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Nutritive Value of Potato




  •  Potato is the natures nearest approach after milk as whole food.Except fat, it contains all the essential minerals, amino acids and starch, which is a good source of sugar and rich source of energy. Potato has provided itself as the best alternative food during wars and also peace. It has high production potential per unit area and also time. Next to maize, potato has drawn attention of researchers for increase in productivity and production.

    Though potato is one of the few foods capable of nourishing the entire population of this world, most of the population in our country is unaware of the nutrient value of the potato. Unless the nutritive value of the potato is known, people will not be able to appreciate the value of potatoes. Potato is known to everyone as a supplier of energy but its ability to supply vital nutrients is vastly under estimated.

    Developing countries today produce 37 % of world’s total output of potatoes. This spectacular growth of potato in developing countries, affirms its increasing importance as a source of food for the ever growing populations, rural employment, and income. In order of importance for food production in comparison to 20 other major food crops on fresh weight basis, potato ranks 6th in the developing countries, 4th in developed countries, 4th in the world and 6th in Pakistan.

    Potato is a short duration crop with higher potential yield as compared to any cereal crop. Also it possesses high protein calorie ratio (17 g: 1000 K. cal). Potatoes allow farmer to harvest up to 80 % of dry matter. Potatoes yield sustainability more edible energy, protein and dry matter per unit area and time than many other crops. High yield of potato per unit land area and time is an especially valuable trait in developing areas where the climate permits more than one crop to be grown in the same field each year. The wide flexibility in its planting and harvesting dates makes the crop most suitable for inclusion in intensive cropping systems. Potato is a highly nutritious food. It provides carbohydrates, proteins, minerals, vitamin C, and a number of B-group vitamins and high quality of fibers. The net protein utilization and biological value of potato protein (about 71 % that of whole egg), is better than that of wheat (53 %), maize (54 %), peas (48 %), beans (46 %) and is comparable to cow’s milk (75 %). In Pakistan, potato can be grown through out the year, in one or the other part of the country. Its adaptability is so wide that traditional agricultural seasons do not restrict its cultivation. Potato has high employment generation potential during crop raising post harvest handling and processing.

    Potato is a nourishing and wholesome food. Its low energy density is advantageous when eaten without much added fat. Potato protein is superior to that of cereals and rich in essential amino acids like lysine. To a large portion of our population to whom citrus fruits are out of reach, potato remains a cheap and rich source of vitamin C. There is misconception that potato causes obesity. Infact, being a low energy and low fat food it simply can not cause fattening. Because of its food qualities like easy digestibility and the ability to blend well with almost all foods of plant and animal origin, potato is readily accepted as a major part of a meal, and meal with out potato is considered incomplete. As per estimate, an adult’s total daily energy requirement of 2550 calories could be met by consuming about 3.3 kg of boiled potatoes. Potato has very high biological value next to egg and higher than soybean, maize, wheat flour, peas and beans. Dry matter production in potato is 47.6 kg /ha/day whereas in wheat and rice it is 18.1 and 12.4 kg/ha/day, respectively. Similarly potato produces 3 kg of edible protein per day as compared to 2.5 and 1.0 kg in wheat and rice respectively.

    The water content of potato tuber is about 80 % and the dry matter content is around 20 %. A major portion of the dry matter is starch. Carbohydrates consisting of starch and sugars constitute 16 % on fresh weight basis. Crude protein content is 2% and the fat content is very low (0.1 %). The ash consisting of minerals constitutes 1 %. In addition potato tuber contains fiber, vitamins, and glyco-alakaloids in small quantities.

    Potato is a low energy food. It has been estimated that 100 g of potato dry matter provides about 310 kcal and a boiled potato provides about 69 kcal per 100 g potato consumed. However, when potatoes are consumed in fried form or processed form in which fat has been added, potato becomes rich in energy because the fat adds considerably to the energy value. When compared to other staples, the energy value of potato is the lowest. However, on a dry weight basis, it becomes comparable. The energy value of potato is lesser than sweet potato and other tuber and root crops, although the productivity of potato in terms of dry matter production is higher than that of yam and cassava. The caloric value of potato is many times lower   than that of some food products of animal origin. Potatoes can supplement meat and milk products, improving their taste, lowering the total energy intake and reducing the cost of food.

