What are the sweet potato processing technologies?

Sweet potato can be divided into fresh sweet potato and dried sweet potato. Starch production from fresh sweet potatoes generally belongs to rural manual production, and sweet potato starch factories mostly use dry sweet potatoes as starch raw materials, so it belongs to industrial production.

1. Production of fresh sweet potato starch

The technological process is as follows: raw material selection-water washing-crushing-grinding and filtering-pulp mixing-skimming-grinding-drying.

The production process is as follows:

(1) Selection of raw materials: Due to different sweet potato varieties, their quality and starch content are also different, even if the same variety is in different producing areas, its quality is also very different. In order to choose a good variety of processed starch, it is required that sweet potatoes with high starch content and diseases are not only unsuitable for starch processing raw materials, but also spread to other potato pieces during storage, which are easy to rot and cause losses, so it is necessary to remove the diseased potato pieces.

(2) Water washing: pour fresh potatoes into a vat, add clear water, turn them over manually, take them out after washing, and drain the remaining water.

(3) Crushing: the fresh potatoes drained are crushed by a crusher.

Break into small pieces, the size is less than 2 cm, so as to facilitate grinding.

(4) Grinding and filtering: this is the main link in the production of sweet potato starch, which affects the product quality and starch yield. Sending fresh potato pieces into a stone mill or emery mill, adding water and grinding into mashed potatoes, wherein the ratio of fresh potato weight to water is 1: 3-3.5. Then pour the mashed potatoes into a sieve with a pore size of 60 meshes and filter.

(5) Sizing: put the filtered starch milk into a vat, and then add acid pulp and water in proportion to adjust the acidity and concentration of the starch milk. The acidity and concentration of starch milk are closely related to the precipitation of starch and protein. If the content of lactic acid in starch is too large, starch and protein will precipitate at the same time, resulting in unclear separation of starch. If the acid is too small, both protein and starch do not precipitate well, and they are emulsified and cannot be separated. According to the production experience, the optimum ph value of acidic pulp is 3.6-4.0. The concentration of starch milk in the tank is 3.5-4.0 Baume, and the amount of acid slurry is 2% of starch milk. After adding acid pulp, the ph value of starch milk is 5.6. If the temperature is high and the fermentation is fast, the amount of acid slurry can be reduced appropriately.

(6) skimming and sitting in the tank: after mixing the slurry, let it stand for about 20-30 minutes to complete precipitation, and then skim and sit in the tank. Take out the mixed solution of clear swill, protein, fiber and a small amount of starch on the upper layer, leaving the starch on the bottom layer. After skimming, the bottom starch was added with water to make starch milk, which made the starch precipitate again. In the process of precipitation, it plays the role of acid slurry fermentation, which is called sitting in the tank. Control the temperature and time when sitting in the cylinder. The sitting temperature is about 20 degrees Celsius. It must be kept warm in cold weather or hot water can be used when adding water to stir. Tank fermentation must be fully developed and properly stirred in the fermentation process to promote the completion of fermentation. Generally, the sitting time is 24 hours, which can be shortened in hot weather. After fermentation, starch precipitates.

(7) Skimming filtration: The acid pulp produced by sitting in the tank is called bidirectional pulp, which is mainly used in the acid pulp method. The normal fermented sour pulp has a clear aroma and the color of the pulp is as white as milk. If the acid pulp is under-fermented or over-fermented, the color and fragrance are poor, and the effect is not good when it is used for pulp mixing. Skimming is skimming off the acid pulp above for pulping. After skimming the starch, sieve with a 120 mesh fine sieve. The screened material is fine slag, which can be used as feed. The undersize is starch, which is transferred to a small bucket. After the starch is transferred to a small bucket, it is washed with water and left for about 24 hours to prevent fermentation.

(8) Flocking: After starch is precipitated in a small tank, the upper liquid is small slurry, which can be used together with acid slurry or as grinding water. After skimming the starch, there is a layer of gray oily powder on the surface of the starch, which is impure starch containing protein. The oil powder on the surface of starch can be washed off with water, and the eluent can be used as a nutrient for cultivating acidic pulp. The bottom starch is taken out with a shovel, and fine sand may adhere to the bottom of the starch and should be brushed off.

