100 | 4.5 | 4 | Puffed products are tiny, dense and soft
Cited from: (Tan, 1996)
Table 11.2.4 Effect of rice and millet addition on puffing
Proportion of raw materials (g)
Sweet potato | Rice | Millet | Puffing effects
100 | 0 | 0 | Khaki color, strong sweet potato smell, bitter taste
80 | 20 | 0 | Yellow color, sweet potato smell, no bitter taste, good | comprehensive flavor
60 | 40 | 0 | Light yellow color, light sweet potato smell, no bitter taste
80 | 0 | 20 | Orange color, sweet potato smell, no bitter taste, good comprehensive flavor
60 | 0 | 40 | Golden yellow color, light sweet potato smell, no bitter | taste
Cited from: (Tan, 1996)
Quality standards of the puffed sweet potato food mainly include physico-chemical, microbiological and sensory parameters, which are given in Table 11.2.5.
Table 11.2.5 Quality standards of the puffed sweet potato food
S. No. | Quality parameters | Attributes
1 | Sensory attributes | Yellow or orange color • Loose and porous • Particles are well-distributed • Product tastes crispy • Palatable flavor • No bitter taste or other strange flavor • Moisture content: 3%
2 | Physicochemical parameters | Carbohydrate content: 78 % • Fat content: 9 % • Protein content: 4.7 %.
3 | Microbial index | Absence of pathogens
11.2.7 Airflow Puffed Sweet Potato Chips
Airflow puffing technology removes part of the water from fruit and vegetable materials in advance, then the fruit and vegetables are heated inside a pressure tank to achieve a relatively low-temperature but high-pressure state. After insulating the target temperature for a certain period of time, release the pressure instantly so that the water inside the materials produces the effect of “flash evaporation”. In this way, a puffing effect is achieved. The fruit and vegetable puffed crispy product is produced by continuous dehydration under vacuum. The equipment for airflow puffing, shown in Figure 11.2.15, is composed of expansion tanks, heating elements, rotating, safety and vibration damping devices, frame and a base. A certain amount of material is loaded into the cover-sealed steel tanks, heated by gas, and rotated continuously at a certain speed. When the tank temperature reaches above 100 °C, moisture on the surface of the material vaporizes, resulting in the increase of pressure inside the tank. When the pressure reaches 0.8–1.25 Mpa, the lid of the tank is suddenly opened and the pressure is released. The volume of the material then increases by nearly ten-fold. Compared to hot air drying and freeze drying, the airflow puffed drying processing of sweet potato chips have the advantages of energy saving, high drying efficiency, and could be applied to large-scale production. The airflow explosion puffing machine is presented in Figure 11.2.16.
Figure 11.2.15 Extrusion puffed sweet potato crisps.
Figure 11.2.16 Airflow explosion puffing machine.
When processed with airflow puffed processing equipment only, the pressure of produced vapor is too low, hence there is the possibility that the sweet potato may not achieve the expected puffing effect. However, before the airflow puffing process, if the sweet potato is pre-treated by the vacuum microwave puffing processing method, the expected puffing state of the sweet potato product can be achieved. The processing steps of airflow puffed sweet potato chips are given in Figure 11.2.17.
Figure 11.2.17 Processing steps of airflow puffed sweet potato chips.
The specific treatments include:
1. Washed and cleaned selected sweet potatoes are peeled, sliced into pieces of 5 mm thickness and then blanched at 95 °C (water bath) for 3 min. Remove them from the water and drain the surface water off, then place in the hot air drying oven in a single layer (initial moisture content of the sweet potato is 79.9 %). After hot air drying at 65 °C, the moisture content decreases to 37 %. Place the slices into a sealed container at 5 °C and store for 12 h to soften and mellow to obtain pre-dried sweet potato chips.
2. Place pre-dried sweet potato chips inside the vacuum microwave dryer chamber in a single layer, after dialling a microwave power of 27.4 Watt, vacuum degree of 0.085 MPa, and 34 s continuous heating, to obtain the pre-puffed sweet potato chips (moisture content is 18 %).
3. Place 2,000 g pre-puffed sweet potato chips inside the puffing tank of the airflow puffing equipment. The pressure difference between the puffing tank and vacuum tank is 110 kPa, which should remain for 6 min under certain puffing temperatures, then the pressure will instantly drop to the vacuum state (0.095 MPa). Then decrease the temperature to the degree that is needed for vacuum drying of the chips for a certain period of time. Then move the chips to a sealed container and cool at 10–20 °C for 45 min. The final product, airflow puffed sweet potato chips with a moisture content of 3 %, is obtained. The last step of the processing procedure is nitrogen charging and packaging of the chips with a volume fraction of N2 of 99.99 %, and a pressure of 0.5 Mpa and charging for 2 s. The airflow puffed sweet potatoes are presented in Figure 11.2.18.
Figure 11.2.18 Airflow puffed sweet potatoes.
4. The precautions during the processing may include:
i. Puffing temperature, vacuum drying temperature and vacuum drying time have significant influences on the hardness index and crispness index. The puffing temperature has significant effects on the lightness L* of sweet potato chips, while the effects of vacuum drying temperature and vacuum drying time are not significant.
ii. The airflow puffed sweet potato chips can be produced with optimum quality based on the selected range of hardness. The optimum processing parameters under the selected range of hardness (0.2143 N < Y1 < 0.2857 N) is shown in Table 11.2.6. The sweet potato chips produced under the processing conditions are assured to have a crispy taste and satisfactory color.
Table 11.2.6 Optimum processing parameters for air puffed sweet potato chips
S. No. | Processing parameters | Range
1 | Puffing temperature | 91 °C
2 | Vacuum drying temperature | 75 °C
3 | Vacuum drying time | 47 min
11.2.8 Aromatic and Crispy Sweet Potato Chips
The processing steps of aromatic and crispy sweet potato chips are illustrated in Figure 11.2.19.
Figure 11.2.19 Processing steps of aromatic and crispy sweet potato chips.
The key processing steps include:
1. Pretreatment of raw materials: Wash and clean the qualified sweet potato and potato (at the ration of 6: 1) and pre-boil in boiling water for 10–20 minutes. After peeling and slicing, re-boil them until completely cooked. Sodium sulfite, 0.05 %, is added into the pre-boiling water in advance.
2. Slurring: Crush thoroughly cooked potato and sweet potato slices into a slurry. If necessary, add a small amount of water, but not too much.
3. Stirring and fermentation: Add 0.4 % dry yeast, 8 % sucrose and 0.2 % salt into mixed slurry and ferment for 2 h at 28 °C.
4. Drying: Dry the fermented slurry at 80 °C for 60–80 min (until it can be flaked). Keep turning during the drying process to prevent burning.