In which food would you find lactic acid bacteria? Mention some of their useful applications.

Hint: Lactic acid bacteria perform a special type of respiration where the pathway progresses in the absence or insufficient quantities of oxygen. The lack of sufficient oxygen leads to the release of different products which may be lactic acid or ethyl alcohol.

Complete answer:
Lactic acid bacteria are a group of gram-negative and acid-tolerant bacteria that produces lactic acid as their major metabolic end-product of respiration. These bacterial species oxidize NADH by transforming pyruvate into lactate with the help of the enzyme ‘lactate dehydrogenase or LDH.’ This process is known as lactic acid fermentation. Genera like Lactobacillus, Lactococcus, and Streptococcus, etc. They are found in fermented foodstuffs such as curd, yogurt, wine, and cheese. Their growth in milk converts the milk into curd as the lactic acid produced by them leads to the denaturation of proteins. This imparts the thick consistency to curd due to the coagulation of proteins. Some of their useful applications include:
-They are used as probiotics wherein their intake as supplements helps improve the microbial populations of our gut.
-They also synthesize Vitamin B12 and thus improve the nutritional quality of fermented products.

Additional Information: Fermentation is an enzyme-mediated energy liberating process in the absence or insufficient amounts of oxygen. This leads to the incomplete breakdown of the substrates and thus water is not produced as the end product in this process. They are of two types- alcoholic fermentation and lactic acid fermentation.
In alcoholic fermentation, the pyruvic acid formed at the end of the glycolysis is converted to alcohol by using two enzymes, pyruvic acid decarboxylase, and alcohol dehydrogenase. The end products are two moles of ethyl alcohol, carbon dioxide, and ATP. E.g. plants, yeasts.

Note: -Lactic acid accumulates in the muscle cells during extensive exercise and the oxygen demand becomes insufficient. Its accumulation leads to minor muscle cramps and pain in the body.
-In alcoholic fermentation, Yeast poison to death when the concentration of alcohol reaches about 13%.