(9) drying: wet starch is obtained through the above process. Drying is necessary to facilitate storage and transportation. Generally, it is dried or sent to a drying room for drying.

2. Dry sweet potato to produce starch

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

The production process is as follows:

(1) Pretreatment of raw materials: After fresh sweet potatoes are harvested, they are usually cut into thin slices or filaments, dried or roasted to make dried sweet potatoes. This kind of dried sweet potato will inevitably be mixed with various impurities during processing and transportation, so it must be pretreated. There are two kinds of pretreatment: dry method and wet method. The dry method adopts screening equipment and air separation equipment. Wet treatment is to use a washing machine or a bucket.

(2) Soaking: In order to improve the yield of starch, it can be soaked in alkaline water. Generally, saturated lime milk or 1% dilute lye is used to add soaking water to make the ph value10-1. The soaking time is about 12 hour, and the temperature is controlled at 35-40 degrees Celsius. The water content of soaked sweet potato chips is 60%.

(3) Crushing and grinding: the soaked sweet potato slices enter a hammer mill with water to be crushed to a certain fineness, and then are discharged out of the machine through a sieve hole. In the process of crushing, the instantaneous temperature of dried potato increases, and some starch is easy to gelatinize when heated, which affects the separation of starch and potato residue during screening. It is not easy to precipitate when it is separated in the launder, which leads to the increase of secondary powder and affects the yield of good powder. In order to prevent starch from being easily gelatinized after primary crushing, the technological process of twice crushing and sieving can be adopted. That is, the dried potatoes are crushed and sieved for the first time and crushed and sieved for the second time. In the crushing process, in order to reduce the instantaneous temperature rise, the slurry concentration is adjusted according to the different particle sizes of each pass, the first pass is 3-3.5 Baume, and the second pass is 2-2.5 Baume. At the same time, the homogenizer is used to control the feeding of dried sweet potatoes, balance the powder slurry, avoid the overload phenomenon of the pulverizer, and is also beneficial to the separation of the launder.

(4) Sieving: The sweet potato paste obtained by crushing the dry sweet potato is also called feed liquid, and it must be sieved to separate the dregs, that is, fibers. Usually, a flat shaking screen is used, and the feed liquid enters the screen surface, which requires uniform screening and continuous water addition. Starch enters the pulp storage tank through the sieve holes with water, while the dregs remain on the sieve surface and are discharged from the tail of the sieve. The screen is 120 mesh nylon cloth. In the screening process, sticky substances such as pectin stick to the screen surface, which affects the separation effect of the screen. Therefore, the screen should be cleaned frequently to ensure the smoothness of the mesh.

(5) Flow channel separation: At present, starch after fine screening is generally separated from protein through flow channels. Its advantages are convenient construction, less steel consumption, energy saving and stable operation. Disadvantages are large land occupation, intermittent operation, low separation efficiency, large starch loss and poor working conditions.

(6) Alkali and acid treatment and cleaning: In order to further improve the purity of starch, in the process of cleaning starch, alkali and acid treatment are needed. Alkali, acid treatment and cleaning of starch are all carried out in a cement pool with a stirring device. The size of the cement pool is determined according to the production capacity, and the rotation speed of the agitator is about 60 revolutions. Starch milk from the washing tank is first treated with alkali to remove impurities, such as alkali-soluble protein and pectin. The method of alkali treatment is to slowly add 1 Baume dilute lye into the starch milk, control the ph value to 12, start the agitator at the same time, fully mix evenly, stop stirring after half an hour, discharge the upper waste liquid after the starch is completely precipitated, and inject clean water twice to make it close to neutrality. In the process of alkali treatment, sodium chlorate solution with a concentration of 35 Baume degree can also be added, and the dosage does not exceed 0.4% of the weight of dry starch. Because sodium hypochlorite is a strong oxidant, it has strong bleaching and sterilization effects, thus achieving the purpose of whitening and anticorrosion. Starch treated and washed with alkali and sodium hypochlorite is then treated with acid. Its purpose is to dissolve acid-soluble protein, neutralize the residual alkalinity during alkali treatment, and also inhibit the activity and reproduction of microorganisms. Industrial hydrochloric acid can be used for acid treatment, and it must be slowly added and fully stirred during operation, and the ph value of starch milk should be controlled at about 3. Prevent local acid, resulting in starch loss. After adding acid to the starch milk, the precipitated starch was treated with alkali to remove the upper waste liquid, and then washed with water. Finally, the starch was slightly acidic, and the ph value was about 6, which was beneficial to the storage and transportation of starch.

