Lactic Acid Fermentations

Keith H. Steinkraus

Lactic acid bacteria perform an essential role in the preservation and production of wholesome foods. The lactic acid fermentations are generally inexpensive, and often little or no heat is required in their preparation, making them fuel efficient as well. Foods fermented with lactic acid play an important role in feeding the world's population on every continent.

Lactic acid bacteria perform this essential function in preserving and producing a wide range of foods: fermented fresh vegetables such as cabbage (sauerkraut, Korean kimchi); cucumbers (pickles); fermented cereal yogurt (Nigerian ogi, Kenyan uji); sourdough bread and bread-like products made without wheat or rye flours (Indian idli, Philippine puto); fermented milks
(yogurts and cheeses); fermented milk-wheat mixtures (Egyptian kishk, Greek trahanas); protein-rich vegetable protein meat substitutes (Indonesian tempe); amino acid/peptide meat-flavored sauces and pastes produced by fermentation of cereals and legumes (Japanese miso, Chinese soy sauce); fermented cereal-fish-shrimp mixtures (Philippine balao balao and burong dalag); and fermented meats (e.g., salami).

Lactic acid bacteria are generally fastidious on artificial media, but they grow readily in most food substrates and lower the pH rapidly to a point where competing organisms are no longer able to grow. Leuconostocs and lactic streptococci generally lower the pH to about 4.0 to 4.5, and some of the lactobacilli and pediococci to about pH 3.5, before inhibiting their own growth.

In addition to producing lactic acid, lactobacilli also have the ability to produce hydrogen peroxide through oxidation of reduced nicotinamide adenine dinucleotide (NADH) by flavin nucleotide, which reacts rapidly with gaseous oxygen. Flavoproteins, such as glucose oxidase, also generate hydrogen peroxide and produce an antibiotic effect on other organisms that might cause food spoilage; the lactobacilli themselves are relatively resistant to hydrogen peroxide.

Streptococcus lactis produces the polypeptide antibiotic nisin, active against gram-positive organisms, including S. cremoris, which in turn produces the antibiotic diplococcin, active against gram-positive organisms such as S. lactis. Thus, these two organisms compete in the fermentation of milk products while inhibiting growth of other gram-positive bacteria.

Carbon dioxide produced by heterofermentative lactobacilli also has a preservative effect in foods, resulting, among others, from its flushing action and leading to anaerobiosis if the substrate is properly protected.

Brining and lactic acid fermentation continue to be highly desirable methods of processing and preserving vegetables because they are of low cost, have low energy requirements for both processing and preparing foods for consumption, and yield highly acceptable and diversified flavors. Depending on the salt concentration, salting directs the subsequent course of the fermentation, limiting the amount of pectinolytic and proteolytic hydrolysis that occurs, thereby
controlling softening and preventing putrefaction. Lactic acid fermentations have other distinct advantages in that the foods become resistant to microbial spoilage and toxin development. Acid fermentations also modify the flavor of the original ingredients and often improve nutritive value.

Because canned or frozen foods are mostly unavailable or too expensive for hundreds of millions of the world's economically deprived and hungry people, acid fermentation combined with salting remains one of the most practical methods of preservation, often enhancing the organoleptic and nutritional qualities of fresh vegetables, cereal gruels, and milk-cereal mixtures.