taste sweet and have a poor texture when cooked, and in general if the sugar content is above 2 per cent the tubers are unsuitable for processing into food products.
Potato starch is large-grained, contains 25 per cent amylose and 73 per cent amylopectin, and has a high phosphate content. Unless specially treated it has a characteristic odour. Its approximate composition is: water 18-22 per cent; protein 0.08-0.18 per cent; fat 0.11-0.18 per cent; ash 0.2-0.4 per cent. The grains are up to 100 microns in length, with the appearance of oyster shells, but there is considerable variation in the size according to the cultivar and the temperature at which the tubers are stored.
The tubers also contain a number of phenolic compounds, which are partially responsible for certain types of discoloration in raw or processed products. In addition, potatoes contain about 0.01-0.1 per cent on a dry weight basis of a steroid alkaloid, solanine. This is mainly concentrated in the skin, particularly around the eyes, and exposure to light increases the amount present (green areas on the skin are evidence of exposure to light). Solanine is responsible for the outbreaks of potato poisoning which have occurred from time to time and potatoes containing amounts in excess of 0.1 per cent are generally considered unfit for human consumption.
Processing
Canning - the potatoes are washed, peeled with lye, steam or by abrasion, or a combination of these methods, sorted for defects and size trimmed and sliced if necessary. They are then put into cans and boiling water or a 1.5-3 per cent salt solution added; calcium salts may be added to improve the texture (not more than 0.015 per cent of the net weight of the finished product). The cans are heated to above 70°C, closed and heat processed, normally for 20-55 minutes at 114-120°C, depending upon the size of the can used, and cooled immediately to 37°C.
Dehydration - potatoes are dehydrated to give various products, such as dice, flakes or granules. In the preparation of dehydrated diced potatoes the peeled tubers are diced and the enzymes inactivated by blanching in steam or boiling water. The diced potatoes are next sulphited and sometimes treated with calcium chloride to improve their texture, and then dried on trays or conveyor driers. The rate of drying is dependent upon the size of the pieces of potato, but normally the operation takes 6-8 hours for pieces not more than 5 mm thick. Potato flakes and granules are prepared by various techniques in which cooked, mashed potatoes are dried: drum driers of the single drum type are used for flakes; air lift driers, spray driers, fluidised bed, or other suitable driers are used for granules and 'mashed potato powder'. In some processes 'adding back' is practiced, when previously-dried granules are added to the moist mash to bring it to a suitable consistency for drying. In both the preparation of the mash and the drying process, conditions have to be carefully controlled to avoid breakage of the potato cells, as free starch causes unwanted stickiness in the rehydrated product. Small quantities of additives are usually employed in the process, eg sodium sulphite and bisulphite to give 150-200 ppm in the product to retard oxidative changes during processing and non-enzymic browning during storage; antioxidants and emulsifying agents may be added to minimise oxidative changes during storage and to improve the texture of the reconstituted product respectively. Skim milk powder is also a frequent constituent of dehydrated mashed potato products. A moisture content of 5-6 per cent is usually considered satisfactory in the finished material, which, if properly packaged, will retain good quality for a year or more. Transparent packs should, however, be avoided as rancidity develops if the dehydrated product is exposed to light.
Starch - potato starch is prepared either by a batch process or a continuous process, depending upon the scale of operation. The batch process nor mally takes about 3 days and yields on average about 71 per cent of the starch content of the potatoes, while the continuous process takes only a few hours and yields on average approximately 80 per cent. In the batch process the potatoes are first cleaned in running water and then passed through a rasping machine, which reduces them to pulp. The paste is sprayed on vibrating or revolving screens and the starch milk washed through the screen. The milk contains not only starch granules, but other constituents such as fibre, protein, gums, etc and these are removed by repeated washing with water and settling in tanks. When the starch is sufficiently pure it is dried in steam-heated ovens, pulverised and packed for sale. The continuous process is similar to the batch process as far as screening, but then the starch milk passes from the shaker screen to a highspeed centrifugal separator, where the water, protein, etc are removed. The starch milk from the centrifuge is then screened again through a fine-mesh screen and passed on to starch tables for settling. Sometimes this operation is eliminated by the use of slurry separators. Finally the starch is dewatered to about 40 per cent moisture by rotary filters before being dried in hot-air driers.
Chu�o - (or chu�a) is the name given to potato and a number of other Andean root crops which have been preserved in a particular manner. It is of great antiquity and is important in the nutrition of the inhabitants of the Andean highlands. The product has a long storage life - it has been reported that chu�o was found in 1925 in perfect condition in pre-lnca tombs. Chu�o preparation requires the climatic conditions that exist in parts of the Andean altiplano, with very low night temperatures (-10 to - 20°C), high day temperatures (20-25°C) and low RH (30-40 per cent).
