1.9.2 Sweet Potato
The dietary fiber content, particularly hemicelluloses, in sweet potato (variety Tinipa) has been reported to be 4.5 % of the total carbohydrate, which is twice the amount of free sugars (2.41 %) (Roxas et al., 1985). In-vitro degradation of hemicellulose by intestinal bacteria may result in increased breath production of hydrogen, one of the gases produced during flatus production. Thus, a high level of food fiber has a great potential for inducing flatulence (Salyers et al., 1978). On the other hand, raffinose in sweet potato is also considered one of the sugars, responsible for flatulence (FAO, 1990). However, further research is required to verify the role of crude fiber/raffinose in foods including sweet potato in producing flatulence. The sugars which occur in plant tissues, stachyose, raffinose and verbascose, are not digested in the upper digestive intestinal tract, and therefore are fermented by colon bacteria to yield the flatus gases, hydrogen and carbon dioxide (FAO, 1990). The level of sugars present depends upon the cultivar. Lin and Chen (1985) established that sweet potato shows trypsin inhibitor activity (TIA) ranging from 20–90 % in different varieties.
A major anti-nutrient of sweet potato is the presence of trypsin and proteinase inhibitors. Inactivation of trypsin inhibitors by heat treatment improves the protein quality and thereby increases the nutritive quality of the sweet potato (Senanayake et al., 2013). Roasting greatly lowers the level of trypsin inhibitor activity compared to boiling. The highest level of trypsin inhibitor activity is recorded in the raw tubers, and the reduction is observed upon processing (Omoruyi et al, 2007). The trypsin inhibitor content of sweet potato can be correlated with the protein content. Heating to 90 °C for several minutes completely removes trypsin inhibitors. TIA in sweet potato may be a contributory factor in the disease enteritis necroticans (Lawrence and Walker, 1976). However, this appears doubtful because sweet potatoes contain anti-nutrients, but these occur at very low levels, and most of the time our bodies are perfectly able to process them.
In response to injury, or exposure to infectious agents, sweet potato produces certain metabolites. Fungal contamination of these tubers by Ceratocystis fimbriata and several Fusarium species leads to the production of ipomeamarone, a hepatoxin and other metabolites like 4-ipomeanol, pulmonary toxins (FAO, 1987). Baking destroys only 40 % of these toxins. The peeling of blemished or diseased sweet potatoes from 3-10 mm beyond the infested area is sufficient to remove most of the toxin (Catalano et al., 1977). Various methods of processing such as soaking and cooking have an effective result in reducing the anti-nutrients of foods. Hydrocyanic glycoside, a toxic compound in sweet potato, can be easily destroyed by cooking (Ojo and Akande, 2013).
1.9.3 Taro
Taro is inedible when raw and considered toxic due to the presence of calcium oxalate crystals, typically as raphides. Foods produced from taro suffer from the presence of acrid factors, which may cause itchiness and considerable inflammation of tissues to consumers. Even raw leaves and petioles can cause acridity. The intensity of the acridity varies considerably among taro cultivars. Also for the same cultivar, environmental stress (such as drought or nutrient stress) during the growing season may result in higher levels of acridity.
Presumably, itchiness arises when the calcium oxalate crystals are released and inflict minute punctures to the skin when in contact with it. Bradbury and Holloway (1988) suggested that the crystals have to interact with a certain chemical on the raphide surface before acridity is experienced. The acridity factor can be reduced by different unit operations such as peeling, grating, soaking and fermentation (Pena et al., 1984). Removal of the thick layer of skin may help to remove acridity. Acridity in taro root can be minimized by cooking, especially with a pinch of baking soda and by steeping taro roots in cold water overnight. Kaushal et al. (2012) compared the anti-nutrients in taro, rice and pigeon pea flours. Phytic acid and total polyphenol content for taro flour was 107.3 mg/100 g and 577.21 mg/100 g, respectively. The total polyphenol content in the noodles prepared from 100 % taro flour was observed to be 577.21 mg/100 g (Kaushal and Sharma, 2014).
1.9.4 Yam
The edible matured yam generally does not contain any toxic principles (Coursey, 1983). Wild forms of D. dumetorum contain bitter principles, and hence are referred as bitter yam. The bitter principle is the alkaloid dihydrodioscorine, while that of the Malayan species, D. hispida, is dioscorine (Palaniswami and Peter, 2008). There are water-soluble alkaloids which, on ingestion, produce severe and distressing symptoms. The contents of the anti-nutrients (cyanide, oxalic acid, tannin, sapogenin and alkaloid of species) in wild yam are well below the FAO/WHO safety limits (Sahore et al., 2006).
The bitter principles of D. bulbifera (called the aerial or potato yam) include a 3-furanoside norditerpene called diosbulbin (FAO, 1990). Such substances are toxic and the extract finds its application in immobilizing fish to facilitate capture. The toxicity of the extract may be due to saponins. The detoxification methods for bitter cultivars may involve water extraction, fermentation and roasting of the grated tuber. Boiling possesses both a positive and negative effect on water yam. A cooking time of between 30 and 60 min at 100 °C is recommended for D. alata (Ezeocha and Ojimelukwe, 2012). The anti-nutritional factors of yams decrease greatly during boiling rather then during than baking (Kouassi et al., 2010).
1.9.5 Elephant Foot Yam
The edible, mature, cultivated elephant foot yam does not contain any toxic principles (www.wikipedia.com). Calcium oxalate is present as a fine crystal resulting in itching of fingers and pricking sensation of tongue and throat. However, calcium oxalate is easily broken down thoroughly either by cooking or by complete drying. Under either of these conditions, it is safe to eat. It can also be consumed after it is washed well and boiled in tamarind water or butter milk.
1.10 Applications of Tropical Roots and Tubers
The various applications of tropical roots and tubers include the following:
1.10.1 Animal Feed
Nearly half of the sweet potatoes produced in Asia are used for animal feed. The vines have a lower carbohydrate content but higher fiber and protein and their principle nutritive value is a source of vitamins and protein. The sweet potato vines can serve as a nutritive and palatable feed for cattle. The unmarketable and poorly developed tubers can also be utilized in animal feed. Cassava chips are utilized as cattle feed and poultry feed. In the animal feed industry, cassava is one of the most abundantly used food ingredients in place of cereal grain. In some parts of the world, sweet potato and cassava tubers, taro corms and petioles are chopped, boiled and fed to pigs. However, sweet potato vines and cassava leaves are also used for feeding cattle and pigs. Taro peels and wastes are also fed to domestic livestock in various countries.