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Although a valuable food, the potato has toxic, or potentially toxic, constituents: glycoalkaloids, proteinase inhibitors and lectins. These have been the subject of much research and debate, particularly in recent years. In this chapter, each toxic component group is reviewed, and its structure and probable function within the general physiology of the potato plant described briefly. Emphasis, however, is on the current consensus of opinions regarding nutritional and physiological significance of these components for human beings (for other reviews, see Jadhav & Salunkhe, 1975; Maga, 1980; Morris & Lee, 1984).
Chemical structure and content in the tuber
The Solanaceae family is recognized for the numerous alkaloids found among its various member species. Alkaloids are nitrogen-containing organic compounds occurring in plants, as well as in a small number of animal products (Robinson, 1974). As a result of the diverse pharmacological properties of alkaloids, many plants have long been used as drug sources; some were prescribed for their curative or beneficial effects; many others have become well known for their poisonous, aphrodisiac, narcotic or hallucinogenic attributes.
Two such narcotic and hallucinogenic plants, the mandrake and deadly nightshade, are related to the potato. When first introduced into Europe, potatoes may have been shunned because of their ‘guilty association’ with such notorious relatives (Rhoades, 1982). Under normal conditions of human consumption, the amounts of potato alkaloids ingested are not harmful. Sometimes, however, alkaloid quantities can increase to toxic, and in rare instances fatal, levels. It is necessary, therefore, to understand how such toxic levels arise and what can be done to prevent them.
The potato has spread around the world during the past 400 years and adapted to a wide variety of environments and an equally diverse range of human tastes and preferences. In some tropical developing countries, it is a common vegetable, while elsewhere, consumption ranges from 1 kg to more than 100 kg per person annually. To some, the potato is the ‘bread of life’, while to others it is taboo. This chapter addresses the great diversity in potato consumption patterns. It discusses important issues concerning the potato's role in developing countries and its potential for the future.
There is a growing realization amongst food planners that ‘programmes aimed at increasing the production of food, even if they are successful, must be accompanied by efforts designed to affect the distribution of incomes and patterns of diet’ (Berg, 1981). The potential for increasing consumption of a food item is largely determined by the extent to which its role in the diet can be altered according to changes in supply or cost. Hence, it is essential to consider not only the production, storage and marketing of the potato, but also consumption behaviour.
This chapter was written with the following questions in mind:
How much potato is currently consumed in the tropics and by whom?
How are potatoes consumed and what factors regulate potato preferences and consumption patterns?
What is the potential for greater potato utilization?
It is hoped that the answers suggested below will stimulate other researchers to probe more deeply into the complexities governing potato consumption in developing countries.
The nutritional value of the potato was considered in Chapter 2, where brief mention was made of the changes in nutrient content of raw potato that result from the various fates of the tubers after harvesting. Goddard & Matthews (1979) have stressed the need for data on the nutrient content of food in the form in which it is actually consumed so that planners may correctly assess intake of nutrients in order to provide a balanced diet. The purpose of this chapter, therefore, is to review current literature pertaining to the changes taking place in potato nutrient content as a result of storage, cooking or processing.
Not all of these changes are adverse or even very significant. Nutrient losses, however, do occur to a varying extent depending on the operation involved. It should be remembered, however, that post-harvest handling of some kind is often essential; the potato has to be cooked before consumption, and storage and processing are frequently needed to prevent seasonal gluts and to increase the availability of potatoes to consumers throughout the year. Some nutritional losses are therefore inevitable. The major points and extents of loss are given below as guides for workers in the fields of storage, nutrition, dietetics, catering and processing and to indicate possibilities for prevention or reduction of such losses.
Other vegetables also undergo adverse nutritional changes after harvesting. However, the potato's skin acts as a barrier, preventing or reducing leaching of nutrients into the cooking water. The skin itself is a source of some nutrients and may be consumed. In contrast, many other vegetables lack a protective skin and are subject to leaching losses during cooking.
As established in the previous chapter, potato is not a rich source of energy (approx. 335 kJ (80 kcal)/(100 g), but it supplies high-quality protein. This is of considerable importance in developing countries where energy supplies tend to be more readily available than protein. The nitrogenous constituents of the potato tuber have a high nutritional value compared with many other vegetable crops and there is a wealth of literature devoted to the subject.
Part 1 of this chapter addresses the factors affecting the composition and quality of tuber N and hence its contribution to the diet; Part 2 assesses ways of measuring the nutritional value. The last part discusses the possibilities for reclamation of valuable protein from waste processing. This may be of use to developing countries in planning potato processing operations.
