Yogurt, sauerkraut, pickles, none of these products would exist without lactic acid fermentation. This is a series of chemical reactions that transforms your ingredients into something pretty different (and delicious) thanks to lactic acid bacteria.
In this post we’ll explore lactic acid fermentation in some more detail, helping you understanding what happens when these bacteria set to work!
What is lactic acid fermentation?
In order for any organism to survive they have to be able to extract energy from an energy source. Fermentation is a way of getting this energy, just like respiration (used by plants and animals). Bacteria and yeasts (for example in beer brewing) commonly use fermentation as their energy source.
There are several types of fermentation, lactic acid fermentation is a type in which lactic acid is formed as a result of the fermentation process. Sugars, for example lactose or glucose, are fermented, along the way energy is released and lactic acid is produced. Lactic acid fermentation does not require oxygen to take place.
Lactic acid fermentation doesn’t just happen in bacteria, it also occurs in humans! If you’ve sported extensively and feel your muscles ‘turn sour’, they literally turn sour. There isn’t enough oxygen anymore and your body has reverted to using lactic acid fermentation to give you that extra energy.
Lactic acid bacteria
Various bacteria use lactic acid fermentation to transform sugars into lactic acid, most of them fall under the appropriately named group of lactic acid bacteria. Within this group there are again several types, for example Lactobacillus, Leuconostoc or Lactococcus. The bacteria might be added to food on purpose for lactic acid fermentation, but also occur naturally in a variety of areas, including in the human body and on most plants.
Different lactic acid bacteria have different optimum living conditions. Some can grow at temperatures as low as 0°C, whereas others stop growing at 10°C.
Lactic acid is an acid which means it will reduce the pH-value of a liquid, you can see the structure below. The -COOH group on the right side of the molecule is what makes lactic acid and acid. The proton (H+) of this group leaves the group quite easily to sit in the rest of the water around it. As a result, the concentration of protons increases, which leads to a decrease of the pH-value (see more here).
The lactic acid fermentation reaction
There are various types of lactic acid bacteria and there are also different ways in how they transform sugars into lactic acid. Also, it depends on the types of sugars available for the bacteria, how the reaction will look like. The simplest, most commonly described mechanism is the one that starts with glucose as the energy source.
Glucose can be fermented into lactic acid through two main pathways: homofermentative & heterofermentative. We’ll simplify the pathways here, in reality there are a lot more steps in between within the cells. Also, bacteria have a lot of other reactions going on while harvesting the energy during fermentation. Flavour molecules are formed for instance and these reaction mechanisms tend to be so complex and dependent on the product, environment and bacteria that it’s impossible here to create a full overview of all what’s going on.
Reaction scheme 1: Glucose – Homofermentative
In the homofermentative mechanism of lactic acid fermentation 1 molecule of glucose is broken down into 2 lactic acid molecules. This reaction results in the release of energy which can then be used by the bacteria. In a reaction scheme that looks as follows:
C6H12O6 → 2 C3H6O3
Reaction scheme 2: Glucose – Heterofermentative
Not all lactic acid bacteria are able to break down the glucose that efficiently. Instead, apart from lactic acid ethanol is formed as well:
C6H12O6 → C3H6O3 + C2H6O (ethanol) + CO2 ; again energy is released which is stored by the bacteria, but only half that of the homofermentative reaction.
Fermentation of larger sugars, lactose
But not all products in which lactic acid fermentation takes place contain glucose. Take for example milk. The sugar in milk is lactose. Lactose is a so-called disacharide and is made of one molecule of glucose and one molecule galactose. When the right lactic acid bacteria are used, they will have an enzyme that can break down the lactose in glucose and galactose. The glucose can then be fermentedly through one of the pathways described above. The galactose itself can be transformed into glucose through a series of reactions and can then also be fermented.
Lactic acid fermentation in food
Lactic acid fermentation in foods are sometimes desirable, but in other cases highly undesirable.
Lactic acid bacteria causes spoilage of meats as well as vegetables. The production of the lactic acid makes the products turn sour and unappetizing. This way lactic acid bacteria are a good indicator for spoilage, since they aren’t harmful themselves, they prevent us from eating something that is spoiled and might contain pathogenic micro organisms that you don’t necessarily taste.
Desirable lactic acid bacteria in food
In the case of yogurt, pickles, olives, sauerkraut, sour cream and sourdough, we want the lactic acid bacteria to do their job. The lactic acid bacteria help preserve the foods. Just have a look, yogurt & sour cream are a preserved form of milk, and sauerkaut is a preserved form of cabbage. Apart from preservation they also cause desired changes in taste and texture!
Pessione Enrica, Lactic acid bacteria contribution to gut microbiota complexity: lights and shadows, Frontiers in Cellular and Infection Microbiology, Vol. 2, 2012, p. 86, DOI 10.3389/fcimb.2012.00086, link