Processing Methods and Technical Key Points for Sweet Potato Chips
1. Process flow: raw material selection -- cleaning -- slicing -- color protection -- dehydration -- vacuum frying -- de-oiling -- cooling -- packaging
2. Operating key points:
(1) Raw material selection: select fresh, plump, firm-fleshed oval sweet potatoes that are free from mold, rot, pests, diseases and mechanical damage.
(2) Cleaning: first soak the sweet potatoes in clean water for 1-2 hours, then remove the sand, mud and debris from the tubers by hand or with a washing machine, then remove the skin layer of the sweet potatoes with a stainless steel knife and quickly immerse them in clean water.
(3) Slicing and color protection: because sweet potatoes are rich in starch and have a high solids content, the slice thickness should not exceed 2 mm. After slicing, starch quickly exudes onto the surface of the sweet potato slices, and browning can occur if they are left in the air for a long time, so they should be immediately placed in boiling water for 2-3 minutes, then removed and placed in water to cool. Heat treatment mainly has the following effects: destroying enzyme activity and stabilizing color; removing the starch exposed on the surface after the tissue is sliced, to prevent part of the starch from dissolving into the cooking oil during frying and affecting oil quality; and preventing the slices from sticking to one another during frying. Heat treatment is the alpha-gelatinization process of starch, and can prevent, during frying, the gradual heating of the oil from causing starch gelatinization to form a colloidal isolation layer, which would affect dehydration of the internal tissue and reduce the dehydration rate. Fried sweet potato chips that are not treated by this step are hard and have a poor texture.
(4) Dehydration: before frying, the sweet potato slices need to be dehydrated to remove surface moisture from the chips. Equipment that can be used includes perforated rotating drums, rubber sponge squeeze rollers, vibrating mesh conveyor belts and centrifugal separators.
(5) Vacuum frying: vacuum frying technology overcomes the disadvantages of high-temperature frying and can better preserve the nutrients and color of sweet potato chips, making them crisp and fragrant, tender but not greasy. A vacuum frying system includes a working section and an auxiliary section. The working section mainly completes the vacuum frying process, including the frying tank, storage tank, vacuum system, heating section, etc. The auxiliary section mainly completes processes such as adding oil, discharging waste oil, and cleaning containers and pipes (including the oil storage tank and alkali liquor tank). The vacuum system can use a water-ring vacuum pump to create a vacuum degree of more than 93.3 kPa (700 mmHg) in the vacuum tank. This vacuum degree can lift oil from the storage tank to the frying tank at a maximum height of more than 10 m. However, if it is used as a vacuum material suction device, factors such as flow resistance, flow velocity and fluctuations in vacuum degree should also be taken into account. During vacuum frying, first pour edible oil into the oil storage tank to 1/3 of its volume and heat it to 95 degrees Celsius; place the hanging basket holding the sieve tray of sweet potato slices into the frying tank and lock the tank lid. After closing the vacuum valve of the oil storage tank, evacuate the frying tank, open the oil passage connecting valve between the two tanks, and the oil is pressed from the storage tank into the frying tank; close the oil passage connecting valve, heat, keep the oil temperature at 90 degrees Celsius, raise the vacuum degree to 86.7 kPa within 5 minutes, and raise the vacuum degree to 93.3 kPa within 10 minutes. During this process, a large amount of foam can be seen forming and the sweet potato slices float up, and the vacuum degree can be controlled according to actual conditions, with the limit being that violent boiling does not occur. When the foam has basically disappeared and the oil temperature begins to rise, heating can be stopped. Then, while maintaining the frying vacuum degree, open the oil passage connecting valve, and all the oil in the frying tank flows back into the storage tank under gravity. Then close the vacuum valves of each tank body, then turn off the vacuum pump, and finally slowly open the valve connecting the frying tank to the atmosphere so that the pressure inside the tank matches atmospheric pressure.
(6) De-oiling and cooling: while hot, place the sweet potato chips in a centrifuge and de-oil by centrifugation at 1200 rpm for 6 minutes, then spread them out to cool.
(7) Grading and packaging: bag and seal the product according to shape and color conditions. Vacuum nitrogen-filled packaging is best, maintaining the moisture content of the finished product at about 3%, so as to ensure the quality of the finished product.