Part 1: Composition of tuber nitrogen
Factors affecting total tuber nitrogen
The average contents of total protein in potato are approximately 2% (FWB) and 10% (DWB). Total protein is Kjeldahl N x 6.25, according to van Gelder (1981), although conversion factors of 5.7 and 7.5 have been suggested (Vigue & Li, 1975; Desborough & Weiser, 1974). Wide ranges of crude protein contents have been reported, e.g. 11.6% to 16.1% (DWB) between different species of Solarium and 9.5% to 14% (DWB) between different varieties of S. tuberosum (Hoff et al., 1978; see also Espinola, 1979; Snyder & Desborough, 1980; International Potato Center, unpublished data). As the potato absorbs little water on boiling or steaming, the total protein content of boiled, unpeeled potato is virtually identical with that of the raw, uncooked tuber.
The potato (Solarium spp.; Figure 1) is grown in 79% of the world's countries (FAO, 1986). It is second only to maize in terms of the number of producer countries and fourth after wheat, maize and rice in global tonnage. Its importance in European countries, the USSR, North America, Australia and the Andean countries of Latin America is well known. Less widely recognized, however, is the rapid growth rate of potato production in developing countries.
FAO statistics show that the percentage increase in potato production from 1961/65 to 1979, for all developing market economy countries, was greater than 99%, while that of cereals and other roots and tubers was, respectively, only 47% and 44% (International Potato Center, 1981). Potatoes are one of the most efficient crops for converting natural resources, labour and capital into a high quality food (Horton, 1981). They can yield more nutritious food material more quickly on less land and in harsher climates than most other major crops; and the edible food material can be harvested after only 60 days.
Current and future roles
Though potatoes occupy a smaller area in most developing countries than do other major food crops, their increasing popularity has caused planners and policy makers to take a closer look at the current and future roles that potatoes may play in national food production systems.
In recent years, the enormous potential for agronomic improvement in food plants through plant breeding has been increasingly recognized. In the case of potatoes, greater attention is being focused on ways to increase production, improve storage methods and facilitate marketing.
The idea for this book arose from the large number of requests to the International Potato Center (Centro Internacional de la Papa) for information on consumption and nutritional aspects of potatoes. There was clearly a need for an up-to-date review, particularly in respect of developing countries. Within its mandate to disseminate information on potatoes, the International Potato Center funded this review of the potato's nutritional value. The work was part of a larger three-year project on potato consumption and utilization in developing countries carried out by Dr Susan Poats.
Over 700 titles concerning various aspects of the potato as a food were collected, and Chapters 1 to 5 survey this literature. Because there are few data available on potato consumption in developing countries, Chapter 6 presents the results of some case studies in the tropics by Dr Poats.
I hope that the book provides useful information for, and stimulus to, the work of all those concerned with the greater exploitation of the potato as a food contributing significantly to the health and well-being of humankind. It may also be of value to the interested casual reader who simply wishes to learn more about the dietary role of potatoes.
Since the latter half of the twentieth century, the rate of increase in potato production in developing countries has outpaced growth rates of most other major food crops. When this book was first published in 1987, the potato was being produced in 132 of the 167 independent countries of the world and more than three billion people, 75 per cent of the world's population at the time, inhabited the 95 developing countries that produced potatoes. As potato cultivation continues to expand at a rapid pace, particularly in the tropics, more attention has been directed towards understanding and improving the nutritional contribution potatoes can make to the human diet. The book reviews the knowledge about the nutritional value of the potato and its role in the nutrition of both children and adults. The synthesis presented in this volume will be of value to students and research workers in nutrition and food science in both developed and developing countries. Dieticians, nutritionists, policy makers and aid personnel involved in agricultural and rural development will also find this book informative and of practical use.
Potatoes are thought to have originated in the Andean highlands of South America (Figure 2.1), and for thousands of years they have been used to maintain and support the growth and health of large numbers of humans. Salaman (1949) asserts that, through discovery of the potato, the ensuing cultivation of frost-hardy types, and the development of methods of preserving tubers, man was able to live at greater altitudes and thus gain mastery of the continent of South America. The dependence of the Irish and Scots on potatoes as their principal source of nourishment and the resulting famine in 1846–47, when the potato crop failed due to blight, are well documented (Woodham-Smith, 1962; Salaman, 1949). Anthropologist Fürer-Haimendorf (1964) has argued that the introduction of the potato into the Sherpa Khumbu region of Nepal stimulated population growth and provided the agricultural surplus necessary for the rise of the elaborate Buddhist civilization in the northern part of the country.
However, although the potato has been shown to be a source of good-quality protein, to have a favourable ratio of protein calories to total calories and to be an important source of vitamins and minerals, its overall value in the diet nowadays is generally greatly underestimated. This chapter demonstrates the value of potato, particularly for consumers in developing countries, where diets are principally made up of basic regional foods. Data are from nutrition studies, interpretations of experimental results and discussion of the nutritional quality of the potato relative to known requirements or recommended levels of nutrient intake, drawn from WHO (1985), FAO/WHO (Passmore et al., 1974) and from the US National Research Council (1980).
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