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Almost all lactic acid available on the market is produced by fermentation. During fermentation, a suitable carbohydrate is converted to lactic acid by microorganisms. Although some of the microorganisms used, such as the mold Rhizopus, need oxygen for growth, the actual conversion of sugars to lactic acid is carried out without oxygen. As a matter of fact, the complete oxidation of a sugar to carbon dioxide and water is energetically much more favorable, so lactic acid is mainly formed under anaerobic conditions. Indeed, most lactic‐acid‐producing microorganisms are inactive when oxygen is continuously present in high amounts [29]. Upon entering the cell, the sugar is first converted to pyruvate by several enzymatic steps. This conversion yields chemical energy in the form of ATP (adenosine triphosphate) and reducing equivalents (NADH); see the reaction in Figure 1.3.

To recycle these reducing equivalents, microorganisms convert the pyruvate into the more reduced lactic acid; see the reaction in Figure 1.4.

In other words, lactic acid is mainly produced to keep the cellular processes going [30]. The chemical energy obtained is used by several processes elsewhere in the cell, for example cell growth, maintenance, and sometimes even motility.

The reaction in Figure 1.3 takes place in the so‐called homofermentative lactic acid bacteria (LAB). Homofermentative bacteria have almost exclusively lactic acid as a fermentation product, in contrast to heterofermentative bacteria that produce a mixture of lactic acid, acetate, CO₂, and acetate or ethanol [31]. Heterofermentative bacteria were believed to use exclusively the so‐called phosphoketolase pathway, and homofermentative bacteria were believed to use exclusively the glycolysis (Figure 1.3) that splits C₆ into two C₃ molecules [31].

FIGURE 1.2 Lactic acid condensation reactions: interchange between lactide, oligomers, and poly(lactic acid).

FIGURE 1.3 During conversion of glucose to pyruvate, chemical energy (ATP) is generated, as well as reducing equivalents (NADH).

FIGURE 1.4 Lactic acid formation from pyruvate: reoxidation of NADH and NAD takes place; NAD can be used again in the reaction of Figure 1.3.

The phosphoketolase pathway is a route where a C₆ is transformed to a C₅ sugar (and CO₂) and split into a C₂ and a C₃ molecule. The C₃ molecule is then converted to lactic acid, whereas the C₂ molecule is converted to acetate or ethanol. In the same traditional view, C₅ sugars were regarded as leading to this heterofermentative metabolism, which is less interesting from the point of view of industrial production as a lot of acetic acid or ethanol is produced simultaneously. Although some bacteria seem to fit well in this paradigm, more recent literature has shown that this view is oversimplified and somewhat obsolete for a number of reasons.

 Some heterofermentative bacteria are shown to have both pathways active at the same moment and produce mostly lactic acid under certain circumstances [32, 33].

 Pentoses can lead exclusively to lactic acid as a fermentation product [34].

 Lactic‐acid‐producing organisms that do not have a phosphoketolase pathway can still produce acetate or ethanol, formed by the usual mixed acid fermentation, via pyruvate. This is the case for, for example, Lactococcus lactis [35].

The reason why even heterofermentative bacteria prefer to produce mostly lactic acid is related to the fast generation of chemical energy and thus fast growth and acidification of the environment [32, 36, 37]. The fast growth and acidification gives lactic acid bacteria a competitive advantage and that is exactly why lactic acid bacteria are so troublesome in ethanol fermentations [38]. Lactic acid production is certainly not restricted to bacteria or fungi. Higher organisms, including humans, also use lactic acid formation for fast supply of energy in muscles when needed [39].

The uniformity in this biochemistry is in sharp contrast with the degrees of freedom one has in choosing the microbes, the acid‐neutralizing agent, nutrients, and carbohydrates needed for industrial lactic acid fermentation. Only delicate weighing of the pros and cons of every possibility leads to an economically feasible fermentation.