Tuber crops form an important group of staple serving as secondary food for one fifth of world population of tropics and sub-tropics. It is the third important group of food materials after cereals and pulses.
The North Eastern States of India also have a wide variety of tropical roots and tuber crops. The major tuber crops grown in India are cassava, sweet potato, yam and aroid. These crops are the cheap source of dietary energy in the form of carbohydrates and certain amount of protein, fat, fiber, vitamin A and C.
They are staple foods in many parts of the tropics being the major source of their carbohydrate intakes. These carbohydrates are mostly starches found in storage organs, which may be enlarged roots, corns, rhizomes or tubers. Several of these crops have been termed under-exploited and deserving of considerably more research input.
POTATO AND BANANA CHIPS
Potato chips are one of the popular snack foods consumed throughout the country. Chips are mostly prepared by small scale artisans/confectioners, and sold in plastic film or paper packs. Recently, however some of the companies have installed plants having relatively large capacities and have started marketing chips in attractive packs. The fat content generally ranges between 25-40% and moisture content between 2-4%. Potatoes having high total solids, low reducing sugars and low amylase activity are most suitable for chips manufacture.
The presence of hydrocyanic glucosides (HCN) in all plant parts presents some problems in marketing cassava. Selections have been made from both chance seedlings and in breeding programs which are low in HCN. Peeling and boiling in water are common methods of removing a large proportion of the HCN in the roots. Other post-harvest problems with cassava include proper handling and storage of cuttings under frost-free conditions. Roots are usually peeled and boiled or baked. Commercial processing of cassava is limited to packers of frozen, peeled roots, which are marketed in India.
Sweet potato is one of the World’s most important food crops and an important staple food crop in many countries. It is very valuable in the diet of rural poor in the tropics. It is a low-input crop and it is used as a vegetable, a dessert, source of starch and animal feed. In North-Eastern India, sweet potato is eaten as a substitute for yam as a result of lower cost of production. Sweet potato is an important food crops. Sweet potatoes are either baked or boiled. The roots and leaves are sources of carbohydrates, proteins and minerals. They are eaten as a vegetable after boiling baking, or frying and sometimes sliced and sun-dried to produce chips, which are ground into sweet potato flour.
Several types of yams are grown in the tropics and subtropics. Some, which will not be discussed here, are grown only for medicinal purposes. Of the edible species, Dioscoreaalat L, known as the Greater Yam; D rotundata Poir, the White Yam are the most common varieties. The tuber flesh varies from white to yellow and it ranges between 15 to 40% starch. Tubers have a distinct dormancy period, which can be extended with curing and the application of gibberellic acid. This makes yams ideal for long distance shipment and export. Yams are usually baked or boiled and mashed. Unless the production expenses are reduced, little potential exists for commercial processing of yams into items like potato chips or French fries.
Corns which contain 25 to 35% starch are plagued by the presence of an acrid factor, which causes itchiness and considerable inflammation of tissues. Cooking removes most of the acrid factors. Shelf life of harvested corns varies considerably between taro and cocoyam and depends on the care taken during the harvesting and packaging process. Cocoyam has a considerably longer shelf life of several weeks. This can be extended further with curing and refrigerated storage. Corns are usually peeled and boiled. Processing is limited to the production of deep-fried chips.
VALUE ADDED PRODUCTS FROM TUBER CROPS
CASSAVA FLOUR AND STARCH
Separation of the starch granules from the tuber in as pure a form as possible is essential in the manufacture of cassava flour. The granules are locked in cells together with all the other constituents of the protoplasm (proteins, soluble carbohydrates, fats and so on), which can only be removed by a purification process in the watery phase.
Similar to chips in appearance, but generally thicker and longer, they are often 12-15 cm long and can jam the mechanism of handling equipment. They are produced mainly in Africa where local processors prefer to produce longer roots because of the domestic demand mainly for products suitable for human consumption, as cassava is part of the staple diet. Once processed into chips the product becomes inedible, and the producer wants to conserve the local market.
The pellets are obtained from dried and broken roots by grinding and hardening into a cylindrical shape. The cylinders are about 2-3 cm long and about 0.4-0.8 cm in diameter and are uniform in appearance and texture. The production of pelleted chips has recently been increasing as they meet a ready demand on the European markets. They have the following advantages over chips: quality is more uniform; they occupy 25-30 percent less space than chips, thus reducing the cost of transport and storage; handling charges for loading and unloading are also cheaper; they usually reach their destination sound and undamaged, while a great part of a cargo of sliced chips is damaged in long -distance shipment because of sweating and heating. Pellets are produced by feeding dried chips into the pelleting machine, after which they are screened and bagged for export. There is usually about 2-3 percent loss of weight during the process.
This product is the powdered residue of the chips and roots after processing to extract edible starch. It is generally inferior in quality to chips, pellets and broken roots, has a lower starch content and usually contains more sand. The use of cassava meal in the European Economic Community has declined with a shift to the other cassava products during the last few years. However, there will remain some demand for this product, especially by small scale farmers who produce their own feedstuffs. Since it does not require grinding and thus can be readily mixed with other ingredients.
The modification techniques, namely, chemical, physical and enzymatic modification, the chemical method is the most important changed in its physical and /or chemical properties. Modification can be as simple as sterilizing products required for the pharmaceutical industry to highly complex chemical modification to confer properties totally different from the native starch. A simple modification process is represented by washing, air classification, centrifugation and pre-gelatinization. Pre-gelatinization can be done in many forms from boiling in crude pots to drum dryers to modern multi-screw extruders.
Modified starch products are used in the food, paper and textiles industries. Modified starches increase the acceptability and palatability of many processed foods to consumers. Modified starches are also used to reduce costs of established food products. More expensive ingredients such as tomato solids, fruit solids, or cocoa powder can be extended with combinations of modified food starches, flavours, and other inexpensive food substances.
MONOSODIUM GLUTAMATE (MSG)
This product is used extensively in many parts of the continent in powder or crystal form as a flavouring agent in foods such as meats, vegetables, sauces and gravies. Cassava starch and molasses are the major raw materials used in the manufacture of MSG in the Far East and Latin American countries. The starch is usually hydrolyzed into glucose by boiling with hydrochloric or sulfuric acid solutions in closed converters under pressure. The glucose is filtered and converted into glutamic acid by bacterial fermentation. The resulting glutamic acid is refined, filtered and treated with caustic soda to produce monosodium glutamate, which is then centrifuged and dried in drum driers.
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