11.2.5 Instant Nutritious Sweet Potato Chips
The technological process of instant nutritious sweet potato chips is shown in Figure 11.2.11.
Figure 11.2.11 Technological process for instant nutritious sweet potato chips.
The operating procedure include:
1. Cleaning and washing: Select high-quality fresh sweet potatoes and place them in water for 0.5–1 h, then wash off surface slit and drain for drying.
2. Peeling: The skin of sweet potato contains a variety of pigments, which will significantly influence the color of the final product. Therefore, the peeling process has to be carried out before the following operating procedures. Mechanical or alkali peeling is preferred. In case of alkali peeling, concentration of alkali solution is in between 0.5 and 1.0 %.
3. Slicing: Sweet potato is sliced with a fruit and vegetable slicer, for the purpose of thorough steaming and cooking. The appropriate thickness is about 1 cm.
4. Color protection: Immediately place the sliced sweet potato into a 0.1 % citric acid solution. To prevent oxidation and discoloration, it is better to minimize the period of exposure to air.
5. Slurring: After color protection, blanch the sweet potato slices in boiling water. Then grind the sweet potato with a grinding wheel and sift through an 80-mesh sieve. Later, appropriate amounts of other ingredients, such as milk powder, sugar, palm oil and emulsifier are added and stirred together according to the desired flavor of the final products.
6. Homogenization: All the well-mixed ingredients are added to a colloid mill for homogenization. The diversity of ingredients will influence the final mixing effect, especially the oil ingredients, which are not easily soluble in water. To overcome this problem, the colloid mill is used to grind the slurry into fine particles to approach emulsification and effectively improve the product quality.
7. Pre-gelatinization: The surface temperature of a common steam drying roller is over 140 °C. When the slurry is transferred onto the reservoir of the steam drying roller and accumulated to a certain amount, the pre-gelatinization reaction then occurs.
8. Roller drying: This is the key process of producing instant sweet potato chips, and the product flavor is determined by this process. The steam roller drying has strong thermal stability. If rotation speed and temperature are co-ordinated well, the gelatinization degree and drying effect are easy to control.
9. Granulation and packaging: The size of sweet potato chips is controlled by the adjustment of the granulator mesh density. After granulation, the product is packed according to different requirements and divided into small, medium and large pieces. The appearance of instant sweet potato chips is shown in Figure 11.2.12.
Figure 11.2.12 Instant sweet potato chips.
11.2.6 Puffed Sweet Potato Food
With high market demand, puffed snacks are one of the most popular products favored by consumers. So far, production of this kind of food has mainly used grains (rice, corn, milt and wheat) as raw materials. Puffed sweet potato food is a new category of deep processed food produced by screw extrusion and puffing. Compared to grain puffing, sweet potato puffing is more difficult to puff and mold, easily burns inside the extruder barrel, has poor flavor, bitter taste and so on.
Extrusion technology is one of the advanced processing methods in the modern food engineering field. The mechanism is the melting of materials into a molten state under high pressure and temperature by shearing and rubbing inside machine. Once the materials are forced to squeeze out through a die hole, the pressure immediately drops to a normal value, water is instantaneously evaporated, and the materials are then correspondingly inflated. The extrusion technology belongs to one of the non-fried processing methods. With the advantages of high production efficiency, low energy consumption, good product quality and waste-free features, extrusion technology widely applied in starch-based food (like corn) is a scientific and nutritious processing method (Li and Mu, 2005). The double screw extrusion machine is presented in Figure 11.2.13.
Figure 11.2.13 Double screw extrusion machine.
Based on investigations, crisp and delicious puffed sweet potato food is prepared by mixing 80 % sweet potato and 20 % rice or millet with addition of 2.5 % water and 2 % vegetable oil, puffing in a screw extrusion machine, and seasoning. The specific processing is given in Figure 11.2.14.
Figure 11.2.14 Preparation of crisp and delicious puffed sweet potato food.
The operating procedure includes:
1. Selection and cleaning of raw materials: The selected fresh sweet potato are cleaned and washed.
2. Shredding and drying: The moisture content of fresh sweet potato is very high, and so not suitable for puffing. Therefore, dehydration treatment is necessary. To accelerate the dehydration process as well as to facilitate further crushing procedures, sweet potato needs to be first shredded into 3–4 mm strips, then dried to the moisture content of 11 % by oven or sun drying. The dried materials can be stored for a relatively long period under moisture-proof condition.
3. Material crushing: The materials added into the screw extrusion machine should be presented as an appropriate granulated size, because powdery material is easily aggregated and burnt inside the screw roller, while larger-sized material causes transportation problems and puffing inhomogeneity, and even increases the wear and tear of the equipment. Therefore, raw materials should be crushed into 2–3 mm particles.
4. Ingredients preparation: Ingredients preparation is the process of adding an appropriate amount of water, vegetable oil and rice or millet to the sweet potato before puffing, to ensure smooth puffing and also provide the product with a better color and taste. The effect of different additives on the puffing effect is given in Tables 11.2.3 and 11.2.4 respectively:
5. Puffing: Firstpre-heat extrusion machine,then gently feed raw materials, and later gradually increase the materials feeding rate, when normal operational level is achieved. Continue feeding at a constant speed, and collect the puffed products, which are well-distributed into strip shapes. The extrusion puffed sweet potato crisps are shown in Figure 11.2.15.
6. Seasoning: Evenly spray the proportionally prepared various seasonings onto the surface of the puffed products, to yield the product in a variety of flavors.
7. Drying: After seasoning, place the puffed products at 120 °C for drying (until the moisture content is <3 %), to make the product crisp.
8. Packaging: Choose good airtight plastic packaging materials for product packaging to prevent damage during transportation and good shelf life. Packaging should be carried out under sterile conditions.
Table 11.2.3 Effect of water and oil addition on puffing
Proportion of raw materials (g)
Sweet potato | Water | Vegetable oil | Puffing effects
100 | 0 | 0 | Puffed products are easily cracked, shapeless, burnt
100 | 2.5 | 2 | Good puffing effect, puffed products are well mixed and loose