Methods and Key Technical Points for Making Alcohol from Sweet Potato

Alcohol, also called ethanol, is an important chemical raw material. It can be used to make various organic reagents and solvents, and is also a raw material for making dyes, coatings, medicines, spices, synthetic materials, detergents, and more.

1. Process flow: making koji - making yeast starter - cooking and gelatinization - saccharification and fermentation - distillation

2. Key points of process operation:

① Making koji: The saccharifying agent used in this process is mostly cultured on a solid medium. Add 5%-10% rice husk or fine rice bran to wheat bran as the medium, and culture Aspergillus niger or Aspergillus flavus as the saccharifying agent. This is called the bran koji method. The Aspergillus in the bran koji method is generally expanded and cultured step by step through several stages, including test tube culture, petri dish culture, seed koji making, and koji making, until it meets production needs.

Test tube culture: Commonly used Aspergillus niger strains (such as Aspergillus niger Qingyan No. 2) are provided by specialized research institutes. There are many kinds of media used for test tube culture, and millet medium can be used. Dispense the millet medium into cleaned and sterilized test tubes, about 10 ml per tube, and steam and sterilize three times at intervals of 1 day and night each time, steaming for 1 hour each time. After the last sterilization, place the test tubes slanted overnight, use an inoculation needle to inoculate a small amount of Aspergillus niger strain, and at a temperature of 30-32°C, it will grow vigorously after 5-7 days. After passing inspection, it can be provided for expanded culture.

Petri dish culture: This can be done in petri dishes or Erlenmeyer flasks. The medium can be made from 5%-10% rice husk or fine rice bran. Soak wheat bran and rice husk in 1-1.2 times their amount of water for 2 hours, steam in a pot for 30-40 minutes, then respectively put into sterilized petri dishes or Erlenmeyer flasks, with a material thickness of 0.2-0.3 cm, and continue steaming for 30 minutes to form the medium. When the medium cools to below 40°C, in a sterile room use an inoculation needle to inoculate a small amount of Aspergillus niger cultured in test tubes, stir thoroughly, place in a constant-temperature incubator, and culture at 30-32°C for about 3-5 days, and the strain will grow vigorously. After passing inspection, pour out the water in the petri dishes or Erlenmeyer flasks and dry at 35-40°C. After drying, store in a cool, dry place for later use.

Making seed koji: Seed koji, also called starter koji, is the Aspergillus strain used for making koji. Seed koji should be made in a dedicated seed koji room. One day before inoculation, first place the koji trays and tools in the room, seal it, burn formaldehyde for chemical sterilization, and after about 1 day and night open the skylight to let the chemical gas escape. The raw materials for making seed koji can be wheat bran, rice, millet, sorghum, and others. A commonly used formula is: wheat bran 70%-80%, rice husk or rice bran 10%, fresh distiller's grains with about 60% moisture 20%-30%. Add water to the raw materials and mix well, with the amount of added water being 75%-85% of the material amount, leave to moisten for 1 hour, sieve, then steam in a pot for 45-50 minutes. After steaming, remove from the pot, cool to 37-38°C, inoculate with the strain cultured in petri dishes at an inoculation amount of 0.1%-0.2%, and shovel evenly, pile at 32-35°C for 4-6 hours, turning and mixing once in the middle. Then divide the material into koji trays, with a thickness of about 1 cm, and stack the koji trays at a temperature of about 30°C. At this time the material temperature is 28-30°C. After 5-7 hours, the material temperature gradually rises to 32-34°C, and at this time the stacking form of the koji trays should be changed. After that, break up the caked koji material three times every 3-4 hours, and keep the material temperature not exceeding 35-36°C. From inoculation, after 48-50 hours, the spores have grown, and the material temperature begins to drop or no longer rises, so the room temperature can be raised, the skylight of the koji room opened, and drying carried out at 35-40°C for 5-6 hours. After drying, store the seed koji in a cool, dry place for later use.

Making koji: Koji is a saccharifying agent that can be directly put into production use. Making koji should be carried out in a dedicated koji room. The raw materials for making koji are generally mixed with 5%-10% rice husk or wheat bran, and may also be mixed with 20%-30% fresh distiller's grains. During production, add water to the raw materials and mix well. The amount of added water should be determined by the moisture content of the wheat bran and distiller's grains, and can generally be calculated using the following formula.

Weight of added water = weight of raw materials * (required moisture % - raw material moisture %) / (1 - required moisture %)

The required moisture content is generally 45%-50%. Sieve the material mixed evenly with water, leave for 1 hour to let the water soak through the raw materials, then steam in a pot for 45-50 minutes, take out and sieve, spread out and cool to 37-39°C, inoculate with seed koji at an inoculation amount of 0.3%-0.5%, and mix well. The inoculated koji material is sent into the koji room. Pile at 28-30°C for 4-6 hours, and turn and mix once. Then pile again for 1-2 hours, mix well, and respectively put into koji trays with a thickness of 2-2.5 cm. Then stack each tray into a square column shape. After 4-5 hours in the trays, the material temperature can rise to 36-37°C, and at this time carry out tray pulling. So-called tray pulling is to pull apart the koji trays stacked in a square column shape to a certain distance, forming a pinwheel shape, to facilitate air circulation and discharge the carbon dioxide produced by Aspergillus. After 5-7 hours of tray pulling, the material temperature rises rapidly, reaching 41-42°C, and at this time carry out tray turning. So-called tray turning is to invert an empty koji tray onto a koji tray containing koji material, then flip them mutually so that the koji material falls from the original tray into the empty tray, so that the koji material originally pressed underneath grows Aspergillus. After tray turning, the koji trays are still stacked in a pinwheel shape. According to changes in material temperature, adjust the positions of the koji trays in a timely manner to keep the material temperature in each tray basically consistent. During the koji-making process, the temperature of the koji room should be maintained at 28-33°C. Generally, from the start of inoculation, after 40-45 hours, the koji can be taken out. The finished bran koji should be put into production use immediately and should not be stored for a long time.

