Lactic acid (also known as milk acid) is a carboxylic acid, thus organic, and is roughly ten times more acidic than acetic acid (vinegar). Attracting and holding water from the surrounding environment, referred to as hygroscopy, is how lactic acid keeps moisture within the foods that it interacts with. Lactic acid comes in two forms, known as D and L isomers, which have identical chemical compositions, but differ in their optical activity. Simply put, the D isomer in solution interacts with polarized light by rotating it counter-clockwise while the L isomer rotates it clockwise and this difference is due to the central carbon’s subgroups mirroring one another in spatial positioning (please see below D- and L-lactic acid’s Molecular Structures).
There are two main processes by which lactic acid is formed. The first, through chemical synthesis, results in a racemic mixture of lactic acid, which means that a 1:1 ratio in concentration of its two isomers is the end result. The second, known as carbohydrate fermentation, results in a dominant concentration of one of the two isomers due to the work of a specific strand of lactic acid bacteria (LAB). There are different types of LAB and some will yield higher concentrations of the L isomer, while others will yield higher concentrations of the D isomer.
L-lactic acid is the generally-sought-after isomer with respect to commercial production as it is this isomer that is biologically important with its role in normal human metabolism and exercise. While L-lactic acid is metabolized in its entirety by human glycogen synthesis and rapidly migrates away from the intestine, D-lactic acid remains for a longer period as it can’t be as easily metabolized. Furthermore, it is L-lactic acid that gives the wanted characteristics of milk, cheese or yogurt such as added texture and increased flavour.
A Swedish chemist by the name of Carl Scheele refined lactic acid from sour milk near the end of the 18th century only to be followed by another Swedish chemist, Jöns Berzelius, and his breakthrough discovery in 1808. What Berzelius found was that this same chemical compound, specifically L-lactate, was what muscles produced during high-intensity exercise. It wasn’t until 1856 that the gifted mind of renowned French chemist and microbiologist, Louis Pasteur, was able to find the first known LAB, Lactobacillus. Less than four decades later, lactic acid began to be commercially produced as through a German pharmacy, Boehringer Ingelheim, which is currently a top 20 global company in its field.