Sweet potato is highly versatile in having a wide cultivation zone, ranging from temperate to humid tropics, and in being used in various food preparations. In Malaysia, being able to adapt (with the required agronomic amendments) to marginal soils, it can compete favourably with other crops for arable land. A range of available processing technologies also provide the means for increasing its popularity as a supplementary staple, so that consumers do not tire of eating sweet potato just boiled, steamed, baked or fried. Thus, there are ample opportunities for investing in sweet potato for down-stream processing into wholesome food products.
Rice is a staple food in Malaysia – as in many other Asian countries. It plays an important role in the human diet as a source of carbohydrate. Currently, Malaysia is about 70% self-sufficient in rice production, having to import the remaining 30%. This has worked very well until recently with the unprecedented (since World War II) worldwide shortage in rice supplies, leading to prices escalating by two to three times.
The Malaysian government is currently taking urgent action to increase local rice production by expanding the cultivated area as well as establishing new granary areas. This, especially the latter which requires infrastructural development, will take time. In the meanwhile, we should explore the possibility of supplementing rice intake by considering other carbohydrate sources such as sweet potato.
For too long root crops have remained underground; it is now time to reveal their qualities. To make more meaningful comparisons, data on the nutrient compositions of sweet potato and cassava – the two leading root crops in Malaysia – have been converted to a dry basis (at 12% moisture content) for their comparison with white rice.
Table 1. Nutrient composition of cassava and sweet potato compared with white rice (100 g dry weight basis).
FROM TABLE 1, IT IS EVIDENT THAT:
• Energy from the root crops is just slightly lower than from rice;
•Protein content of cassava is low, but that in sweet potato is comparable to the content in white rice (unpolished brown rice will have a higher protein content);
• White rice has practically no dietary fibre (the high-fibre bran having been removed during polishing), whereas dietary fibre content in sweet potato is quite high;
• White rice has low contents of Ca, Mg, K and Cu, but has a higher Mn content than the root crops;
• Vitamin content in white rice is also low – especially for vitamins A, C (completely absent) and E, thiamin, riboflavin, niacin and total folate;
• The low protein content of cassava is also reflected in its contents of amino acids. Sweet potato is better but is much lower in certain amino acids (e.g. the essential amino acids ile, leu, met and val) compared with white rice;
• Orange-fleshed sweet potato contains a high level of B-carotene. When ingested, B-carotene is converted to vitamin A in the body.
Thus, roots crops need not be inferior in nutritive value compared to rice, particularly in terms of providing energy, dietary fibre, minerals and vitamins. Dietary fibre has positive effects against diabetes, constipation, and possibly colorectal cancer. Potassium is effective against hypertension, and provides protection against cardio-vascular disease. Calcium builds strong bones, while iron is important for women in their child-bearing years. Vitamins A, C and E are powerful antioxidants which act against defects in the unborn foetus, certain cancers and the ravages of ageing. (Another powerful antioxidant is anthocyanin, the presence of which is manifested as purple in sweet potato). Vitamin E also reduces the risk of cardio-vascular disease and stroke.
The biggest shortcoming of the root crops is their deficiency in certain essential amino acids. However, Malaysians have access to many other protein sources than rice itself... so it is not a big concern.
When comparing the root crops with white rice in terms of their glycaemic indices or GI (Table 2), it may be seen that cassava and sweet potato have low GI (46 and 50, respectively) whereas white rice has a GI of 70. GI measures the rate at which an ingested food is converted to glucose in the blood, with glucose having a GI of 100 and white bread of 96. This reinforces the fact that cassava and sweet potato are more suitable foods for diabetics.
Table 2. Glycaemic indices (GI) for white rice, cassava and sweet potato.
VERSATILITY OF SWEET POTATO
Sweet potato (Ipomoea batatas (L.) Lam.) is a member of Convolvulaceae, or the morning glory family. Species from this family share the common traits of having latex in the plant sap, simple leaves which are arranged alternately around the stem, complete flowers with a superior pistil, five stamens and the characteristic trumpet-shaped corolla, and a fruit which is a capsule (Lawrence 1951). Kangkong (water convolvulus or water spinach) with its scientific name of I. reptans (= I. aquatica), and various ornamental forms of Ipomoea (e.g. I. purpurea) are close relatives of sweet potato, being in the same genus.
The centre of origin of sweet potato is in the Andean mountains of Peru and Colombia (de Candolle 1886). New Guinea is also postulated as a secondary centre of diversity, with evidence showing that the species had reached the highlands 1,200 years ago. More than 5,000 cultivars were discovered in these isolated ecological conditions, growing (uncharacteristically) at altitudes up to 2,800m. The cultivars from Papua New Guinea have been found to be substantially different from germplasm in South America (Zhang et al., undated).
