Technical Introduction to Note #81 Tropical Root Crops What’s Inside: Perspective and Distinctions Potato Cassava Sweet Potatoes Yams Aroids Adapted from a video produced by ECHO in 1993 featuring Dr. Frank Martin (1928 – 2014). Revised by ECHO staff in 2015. Photos (right) by ECHO staff Dr. Martin Price, co-founder of ECHO, first met Dr. Frank Martin when the latter was director of the US Department of Agricul- ture’s Research Station in Mayaguez, Puerto Rico. When Dr. Price began at ECHO, one of the first things he did was buy a plane ticket to visit Dr. Martin in Puerto Rico. Dr. Frank Martin is best known for his research on the sweet potato and other tropical root crops, but his knowledge is much broader than that. He has written on many topics, both for ECHO and for numerous technical refereed journals in the scientific community. Content for this publication is derived primarily from a video by Dr. Frank Martin; however, it is supplemented with information gleaned from a number of other sources. References for these sources are listed in the references section. Citations for each reference include the URL address for those interested in accessing additional information on the Internet. PERSPECTIVE AND DISTINCTIONS Importance of Roots and Tubers Tropical root and tuber crops are consumed as staples in parts of the tropics and should be considered for their potential to produce impressive yields in small spaces. They provide valuable options for producing food under challenging growing condi- tions. Cassava and taro, for instance, are excellent choices for drought-prone or swampy areas, respectively. In this document, tropical root crops are compared both to enable people to recognize and appreciate them as well as to familiarize readers with their strengths and weaknesses in different tropical environments. Though tropical root crops initially seem to be very similar in their uses, they exhibit important differences. To start with, consider the entire group of roots and tubers in relation to other groups of plants. Approximately 30,000 edible plant species have been documented (FAO 1998). Yet 90% of the world’s food supply depends on only a small number of crops— 7 to 130 depending on how the data are analyzed (FAO 1998; Prescott and Prescott 1990). All edible plants can be classified into a few categories based on use. Copyright © ECHO 2014. All rights reserved. This document may be reproduced for training purposes if distributed free of charge or at cost and credit is given to ECHO. For all other uses, contact ECHO for written permission. Page | 2 The first and most important category of edible plants is thecereal grains, including wheat, rice, and corn. These three crops provide over half of the world’s dietary energy (calories). The cereal grains play a very important part in the diet. They contribute a lot of food energy in the form of carbohydrates, and are a good source of the B vitamins (e.g., thiamine, riboflavin, niacin, and folic acid). Grain legumes constitute the next most important group. Grain legumes produce an edible seed, and include beans and their relatives such as soybeans. In these crops the protein content is much higher. The carbohydrate content is also fair, but not as high as in cereal grains. Some grain legumes have an oil content that is another important source of calories. The B vitamin content is very good, and sometimes the vitamin E content is also fair. Grain legumes are body building foods that can be eaten as a substitute for meat when they are part of a balanced diet. Vegetables are the next major category. Edible leaves are an important type of vegetable, and are the most abundant in the tropics. Leaves of at least 4,000 plant species are eaten. Of all the vegetables and fruits, leafy vegetables are probably the best source of vitamins A and C, protein, and minerals. Roots and tubers are also classified as vegetables. Many people despise these crops and view them as “poor man’s crops,” but their value as a food crop extends to all people. Roots and tubers mainly contribute carbohydrates, starches and some sugars to the diet, but they all contain other nutrients. Often these other nutrients make up for their otherwise relatively poor nutritional status. As an energy food, they provide much-needed calories for carrying out farm-related tasks such as planting and weeding; at least 4000 kilocalories are needed for a full day of diligent farm work (EDN 121). Additionally, the leaves of some of these crops, as explained later in this document, are edible and provide protein, minerals and vitamins (Hahn 1984). See FAO 1990 for detailed information on the nutritional quality of tropical root and tuber crops. Botanical Distinctions Roots and tubers look a great