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11. Sweet Potato: Technological Interventions
11.1 Sweet Potato Flour and Starch
Maninder Kaur1 and Kawaljit Singh Sandhu2
1 Department of Food Science and Technology, Guru Nanak Dev University, Amritsar, India
2 Department of Food Science and Technology, Chaudhary Devi Lal University, Haryana, India
11.1.1 Introduction
Sweet potato (Ipomoea batatas L.) is a dicotyledonous plant that belongs to the family Convolvuceae. It is believed to have its center of origin in tropical America (Salawu and Mukhtar, 2008). However, sweet potato is now cultivated wherever there is sufficient water to support its growth. China was the leading producer of sweet potatoes, with an annual production of 79090068 tons. The Asian countries contribute more than 80 % of total world production of sweet potato. In China, Vietnam, Indonesia, Thailand and India, sweet potatoes are important food crops grown throughout those countries (Tan et al., 2009). According to FAO (2013), the annual production of sweet potatoes world-wide was estimated at approximately 110746162 tons.
It is grown in more than 100 countries in tropical, sub-tropical and temperate climates and is the major staple food in Africa, Asia, the Caribbean and South America, where it is an important source of carbohydrates, vitamins A and C, fiber, iron, potassium and protein (Woolfe, 1992). It is a perennial crop (Shekhar et al., 2015) and classified into soft-fleshed, firm-fleshed and white-fleshed (Farley and Drost, 2010). The crop has limited production costs and does well even under marginal conditions (poor soils with limited water supplies). The composition pattern of sweet potato is presented in Table 11.1.1.
Table 11.1.1 Composition of sweet potato
- | Sweet potato tuber | Sweet potato peels | Sweet potato leaves
Dry matter | 28.72 | ― | 12.45
Crude Protein (%) | 5.24 | 4.64 | 24.65
True Protein (%) | 3.81 | ― | 22.58
Ether Extract (%) | 0.46 | ― | 3.58
Crude Fiber (%) | 0.41 | 3.80 | 9.1
Nitrogen free Extracts (%) 91.49 | 74.70 | 51.2
Reference | Oyenuga (1968) | Faramarziet al. (2012) | Oyenuga (1968)
Being the fifth-most important food crop in developing countries, sweet potato is rich in dietary fiber, minerals, vitamins and antioxidants such as phenolic acids and P-carotene (Ishida et al., 2000). Phenolic acids, primarily chlorogenic (a family of esters formed from certain cinnamic acids and quinic acid), isochlorogenic and cinammic, and the anthocyanins, cyanidin and peonidin aglycones, are present in sweet potato (Oki et al., 2002). Hou et al. (2001) reported that the 33 kDa trypsin inhibitor, one of the major sweet potato root storage proteins, had scavenging activity against the 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical and the hydroxyl radical.
Philpott et al. (2003) reported that hydroxycinnamic acid was the major antioxidant component of sweet potatoes. Sweet potato phenolics were found to inhibit the growth of human colon, leukemia and stomach cancer cells (Kurata et al, 2007) and to ameliorate diabetes in humans (Ludvik et al., 2008). Color and variety can influence levels and profiles of phenolics as well as of anthocyanins (Steed and Truong, 2008) and carotenoids (Van den Berg et al., 2000). Purple-fleshed sweet potatoes have an intense purple color due to the accumulation of anthocyanins (Terahara et al., 2004).
11.1.2 Sweet Potato Flour
Currently, sweet potato is being utilized in various forms in different parts of the world. These uses can be adapted to boost production and consumption of the crop. Sweet potato roots are bulky and perishable, therefore the roots can be dried and ground to produce flour. Sweet potato, either fresh, grated, cooked or mashed, or made into flour, could replace the more expensive wheat flour in making buns, chapattis, mandazis, etc. (Hagenimana et al., 1998). The flour is used as a dough conditioner for bread, biscuit and cake processing (it may substitute for up to 20 % of wheat flour), as well as in gluten-free pancake preparations (Shih et al., 2006). Sweet potato flour can add natural sweetness, color and flavor to processed food products. It can also serve as a source of energy, nutrients and minerals and contributes to the daily nutrient needs for P-carotene, thiamine, iron, vitamin C and protein. Sweet potato flour provides 14–28 % of the dietary reference intake (DRI) for magnesium and 20–39 % for potassium (Van Hal, 2000). When sweet potato flour in whole or in part replaces wheat flour to form bread, additives are selected to improve bread volume, texture, flavor, shelf life and overall quality.
11.1.3 Basic Steps in Production of Sweet Potato Flour
Sweet potato flour processing methods involve several unit operations including selection of raw material, cleaning and trimming, washing and brushing, slicing, drying, milling, packaging and storage. The specific combinations of these unit operations can give different products to consumers. The general steps of preparing sweet potato flour (Figure 11.1.1) are as follows:
The quality factors like size of sweet potato tubers, conformity to the variety, and tolerance limits for under-sized and over-sized sweet potato tubers, can mainly be taken into consideration during the selection of raw materials. In the processing of flour using sweet potato, experiments have shown that there is no need of peeling since the root’s skin is extremely thin and has a very minor influence on the color and final composition of the product.
Figure 11.1.1 Schematic representation of sweet potato flour production.
The washing and brushing steps are the most critical in the production of sweet potato flour. It has been noted that the skin has a protective effect on water diffusion from the roots towards the surface, and consequently slices having a portion of the skin take a long time to dry and their structure becomes harder and not easily breakable. Sweet potato roots should be submerged in water so that impurities which adhere to the skin can be freed. The second washing is for cleaning and brushing concomitantly with removal of possible damaged portions of the roots escaped from the previous step. The third washing is for fine cleaning with very clean water. Pre-drying of clean and washed roots is desirable to reduce the moisture at the surface of the roots and improve the cleanliness of the end product. The sweet potato slicing separates roots into small physical sizes and increases their drying surface. The slicing of sweet potato roots sometimes may become a tedious exercise, but it has been observed that the slicing or chipping gives a sweet potato flour of high-quality color. The sweet potato grating induces a lot of enzymatic browning reactions and the fresh grated product is quickly subjected to an undesirable spontaneous fermentation. Sweet potato balanced chemical composition makes fresh sweet potato a suitable medium for the growth of a wide range of microorganisms and other agents involved in the deterioration of the fresh roots.