Lactic acid is a simple organic acid with the chemical formula CH3CH(OH)COOH. It forms in the body during intense exercise when muscles work without enough oxygen, and it also comes from bacteria turning sugars into acid in foods like yogurt.
This acid exists in two mirror-image forms called enantiomers: L-lactic acid and D-lactic acid. L-lactic acid is the natural one found in humans, animals, and mammals. It comes from processes like fermentation by bacteria such as Lactobacillus. D-lactic acid is less common in nature and mostly made in labs or by certain other bacteria.
The main difference between them is their structure and how the body handles them. L-lactic acid fits perfectly into human enzymes and pathways. During glycolysis, the breakdown of glucose for energy, pyruvate turns into L-lactic acid when oxygen is low. This happens in muscle cells fast for quick energy. Later, the liver can convert it back to glucose through the Cori cycle.
D-lactic acid does not match these human pathways as well. It can build up if eaten in large amounts or from certain gut bacteria, sometimes causing issues like digestive problems. In chemistry, L-lactic acid is a clear to yellowish syrupy liquid with a mild sour taste. It boils at around 122 degrees Celsius under low pressure and has a density of about 1.2 grams per milliliter.
Both forms are safe in small amounts but can irritate skin or eyes if concentrated. Lactic acid overall is nontoxic and breaks down easily in the environment. People use it in foods as a preservative, in cosmetics for moisture, and in industry for cleaning or pH control.
Fermentation makes most lactic acid. Bacteria eat sugars and spit out lactic acid, lowering pH to stop bad germs from growing. This is why yogurt tastes tangy. Homolactic fermentation makes mostly lactic acid, while heterolactic makes some plus ethanol or other things.
In short, L-lactic acid is the body’s choice for energy and natural processes, while D-lactic acid is its twin that does not fit as neatly.
Sources
https://www.chemicalbook.com/ChemicalProductProperty_EN_CB1138319.htm
https://www.sigmaaldrich.com/US/en/product/aldrich/w261106
https://en.wikipedia.org/wiki/Fermentation
https://pubchem.ncbi.nlm.nih.gov/compound/L(+)-lactic%20acid
https://www.fishersci.com/store/msds?partNumber=AAJ61871MD&productDescription=D-LACTIC+ACID+250MG&vendorId=VN00024248&countryCode=US&language=en
https://www.ulprospector.com/en/na/PersonalCare/Detail/15695/353679/Sodium-L-Lactate-Personal-Care-Grade
