Lactic acid (IUPAC systematic name: 2-hydroxypropanoic acid), also
known as Milk Acid, is a chemical compound that plays a role in
several biochemical processes. It was first isolated in 1780 by a
Swedish chemist, Carl Wilhelm Scheele, and is a carboxylic acid
with a chemical formula of C3H6O3. It has a hydroxyl group adjacent
to the carboxyl group, making it an alpha hydroxy acid (AHA). In
solution, it can lose a proton from the acidic group, producing the
lactate ion CH3CH(OH)COO. It is miscible with water or ethanol, and
is hygroscopic.
Lactic acid is chiral and has two optical isomers. One is known as
L-(+)-lactic acid or (S)-lactic acid and the other, its mirror
image, is D-(-)-lactic acid or (R)-lactic acid. L-(+)-Lactic acid
is the biologically important isomer.
In animals, L-lactate is constantly produced from pyruvate via the
enzyme lactate dehydrogenase (LDH) in a process of fermentation
during normal metabolism and exercise. It does not increase in
concentration until the rate of lactate production exceeds the rate
of lactate removal which is governed by a number of factors
including: Monocarboxylate transporters, concentration and isoform
of LDH and oxidative capacity of tissues. The concentration of
blood lactate is usually 1-2 mmol/L at rest, but can rise to over
20 mmol/L during intense exertion.
Industrially, lactic acid fermentation is performed by
Lactobacillus bacteria, among others. These bacteria can operate in
the mouth; The acid they produce is responsible for the tooth decay
known as caries.
In medicine, lactate is one of the main components of Ringer's
lactate or lactated Ringer's solution (Compound Sodium Lactate or
Hartmann's Solution in the UK). This intravenous fluid consists of
sodium and potassium cations, with lactate and chloride anions, in
solution with distilled water in concentration so as to be isotonic
compared to human blood. It is most commonly used for fluid
resuscitation after blood loss due to trauma, surgery, or a burn
injury.
Lactic acid is primarily found in sour milk products, such as:
Koumiss, leban, yogurt, kefir and some cottage cheeses. The casein
in fermented milk is coagulated (curdled) by lactic acid.
Although it can be fermented from lactose (milk sugar), most
commercially used lactic acid is derived by using bacteria such as
Bacillus acidilacti, Lactobacillus delbueckii or Lactobacillus
bulgaricus to ferment carbohydrates from nondairy sources such as
cornstarch, potatoes and molasses. Thus, although it is commonly
known as "milk acid", vegan products can contain lactic acid as an
ingredient.
Lactic acid may also be found in various processed foods, usually
either as a pH adjusting ingredient, or as a preservative (either
as antioxidant or for control of pathogenic micro-organisms). It
may also be used as a fermentation booster in rye and sourdough
breads. [3]
Lactic acid is also present in wheat beers, especially lambic, due
to the activity of Pediococcus damnosus. [4]
Lactic acid is widely used for inhibiting pathogenic bacteria like
E. Coli, Salmonella, Campylobacter and Listeria on animal carcasses
like beef, pork and poultry during the slaughtering process.
Potassium lactate, sodium lactate and calcium lactate are the
neutralized salts of lactic acid. Potassium lactate is used in many
fresh and cooked meat products for shelf life control, color
preservation and reduction of sodium content. Sodium lactate has a
mild saline taste and is therefore suitable for flavour enhancement
in meat products as well. Sodium lactate is being produced as
liquids as well as powders. Calcium lactate is popular for
fortification and improved texture in emulsified meat products like
frankfurters.
