After fresh sweet potatoes are harvested, they are usually cut into flakes or shreds, and dried in the sun or by fire to make dried sweet potatoes. Such dried sweet potatoes are inevitably mixed with various impurities during processing and transportation.
1. Pretreatment: The dried sweet potatoes sent to the sweet potato starch workshop for processing contain various impurities, so they must be pretreated.
There are two types of pretreatment: dry method and wet method. The dry method is to use screening equipment and winnowing equipment. Wet processing is done with a washing machine or a wash bucket. Before crushing, it is best to go through magnetic separation equipment to remove metal impurities and ensure safe production.
2. Soaking: Soaking in alkaline water can be used to increase the yield of starch. The effect of soaking in alkaline water is:
(1) The fiber expansion of sweet potato slices increases, so that the fibers and starch are easily separated when the sweet potato slices are broken, and the starch granules are less broken.
(2) Make pigments and colloidal substances easy to ooze out. When sieving, reduce slag sticking to the sieve holes.
(3) Keep alkaline and inhibit microbial activity.
(4) When the starch milk is separated in the launder, the performance of depositing starch is good, and the powder is hard.
Soaking generally uses saturated lime milk or 1% dilute lye to soak in water, so that the pH value is 10-11; the soaking time is about 12 hours; the temperature is controlled at 35-40°C. After soaking, the water content of sweet potato chips is 60%.
3. Crushing and Grinding: The soaked sweet potato slices enter the hammer mill with water for crushing to a certain fineness, and then pass through the sieve holes and are discharged out of the machine. If the speed of the pulverizer exceeds 3,200 rpm, the hammer is vulnerable, the dynamic balance performance is poor, and the operation is unstable; the instantaneous temperature of the dried potato rises during the pulverization process, and part of the starch is easily gelatinized by heat, so that when sieving , affecting the separation of starch and potato dregs; it is not easy to settle when the launder is separated, resulting in an increase in the number of times and affecting the powder yield.
In order to prevent starch gelatinization in one crushing process, a process of two crushing and separate sieving can be adopted. That is, the dried potato is sieved after the first crushing process, and then sieved after the second crushing process. During the crushing process, in order to reduce the instantaneous temperature rise, adjust the slurry concentration according to the different particle sizes of each crushing stage. At the same time, a homogenizer is used to control the intake of dried sweet potatoes, to balance the slurry, to avoid the overload of the pulverizer, and to facilitate the separation of the launder.
In order to improve the crushing efficiency of slag, grinding treatment is generally adopted, which has a certain effect on reducing the temperature rise of the feed liquid, reducing the powder content of the slag, increasing the starch yield and saving maintenance costs.
4. Sieving: The sweet potato paste obtained after crushing dried sweet potatoes is also called feed liquid, which must be sieved to separate out the slag, that is, fibers. Usually, a flat shaking sieve is used. The feed liquid enters the sieve surface, and it is required to be evenly sieved, and the water is continuously poured. The starch passes through the sieve hole with the water and enters the slurry storage tank, while the slag remains on the sieve surface and is discharged from the tail of the sieve. The size of the sieve hole should be determined according to the particle size of the material in the feed liquid. The sieve configuration of the sieving equipment using two crushing processes is: the first and second sieves are made of 80-mesh nylon cloth, and the powder and slurry obtained from the second sieve are combined and then finely sieved with 120-mesh nylon cloth. Its purpose is to further separate fine slag and improve fiber separation efficiency. Due to the different particle size and sieving requirements of the secondary crushing, the secondary sieving area should be properly equipped and shared reasonably. During the sieving process, the viscous substances such as pectin in the feed liquid stay on the sieve surface, which affects the separation effect of the sieve. Therefore, the screen cloth should be washed frequently to ensure the smooth opening of the screen.
5. Launder separation: currently, launders are generally used to separate proteins from starch after fine sieving. Its advantage is that it is easy to construct, uses less steel, saves kinetic energy, and is more stable in operation. The disadvantage is that it occupies a large area, intermittent operation, low separation efficiency, large starch loss, and poor working conditions.
6. Alkali, acid treatment and cleaning: In order to further improve the purity of starch, it is also treated with alkali and acid during the cleaning process.
Starch alkali, acid treatment and cleaning are all carried out in the cement pool with stirring device. The size of the cement pool is determined according to the production capacity, and the speed of the agitator is about 60 rpm.
The starch milk from the current tank is first treated with alkali to remove impurities such as alkali-soluble protein and pectin in the starch. The method of alkali treatment is to slowly add 1 degree of Baume dilute lye into the starch milk, control the pH value to 12, start the agitator at the same time, mix well, stop stirring after half an hour, and wait until the starch is completely precipitated. , discharge the upper waste liquid, and inject clean water to wash it twice to make it close to neutral.
In the alkali treatment process, a sodium hypochlorite solution with a concentration of 35 degrees Baume can also be added, and the dosage should not exceed 0.4% of the dry starch weight. Because sodium hypochlorite is a strong oxidant, it has strong bleaching and bactericidal effects, so that it can achieve the purpose of whitening and antisepsis.
The starch treated with alkali, sodium hypochlorite and washed is then treated with acid. Its purpose is to dissolve acid-soluble proteins, neutralize the residual alkalinity during alkali treatment, and inhibit the activity and reproduction of microorganisms.
Industrial hydrochloric acid can be used for acid treatment, and it must be added slowly during operation, fully stirred, and the pH value of the starch milk should be controlled at about 3. Prevent local overacid, resulting in starch loss. The acid-added starch milk is treated with the same method of alkali treatment to precipitate the starch, remove the upper waste liquid, add water to wash, and finally make the starch slightly acidic, that is, about pH6. In order to facilitate the storage and transportation of starch.
7. Centrifugal separation and drying: The starch obtained after washing has a high water content, and must be dehydrated by a centrifuge to reduce the water content of the starch to below 45%. Some factories directly leave the factory or use as wet starch.
In order to facilitate storage and transportation, it is best to dry the dehydrated wet starch to reach the required starch water content. Usually the moisture content of sweet potato starch is 12-13%.
Air drying can be used for starch drying. Because it has the advantages of fast drying speed, high efficiency, large equipment production capacity and low equipment cost.