Thus, it is not surprising that sweet potato has a cultivation range from 40°N to 32°S (Kay 1987), and is planted in Japan and Korea down to New Zealand, covering temperate as well as tropical regions, in both the developing and developed countries. The world's largest producer of sweet potato is China with 107,176,100 tonnes per year and accounting for 83% of total production. Uganda (with 2,650,000 tonnes) and Nigeria (2,516,000 tonnes) rank as a poor second and third (FAO 2004). While not strictly a primary staple (except in parts of Polynesia), sweet potato is eaten as a supplementary food.
Sweet potato is prepared in a variety of ways depending on the wide ranging cultures of those who plant it. The simplest methods are by boiling, steaming, baking and frying (with or without batter). In USA, sweet potato features prominently on Thanksgiving Day in the southern states during which sweet potato pie is invariably served. In Japan, where there has been a long tradition of eating sweet potato; it features weekly in most homes as one of the ingredients in tempura. The Japanese love affair with sweet potato has bloomed in many creative ways in using it – in bread, cakes, biscuits, noodles, jam and spreads, ice cream, confectionery, snacks, straight and mixed juices, yoghurt drinks (Yamakawa et al. 1998), to name some.
Although sweet potato shoots and leaves have a much higher protein content (28 – 32% dry matter basis) than the roots (AVRDC 1985), they are only eaten in South-east Asia. Just like kangkong, sweet potato shoots can serve as a green vegetable, and be cooked in a similar fashion, e.g. stir-fried with ground chilli and belacan (shrimp paste) or with fermented beancurd. In Japan and Korea, only the petioles of sweet potato are eaten. Research in Japan has shown high phytochemical activity in sweet potato leaves which can benefit human health (Ishiguro and Yoshimoto 2005, Yoshimoto et al. 2005). In most countries, shoots and leaves are considered as crop residues, and at best are sometimes fed to ruminant livestock (Mai 2005).
WAYS OF CONSUMING SWEET POTATO
From the points of view of its nutritional value and adaptability to planting in Malaysia, sweet potato can take on the role as a supplementary staple so that we do not have to depend entirely on rice. To encourage people to eat sweet potato, there are many ways of preparing it other than just boiling, steaming, baking or frying. To digress a little, most people are unaware that we are also dependent on wheat as a secondary (but less important) staple. This is easily proven: many of us have in our daily diet bread, buns, cakes, biscuits, noodles, roti canai and chappati, all of which are made from wheat flour. Annual imports of wheat amount to around RM 800 million, and this figure has probably gone up with the recent increases of wheat price in the world market (Just like in the case of rice).
The good news is that flour can be made from sweet potato to replace a portion of the wheat flour now used for bakery products. For example, the level of substitution when using sweet potato flour is as follows:
• 100% for cakes
• 60% for muffins
• 50% for cookies or biscuits
Sweet buns can be made directly from sweet potato and can substitute 50% of wheat flour in the original recipe.
Other products which can be made from sweet potato include premix flours for traditional kuih or Malay cakes such as cek mek molek, onde-onde, bingka and keria (sweet potato doughnut). The flour can also be used to make extruded snacks as well as breakfast "cereal" (somewhat like rice krispies) which is served with milk.
Sweet potato can also be made into fries, traditionally prepared from imported Irish potato. Fries are sold in fast food restaurants and as frozen fries in supermarkets for home consumption. They are a favourite among Malaysian youth. It has been estimated that about half the imported Irish potato (valued at RM 60 million per year) are made into fries. Other products made directly from sweet potato are nuggets and breaded sweet potato. Meat or vegetable pies and cheese bakes are yet other ways of replacing Irish potato with sweet potato.
A crucial prerequisite for the birth of such an industry is the availability of a suitable variety, and this has been addressed with the launching of VitAto, by the Minister of Agriculture and Agro-based Industries in June 2007. (VitAto is so named for its intrinsically high Vitamin A content, with "to" referring to "sweet potato"). VitAto is the result of a 10 years breeding programme by MARDI, which involved the testing of selected clones over a number of seasons in various agro ecologies. It was found that VitAto performs well even on marginal soil such as the sandy beach on the East Coast of Peninsular Malaysia.
Malaysia can become self-sufficient in rice if several measures are taken:
1. Current rice production areas are expanded.
2. The per capita consumption of rice is reduced. Several diets (including the Atkin's diet) have shown that reducing daily carbohydrate intake can bring about health benefits.
3. If we replace the rice eaten at lunch or at dinner with sweet potato just once a week, this will cut down on rice demand by 14%, bringing the current self-sufficiency level to 84%. By eating sweet potato instead of rice twice a week, Malaysia will reach 98% self-sufficiency at current rice production levels.
By S.L Tan, Rice & Industrial Crops Research Centre, MARDI (Malaysian Agricultural Research and Development Institute).