What are the sweet potato processing techniques?

Sweet potato can be divided into two kinds of fresh sweet potato and dried sweet potato. Fresh sweet potato production starch generally belongs to the rural handmade production, while most of the sweet potato starch factory is dried sweet potato as starch raw material, so it belongs to industrial production.

1. Fresh sweet potato starch production

The process is as follows: raw material selection - washing - crushing - grinding and filtering - slurry - skimming cylinder and sitting cylinder - skimming slurry - powdering - drying.

The production process is as follows:

(1)Selection of raw materials: Due to different varieties of sweet potato, its quality and starch content are also different, even if the same variety, in different places of origin, its quality is also very different. To choose a good processing starch varieties, requires high starch content, with the disease of sweet potato is not only unsuitable for starch processing raw materials, and in the storage will be infected to other pieces of potatoes, easy to rot and cause losses, so the diseased potatoes should be removed clean.

(2) washing: fresh potatoes are poured into the tank with water, and washed by hand, removed after washing, and drained off the remaining water.

(3) crushing: after draining the fresh potatoes with a crusher

Broken into pieces, the size of the block is 2 cm or less, in order to facilitate the mill.

(4) grinding and filtering: this is the main link of sweet potato starch production, affecting product quality and starch output rate. Fresh potato pieces will be sent to the stone mill or macadamia mill with water grinding into potato paste, the weight of fresh potatoes and the proportion of water is 1:3-3.5. then the potato paste dumped into the aperture of 60 mesh in the sieve for filtration.

(5)Mixing: the starch milk obtained by filtration is put in a big tank, then add acid pulp and water to adjust the acidity and concentration of starch milk according to the proportion. The acidity and concentration of starch milk is closely related to the precipitation of starch and protein. If the acidity of starch milk is too large, the starch and protein will precipitate at the same time, making the starch separation unclear. If the acidity is too small, the protein and starch will not be well precipitated, and the starch will be emulsion, which can not be separated. According to the production experience, the best ph value of acid pulp is 3.6-4.0. the concentration of starch milk in the vat is 3.5-4.0 boe, the amount of acid pulp is 2% of the starch milk, and the ph value of starch milk after adding acid pulp is 5.6. if the temperature is high, the fermentation is fast, and the dosage of acid pulp can be reduced accordingly.

(6) skimming and sitting tank: about 20-30 minutes after the slurry, so that the precipitation is complete, you can skim the tank. The upper layer of clear slop and protein, fiber and a small amount of starch mixture removed, leaving the bottom layer of starch. After skimming the bottom layer of starch in the tank, add water to mix, make starch milk, so that the starch precipitation again. In the process of precipitation, it is called acid pulp fermentation, which is called sitting in the tank. When sitting in the tank, the temperature and time should be controlled. Sitting temperature is about 20 degrees Celsius. When it is cold, it must be kept warm or hot water can be used when adding water to mix. Sitting tank fermentation must be hair through, in the fermentation process, appropriate stirring, to promote the completion of fermentation. General sitting time is 24 hours, hot days can be shortened accordingly. Fermentation is complete, starch precipitation.

(7)Skimming and filtering: The sour pulp generated by sitting in the tank is known as the second and the pulp, that is, the main use of sour pulp in the sour pulp method. Fermentation of normal sour pulp has a clear flavor, pulp color white as milk. If the fermentation is insufficient or over-fermented, the color and aroma are poor, and the effect is not good when used for blending. Skimming is to skim out the upper layer of sour pulp as a pulp. The starch after skimming is sieved with a fine sieve of 120 holes. The upper part of the sieve is fine residue, which can be used as feed. The bottom of the sieve is starch, which is transferred to the small tank. After the starch is transferred to the small tank, water is added to rinse the starch, which needs to be left for about 24 hours to prevent fermentation.

(8) starter: after the starch settles in the small tank, the upper liquid layer is a small slurry, which can be used in conjunction with acid slurry or as water for grinding. After skimming off the small pulp, there is a layer of gray oil powder on the surface of starch, which is impure starch containing protein. Oil powder can be washed away from the surface of starch with water, and the washed out liquid can be used as the nutrient material for cultivating acid slurry. The bottom layer of starch can be taken out with a spatula, and the bottom of starch may be adhered by fine sand, which should be brushed away.