    The role of carbohydrates in human nutrition is to provide energy. Potato carbohydrate consists of mainly starch. Starch constitutes a major part of tuber dry matter. On an average potato contains about 14 % starch on fresh weight basis. During cooking, if the cells are ruptured, the release of starch from the cells makes the cooked potato sticky.

    Sucrose, fructose and glucose are the major sugars in potato. Reducing sugars and sucrose occur in the tuber in only small amounts. In many vegetables, high sugar contents contribute to the flavor. Vegetables like cabbage, carrots, eggplant, radish and turnips may contain up to 20-30 % soluble sugars on dry weight basis but still taste good. This is not the case with potatoes. A high sugar content gives cooked potatoes a “sickly sweet” flavor and such potatoes are not acceptable as food. Sugars are of considerable importance in processing since they determine the color of the processed products like chips and French fries. Sugar content of potato can be reduced by a process called “reconditioning”, which involves storage of cold stored potatoes for 2 or 3 weeks at moderate temperatures (20 C).

    Potato is a rich source of protein. This is considerable importance for a country like Pakistan, where energy supplies are more readily available than protein supplies. The average protein content in potato is approximately 2 % on a fresh weight basis. The total nitrogen of potato tubers can be divided into soluble protein, insoluble protein and soluble non protein nitrogen which are composed of free amino acids, the amides asparagines and glutamine and small amounts of nitrate nitrogen, nucleic acids and alkaloids. The insoluble protein fraction is present mainly in the peel. Soluble potato protein contains substantial level of the essential amino acids. Most of the non protein nitrogen fraction is in the form of free amino acids and amides. Potato free amino acids are totally available for absorption. Potato produces more edible protein per unit area and time than the other major roots and tubers. It is also comparable with that of the cereals on dry weight basis as well as cooked basis.

    The lysine content in potato is similar to that of a typical animal protein. With its high lysine content, potato can supplement diets which are limiting in lysine. Potato protein in combination with egg protein which is rich in sulphur containing amino acids including methionine is of high biological value. The mixture of these two proteins in the proportion of 65:35 can furnish the daily requirement in an amount which is 30 % lower than that provided by egg alone.

    Potato protein has an adequate ratio of total essential amino acids to total amino acids and a balance among individual essential amino acids concentrations to meet the needs of infants and small children. However, potato protein is reported to have relatively low digestibility when fed to infants. Potato protein is of sufficiently high quality for maintenance purposes in human beings.

    Contrary to the common misconceptions, potatoes contain very little fat. The average fat content of potato is 0.1 % on fresh weight basis. This is too low to have any nutritional significance. However, the little fat present in potato contributes towards potato palatability. It has been found that about 60-80 % of the total fatty acid content in potato is composed of unsaturated fatty acids and linoleic acid is the predominant nutritive value of the fat present in potato. Fat content of the potato is very important in processing. Because of the susceptibility of fat to enzymatic degradation and non enzymatic auto-oxidation, “off flavors” and rancidity problems could arise in dehydrated and other potato products.

    Dietary fiber consists of mainly of the un-assimiable materials like cellulose, hemi cellulose, pectins and lignin. The dietary fiber from potato tubers comes mainly from its cell walls which constitute about 1.2 % of the fresh weight of the tubers. The raw potato dietary fiber content ranges from 1 g to 2 g/ 100 g fresh weight. Unpeeled potatoes contain higher dietary fiber than peeled potatoes, therefore, consumption of whole tuber increases dietary fiber intake. The dietary fiber content of fresh potatoes (2.1 g/ 100 g fresh weight) is comparable to that of sweet potato (2.5 g/ 100 g fresh weight) but lower than that of other roots and tubers. Only on dry weight basis, potatoes and cereals contain similar amounts of dietary fiber. Boiled potato flesh has dietary fiber content similar to that of coked white rice. When calculated on the basis of   100 g carbohydrate consumed, the fiber content of potato is much higher than other crops.

