Figure 11.3.7 Sweet potato healthcare tea.
11.3.6 Sweet Potato Shoot-tip Canning
The technological process of preparing sweet potato shoot-tips is given in Figure 11.3.8.
Figure 11.3.8 Technological process of preparing sweet potato shoot-tips.
The processing include:
1. Material selection and treatment: Clean the fresh sweet potato shoot-tips by removing any worm-eaten, spotted, withered and broken leaves. After evenly cutting and then draining, soak the product in 0.2 % Na2SO3 color protection solution for 18–24 h. After that, rinse in water until the slimy feeling disappears, then can the treated sweet potato shoot-tips, and inject liquid (a solution consisting of different ingredients and flavors) at 80–90 °C into the above-mentioned can.
2. Exhausting and sealing: After canning, the exhausting process is taken at 80 °C for 10 min, then sealed.
3. Sterilization and cooling: Sterilize at 115 °C for 3 min and then rapidly cool to 40 °C or below. The sterilization formula is 5-3-5 min/115 °C.
4. Inspection and packaging: After removing broken or swelled cans, sampling inspections of sensory, physicochemical and microbial indexes are taken.
11.3.7 Sweet Potato Beer
Fresh sweet potato beer (Figure 11.3.9) can be prepared by carrying out mixed fermentation with Saccharomyces cerevisiae and Saccharomyces ellipsoideus. This beer has a pleasant sweetness and sourness and pure flavor, along with a rich and white head of foam.
Figure 11.3.9 Sweet potato beer.
The technological process of preparing sweet potato beer is given in Figure 11.3.10.
Figure 11.3.10 Technological process of preparing sweet potato beer.
The processing include:
1. Liquidation: Add 0.1 % α-amylase and then liquidate at 70 °C, pH 5.6 for 60 min.
2. Saccharification: Add 0.1 % glucoamylase, then saccharify at 55 °C, pH 4.5 for 30 min.
3. Mash adjustment: Adjust the concentration of soluble solids in the slurry to 4–5%, with pH 4.8–5.2, which contains 1 % dry beer hops.
4. Inoculation and fermentation: Add 2–3% Saccharomyces cerevisiae liquid culture medium and 2–3% (w/w) immobilized Saccharomyces ellipsoideus seeds. Ferment for 3–4 days at 10–12 °C. When the soluble solid content reaches 2–2.5 %, terminate fermentation.
5. Fermentation after blending: Add jujube juice with the soluble solid content of 5.5–6%, adjust pH between 4.2 and 4.3, then ferment at 10–12 °C for 2–3 days. When the soluble solid content reaches 2–2.5 %, terminate fermentation.
6. Clarification: Cool and keep the container temperature at about 0 °C for 12 h, then remove supernatant. Light transmittance of the liquid should be over 95 %.
7. Pasteurization: The final pH of the clarified liquid is in between 4.0 and 4.2, which means an acidic food. The product should be pasteurized at 70 °C/30 min.
11.3.8 Purple Sweet Potato Juice
Purple sweet potato juice can be prepared by the enzymatic method by taking purple sweet potato as raw material. The processing into juice is illustrated in flow chart (Figure 11.3.11).
Figure 11.3.11 Preparation of purple sweet potato juice.
The optimum processing conditions of enzymatic (amylase) and secondary hydrolysis are given in Table 11.3.2 whereas the optimum processing conditions of the secondary enzymatic hydrolysis are given in Table 11.3.3.
Table 11.3.2 Optimum processing conditions of enzymatic (amylase) hydrolysis
S. No. | Processing conditions | Optimum value
1 | Ratio of ingredient and water | 1: 4
2 | Temperature | 80 °C
3 | pH | 5.5
4 | Amylase additive dosage | 0.15%
5 | Enzymatic reaction time | 40 min
Table 11.3.3 Optimum processing conditions of the secondary enzymatic hydrolysis
S. No. | Processing conditions | Optimum value
1 | Temperature | 50 °C
2 | pH | 4.5
3 | Glucoamylase additive dosage | 0.1%
4 | Pectinase additive dosage | 0.04%
5 | Enzymatic reaction time | 20 min
By following the given optimum processing conditions, a clear purple sweet potato juice (Figure 11.3.12) can be prepared with a color value of 0.16 (530 nm) and total solid content of 7–9 Brix. The fermentation method should be carefully controlled to prevent undesirable microorganisms and unwanted biochemical reactions.
Figure 11.3.12 Purple sweet potato juice.
11.3.9 Sweet Potato Whole Flour
Sweet potato whole flour (Figure 11.3.13) is one of the dehydrated sweet potato products. It is prepared by using fresh sweet potato as the raw material, processed by selecting, cleaning, peeling, slicing, color protection, blanching, cooling, drying, and grinding. The final product contains all the powdered dried contents except the sweet potato skin and is generally known as sweet potato whole flour. The sweet potato whole flour retains its original nutrients, color, and flavor of the sweet potato, and contains all the dried contents, such as starch, protein, carbohydrate, fat, fiber, vitamins and minerals, except the skin. The loss ratio of the healthcare functional components in sweet potato is extremely limited, while its utilization rate is in between 85 and 90 %. Moreover, after rehydration, the sweet potato whole flour presents a similar appearance to steamed and mashed fresh sweet potato, and has the same nutrients and flavor of fresh sweet potatoes. In addition, sweet potato whole flour is convenient to use, and easy for digestion and nutrients absorption.
Figure 11.3.13 Sweet potato whole flour. (a) Purple sweet potato whole flour. (b) Yellow flesh sweet potato whole flour.
Recently, in Japan, the United States and other developed countries, sweet potato whole flour (Figure 11.3.13) is used as the primary product of sweet potato dried products. The FOB (free on board) value of 100–120 mesh sweet potato whole flour in international markets is about $2.000-2.500 per ton, which is 2–4 times the sweet potato starch value. Moreover, appreciation rate of the food mainly prepared by sweet potato whole flour is 10–20 % over sweet potato starch food (silk noodles/starch sheets/starch noodles). Therefore, a large number of peeled, steamed and dried sweet potato chips from Shandong, China are imported by Japan each year, while US companies have built factories in Kunshan, Jiangsu Province, to produce sweet potato whole flour by using the sweet potato source in China. Nevertheless, China’s domestic market has limited knowledge of the sweet potato whole flour, so its application still needs further exploration in the future.
11.3.10 Sweet Potato Healthcare Food
The sweet potato is rich in nutrients and functional components such as protein, dietary fiber, pectin, β-carotene, polyphenols, anthocyanins, β-amylase, and so on. Therefore, it may have broad market prospects if developed and applied to health food. The following health foods can be obtained from sweet potato: