Types of Fermentations
A common way to categorize fermentations is by the main by-product: alcohol and carbon dioxide (yeast), acetic acid (Acetobacter bacteria), lactic acid (bacteria such as Leuconostoc, Lactobacillus, Lactococcus, Enterococcus, Pediococcus, and Streptococcus), propionic acid (Propionibacterium freudenreichii bacteria), and ammonia and fatty acids (Bacillus bacteria and molds).
Food fermentation is usually described as being carried out by one (or a mixture) of three types of organisms-- bacteria, yeast, and mold. If you want to be technical, however, there are two types of organisms that ferment foods: bacteria and fungi. Yeast and mold are actually two types of fungi, with mold being a stringy fungus and yeast being generally in a more round shape. Relatively speaking, there are not many species of microorganisms that ferment food.
Bacteria, especially lactic acid bacteria (LAB), ferment most of the things we eat. LAB are so named because one of their main by-products is lactic acid. LAB create all fermented milks and most fermented vegetables. They also ferment one of the largest calorie sources in the world, cassava, to produce gari and fufu. LAB produce sorghum beer. LAB also make the following: sourdough bread and bread-like products made without wheat or rye flours (Indian idli, Philippine puto); tempeh (from soybeans); 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).
LAB are anaerobic, meaning they thrive in conditions with no oxygen, such as is in our large intestine or under brine, which triggers their growth, and the beginning of fermentation. These bacteria take sugars in the food, and break them down, creating various by-products. There are two types of LAB bacteria, but they both produce lactic acid. The difference is that one type (homofermenters) produce almost entirely lactic acid when they break down glucose. The other type (heterfermenters) produce ethanol and carbon dioxide, in addition to lactic acid.
The Acetobacter species of bacteria oxidize alcohol to acetic acid, i.e., vinegar.
Bacillus species (Bacillus subtilis, B. licheniformis and B. pumilus) bring about alkaline fermentation. Alkaline fermentations are more common with protein-rich foods such as soybeans and other legumes. Bacillus subtilis is the dominant species causing the hydrolysis of protein to amino acids and peptides and releasing ammonia, which increases the alkalinity and makes the substrate unsuitable for the growth of spoilage organisms.
Yeasts are best known as being used to make bread and alcohol. Yeasts are aerobic, meaning they need oxygen to grow. They can ferment (but not grow) without oxygen, however. Yeasts ferment sugars in the food substrate (such as grains and fruits) to produce alcohol and carbon dioxide as by-products. The carbon dioxide produced by bread yeasts creates gas pockets, causing bread to rise.
Molds are involved in a limited number of fermented foods. Molds, however, have the greatest variety of enzymes. They are aerobic, requiring oxygen to grow. (Pederson at 31). Mold helps "ripen" many cheeses. Ripening is a secondary fermentation. The mold, Penicillium roqueforti, for example, gives blue cheese its distinctive taste and blue veins. Other cheeses ripened by mold are brie, Camembert, and Roquefort. (Frias at 275-301). Tempeh, made from soybeans, is made using mold. Mold is also involved in natural olive fermentation. (Frias at 519-20). Some sausages are also "ripened" using mold.
How Microorganisms Ferment Food
Fermentation is the process by which enzymes break down food. These enzymes come from two, sometimes three sources, depending on the type of fermentation: 1.) enzymes naturally part of the food; 2.) enzymes from microorganisms that--due to environmental sources such as air, water, soil, handling, etc.--end up on the food; and 3.) enzymes from the fermenting microorganisms that are added to the food. Instances where there are only two sources of enzymes is when the food is fermented spontaneously, meaning that conditions are created to select for the growth of some of the naturally occurring microorganisms. (Pederson at 30) If we then eat the food, a fourth source of enzymes are produced by our gut bacteria, which also ferment the food.
Getting the desirable microorganisms you want to ferment the food, and thereby create the desired end product that is properly preserved, requires creating conditions under which the desired microorganisms thrive more so than others. This means using clean utensils and containers, and keeping the fermentation at optimal temperature for your microorganisms. For LAB, it means adding salt, removing access to air by assuring the food is covered in brine, and oftentimes cutting the vegetables to allow the bacteria better access to the food. (Pederson at 82-83).
Nearly all types of fermentations are the result of more than one species of microorganism. Usually, a species will ferment until it runs out of food source, or the accumulation of its by-products (e.g., alcohol) inhibits its further growth, at which points another species may take over. Usually, bacteria initiate fermentation, followed by yeasts, and then molds, if the latter two are part of the process. This is due to surface area to mass ratio; bacteria have the largest such ratio, allowing them to take up nutrients most rapidly. Then yeast. Then mold. Sometimes fermentation is a symbiotic relationship, happening simultaneously rather than in stages. One example is kefir where we see a symbiotic relationship between yeast and bacteria. Yeasts produce vitamins, amino acids, and other essential growth factors for bacteria. Bacteria, in turn, produce by-products that the yeast use for energy. (Farnworth at 92).
How Fermenting Microorganisms Naturally Preserve Food
Fermenting microorganisms naturally ferment food by producing various antimicrobial compounds that inhibit the growth of, or kill, other microorganisms that would spoil the food. LAB, for example, produce organic acids, hydrogen peroxide, carbon dioxide, diacetyl, and broad-spectrum antimicrobials such as reuterin and bacteriocins. Organic acids include lactic, acetic, and propionic. These acids result in a low pH, which is beyond the tolerance of most other bacteria, especially pathogenic ones. (Pederson at 76-77). This acidity apparently keeps unwanted bacteria, yeasts, and molds at bay by obstructing the function of their cell membranes. Alcohol, which is one of the main by-products of yeast fermentation, also acts as an antimicrobial, inhibiting the growth of many microorganisms. For LAB fermentation, the addition of salt also acts as a preservative, in addition to giving an advantage to LAB, which can tolerate high salt levels. The beauty of LAB, and fermenting microorganisms in general, is how they are able to convert carbohydrates in food to create these antimicrobial substances, yet affect only minor changes on the food itself, preserving, and often increasing, the nutritive quality of the food. (Pederson at 42).
Fermenting bacteria, however, are not the only source of antimicrobial and bacteriocidal (bacteria-killing) chemicals present during fermentation. Cabbage and other brassicas naturally contain chemicals that inhibit the growth of, or even kill, unwanted bacteria. In fact, these chemicals have been found to be 20 to 100 times more effective than sodium benzoate, which is a common preservative added to packaged foods. The natural preservatives found in cabbage, sauerkraut, and brassicas generally, are destroyed by heat, however, which happens during pasteurization. This is the reason your store-bought, pasteurized, "shelf-stable" sauerkraut is void of any beneficial probiotic bacteria, and also a reason why it contains preservatives such as sodium benzoate.
1. Farnworth, Edward R. Handbook of Fermented Functional Foods. Taylor and Francis Group, 2008.
2. Frias, Juanas, et al. Fermented Foods in Health and Disease Prevention. Academic Press, 2017. (Available online, here.)
3. Pederson, Carl S. Microbiology of Food Fermentations, 2nd ed. AVI Publishing Co., Inc., 1979.
● Types of Fermentations
● How Microorganisms Ferment Food
● How Fermenting Microorganisms Naturally Preserve Food