deal alike and can be difficult to distinguish. However, from a botanical standpoint, the distinction is simple. Even though most roots and tubers develop below the ground, they differ in terms of where they originate. The root is an enlarged storage organ that comes from root tissue, while the tuber is an enlarged storage organ that comes from stem tissue. In this document, roots and tubers are classified together because of their similar uses, regardless of the botanical distinction. Both roots and tubers are sources of and reservoirs of energy (mostly in the form of starch) for the growth of the plant itself. They are most commonly found in plants that have to go through a long dry season. Out of two or three thousand existing edible roots and tubers, only about 25 species are considered to be of primary importance. Examples include potato, sweet potato, cassava, yams and aroids. The significance of these varies between regions. Cassava, for example, only provides 1.6% of the world’s plant-based calories; however, it accounts for over half of plant-derived calories in Central Africa (FAO 1998). Each of these five aforementioned important root and tuber crops will be discussed below, with information given on life cycles, propagation and storage, leaf usage, and food processing. INDVIDUAL CROPS Potato, Solanum tuberosum It might seem a little out of place to talk about the potato (Fig. 1) among tropical root crops, and yet the potato is a tropical crop. It origi- nated in the cool tropical highlands. It has since been spread to other parts of the world, where it is now even more important than in the tropics. Nonetheless, some major producers of the potato are in the tropics. In the highlands of both Peru and Ecuador, potatoes are a staple food and are produced for the whole country. Life Cycle, Propagation and Storage The potato plant grows very rapidly, and has the shortest growing season of the roots and tubers described in this document. Some potatoes can be produced in only two months. However, potatoes are not easy to grow in the tropics because they have many disease and insect problems; this is especially a problem for a crop which is propagated through its roots. Special seed potatoes can be produced under condi- tions that keep them relatively free from viruses and insect pests, but they are difficult to obtain and grow in the tropics. Where seed potatoes are produced in the tropics, they are grown in the most elevated and cool portions where potatoes can be grown. Most countries in the tropics do not produce their own seed potatoes; instead, they import them from places such as Holland. Potatoes can be stored for four to five Figure 1: Potato tuber. Photo by ECHO staff. months if they are kept in cool, dark, humid and ventilated conditions (Hunt 1985). 17391 Durrance Road, North Fort Myers, Florida 33917, USA | 239.543.3246 | ECHOcommunity.org Page | 3 Leaves The leaves of the potato contain a poison called solanine. They should not be eaten. Processing Starch can be obtained by grinding potatoes, placing that ground material in water, and agitating it. The starch will separate from the other cellular material and settle to the bottom of the container, after which the water (with other materials from the cells) can be poured off. A very clean starch can be obtained by repeating this process several times and allowing the starch to settle out each time. Flour can be made from the entire potato tuber. To make flour, either cook, mash, and dry the material; shred, blanch and then dry; or just grind and dry without any cooking. Potato flour can be made industrially; usually an antioxidant substance is added to the mixture to prevent oxidation and the production of off colors. Cassava, Manihot esculenta Cassava can be grown almost anywhere in the tropics, but is very frost-sensitive. Cassava can survive in dry conditions for a very long period of time and still go on to produce a good crop. It can be an extremely heavy yielder. Life Cycle, Propagation and Storage Cassava is propagated from a piece of the stem (Fig. 2), about a foot long and fairly thick. The cassava stick is either buried in the soil or placed in the soil at an angle, with about a third above the level of the soil. The stem is resilient and keeps well. Even under relatively poor conditions, the stem will eventually produce roots, and some of those roots will produce cassava. Cassava takes a long time to mature. A few varieties can be harvested in eight months, but harvest after a year is not considered excessive. Some varieties are harvested at a full year and a half. Cassava is uniquely flexible because it can remain in the soil in a living condition for a very long period of time and can then still be used as food.