10. Heating and cooling: Heat the sealed jelly in 85 °C hot water for 10 min, then quickly cool to 38–42 °C, to ensure retaining the color and flavor. Moreover, in the process, hot water soaking and cold water spraying methods can be carried out to prevent the growth of heat-resistant bacteria.
11. Drying: Clean and dry the cooled product with a clean, dry towel, to prevent mildew during storage and marketing. During processing, the moisture remaining on the jelly cup surface can be evaporated by the hot air drying method at 50–60 °C.
12. Packaging: After inspection for quality, the jelly can be packaged.
Figure 11.3.2 Processing steps of sweet potato jelly.
Based on investigations, it can be concluded that larger the proportion of sweet potato as an ingredient, the greater should be the natural flavor of sweet potato. However, if the proportion of sweet potato is too large, the particular taste of the jelly is unacceptable. It could be estimated that more sweet potato will lead to a larger amount of starch content in the total recipe and less moisture content, therefore the taste of the jelly would be influenced. In contrast, if the proportion of sweet potato is smaller, the natural flavor is weaker. Therefore, it is important to determine the appropriate proportion. As shown in the following Table 11.3.1, the optimum amount of sweet potato content is in between 25 and 35 %.
In short, after optimization, take konjac refined flour and carrageenan as the gelling agent. When the ratio of konjac refined flour and carrageenan is 1.25: 1, but the total use of two gelling agents accounts for 0.9 % and the amount of sweet potato ingredient is in between 25 and 35 %, the quality of sweet potato jelly (Figure 11.3.1) is the highest.
Table 11.3.1 Effect of ingredient content (%) on the quality of sweet potato jelly
Sweet potato | Water | Quality status | Results
15 | 85 | Good sweet potato taste, good elasticity and fragility, extremely delicate texture | Better
25 | 75 | Good sweet potato taste, good elasticity and fragility, delicate texture | Good
35 | 65 | Good sweet potato taste, good elasticity and fragility, delicate texture | Good
45 | 55 | Strong sweet potato but rough taste, good elasticity, poor fragility, and sticky | Poor
Source: Zhang (2013)
11.3.3 Instant Sweet Potato Noodles
The manufacturing of instant sweet potato noodles mainly includes sweet potato starch as the main ingredient. After cleaning, extruding and gelatinizing by single screw, as well as drying and molding, seasoning is added. The instant sweet potato noodles that are edible after 3 min soaking in boiling water can be prepared by the following processing steps:
1. Starch treatment: Select sweet potato starch and potato starch of high quality, and soak in water, then remove sand with a sand-removing machine. Sift through a 120-mesh sieve to remove coarse residues and then add the acid solution, which accounts for 30 % of the dry starch amount. After thorough stirring, soak for 6–8 h in the acid solution to bleach, then thoroughly rinse with water three times.
2. Flour mixing: Pour the prepared sweet potato starch and potato starch into a flour mixer at a certain ratio, then add a small amount of vegetable oil and sodium alginate; stir for 8–12 min to ensure dough of moderate moisture.
3. Extruding and gelatinization: Directly put the prepared dough into a single-screw extruding machine, pass the sweet potato noodles through a 0.8 mm sieve, and soak the shaped sweet potato noodles in water at 95 °C, which is essentially known as starch dextrinization, also known as α-dextrinization. After this process, the noodles are cooled with an air blower.
4. Retrogradation and loosening: Gelatinized sweet potato noodles are retrogradated at room temperature for 4–5 hours, or frozen in cold storage (–4 °C) for 2–3 hours, then loosened in a loosening machine.
5. Drying and packaging: When the sweet potato noodles are loosened, drained off the water, and evenly spread onto a tray for hot air drying in a drier. The noodles are dried at 65–75 °C until the moisture reaches a level of 12 %. Seasoning is then added and the product is finally prepared by sealing and shrink packaging (Figure 11.3.3).
Figure 11.3.3 Instant sweet potato noodles.
11.3.4 Quick-frozen Sweet Potato Product
The technological process of preparing quick-frozen sweet potato product is given in Figure 11.3.4.
Figure 11.3.4 Technological process of preparing quick-frozen sweet potato.
The operating procedure includes:
1. Materials check and acceptance: Select the sweet potato with uniformly round shape, yellow and orange flesh, and showing no pathological changes and mechanical damage.
2. Slitting: Slit the sweet potato into strips with a cross-section area of 2.8 × 2.8 cm2 and length of 8–10 cm with a multi-function cutter.
3. Steaming: After cleaning, evenly spread the sweet potato strips onto a steam- ing tray and drain off water, then steam for 3–5 minutes at a pressure of 0.2 MPa.
4. Quick freezing: It is recommended to use a fluidized continuous quick-frozen machine. The materials are loaded when the quick-frozen temperature drops down to –40 °C. Adjust the operating speed within 25–30 min, so that the core temperature of the product is controlled between –18 and –25 °C to avoid cracking on the surface and touching each other.
5. Cold storage: Send packaged frozen sweet potato chips to a freezing store, where the temperature is maintained at -18 °C or lower. During cold storage, temperature fluctuations should be controlled within 2 °C. The different frozen products from sweet potatoes are shown in Figure 11.3.5.
Figure 11.3.5 Quick-frozen sweet potato products.
11.3.5 Sweet Potato Healthcare Tea
The technological process of preparing sweet potato healthcare tea is given in Figure 11.3.6.
Figure 11.3.6 Preparation of sweet potato healthcare tea.
The processing include:
1. The selection and treatment of sweet potato leaves: Select fresh and tender sweet potato leaves, remove any worm-eaten, mildewed and rotten leaves and also any leaf spots. After removing stalks, clean the leaves in water, then chop into 0.5 × 0.5 cm square pieces.
2. Enzyme inactivation: Decoct Perilla frutescens and Pericarpium citri reticulatae into an enzyme inactivator, then spray on to the chopped leaves at a ratio of 1.25 %, in order to inactivate the polyphenol oxidase in the leaves.
3. Drying: Dry the treated sweet potato leaves in an oven at 25–40 ∘ C for 30 h.
4. Heating tea: Fry the commercially available tea leaves in a tea-frying pot, which will reduce the moisture content of the tea to 6–7 %.
5. Blending and crushing: After blending the tea and sweet potato leaves at a ratio of 3: 2, crush them in a hammer mill, then sift through a 16-mesh sieve. The sweet potato healthcare tea is shown in Figure 11.3.7.