A Detailed Guide to the Extraction and Processing Methods of Sweet Potato Starch

Sweet potatoes can be divided into two types: fresh sweet potatoes and dried sweet potatoes. Starch production from fresh sweet potatoes generally belongs to rural manual production, while sweet potato starch factories mostly use dried sweet potatoes as the starch raw material, so it belongs to industrial production.

1. Starch production from fresh sweet potatoes

The process flow is as follows: raw material selection - washing - crushing - grinding and filtering - slurry blending - skimming the vat and settling the vat - skimming the slurry - removing the starch - drying.

The production process is as follows:

(1) Raw material selection: Because sweet potato varieties differ, their quality and starch content also differ. Even for the same variety, in different producing areas, their quality also varies greatly. A good variety for starch processing should be selected, requiring high starch content. Diseased sweet potatoes are not only unsuitable as raw material for starch processing, but during storage can also infect other tubers, and are prone to rotting and causing losses, so diseased tubers must be removed completely.

(2) Washing: Put the fresh sweet potatoes into a vat, add clean water, and wash them by turning them over manually. After washing, take them out and drain off the remaining water.

(3) Crushing: Use a crusher on the drained fresh sweet potatoes

to break them into pieces, with a piece size of 2 cm or less to facilitate feeding into the mill.

(4) Grinding and filtering: This is the main step in sweet potato starch production, affecting product quality and starch yield. Feed the fresh sweet potato pieces into a stone mill or emery mill, add water, and grind them into a sweet potato paste. The ratio of fresh sweet potato weight to added water is 1:3-3.5. Then pour the sweet potato paste into a sieve with an aperture of 60 mesh for filtering.

(5) Slurry blending: Put the starch milk obtained by filtering into a large vat, then immediately add sour slurry and water in proportion to adjust the acidity and concentration of the starch milk. The acidity and concentration of the starch milk are closely related to the precipitation of starch and protein. If the acidity of the starch milk is too high, starch and protein precipitate at the same time, making the starch separation unclear. If the acidity is too low, then both protein and starch precipitate poorly, forming an emulsion that cannot be separated. According to production experience, the optimal pH value of the sour slurry is 3.6-4.0. The concentration of the starch milk in the large vat is 3.5-4.0 Baume degrees, and the amount of sour slurry added is 2% of the starch milk. After adding the sour slurry, the pH value of the starch milk is 5.6. If the temperature is high and fermentation is fast, the amount of sour slurry used may be reduced as appropriate.

(6) Skimming the vat and settling the vat: After slurry blending, let it stand for about 20-30 minutes to allow precipitation to complete, and then the vat can be skimmed. Remove the upper clear swill water and the mixed liquid of protein, fiber and a small amount of starch; what remains in the bottom layer is starch. Add water to the bottom-layer starch after skimming the vat and mix it to adjust it into starch milk so that the starch precipitates again. The sour slurry fermentation that occurs during the precipitation process is called settling the vat. During settling the vat, temperature and time should be controlled. The temperature for settling the vat is about 20°C. When the weather is cold, insulation must be used, or hot water can be used when adding water and mixing. The fermentation during settling the vat must be thorough, and during the fermentation process it should be stirred appropriately to promote completion of fermentation. Generally, the time for settling the vat is 24 hours, and in hot weather it can be shortened accordingly. After fermentation is complete, the starch precipitates.

(7) Skimming the slurry and filtering: The sour slurry produced by settling the vat is called second-grade slurry, which is the main sour slurry used in the sour slurry method. Normally fermented sour slurry has a fresh fragrance, and the slurry color is as white as milk. If the sour slurry is insufficiently fermented or over-fermented, both its color and fragrance are poor, and the effect when used for slurry blending is not good. Skimming the slurry means skimming off the upper sour slurry for use in slurry blending. The starch after skimming the slurry is screened with a fine sieve with 120 mesh openings. The material retained on the sieve is fine residue, which can be used as feed. The material passing through the sieve is starch, which is transferred to a small vat. After the starch is transferred to the small vat, add water to rinse the starch, and it needs to stand for about 24 hours to prevent fermentation from occurring.

(8) Removing the starch: After the starch precipitates in the small vat, the upper liquid is small slurry, which can be used together with the sour slurry, or used as grinding water. After skimming off the small slurry, a layer of grayish-white oil starch remains on the surface of the starch, which is impure starch containing protein. The oil starch can be washed off the starch surface with water, and the wash liquid can be used as nutrient material for culturing sour slurry. The bottom-layer starch is taken out with a shovel. Fine sand may adhere to the bottom of the starch, and it should be brushed off.

(9) Drying: Through the above process, wet starch is obtained. To facilitate storage and transportation, it must be dried. Generally, it is sun-dried or sent to a drying room for oven drying.

2. Starch production from dried sweet potatoes

The process flow is as follows: raw material pretreatment - soaking - crushing and grinding - sieving - chute separation - alkali and acid treatment and washing - centrifugal separation and drying.

The production process is as follows:

(1) Raw material pretreatment: After fresh sweet potatoes are harvested, they are usually cut into slices or strips, and after sun drying or fire drying, they are made into dried sweet potatoes. This kind of dried sweet potato inevitably mixes with various impurities during processing and transportation, so it must undergo pretreatment. There are two types of pretreatment: dry method and wet method. The dry method uses screening equipment and air separation equipment. The wet method uses a washing machine or washing tub.

(2) Soaking: To increase the starch yield, alkaline water soaking can be used. Generally, saturated lime milk or 1% dilute alkali solution is added to the soaking water to make the pH value 10-11. The soaking time is about 12 hours, and the temperature is controlled at 35-40°C. After soaking, the water content of the sweet potato slices is 60%.

(3) Crushing and grinding: The soaked sweet potato slices enter a hammer-type crusher with water for crushing, reaching a certain fineness, and are discharged from the machine through the sieve openings. During the crushing process, the instantaneous temperature of the dried sweet potatoes rises, and part of the starch is easily gelatinized by heat, so that during sieving it affects the separation of starch and sweet potato residue; during chute separation it is also not easy to precipitate, resulting in an increase in secondary flour and affecting the yield of good flour. To prevent one-time crushing treatment from easily causing starch gelatinization, a process flow of two-stage crushing with separate sieving for each stage can be used. That is, the dried sweet potatoes undergo first-stage crushing, are sieved, then undergo second-stage crushing treatment, and are sieved again. During the crushing process, in order to reduce the instantaneous temperature rise, adjust the flour slurry concentration according to the different crushing fineness of each stage: 3-3.5 Baume degrees for the first stage and 2-2.5 Baume degrees for the second stage. At the same time, a material equalizer is used to control the feed amount of dried sweet potatoes, balance the flour slurry, avoid overload of the crusher, and also facilitate chute separation.

(4) Sieving: The sweet potato paste, also called material liquid, obtained after the dried sweet potatoes are crushed must be sieved to separate out the residue, namely fiber. A flat shaking sieve is usually used. The material liquid enters the sieve surface and is required to pass through the sieve evenly, with continuous water spraying. The starch passes through the sieve openings with the water into the slurry storage tank, while the residue remains on the sieve surface and is discharged from the tail of the sieve. The sieve cloth is 120 mesh nylon cloth. During the sieving process, because the material liquid contains viscous substances such as pectin that remain on the sieve surface, the separation effect of the sieve is affected. Therefore, the sieve cloth should be washed and brushed frequently to ensure the sieve openings remain unobstructed.