Two types of chu�o are made, chu�o blanco (white chu�o) and chu�o negro (black chu�o). The critical features, common to both processes, are the freezing of the tubers to allow release of the cell sap on thawing, and skillful trampling to press out the liquid and, in the case of chu�o blanco, removal of the skins without destroying the integrity of the tubers. Freshly-harvested tubers are spread out and allowed to freeze and thaw alternately for 1-3 days. As much liquid as possible is then squeezed out of the tubers by trampling. When chu�o negro is required the trampled tubers are then dried in the sun for about 2 weeks, yielding a black or dark brown product with a strong flavour. Chu�o blanco is obtained by washing the trampled tubers to remove the loosened skins, then leaving them in a running stream
for 1-3 weeks, and subsequently drying them in the sun: a chalky-white crust forms as they dry. Both types of chu�o are consumed whole, broken into small pieces, or pounded into flour and used in soups and stews.
Many of the Andean subspecies of potato are bitter and virtually uneatable when fresh, and the loss of sap by trampling and washing (chu�o blanco) removes a substantial proportion of the bitter glycoalkaloids, though chu�o negro still requires soaking for 1-2 days before cooking to render it palatable. The loss of cell juices during trampling and subsequent washing substantially reduces the protein, ascorbic acid, thiamine and niacin content of the product. Analyses of raw potato, chu�o blanco and chu�o negro from the same potato stock (on a dry weight basis) have been reported as:
Raw potato: energy 1 525 kJ/100 g; protein 9.5 per cent; carbohydrate 84.1 per cent; calcium 41 mg/100 g; iron 3.6 mg/100 g; phosphorus 227 mg/100 g; thiamine 0.45 mg/100 g; riboflavin 0.18 mg/100 g; niacin 6.82 mg/100 g; ascorbic acid 90.9 mg/100 g; glycoalkaloids 30.4 mg/100 g.
Chu�o blanco: energy 1 651 kJ/100 g; protein 2.3 per cent; carbohydrate 94.8 per cent; calcium 112 mg/100 g; iron 4 mg/100 g; phosphorus 66 mg/100 g; thiamine 0.04 mg/100 g; riboflavin 0.05 mg/100 g; niacin 0.46 mg/100 g; ascorbic acid 1.3 mg/100 g; glycoalkaloids 4.2 mg/100 g.
Chu�o negro: energy 1 626 kJ/100 g; protein 4.7 per cent; carbohydrate 92.4 per cent; calcium 51 mg/100 g; iron I mg/100 g; phosphorus 236 mg/100 g; thiamine 0.15 mg/100 g; riboflavin 0.2 mg/100 g; niacin 3.96 mg/100 g; ascorbic acid 2 mg/100 g; glycoalkaloids 18 mg/100 g.
Production and trade
Production - world output increased by 16 per cent over the period 1961-70, but fluctuated considerably during the period 1967-70, and although production increased there was an overall fall in area harvested during the decade 1974-84 (Table 2). The developed countries continue to produce the majority of the world's potatoes but the proportion contributed by developing countries increased from about 14 per cent in 1969-71 to 30 per cent in 1984.
Trade - a considerable tonnage of potatoes enters international trade both as seed potatoes and as 'ware' potatoes for consumption. For example, northern European countries import early potatoes from Mediterranean countries to partially fill the gap before their own crops mature, and many tropical countries that cannot produce potatoes economically import them from Europe and North America (Table 3).
Table 2: Potato - Area and Production in selected countries
Table 3: Potato - Imports to and exports from selected countries ('000 t)
Table 3: Potato - Imports to and exports from selected countries ('000 t) (cont.)
The six major exporting countries are France, Canada, Italy, the Netherlands, the Federal Republic of Germany and Cyprus. The developed countries in general export rather more potatoes than they import: the developing countries import more than they export. Of the developing countries, however, there are some that export more than they import: notable among these is Egypt. About 25 per cent of the Netherlands exports are as seed potatoes, and also a high proportion of those from the Irish Republic and France.
Major influences
In many industrialised countries increasing quantities of potatoes are processed before being sold to the consumer and the production of tubers which meet the stringent requirements of processors as regards size, composition, cultivar, etc are becoming of considerable importance. The potato is also growing in popularity in many tropical countries, where the demand is often being satisfied by imports from more temperate areas, and there is a need to increase the production of potatoes in the tropics by the development of improved cultivars, better adapted to the environment and more resistant to disease. There is also a need to develop improved storage techniques for the tubers, both for human consumption and for use as seed. Recent research and actual production data give real hope for a rapid increase in potato production in the tropics.
With the exception of the Netherlands, most countries have found it increasingly difficult to produce potato starch at a price competitive with maize starch in recent years, owning to the increasing labour costs involved in the cultivation of potatoes and the greater utilisation of small or misshapen tubers for dehydration.
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