Humans have a long history of using lactic acid bacteria. Cows were domesticated in the Middle East around 10,000 years ago during Neolithic era, particularly in Mesopotamia. Archaeologists have also found that ancient people in the Middle East consumed fermented dairy products as early as 8,000 years ago. When milk became one of our foods, our ancestors always stored the surplus milk in clay pots. They discovered that milk tasted sour after a long time storage, and no diarrhea would occur. These cheeses were preserved from generation to generation until Joseph Lister successfully isolated lactic acid bacteria in 1878. This’s foundation for cultivating pure lactic acid bacteria and industrial production.
L-lactic acid fermentation process
They have two isomers, L or D-lactic acid. Only the nontoxic L-lactic acid can be used by ourselves, while the latter ones accumulate in our body to disrupt metabolism. Most of the lactic acid in modern industry comes from microbial fermentation. Heterofermentation is a combination of alcohol and lactic acid fermentation. In practice, less than half of saccharide is converted into lactic acid. Therefore, it’s not favored in industry. Homofermentation converts almost all the saccharide into lactic acid, so it is highly favored in manufacture.
Corn, wheat, and potato starch are raw materials for lactic acid fermentation. They’re hydrolyzed into glucose by enzyme or hydrochloric acid to improve efficiency in a step called saccharification. Lactic acid bacteria can’t synthesize certain amino acids and vitamins, so nitrogen sources, vitamins, growth promoters and trace elements are also necessary. Rice bran, corn steep liquor, and hair hydrolysate are common natural, cheap auxiliary materials.
The culture medium should fill 80% of tank. A space of 30-40 cm should be left at the top to prevent foam overflow. Excess lactic acid is neutralized by CaCO₃ to keep pH above 5, and they contribute most of CO₂ in the tank. They need to be released outside through exhaust valve. Fermentation is complete, when glucose concentration drops to 1 g/L within 1-3 days. Adding lime milk raises pH to around 10, and heating to 90°C sterilizes the mixture.
Calcium lactate crystals were produced by vacuum evaporation. Washing in a centrifuge avoids crystals contact with water for a long time. The crystals are dissolved by 50% sulfuric acid to obtain lactic acid solution again, which is evaporated under reduced pressure to regenerate pure crystals. Some extraction processes skip crystallization and sulfuric acid was added directly, and another companies use ion exchange methods. However, recrystallization is the most common and economical method.
L-lactic acid fermentation in Food, beverages
L-lactic acid fermentation is widely used in food and beverages, such as yogurt, cheese, sauerkraut, pickles and other fermented vegetables. It doesn’t produce toxins but also synthesizes nutrients like folic acid and vitamins. Indigestible proteins and fats are broken down into amino acids, peptides and short-chain fatty acids that are absorbed more easily. Lactic acid fermentation is also a natural food preservation method. The acidic environment inhibits harmful bacteria to extend shelf life. Some bacteriocins were also synthesized by lactic acid bacteria to make themselves the dominant species. Because bacteriocins are broad-spectrum antibiotics with no side effects, recently some companies have started using them to replace artificial preservatives.
When people consume these foods, lactic acid bacteria settle in their intestines. Lactic acid and bacteriocins regulate the balance of gut flora to inhibit pathogenic microorganisms. The proteins for tight junction are also secreted from intestinal epithelial cells to block pathogens and toxins. Intestinal immune cells are activated to fight pathogens and eliminate inflammatory factors.
Lactic acid, a little carbon dioxide, alcohol and esters change the texture and flavor of food, especially short-chain esters that emit fruity aromas. For example, casein coagulates in acidic conditions and milk turn into a thick semi-solid. Lactose, which is difficult for a few people to digest, will also be hydrolyzed during fermentation.
Biodegradable materials: Polylactic acid
Every year, several hundred million tons of polymers are produced from petroleum and other mineral resources. Because these materials are difficult to biodegrade in nature, waste plastics will accumulate in our environment. This is what we call "white pollution." Many countries have enacted plastic bans, therefore it’s urgent to find new biodegradable materials to replace current petroleum products for saving precious non-renewable resources and reducing environmental pollution.
Biodegradable polylactic acid (PLA) is synthesized from natural, non-toxic L-lactic acid. Hydroxyl and carboxyl groups condense into ester polymers with the help of catalysts. PLA resists water and common organic solvents. Its thermoplasticity allows it to be processed like plastic at high temperatures, such as by blow molding, stretching and extrusion. Its strength, gloss, transparency and texture is similar to plastic. Packaging bags, straws, disposable tableware, and even clothing already use PLA.
Although they’re biodegradable, PLA doesn’t decompose immediately in daily life. It still takes 6 months for soil buried PLA decomposition. Carbon, hydrogen and oxygen are the only elements, so final products are just water and carbon dioxide that absorbed by soil directly. They don’t release into atmosphere.
Good biocompatibility makes them popular in the medical field. PLA is combined with other materials to form structures rich in micropores, such as scaffolds, screws and surgical sutures. Tissues grow on their surfaces and inside to gradually decompose them into L-lactic acid. When they’re completely ate by our body, the wound is healed. It avoids removing the implant again.