Processing Flow and Operating Methods for Making Vinegar from Sweet Potato

1. Process flow: raw material treatment -- adding bran koji and yeast culture -- saccharification and fermentation -- acetic acid fermentation -- adding salt and post-ripening -- leaching vinegar -- aging -- sterilization and blending -- finished product

2. Key points of process operation:

① Ingredients: 100 kg dried sweet potato, 50 kg coarse rice husk (rice chaff), 40 kg yeast culture, 175 kg fine rice husk (unified rice husk), 50 kg bran koji, 40 kg acetic acid bacteria seed, 3.75-7.5 kg salt.

② Raw material treatment: grind the dried sweet potato into powder. When mixing with the unified rice husk, add 275 kg water while mixing, so that the water and raw materials are evenly mixed and fully absorbed. After completion, put into a steamer and steam for 1 hour, then let it steep in the steamer for 1 hour. Steam until thoroughly cooked. After steaming is finished, take it out and place it on a clean mixing floor, sift to remove lumps, and at the same time turn and mix while using air circulation to cool.

③ Adding bran koji and yeast culture: in summer, cool the cooked material to 30-33 degrees Celsius; in winter, after cooling to below 40 degrees Celsius, sprinkle in cold water a second time, adding about 125 kg water, turn and mix once, then spread it flat again. Then spread the finely crushed bran koji on the surface layer of the cooked material, then evenly sprinkle on the uniformly stirred yeast culture (that is, yeast culture liquid), turn and mix evenly, and put it into a jar. The moisture of the vinegar mash after entering the jar is about 60%-62%.

④ Saccharification and fermentation: raw material saccharification and alcoholic fermentation are carried out simultaneously. After the vinegar mash enters the jar, spread it flat. Each jar holds about 160 kg. The mash temperature measured with a thermometer should be 24-28 degrees Celsius (in summer not exceeding 28 degrees Celsius, in winter not lower than 24 degrees Celsius). Cover the jar mouth with a straw lid, and the room temperature should be maintained at about 28 degrees Celsius. When the mash temperature rises to 38 degrees Celsius, turn the mash, and generally it should not exceed 40 degrees Celsius. The method of turning the mash is to leave one empty jar for every 10-20 jars, move the vinegar mash whose temperature has risen into the empty jar, then pour the next jar into the newly emptied jar, and turn it all once in sequence. After 5-8 hours, the mash temperature rises again to 38-39 degrees Celsius, and the mash is turned once more. After that, the normal mash temperature is between 38-40 degrees Celsius. After 48 hours, it gradually decreases, and the mash is turned once a day. By the 5th day, the mash temperature drops to 33-35 degrees Celsius, indicating that starch saccharification and alcoholic fermentation are complete. At this time, the alcohol content reaches about 8%.

⑤ Acetic acid fermentation: after the saccharification and alcoholic fermentation processes are finished, add 10 kg rice chaff to each jar (in summer when the temperature is high, the rice chaff can be reduced to 8 kg; in winter when the temperature is low and heating is slow, it can be increased to 12 kg), and 8 kg acetic acid bacteria seed. The mixing method adopts separate mixing of the upper and lower portions. First sprinkle half of the rice chaff and acetic acid bacteria seed into the jar, use both hands to mix the upper half of the mash evenly and pour it into another empty jar; then add the remaining half of the rice chaff and acetic acid bacteria seed into the lower half of the mash, mix evenly, and combine into one jar. On the 1st day, the mash temperature will not rise quickly; on the 2nd and 3rd days, it rises quickly. At this time, the mash temperature is kept at 39-41 degrees Celsius, generally not exceeding 42 degrees Celsius. Turn the mash once a day. After about 12 days, the mash temperature gradually decreases. Take samples daily to determine the acetic acid content. In winter, the acetic acid content is above 7.5%; in summer, above 7%. When the mash temperature drops to about 38 degrees Celsius, acetic acid fermentation is complete.

⑥ Adding salt for post-ripening: the purpose of adding salt is to inhibit the growth of acetic acid bacteria and avoid burning the mash. In summer, add 3 kg salt per jar; in winter, only 1.5 kg is needed. First sprinkle half of the salt onto the vinegar mash, use a shovel to turn and mix the upper half of the mash, and move it into another jar. The next day, mix the remaining half of the salt into the remaining half of the mash, mix evenly, and combine into one jar. After adding salt, leave it for 2 days for post-ripening, so that the alcohol and intermediate products that have not yet become acetic acid are further oxidized into acetic acid, while esterification proceeds to enhance aroma and color.

⑦ Leaching vinegar: so-called leaching vinegar means using water or thin vinegar liquid to dissolve out the acetic acid in the vinegar mash. To avoid turbidity of the acetic acid, pressing is generally not used for leaching vinegar. When leaching vinegar, place the vinegar mash on a vinegar tank or on the false bottom of a wooden barrel. First lay 1-2 layers of reed matting on the false bottom in advance, then add the thin vinegar liquid left from the previous leaching of vinegar, immerse the vinegar mash, soak for 1-2 hours, then open the drainage hole at the bottom of the vessel to obtain vinegar liquid. Repeat leaching with thin vinegar liquid in this way. Each time the thin vinegar liquid obtained is used for the next leaching of vinegar. After leaching vinegar, the standard is that the residual amount of acetic acid in the vinegar mash does not exceed 0.1%, and the acetic acid yield (calculated as 5% acetic acid) is about 6-10 times that of the dried sweet potato.

⑧ Aging: the purpose of aging is to improve the quality of the vinegar, making its color bright and its aroma mellow. Vinegar liquid with an acetic acid content below 5.5% is generally not aged, otherwise it is prone to spoilage. There are two methods for aging vinegar liquid. One is aging the vinegar mash: move the salted and matured vinegar mash into a jar, press it firmly, spread a layer of salt on top, cover it, and seal the top with mud. After standing for 15-20 days, turn the mash once, then seal the jar again, and usually after another 1 month, leach the vinegar. This method is only suitable for winter; in summer it should not be used because burning of the mash easily occurs. The other method is to put the leached vinegar liquid into a jar, cover the top with a lid, and uncover it once every 1-2 days. The time for uncovering is at night in summer and during the day in other seasons. The aging time is generally 30 days in summer and about 60 days in winter.

⑨ Sterilization and blending: put the vinegar liquid into a settling tank for sedimentation, adjust the quality standards. Except for products sold immediately and high-grade vinegar, which do not have preservatives added, ordinary edible vinegar is generally added with 0.1% sodium benzoate as a preservative. Raw vinegar needs to be heated for sterilization. The sterilization temperature is 80-90 degrees Celsius. Finally, it is quantitatively put into jars or bottles to become the finished product.