1. Antioxidant – Reduces oxidation to prevent the formation of free radicals which may be harmful to health
2. Preservative – Prevents and inhibits the growth of unwanted microorganisms which may be harmful
Lactic acid is a normal intermediate in the fermentation (oxidation, metabolism) of sugaris, found primarily in sour milk products, such as koumiss, laban, yogurt, kefir, and some cottage cheeses. The casein in fermented milk is coagulated (curdled) by lactic acid. Lactic acid is also responsible for the sour flavor of sourdough bread. In beer brewing some styles of beer (sour beer) purposely contain lactic acid. Most commonly this is produced naturally by various strains of bacteria, which ferment sugars into acids.
Lactic acid is an ingredient in processed foods and is used as a decontaminant during meat processing. It is produced commercially by fermentation of carbohydrates such as glucose, sucrose, or lactose, or by chemical synthesis. Carbohydrate sources include corn, beets, and cane sugar. It has a E number 270.
It is approved to use as food additive in EU. In US, it is generally recognized as safe food substance.
Scientific References :
1. PubChem: https://pubchem.ncbi.nlm.nih.gov/compound/612
Regulatory References :
1. EU Approved Food Additive 
2. US FDA Food Additives Status List 
3. US FDA Generally Recognized As Safe (GRAS) Food Substances