Definition of lactic acid
Lactic acid, in the form of lactate, is an important intermediate product of the human metabolic process, for instance as a product of the breakdown of sugar as a result of lactic acid fermentation.
In view of their optical chirality, D-(–)-lactic acid [alternatively, (R) lactic acid] is referred to as levorotatory lactic acid, and L – (+) – lactic acid [alternatively, (S) lactic acid] as dextrorotatory lactic acid.
Use and dangerousness of lactic acid
L-(+)-lactic acid is present in sweat, in blood in muscular serum and the kidneys, as well as bile and saliva. The racemic mixture (i.e., a 1:1 mixture of D and L-forms of lactic acid), occurs in dairy products, tomato juice and beer. Fungi also produce lactic acid.
A range of foodstuffs is produced as a direct result of lactic acid fermentation. These include dairy products such as curdled milk, yoghurt, kefir and buttermilk. Other products include lacto-fermented vegetables, such as sauerkraut, borscht and kimchi, as well as sourdough and sourdough products. Silage, which is fresh animal feed preserved through fermentation, is produced as a result of the presence of lactic acid.
When used as a dietary supplement, lactic acid is designated as E270. It is used widely throughout the food and beverage industries, including in the production of baked goods and confectionery, and occasionally in soft drinks. Changing the pH of a foodstuff to a value of 4 or similar causes the food to be preserved as colonization by other microorganisms is largely prevented.
Physician, Dr. Dieter Thierbach, refers to the dextrorotatory form of lactic acid as a natural intermediate product of metabolism. It is quickly and comprehensively broken down in the body by a specific enzyme, L-lactate dehydrogenase.
In a physiological context, dextrorotatory S and L-(+)-lactic acid is formed in sufficient quantities as a result of the presence of glucose in all healthy muscles, organs and erythrocytes. Every human heart is dependent on the constant presence and supply of dextrorotatory (S)-lactic acid to regulate cardiac and circulatory functions. Lactic acid can be broken down in the liver, and around 60% of all lactic acid expended in daily life is converted back in to glucose and glycogen there.
Pharmaceutical technology makes use of lactic acid to convert water insoluble drugs into salts of lactic acids (lactates), which then dissolve more effectively in water.
In the cosmetic industry, lactic acid is used in skin creams and other products used in treating acne. 8%-lactic acid is referred to as Natural Moisturizing Factor (NMF): this creates a hydroscopic film over the epidermis, which has a moisturizing effect on the stratum corneum. Lactic acid also regulates the skin’s physiological pH value as a component of the skin’s own protective acidic layer and thus prevents bacterial colonization of the skin. In low doses, lactic acid reduces the cohesion of the corneocytes of the stratum corneum (the uppermost layer of the epidermis in humans), loosening intercellular connections and thus regulating keratinization. This causes the stratum corneum to become thinner and more flexible, which assists in treating skin conditions such as blemishes, acne, psoriasis and ichthyosis.