5. Flaking and chip dicing: Evenly flake dried slurry into 2–3 mm thick flakes, then cut flakes into small pieces with a uniform size of about 3 × 4 cm2.
6. Baking, cooling and frying: Bake the small slices at 60–70 °C for 3–5 min. After spreading and cooling, fry the sweet potato slices for 0.5 min at 170 ± 2 °C. Remove the slices and drain off the remaining oil. When cooled, the sweet potato slices are packaged. The aromatic and crispy sweet potato chips are presented in Figure 11.2.20.
Figure 11.2.20 Aromatic and crispy sweet potato chips.
11.2.9 Low Temperature Vacuum Fried Sweet Potato Chips
Technological process of low temperature vacuum fried sweet potato chips is shown in Figure 11.2.21.
Figure 11.2.21 Technological process of low temperature vacuum fried sweet potato chips.
The operating procedure and precautions during processing includes:
1. Selection of raw materiaclass="underline" During selection of sweet potato cultivars and pre-treatment of raw materials, high-quality and fresh yellow-core, red-core and purple-core sweet potatoes should be without lesions, mildew, germination and of moderate size. After cleaning with a cleaning machine and peeling, cut into 2–3 mm slices or 6 × 6 × (50–80) mm3 strips with a slicing machine. Then treat with color protection solution, blanch in water (95 °C for 2 min) and drain. Later, sweet potato slices or strips are dipped in 30 % (mass fraction) mixed sugar solution (2 parts maltose and 1 part dextrin) at 50 °C for 1 h and then frozen overnight at -18 °C. The frozen sweet potato slices or strips are then ready for vacuum frying.
2. Color protection treatment: Treat the sliced materials with a certain mass fraction of color protection solution as described above. After the solution is prepared, soak the sweet potato chips for 15 min.
3. Vacuum drying: Certain degree of vacuum, different temperatures and frying times are selected to determine the temperature influence on water and fat of the sweet potato slices. After comparison is made, a relatively ideal frying temperature is chosen. The vacuum degree influences on water and fat of sweet potato slices are determined together by the degree of vacuum and frying time.
4. Vacuum frying and oil removing: At 85 °C and vacuum degree of 0.085 MPa, fry the product for 20 min. Then lift the frying basket off the oil, and remove the remaining oil by vacuum de-oiling centrifugation for 5 min at 350 rpm. The low temperature vacuum frying machine is presented in Figure 11.2.22.
Figure 11.2.22 Low-temperature vacuum frying machine.
The processing procedures demonstrate that satisfactory products are determined by good raw materials. The requirements of the sweet potato materials may depend on shape, appropriate size, smooth skin, well-distributed flesh color, delicate quality, low fiber content, browning resistance, etc.
During the process of vacuum frying, moisture content changes are in accordance with the falling rate drying rule of traditional drying technology. High temperature and high vacuum degrees could improve the sweet potato drying rate. Fat absorption rule is consistent with water evaporation rule, and the enhancement of fat mass fraction is proportional to the water content reduction of sweet potato chips. With increase of drying temperature and vacuum degree, and decrease of breaking forces of sweet potato chips, crispness is gradually improved. Moreover, the fat content of sweet potato processed by regular frying is relatively higher, and would directly affect product color, taste and shelf life, so it is necessary to remove oil in plenty of time. Therefore, after optimization of the factors of moisture content, fat mass fraction and crushing force, it is concluded that the sweet potato chips should be fried under 0.085 MPa vacuum degree at 85 °C for 20 min followed by centrifugation at 350 rpm for 5 min (for fat removal) to achieve full shape and high-quality sweet potato chips. The low temperature vacuum fried sweet potato chips are presented in Figure 11.2.23.
Figure 11.2.23 Low temperature vacuum fried sweet potato chips.
11.2.10 Vacuum Microwave Drying Sweet Potato Chips
Vacuum microwave drying is the combination of two technologies, vacuum drying and microwave drying and also fully exerts the advantages of these two technologies. Under vacuum conditions, heat is easily transmitted in between water or solvent molecules, so that drying time is greatly shortened and production efficiency is improved. The vacuum microwave drying equipment is presented in Figure 11.2.24.
Figure 11.2.24 Vacuum microwave drying equipment.
The processing steps of vacuum microwave dried sweet potato chips are shown in Figure 11.2.25.
Figure 11.2.25 Processing steps of vacuum microwave dried sweet potato chips.
Select fresh sweet potato without mildew and pests, and control the slice thickness within 5.0 ± 0.3 mm. Blanch sweet potato slices at 95 °C (water bath) for 5 min then remove and drain off surface water. Then place slices into a freezer at -18 °C for 12 h, then process with forced air drying for 4 h at 65 °C in a drying oven. When the moisture content of pre-drying sweet potato chips is decreased to 31 %, place the chips into a sealed container at 5 °C for 10 h to attain uniform moisture.
Weigh 10 ± 0.5 g pre-dried sweet potato chips and place inside a vacuum microwave drying equipment chamber. Then open the water circulating vacuum pump and adjust the valve to obtain the desired vacuum degree. Set the appropriate heating power and time of the microwave, and then start microwave heating. Finally, vacuum microwave dried sweet potato chips are obtained.
The precautions during processing include:
1. Moisture content, vacuum degree, microwave power and heating time have extremely significant influences on the puffing ratio and crispness index of the pre-dried sweet potato chips. Chips with high moisture content have a relatively higher puffing ratio, which could result in lower crispness. On the other hand, lower moisture content not only decreases the puffing ratio, but also increases the appearance of stiff pieces.
2. Higher microwave power could increase puffing ratio and crispness of the sweet potato chips, so as to improve the sensory quality. Moreover, a high vacuum degree could not only increase the puffing ratio and crispness of the sweet potato chips, but also reduce the boiling point of water, therefore increase the thermal efficiency.
3. The best vacuum microwave drying technology parameters for sweet potato chips are given in Table 11.2.7.
Table 11.2.7 The parameters for vacuum microwave drying technology
S. No. | Parameters | Value
1 | Vacuum degree | 0.085 MPa
2 | Moisture content of pre-dried sweet potato chips | 31.23%
3 | Microwave power | 771.38W
4 | Heating time | 39 sec
Sweet potato chips with satisfactory quality could be prepared by following these processing conditions. The vacuum microwave dried sweet potato chips are presented in Figure 11.2.26.
Figure 11.2.26 Vacuum microwave dried sweet potato chips.