(9)Drying: After the above process, wet starch is obtained. In order to facilitate storage and transportation must be dried. Generally use sunlight drying or sent to the drying room drying.

2. Dry sweet potato starch production

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

The production process is as follows:

(1) Raw material pretreatment: after fresh sweet potato is harvested, it is usually cut into slices or shreds, and made into dried sweet potato after sun or fire drying. This kind of dried sweet potato is inevitably mixed with various impurities during processing and transportation, so it must be pretreated. There are two kinds of pretreatment: dry method and wet method. Dry method is to use screening equipment and wind selection equipment. Wet processing is to use washing machine or washing drum.

(2) soaking: in order to improve the starch yield can be used alkaline water soaking. Generally saturated milk of lime or 1% dilute lye added to the soaking water, so that the ph value of 10-11. soaking time of about 12 hours, the temperature is controlled at 35-40 degrees Celsius. After soaking, the water content of sweet potato chips is 60%.

(3) crushing and grinding: soaked sweet potato chips enter the hammer mill with water for crushing to reach a certain fineness, and are discharged out of the machine through the sieve holes. Dried potatoes in the crushing process in the instantaneous temperature rise, part of the starch is easy to heat paste, so that in the sieve, affecting the separation of starch and potato residue; separation in the flow tank is not easy to precipitate, resulting in an increase in the second powder, affecting the output rate of good powder. In order to prevent a crushing process is easy to make starch paste, you can use two crushing, subway sieving process. That is, dried potatoes by the first crushing, sieving, and then by the second crushing process, and then sieving. In the crushing process, in order to low transient temperature rise, according to the different particle size of each channel crushing, adjust the concentration of slurry, the first channel for 3-3.5 degrees boe, the second channel for 2-2.5 degrees boe. At the same time the use of equalizer to control the amount of dried sweet potato feed, balanced slurry, to avoid overloading of the crusher phenomenon, but also conducive to the separation of the flow channel.

(4) sieve: sweet potato paste obtained after crushing the dried sweet potato, also known as liquid, must be sieved, separated from the slag, that is, fiber. Usually use flat shaking sieve, liquid material into the sieve surface, the requirement of uniform sieve, constantly dripping, starch with water through the sieve holes into the slurry pool, and the residue retained in the sieve surface from the end of the sieve discharged. The sieve cloth is 120 mesh nylon cloth. In the sieving process, due to the liquid contains pectin and other viscous substances retained in the sieve surface, affecting the separation effect of the sieve, therefore, the sieve cloth should be frequently washed to ensure that the sieve holes are smooth.

(5) flow trough separation: after fine sieve treatment of starch, the current general use of flow trough separation of protein. Its advantages are easy to build, less steel, save kinetic energy, operation is also more stable. Disadvantages are large area, intermittent operation, low separation efficiency, starch loss, poor labor conditions.

(6) alkali, acid treatment and cleaning: in order to further improve the purity of starch, in the process of cleaning starch, but also after alkali, acid treatment. Starch alkali, acid treatment and cleaning are carried out in the cement pool with stirring device, the size of the cement pool according to the production capacity to determine the speed of the stirrer for 60 rpm or so. Starch milk from the flow tank, first by the alkali treatment, the purpose is to remove the starch in the alkali soluble protein and pectin and other impurities. The method of alkali treatment is a degree of dilute lye, slowly added to the starch milk, control ph value of 12, while starting the stirrer, fully mixed, after half an hour, stop stirring, to be completely precipitated starch, discharge the upper layer of the waste liquid, and injected into the water clear twice, so that it is close to neutral can be. In the process of alkali treatment, you can also add sodium chlorate liquid with a concentration of 35 boe, the dosage is not more than 0.4% of the weight of dry starch. Because sodium hypochlorite is a strong oxidizing agent, with strong bleaching and bactericidal effect, so that it can achieve the purpose of whitening and antiseptic. After alkali, sodium hypochlorite treatment and cleaning, the starch is then treated with acid. Its purpose is to dissolve acid-soluble proteins, neutralize the alkaline residue of alkali treatment, and also inhibit microbial activity and reproduction. Acid treatment can be used industrial hydrochloric acid, the operation must also be added slowly, fully stirred, control the ph value of starch milk is about 3. Prevent localized over-acid, resulting in starch loss. After adding acid starch milk with alkali treatment of the same operation method to make starch precipitation, remove the upper layer of waste liquid, add water to clean, and finally make the starch is slightly acidic ph value of 6 or so, in order to facilitate the storage and transportation of starch.

