Many different techniques exist to carry out the drying of food products. The approximate sun-drying time of sweet potato chips is from 2–4 days and the residual humidity should be between 10 and 12 %. For grinding, any hammer mill can be used. Over-drying of slices produces a lot of dust and loss of material during the milling. It is recommended to grind sweet potato dried chips immediately after drying to avoid any risk of rehydration. After grinding, packaging and sealing is done immediately to avoid rehydration and insect infestation. Materials with little permeability to water vapor, such as cellophane, polyethylene or polypropylene should be used. The decision on packaging materials is based on transportation requirements and storage time. As soon as the product is placed in its package it should be sealed immediately, removing as much air as possible from inside the package. This is to avoid direct exposition of the product to the surrounding air and to minimize any insect attack. The packed products should be stored in a fresh, dry and preferably dark place until it is shipped to the consumer.
11.1.4 Methods for Production of Sweet Potato Flour
The sweet potato flour can be produced by washing, peeling, slicing and air drying (60 °C/6 h), followed by grinding and sieving (70 mesh) (Srivastava et al, 2012). The drying of sweet potato flour at high temperatures affects the color. Sweet potato flour has been prepared by different researchers. The chronological progression for the production of sweet potato flour is given in Table 11.1.2.
Table 11.1.2 Chronological progression for the production of sweet potato flour
S. No. | Salient features | Reference
1 | Selection of sweet potato tubers, cleaning, sorting, peeling, washing, blanching (hot water at 98 °C/3 min), slicing (2–4 mm thick), oven drying (70 °C/8h), milling, sieving (300 microns) | Adeyemiand Ogazi (1985)
2 | Selection of sweet potato tubers, washing, peeling, slicing (3 cm thick), conditioning (in water 50 °C/3 h), steeping (72 h, ambient temperature), decanting, washing, sun drying (45 °C), milling (hammer mill), screening (0.5–1 mm) | Ukpabiet al. (2008)
3 | Selection of sweet potato roots, peeling, slicing (2 mm thick), soaking (0.2 % metabisulphite/5 min), tray drying (40 °C/24 h), grinding (hammer mill) and sieving (300 micron) | Mais (2008)
4 | Selection of sweet potatoes, peeling, cutting, immersion (1 % NaCl solution and then immersed in solution containing KMS, 1 % and citric acid, 0.5 % for 30 min), tray drying (55 °C until 7–8% moisture content), storage | Singh et al. (2008)
5 | Selection of sweet potato tuber, cleaning, peeling,shredding (4 × 4 mm cross-section), drying (60 °C/ 5 h), grinding and milling (pin mill, 100–150 pm) | Reungmaneepaitoon (2009)
6 | Selection of sweet potato roots, washing, trimming, peeling, cutting, immersion (in solution containing potassium meta-bisulphite, 0.5 %/15 min), drying (tunnel dehydrator at 60 °C till 10 % moisture content), milling and sieving (80 mesh) | Saeed et al. (2012)
7 | Selection of sweet potato tuber, peeling, washing, slicing (5 mm thick), blanching (95 °C/10 min, cabinet drying (60 °C/8 h), milling (hammer mill), screening (0.8 mm), sweet potato flour, packaging (low density polyethylene bag) and storage | Olapade and Ogunade (2014)
8 | Selection of sweet potato tubers, sorting, washing, peeling, slicing (1–3 mm thick), immersion (in a 0.2 N potassium meta-bisulphite solution for 10 min), draining, blanching (in hot water at 80 °C/5 min), draining, cabinet drying (60 °C/24 h), grinding (hammer mill), sieving (0.3 mm thick) and packaging (in cellophane bag) | Adeyeye and Akingbal (2014)
11.1.5 Properties of Sweet Potato Flour
The processing can affect physico-chemical and functional properties of sweet potato flour. Jangchud et al. (2003) studied and compared the effects of blanching on physico-chemical properties of flours and starches prepared from two varieties of sweet potatoes (Mun-Kai and Negro). The pasting temperature and peak viscosity of starches, respectively, were 74 and 80 °C and 381 and 433 RVU. The pasting temperature (74.0-94.8 °C) of flours was greater than that of starch, depending on the variety and blanching process. However, the peak viscosity (103–120 RVU) of flours was lower than that of the corresponding starches. Partial gelatinization of starch granules was observed as a result of a 1 min blanching. Compositions of starch and flour was found to affect swelling power and solubility.
The rheological properties of the hydrated spray dried sweet potato powders, when compared with sweet potato puree, reveal that spray drying significantly reduced the β-carotene and ascorbic acid contents and all-trans form of beta-carotene was transformed into cis-isomers at the same solid concentration. The viscosity of the reconstituted solutions was much lower than that of the puree at the same solid concentration. Rheologically, the reconstituted sweet potato slurries behaved similarly to pre-gelatinized starch solutions (Grabowski et al., 2008).
Blending sweet potato flour with wheat flour up to the 20 % level produced samples which can be used for production of bakery goods with improved functional properties and reduced retrogradation, staling rate and production time (Adeleke and Odedeji, 2010). Decrease in peak viscosity and an increase in cold paste viscosity was observed following heat-moisture treatment, as compared to native sweet potato (Putri et al., 2014). The changes observed between yellow sweet potato and purple sweet potato indicated that different starches may respond differently to physical modification.
The effects of addition of orange sweet potato flour to tapioca starch indicated that fried extruded fish crackers with 30 % fish, 14 % orange sweet potato flour and 56 % tapioca starch had a high crispness score and were accepted by the trained panellists (Noorakmar et al., 2012). Singh et al. (2008) prepared cookies by utilizing sweet potato flour and wheat flour. Sweet potato flour lowered the spread factor from 6.666-6.150, while increasing its level from 0-100 %. Sensory evaluation revealed that increasing levels of sweet potato flour (60 %) lowered the overall acceptability, because of the distinct flavor developed during baking. Substitution of maize flour with sweet potato flour to the level of 40 % produced acceptable cookies (Adeyeye et al, 2014). The protein content reduced from 6.8–4.4 %, moisture from 5.3–5.0 %, crude fiber from 3.4–2.5 % and fat from 9.8–8.5 % of the composite flours and the cookies
11.1.5.1 Storage
The important parameters like moisture, temperature and hygiene need to be controlled during storage. The substantial losses can be observed in sweet potato flour during storage. The losses may be due to various factors like insects, pests, nematodes, rodents, etc. The water activity of the flour can give a prolonged shelf life. It is therefore very important to control moisture content of the foods during processing and storage. The storage of sweet potato flour in plastic bags covered by aluminium foil was able to preserve the content of total carotenoids over a period of 2 weeks. After this time, the carotenoids degraded rapidly. It was concluded that although sweet potato was protected from light and restriction of gas exchange, the oxidation of carotenoids was rapid, suggesting a need for more studies on other types of packaging and modified atmospheric conditions (Rodriguez-Amaya, 1999).