The microbial fermentation process is divided into aerobic fermentation and anaerobic fermentation based on the requirements of fermentation conditions. Aerobic fermentation methods include surface culture fermentation on liquid, surface fermentation on porous or granular solid media, and oxygenated deep fermentation. Anaerobic fermentation uses deep fermentation without oxygen supply. Therefore, both aerobic and anaerobic fermentation can be achieved through deep culture, which is carried out in cylindrical fermentation tanks with a certain diameter-to-height ratio. According to the operation methods, they can be classified as follows.
1. Batch operation
The substrate is loaded into the tank at one time, inoculated under suitable conditions, and the reaction proceeds. After a certain period, all the reaction products are removed.
2. Fed-batchoperation
Also known as fed-batch operation. It refers to loading a certain amount of substrate into the tank, inoculating under suitable conditions to start the reaction. During the reaction process, specific limiting substrates are continuously added to the reactor to maintain the concentration of limiting substrates within a certain range. At the end of the reaction, all the reaction products are removed.
3. Repeated batch operation
After the batch operation is completed, part of the reaction system is removed, and the remaining part is reloaded with substrate and then operated in batch mode again.
4. Repeated fed-batch operation
After the fed-batch operation is completed, part of the reaction system is removed, and the remaining part is reloaded with a certain amount of substrate and then operated in fed-batch mode again.
5. Continuous operation
After the reaction starts, the substrate is continuously supplied to the reactor, and the reaction liquid is continuously removed at the same time, keeping the reaction process in a stable state and the reaction conditions unchanged over time.
1. Alcoholic beverages
Wine, beer, spirits and other alcoholic beverages are all produced by fermenting glucose into alcohol under anaerobic conditions using brewer's yeast. Spirits are distilled, so the main components of spirits are water and alcohol, as well as some volatile substances that are easily evaporated when heated, such as various esters, other alcohols and a small amount of low-carbon aldehydes and ketones. Wine and beer are non-distilled alcoholic beverages. During fermentation, yeast converts glucose in the fruit juice or fermentation liquid into alcohol, and some of the other nutrients are utilized by the yeast, generating some metabolic products such as amino acids and vitamins, which also enter the fermented wine or beer. Therefore, wine and beer have relatively high nutritional value.
2. Soy sauce
The main raw materials for soy sauce production are soybeans, along with wheat bran, wheat, corn, etc. These raw materials are crushed to form a solid culture medium. Under aerobic conditions, molds that produce protease, such as Aspergillus niger, are used for fermentation. During the growth of microorganisms, a large amount of protease is produced, which hydrolyzes the proteins in the culture medium into small peptides and amino acids. Then, the liquid is washed and adjusted to form the soy sauce product. Therefore, the fermented soy sauce is rich in amino acids and peptides and has a special flavor.
3. Vinegar
The vinegar consumed in daily life, except for white vinegar which is made by blending food-grade acetic acid chemically synthesized, is all produced by acetic acid bacteria under aerobic conditions, converting the alcohol produced by solid-state fermentation into acetic acid. The different flavors of vinegar are due to the differences in the microbial strains or starters used. For instance, lactic acid or other organic acids may be produced during the glucose fermentation process, thus giving vinegar different tastes.
4. Yogurt
Yogurt is produced when milk is fermented by lactic acid bacteria under anaerobic conditions. The bacteria break down lactose and further ferment it to produce lactic acid, other organic acids, aromatic substances, vitamins, and partially hydrolyze proteins. Therefore, yogurt is a nutritious, easily digestible food with less lactose, making it an excellent choice for people with lactose intolerance.
5. Bread
Bread is made by activating active dry yeast, mixing it with flour for fermentation, and then adding various additives before baking. After the flour ferments, the structure of the starch changes, making it easier to digest and more nutritious. Sweet fermented rice, also known as rice wine, is a food product made by fermenting steamed rice with Rhizopus in aerobic conditions. It contains low alcohol concentration and various sugars. Rhizopus produces a large amount of amylase during its growth, which hydrolyzes the starch in rice into glucose. At the same time, it ferments part of the glucose to produce alcohol. Due to the use of different Rhizopus strains, sweet fermented rice with different alcohol content, sweetness and aroma can be produced.
7. Tea Categories
In the fermentation process of tea-based foods, the rational application of microbial fermentation technology can enhance the quality of tea. Take Pu'er tea as an example. Yeast plays a promoting role in the fermentation process of Pu'er tea, which can improve the efficiency and reliability of fermentation. During the yeast fermentation process, many active substances are produced, which in turn affect the quality of Pu'er tea.
8. Legume foods
During the microbial fermentation of legume foods, microorganisms can transform the fibrous substances in legumes into soluble substances, effectively exerting the fermentation function of microorganisms.
9. Enzyme
Enzyme is a product with specific biological activity obtained through microbial fermentation using raw materials such as animals, plants, and fungi.
Research has found that microbial fermentation technology can improve the special flavors of some plant or animal foods. For instance, after the second fermentation by lactic acid bacteria and yeast, the fishy smell of licorice can be removed, making the licorice extract taste sour and refreshing with a wine aroma. In addition, microbial fermentation can affect the content, composition and form of nutrients such as polysaccharides, proteins and fats in food to varying degrees. Meanwhile, for amino acids, total flavonoids and lipids, certain biological transformations can enhance their nutritional value and make their chemical composition more balanced.
With the continuous increase in consumers' demands for food functions, microbial fermentation technology is also constantly developing. Targeted control of food quality will continue to be a research direction for microbial fermentation.