Alves et al. (2011) studied the effect of the type of package and packaging system of the product to preserve carotenoids in bio-fortified sweet potato flour. The flour was packed with and without vacuum in packs with different barriers to oxygen, water vapor and light (PET/Al/LDPE, PETmet/LDPE, LDPE/PA/LDPE and LDPE) and stored at 25 °C/75%RH with and without exposure to light. Losses of 50 % total carotenoids and B-carotene occurred in the flour of the LDPE and PETmet/LDPE without vacuum after 50-day storage and in LDPE/PA/LDPE with vacuum after 90 days with or without exposure to light. Under vacuum and in PETmet/LDPE, contents of carotenoids in flour were slightly degraded and were preserved in PET/Al/LDPE up to 360 days of storage. The major problems faced during storage of sweet potato flour along with their preventive measures are listed in Table 11.1.3.
Table 11.1.3 Problems encountered during storage of sweet potato flour
Problem | Causes |
Problem 1. Brown discoloration
Causes Improper drying due tonon-uniformity in chip sizes and mould growth
Solution
1. Slice the chips into uniform sizes.
2. Use solar dryer with alternative energy source.
Problem 1. Bad or fermented smell
Causes Microbial growth and insufficient drying due to high moisture content in flour
Solution
1. Drying thoroughly until brittle
Problem 1. Weevils and rodents
Causes Storing flour on ground and bushy surrounding
Solution
1. Pack flour in strong plastic bags.
2. Store packed flour in cartoons on racks or raised surface.
3. Ensure storage area is free of pests and rodents.
4. Clear area around store.
11.1.5.2 Utilization
Sweet potato flour can be potentially used for preparation of cakes, cookies, amala, biscuits, bread, sausage, crunchy snacks, etc. It can be used by the baking industry for the production of bread and other bakery products. The use of sweet potato in livestock feed will help to improve livestock nutrition and lead to cheaper meat production. Wheat flour could be substituted with African breadfruit and sweet potato flour by up to the 20 % level in cake-making and 30 % in biscuits with improved nutritional and sensory properties (Eke-Ejiofor, 2013).
11.1.6 Starch
Starches are polymers that naturally occur in a variety of botanical sources such as wheat, potato, pearl millet, corn, sweet potato and many others. They are useful for numerous applications in the food industry and their functional properties depend on the source, but are also affected by other factors such as chemical modifications, system composition, pH and ionic strength of the media (Fama et al., 2005). Starch plays a vital role in developing food products, either as a raw material or as a food additive, such as thickener, stabilizer or texture enhancer (Aina et al, 2012). Starch is a polymeric carbohydrate composed of anhydroglucose units and is not a uniform material and most starches contain two types of glucose polymers, amylose and amylopectin. Amylose is a linear polymer and amylopectin is a highly-branched polymer (Van Soest et al., 1996).
Starch is the major polysaccharide in plants and is in the form of granules that exist naturally within the plant cells. It is semi-crystalline in nature with varying levels of crystallinity. The crystallinity is exclusively associated with the amylopectin component, while the amorphous regions mainly represent amylose (Zobel, 1988a, 1988b). Starch is useful in maintaining the quality of stored food products; it improves moisture retention and consequently controls water mobility in food products.
The root starches have unique functional properties compared to cereal starches. Generally, root and tuber crops are rich sources of starch containing 70–80 % of water, 16–24 % of starch, and less than 4 % of trace quantities of protein and lipids besides other minerals and vitamins (Hoover, 2001). Sweet potatoes are rich in starch (6.9-30.7 % w/b) and starch production is its main industrial utilization (Chen et al., 2004). The starch of sweet potato is composed of a mixture of amylose and amylopectin and is reported to possess an A-type (high swelling) pattern and like those from many other roots and tubers, its starch granules are of medium size with a smooth round oval shape (Moorthy, 2002).
Sweet potato contains 75–80 % amylopectin and 20–25 % amylose and gave 2 peaks when it was thermally treated at temperatures from 0-100 °C, at a rate of 5 °C/min. It was found that the first peak corresponds to the gelatinization of the starches and the second peak could be attributed to the presence of other ingredients and/or disorganization of amylose-lipid complexes (Ahmed and Ramaswamy, 2006). Legumes contain 30–40 % amylose and 60–70 % amylopectin in their starch granules, while most other food starches contain 25–30 % amylose and 70–75 % amylopectin (Hoover and Zhou, 2003). Amylopectin is a much larger molecule than amylose, with average molecular weight at 105 and 106, whereas the average molecular weight for amylose is 104. The comparison of characteristics of sweet potato starch with other starches is presented in Table 11.1.4.
Table 11.1.4 Comparison of characteristics of sweet potato starch with other starches
S. no. | Starch source | Type | Size (pm) | Shape | Reference
1 | Sweet potato | Root | 12–21 | Oval, round and Polygonal | Woolfe (1992)
2 | Cassava | Root | 4-35 | Oval and truncated | Swinkels (1985)
3 | Canna | Root | 5-44 | Oval and polyhedral | Swinkels (1985)
4 | Arrowroot | Root | 9-40 | Oval and truncated | www.fao.org
11.1.7 Basic Steps of Production
11.1.7.1 Selection of Raw Materials
Fresh sweet potato tubers of selected variety are chosen for the extraction of starch. The selection of the variety and harvesting methodology are the important aspects in the selection, because these parameters will directly affect the starch yield.
11.1.7.2 Washing of Raw Materials
The impurities adhering to sweet potato tubers depend largely on weather conditions and soil. The raw tuber is washed with water to remove different impurities together with soil. Sometimes several washing steps are needed to be carried out in order to remove slimy mucilage while extracting sweet potato starch.
11.1.7.3 Extraction
Isolation using Sodium Metabisulfite Starch can be extracted from the sweet potato as described by Vasanthan (2001). Blending of sweet potato with water was done at a ratio of 1: 10 until a smooth slurry was formed. Sodium metabisulfite, 0.01 % (w/v) was added during slurrying. After slurrying, the filtration was carried out with double-layered cheese cloth then filtered through a series of polypropylene screens (250, 175, 125 and/or 75 pm) and centrifuged for 20 min at 5000 × g at 20 °C. Starch settled at the bottom of centrifuge tube was washed with toluene, followed by oven drying at 30–40 °C. The dried starch was ground with a mortar and pestle into a fine powder.