References:
Plants and Society: Pages 223-233.Learning Objectives:
On Food and Cooking; Pages 191-194
TubersMany food products are underground plant storage organs. Many plants live in environments that are variable, such as environments with dry seasons, cold season or regular fires. One adaptation to such types of environment is large underground storage reserves that won't dry out in dry seasons, won't freeze in cold season and/or won't be damaged in fires. These storage organs provide energy and nutrients that allow plants to regenerate following the adverse environmental conditions. They also provide humans a means to cultivate a crop that is not so susceptible to adverse environments. Almost all human cultures have used such plants as a food source.What are tubers?PotatoesTuberous rootsOrigin of potatoesYams
Types of potatoes
History of potatoesOrigin of yams
SaponinsSweet potatoesTaroOrigin of sweet potatoesCassavaOrigin of cassavaOrigin of Taro
Tubers
White Potatoes
The white potato, Solanum tuberosum, is in the nightshade family, which includes plants like green peppers, tobacco and tomatoes. Potato plants store starch and other nutrients in underground stems. The parts of underground stems that thicken as they accumulate the energy reserve, they form TUBERS at the end of the underground stems. Solanum tuberosum is native to South America, where it was a staple crop for civilizations, such as the Incas, in that region. Of the 2,000 species in the Solanaceae (nightshade) family, only 160 have tubers, and only 8 of these have ever been cultivated. There are 6,000 cultivars of Solanum tuberosom, but most cultivars are not grown commercially. In the US, only 50 varieties are grown commercially, with 12 of these accounting for 85% of the total crop. Most of the cultivars grown fall into 4 types: round white, russett, round red and long white. The russet is a pithy type of potato that is very good for baking. Round whites are good for general purpose uses, boiling, baking, and processing into chips, flakes and fries. Round red and long white potatoes are generally called new potatoes, and are the least pithy of potatoes, and are good for boiling, steaming and roasting.
Potatoes were brought to Europe by the Spanish following their conquests in the new world. As they were in the nightshade family, various myths regarding their effects limited their success, such as they may be poisonous, hallucinogenic, caused disease and stimulated lust. It turned out, however, that potatoes were an excellent food source for the poor, and since the vast majority of Europeans were poor, was greatly useful, as 12 tons of potatoes could be produced from 1 acre of land; enough to feed over 10 people for a year. Potatoes are propagated from seed potatoes, small potatoes that are removed from the ends of underground stems (rhizomes). As such, potatoes are propagated asexually; they are all one clone (genetically identical). Being genetically identical, any pathogen to which one potato is susceptible, all are susceptible. Potatoes were used to feed the general population in Ireland, almost all of whom were driven to poverty by British colonial policy. When the potatoes were invaded by a serious pathogen, the late potato blight, the fungus Phytophthora infestans, which attacks the leaves, destroying them, and then attacking and destroying the rest of the plant, a million people in Ireland starved to death, not so much because of the blight, as there was a lot of food available, but because British merchants insisted on selling the food available, and most people had no money.
Potatoes have become so well established as a food crop in Europe that today Europe and republics of the former Soviet Union produce over 70% of the world's potato crop. The potato is the world's fifth leading crop.
Potatoes are about 25% starch and 2.5% protein, with the amino acid composition of proteins well balanced. Potatoes (without sour cream, butter and unfried) contain almost no fat, no cholesterol and are good sources of potassium, B vitamins and vitamin C. Most of the vitamins are in the outer part, so potatoes are more nutritious if eaten with the skins.
Yams
Yams (true yams, not the sweet potatoes we call yams) are in the plant genus Dioscorea in the plant family Dioscoreaceae (the yam family). Yams have been cultivated in Africa for at least 5,000 years, and are staple crops in many parts of Africa, Asia and the Caribbean. Yams are tubers produced by vines, and the tubers can get as large as 88 pounds.
Yams contain medically important compounds called SAPONINS, a type of steroid. These steroids have been used to make cortisol and human sex hormones. One of these compounds, DIOSGENIN, is known to inhibit ovulation, and was important in the development of the birth control pill.
Yams grow in humid tropics, where plants like potatoes don't grow well. The tubers are usually deeply buried, and thus difficult to harvest. As a result, yam cultivation has been replaced with cassava cultivation in many tropical areas.