    Processed forms of potato like French fries and chips are a more concentrated source of fiber. It has been found that 100 g of boiled potato can supply the fiber equal to that found in 35 g of white bread and 25 g of potato chips can supply 1.9 times the fiber found in 35 g of bread. It has been suggested that the potato peel, as a by product of the potato processing industry could be incorporated into bread to increase its fiber content. More than half of the dietary fiber is in the form of pectic substances and the potato dietary fiber can thus help in lowering cholesterol levels. Interest in recent years has been raised in dietary fibers because of the suggestions that it gives protection against cardiovascular diseases, colonic cancer and diabetes.

    The ascorbic acid content of potato is lower than that of sweet potato, and cassava and much more than that in yam. The ascorbic acid content declines when potatoes are stored, cooked or processed. Though potato loses some of its ascorbic acid during storage, substantial amounts remain until it sprouts. Ascorbic acid may be oxidized during the preparation of potatoes or cooking. The oxidation occurs at surfaces. However, the tubers are cooked immediately or are kept under water after peeling the oxidation of ascorbic acid is not large. The loss of ascorbic acid during processing may be considerable. But this is not serious problem since processed products of potatoes can be fortified with ascorbic acid.

    Potato contains more thiamin and niacin than other vegetables and roots and tubers it has lower riboflavin content than the other vegetables and roots and tubers. Thiamin, niacin and riboflavin content of boiled potatoes are much higher than other cooked cereals. Like ascorbic acid, the vitamins of B group are water soluble; therefore, these can easily be leached out and destroyed in cooking process. The B vitamins are essential for general health and growth. They are also needed for carbohydrate metabolism; smooth functioning of the nervous system, normal digestion and health skin. Potatoes have a low fat content therefore; the fat soluble vitamins are either absent or are present in only traces. It is, therefore,

    recommended that mixed diet containing potato should include a source of vitamin A.

    Potato is a good source of phosphorous and contains about 40 mg/ 100 g. of fresh potato tuber. The phosphorous content is more assimiable than the phosphorous present in other food crops because of the relatively small percentage of phytic acid phosphorus in potatoes. The lower phytic acid content of potatoes makes a large part of the potato phosphorous available to human body and is also helpful in allowing greater availability of calcium, iron, and zinc. This property of potato phosphorous is particularly useful in the case of calcium, because though potato is a poor source of calcium, it increases the availability calcium from other foods that are rich in calcium.

    The potassium content of potato is relatively high. Because of the high potassium content of 247 mg/ 100 g. fresh weights on average of potato, it is not included in the diet of patients with renal failure. On the other hand, sodium content of the potato is very low with an average value of about 11 mg/ 100 g. fresh weight of potato. When cooked unpeeled, potatoes don’t absorb salt; therefore, potatoes can be used in the diets given to patients of high blood pressure.

    A quantity of 100 g. of cooked potatoes can supply between 6 % and 12 % of daily iron requirements for children or adult men. The non-haem form of iron present in potatoes and other vegetables is more readily available for absorption by the intestines in the presence of ascorbic acid consumed along with the iron source. Potatoes which are rich in ascorbic acid thus increase the availability iron from the other food source as well. A higher proportion of the iron from potato is soluble than that from other foods such as kidney beans, wheat flour, and bread. For this reason, iron availability from potato is superior to that of other foods.

    Magnesium is another important dietary mineral. It ranges in potato as 21 mg/ 100 g fresh weight. Potato can be used as a good supplement with other foods low in magnesium content like milk. It also contains sufficient quantity of zinc, copper, manganese, molybdenum and chromium. Trace elements like bromine, iodine, cobalt, selenium and aluminium are also found in potato.

     

    Key Reference:  Muhammad Ahmad and Dr. Ahmad Saleem Bhatti

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