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

3. Processing of sweet potato vermicelli

The process of processing sweet potato vermicelli: starch - pulping - powder mixing - powder leakage - cooling, bleaching - freezing - drying - finished product.

(1) pulping: first a small amount of starch with hot water into a thin paste, and then use boiling water to wash people adjusted thin powder paste, and constantly in a direction of rapid stirring, until the paste becomes thick, transparent, uniform, that is, the powder gravy. Make 100 kilograms of dry sweet potato vermicelli, need to use 300 grams of alum, 35 kilograms of boiling water, used to beat the dry starch about 3 kilograms.

(2) adjust the powder: first add 0.5% alum in the powder gravy, mix well and then mix the wet starch and powder gravy, stirring and kneading to no lumps, non-sticky hands, can pull the soft powder can be. Leak powder before you can try to see if the powder is appropriate, such as leakage of the fan is not thick, not fine, not constant that is appropriate. If the next bar is too fast, the phenomenon of broken bars, said that the batter is too thin, should be mixed with dry starch and then kneaded, so that the toughness of the noodles moderate; as the next bar is difficult or too slow, coarse and fine and uneven, the table batter is too dry, you should add some wet starch. It is better to mix the flour at one time. The temperature of the dough is 30-42 degrees Celsius.

(3) leakage of powder: the kneaded powder in the scoop with small holes in the funnel, the funnel hole diameter of 7.5 millimeters, fan fineness 0.6-0.8 millimeters. Squeeze the powder inside the scoop with your hand, through the small hole, the powder is leaking down into the fan. From the leakage scoop below 55-65 centimeters to put a pot of boiling water, fans fall into the pot of boiling water, heat solidification cooking. The temperature of the water should be kept at 97-98 degrees Celsius, boiling water will spoil the vermicelli. When the vermicelli is leaking, use bamboo chopsticks to stir the vermicelli in the pot to prevent the vermicelli from sticking to the bottom of the pot. After the raw vermicelli is leaked into the pot, control the time and master the fire. Cooking time is too short, the fans are not cooked; cooking time is too long, it is easy to rise and paste, make the fans brittle.

(4) cooling, bleaching: fans fall into the boiling water pot; to be about to float, with a small bamboo pole launched, pulled to the cold water tank cooling, the purpose is to increase the elasticity of the fans. After cooling, and then use the bamboo pole around the bundle, put people in the sour milk immersed in 3-4 minutes, pick up cool, and then bleached with water, and rubbed away from each other sticking to rhyme with the fans. The purpose of soaking in sour milk is to bleach away the pigment on the vermicelli, remove the stickiness and increase the smoothness. In order to make fans white, but also available sulfur dioxide fumigation bleaching. Sulfur dioxide can be made by igniting sulfur blocks, fumigation can be carried out in a closed room.

(5) freezing: sweet potato vermicelli bonding strong, poor toughness, so need to freeze. Freezing temperature is minus 8 degrees Celsius to minus 10 degrees Celsius, to reach all the ice until. Then, the fans will be put in 30-40 degrees Celsius water to make it dissolve, with the hand pulling and rubbing, so that all the fans into a monofilament scattered, placed on the rack to dry.

(6) drying: drying racks should be placed in the open sun field, drying should be gently shaking the fan, so that it is uniformly dry, drying can be packaged into bags. Finished sweet potato vermicelli should be white in color, no visible impurities, dry and brittle silk, moisture does not exceed 2%, no odor, cooking and processing has a good toughness, not easy to break, with the unique flavor of sweet potato vermicelli.