Tuberous Roots
Many plants produce roots that develop storage organs that look like tubers. True tubers develop from rhizomes (underground stems). The similar structures that develop from roots are thus called tuberous roots.
Sweet Potatoes
The sweet potato, Ipomoea batatas, is in the morning glory family (Convolvulaceae). Sweet potatoes are tuberous roots. The plant itself is a vine, like the morning glory. Sweet potatoes are propagated asexually from SLIPS that grow up from the roots. Sweet potatoes require a long growing season, and unlike white potatoes, are not tolerant of cold. Columbus brought sweet potatoes back to Europe upon his return.
The sweet potato is thought to be native to tropical SOuth America. Evidence of a very early introduction to Polynesia is evidence that people from Polynesia and South America had some contact prior to the arrival of western Europeans in South America. Today the sweet potato is cultivated throughout the tropics. China is the world leader in sweet potato production, with the sweet potato cultivated throughout east Asia.
The sweet potato can be sliced, dried and ground into a meal that can be used for cooking. The sweet potato is an important part of the diet in Africa. In the US, 2 different cultivars are grown, the orange cultivar is grown on the south, and a yellow cultivar, often called a yam even though it is not a yam, is grown further north. The orange sweet potato is moister and sweeter than the drier, starchier fake yam grown in the northern states. Sweet potatoes are a good source of carbohydrates, having 50% more calories than white potatoes and more carbohydrate present as sugar. Sweet potatoes have less protein than white potatoes. Sweet potatoes are a good source of vitamins A and C.
Cassava
Cassava, Manihot esculenta, is a member of the spurge family (Euphorbiaceae). Cassava is an important staple in tropical countries.. Cassava originated in South America, and Brazil is a leading cassava producer. The Portuguese, who ruled Brazil during the western European colonial era, exported cassava to Africa, where it has become an important crop. Cassava has also been introduced into tropical east Asia, where it is also an important crop. Cassava plants are propagated from stem cuttings ands by seed. Cassava can tolerate dry periods, but must be grown on well drained soils to prevent root rot. Roots can be left in the ground for long periods, but once harvested, must be processed within a day or so.
Cassava grows in nutrient poor soil, and is tolerant of acidic soils and resistant to a lot of pest and pathogens. For these reasons, cassava is promoted as a crop that people in developing countries should grow. The problem with cassava is that it contains high hydrocyanic acid (HCH) levels. HCN is highly toxic, explaining the resistance to pests and pathogens. HCN is very bitter to the taste, and varieties of cassava with lower levels of HCN are considered "sweet". HCN levels vary with environmental conditions, however, and given the right environment, even "sweet" varieties can produce excess amounts of HCN. HCN is generally removed by grating the cassava and boiling it, dumping the water and drying the remaining cassava and making that into some sort of meal. The meal is called farinha in South America, Gari in Africa and made into a bread called casabe in the Caribbean (from which we get the word cassava. We eat cassava meal as tapioca. Another method of removing HCN from cassava is to squeeze the juices from the plant, and then use the pulp for some sort of meal, or even ferment it into an alcoholic beverage.
Cassava is 30% starch, and has moderate amounts of calcium, B vitamins and vitamin C.
Taro
Taro is a CORM, an upright underground stem where the stem is highly thickened into a storage organ. Taro, Colocasia esculenta, is in the arum family (Araceae), which also contains a local plant called Jack in the Pulpit. Taro is a major food source in Hawaii. Taro is prepared for eating by grinding or crushing the corm into a paste called poi. The leaves of taro are called luau, and the poi is traditionally wrapped in luau and cooked. The corm may also be roasted or boiled like potatoes, ground to a flour, chipped, and made into a breakfast food. Taro originated in southeast Asia, and spread throughout the Asia/Polynesia region, including Hawaii. Taro is about 25% carbohydrate, 2% protein and a good source of calcium. Like other plants in this family, taro produces calcium oxalate crystals, that cause a burning and stinging in the throat. However cooking breaks down the calcium oxalate, releasing the calcium. Taro has been introduced throughout the world, being brought from Africa to the new world by slaves. Taro is cultivated throughout the tropics as it grows well in saturated soils. Taro is usually propagated by culturing the tops of the corms (cormels).