② Making yeast starter: The expanded culture of yeast is generally divided into stages such as test tube culture, Erlenmeyer flask culture, Carlsberg flask culture, and making yeast starter.

Test tube culture: First soak barley in cold water for 24 hours, changing the water three times in the middle, then spread it on a bamboo curtain or in a bamboo sieve and let it germinate at room temperature. When the leaf bud length is equal to the barley grain, crush it, add 1.5-2 times water, saccharify at 60°C for 1-3 hours, then filter with a cloth bag. Adjust the sugar degree to 12-13 degrees, add sulfuric acid to adjust the pH to 0.2-0.4, then respectively put the sugar solution into test tubes, about 5 ml per tube, steam and sterilize for about 1 hour, sterilizing once every day and night, for a total of three times. For the last sterilization, leave overnight, and at the same time aseptically inoculate a small amount of yeast cultured in test tubes, and culture at 25-26°C for 22-24 hours. After passing inspection, it can be provided for expanded culture.

Erlenmeyer flask culture: This can be done in Erlenmeyer flasks and flasks. The culture liquid used and the culture method are the same as for test tube culture, except that the expansion culture multiple is 8-10.

Carlsberg flask culture: First stir sweet potato flour with 2 times its amount of water into a slurry, then put water equal to 3 times the weight of the sweet potato flour into a pot and boil. At this time pour in the sweet potato flour slurry, stir and steam for 40 minutes. After discharging, filter with coarse cloth to obtain gelatinized mash. Put the gelatinized mash into a sterilized Carlsberg flask, add seed koji at 8%-10% of the material amount, and saccharify at 60°C for 1-3 hours. Adjust the sugar degree to 12-19 degrees and the pH to 0.3-0.35. Then sterilize in steam or boiling water for 1 hour. After cooling, inoculate with yeast cultured in Erlenmeyer flasks at an inoculation amount of 7%-10%. Culture at 27-28°C for 20-22 hours, stirring once every 4-5 hours. When it grows vigorously and passes inspection, it can be used for making yeast starter.

Making yeast starter: Mix sweet potato flour, fresh distiller's grains, rice husk, and water in a ratio of 100:20:5:50, sift out coarse particles, moisten for 3-4 hours, and steam in a pot for 40 minutes. Then divide the material into sterilized yeast starter culture tanks, add water, and add acid to adjust the pH to about 4.3, then add seed koji at 8%-10% of the material amount, and mix well. When the material temperature is 25-27°C, inoculate with the yeast mash cultured in the Carlsberg flask at an inoculation amount of 5%-7%, and culture at 25-26°C for 8-10 hours. After the yeast starter matures, it can serve as the yeast for production use.

③ Cooking and gelatinization: Crush dried sweet potato, sieve it, add water to adjust it into a 13%-15% slurry, and steam in a pot. The steaming time is related to the strength of the fire and is often judged by experience. Generally, it can be steamed through in about 40 minutes to obtain gelatinized mash.

④ Saccharification and fermentation: Bran koji and yeast starter can be inoculated into the gelatinized mash at the same time. When the gelatinized mash is cooled to 20°C, add 8%-10% bran koji and 3%-5% yeast starter and mix well, and add water according to the ratio of 60%-65% moisture in the material. Under the condition of not exceeding 20°C, put the material into ceramic vats, fermentation tanks, or cellars, seal the opening, and carry out saccharification and fermentation. During the fermentation process, the material temperature gradually rises, generally to 28-35°C, and can reach as high as 40-42°C, with moisture maintained at about 65%. The fermentation time is determined by multiple factors such as the temperature at which it enters the tank, air temperature, heat dissipation of the material, degree of raw material crushing, and starch concentration. Generally, fermentation can be completed in about 3 days. After fermentation, discharge the material and send it for distillation.

⑤ Distillation: Separating refined ethanol from the crude product is carried out in a batch distillation tower. Put the fermentation mash into the distillation still, heat with indirect steam and direct steam, and after boiling keep the material liquid in a boiling state so that volatile components including ethanol vaporize. The steam escapes from the top of the tower and enters the partial condenser to condense into liquid. In the early stage of distillation, all the condensate flows back from the top of the tower into the tower (called tower sealing in production). Its main purpose is to concentrate ethanol. The temperature of the reflux liquid flowing out of the partial condenser should not be lower than 50-56°C, so as not to disrupt the operation of the distillation tower. After the condenser has been sealed in the tower for 1.5-3 hours, ethanol begins to distill out. What distills out first are head-level impurities with a boiling point lower than ethanol, followed by finished ethanol, and finally tail-level impurities with a boiling point higher than ethanol. These should all be separately collected in different containers. When the ethanol fraction has finished distilling, heating can be stopped. After the material liquid cools, open the residual liquid discharge valve at the bottom of the still, release the residual liquid, clean the tower, still, partial condenser, condenser, and other equipment with clean water, then reload the material and carry out the next distillation.