(7) Centrifugal separation and drying: the water content of the starch obtained after cleaning is high, so it must be dehydrated by centrifugal separator to reduce the water content of the starch to below 45%. Some factories leave the factory directly or use it as wet starch. In order to facilitate storage and transportation, dehydrated wet starch needs to be dried to achieve the required starch water content. Usually, the moisture content of sweet potato starch is 12%- 13%. Starch can be dried by airflow, because airflow drying has the advantages of fast drying speed, high efficiency, large equipment production capacity and low equipment cost.

3. Processing sweet potato vermicelli

Processing technology of sweet potato vermicelli: starch-beating-blending-leakage-cooling-bleaching-freezing-drying-finished product.

(1) Beating: firstly, a small amount of starch is mixed into a thin paste with hot water, then the mixed thin paste is washed with boiling water, and continuously and rapidly stirred in one direction until it becomes sticky, transparent and uniform, that is, sticky. To make 100 kg dried sweet potato vermicelli, 300 g of alum, 35 kg of boiling water and about 3 kg of dry starch are needed for pulping.

(2) Powder mixing: firstly, add 0.5% alum into the starch paste, then mix the wet starch with the starch paste, stir and knead until soft and sticky, and brush. You can try it before it leaks to see if the flour dough is suitable. If the leaked vermicelli is neither coarse nor fine, it is appropriate. The following strips are too fast and broken, indicating that the starch paste is too thin, so it should be mixed and rubbed with dry starch to make the surface toughness moderate; It is difficult or too slow below, the thickness is uneven, the surface starch is too dry, and some wet starch should be added. It is advisable to mix powder once. The temperature of the dough is preferably 30-42 degrees Celsius.

(3) Powder leakage: put the kneaded dough into a colander with a small hole, the diameter of the colander is 7.5 mm, and the fineness of vermicelli is 0.6-0.8 mm. Squeeze the dough in the spoon by hand, and through the small hole, the dough will leak down and become vermicelli. Put a boiling pot 55-65 cm below the colander, put the vermicelli into the boiling pot, and heat, solidify and cook. The water temperature should be kept at 97-98 degrees Celsius. Boiling water will spoil the vermicelli. Stir with bamboo chopsticks in the pot to prevent the vermicelli from sticking to the bottom of the pot. After the raw vermicelli leaks into the pot, it is necessary to control the time and the heat. Cooking time is too short, and fans are unfamiliar; Cooked for too long, it is easy to swell and the vermicelli becomes brittle.

(4) cooling and bleaching: after the vermicelli falls into a boiling water pot; When it is about to float, start it with a small bamboo pole and pull it into a cold sink to cool it, in order to increase the elasticity of vermicelli. After air-cooling, wrap bamboo poles into bundles, soak them in acid pulp for 3-4 minutes, scoop them up and cool them thoroughly, then rinse them with clear water and rub off the vermicelli stuck together. The purpose of acid pulp soaking is to remove pigment, viscosity and smoothness from vermicelli. In order to whiten the vermicelli color, sulfur dioxide fumigation and bleaching can also be used. Sulfur dioxide can be prepared by igniting sulfur blocks, and fumigation can be carried out in a closed room.

(5) Freezing: Sweet potato vermicelli has strong cohesiveness and poor toughness, so it needs freezing. The freezing temperature is minus 8 degrees Celsius to minus 10 degrees Celsius until it is completely frozen. Then put the vermicelli into water at 30-40 degrees Celsius to melt it, rub it by hand to disperse all vermicelli into monofilaments, and put it on a shelf to dry.

(6) Drying: The drying rack should be placed in an open threshing field. When drying, gently shake off the vermicelli to make it dry evenly, and then bag it. The finished sweet potato vermicelli should be white in color, without visible impurities, crisp in silk, with no moisture exceeding 2% by weight and no peculiar smell. After cooking, it has good toughness and is not easy to break, and has the unique flavor of sweet potato vermicelli.