Lactic Acid Fermentation

Lactic acid fermentation is caused by some fungi and bacteria. The most important lactic acid producing bacteria is Lactobacillus. Other bacteria which produce lactic acid include leuconostoc mesenteroides, pediococcus cerevisiae, streptococcus lactis, and bifidobacterium bifidus. Lactic acid fermentation is used throughout the world to produce speciality foods:
  • Western world: yogurt, sourdough breads, sauerkraut, cucumber pickles and olives
  • Middle East: pickled vegetables
  • Korea: kimchi (fermented mixture of Chinese cabbage, radishes, red pepper, garlic and ginger)
  • Russia: kefir
  • Egypt: laban rayab and laban zeer (fermented milks), kishk (fermented cereal and milk mixture)
  • Nigeria: gari (fermented cassava)
  • South Africa : magou (fermented maize porridge)
  • Thailand : nham (fermented fresh pork)
  • Philippines : balao balao (fermented rice and shrimp mixture)
The sour taste in different foods is from the lactic acid produced during the process of lactic acid fermentation. This sour taste is often found in all sorts of dairy products such as cheese, yoghurt and kefirs. Lactic acid also gives a sour taste to fermented vegetable like cultured sauerkraut and pickles.

Fermentation of Yogurt

Yogurt is made by the fermentation of milk with friendly bacteria, mainly Lactobacillus bulgaricus and Streptococcus thermophilus. Yogurt fermentation was most likely invented by accident by Balkan tribes thousands of years ago. Yogurt remained mainly a food of eastern Europe until the 1900s, when the biologist Mechnikov created the theory that lactobacillus bacteria in yogurt are responsible for the unusually long lifespans of the Bulgar people.
The milk sugar or lactose is fermented by these bacteria to lactic acid which causes the characteristic curd to form. The acid also restricts the growth of food poisoning bacteria. During the yogurt fermentation some flavours are produced, which give yogurt its characteristic flavor.
Yogurt can easily be made at home with a live yogurt as the starter culture. To make your own yogurt:
Bring the milk to boiling point and cool down to 40- 45°C. Pour this milk in a sterile container and and per liter milk about 100 ml live yogurt. Mix with a sterile spoon and incubate at 40-44°C during 4 to 6 hours or until the yogurt is set. Put the yogurt in the refrigerator. If you worked under hygienic conditions, you can use your own yogurt as a starter for your next batch.

Fermentation of Magou

Magou is very popular in South Africa, especially among the Bantu people. Magou is a lactic acid fermented porridge made from maize. To make magua a 10 percent maize meal slurry is cooked, cooled and inoculated with wheat flour, which contains the bacteria. Magou is also produced on industrial scale and is then packed in cartons. In the industrial process the magou is inoculated with lactobacillus delbreuckii cultures.

Fermentation of Kefir

Kefir fermentation is similar to yogurt fermentation. Yogurt is only fermented by bacteria but kefir fermentation involves the help of bacteria as well as yeasts. These yeast produce some alcohol and carbon dioxide, which gives kefir its typical fizzy aspect. Kefir is inoculated with special kefir grains. These grains are mixtures of bacteria and yeasts in a matrix of proteins, lipids and carbohydrates. Kefir fermentation is done at room temperature, which makes the process easier. On the other hand, not everyone likes the taste of kefir.

Alcohol Fermentation

Unlike lactic acid fermentation, alcohol fermentation is done by yeast and some kinds of bacteria. These microorganisms convert sugars into ethyl alcohol and carbon dioxide. Alcoholic fermentation begins after glucose enters the cell. The glucose is broken down into pyruvic acid through glycolysis. This pyruvic acid is then converted to CO2, ethanol, and energy for the cell. Humans have long taken advantage of this process in making bread, beer, and wine. In these three product the same microorganism is used: the common yeast or Saccharomyces Cerevisae.

Chemical Formula

C6H12O6 -> 2CO2 + 2C2H5OH This formula was created by the French chemist Joseph Louis Gay-Lussac and demostrated that alcohol fermentation could be expressed by one molecule of the sugar glucose is decomposed to yield two molecules of carbon dioxide and two molecules of ethyl alcohol.

Chemical Process

Fermentation consists of glycolysis and reactions that regenerate NAD+ by transferring electrons form NADH to pyruvate. The NAD+ can then be reused to oxidize sugar by glycolysis, which produces a net gain of two ATP molecules by substrate-level phosphorylation. In the following paragraphs these steps of fermentation will be discussed thoroughly in detail.
In all organisms, including yeast, the breakdown of the six-carbon sugar glucose begins with a multi-step pathway known as glycolysis. Glycolysis is an anaerobic metabolic pathway found in the cytosol of all cells, which forms ATP by degrading glucose, and is the first step of fermentation. During glycolysis, glucose is split to form two molecules of the three-carbon compund, pyruvate in the cytosol of the cell. Usually pyruvate is transported to the mitochondrion to undergo further transformations, but without the presence of oxygen the pyruvate remains in the cytosol and undergoes anaerobic changes.
The purpose of this glucose breakdown by yeast is to harvest energy and allow the cell to perform various tasks. These tasks or processes use ATP or adenosine triphosphate as the direct supplier of energy. Before ATP can be harvested or used it must be created. ATP is made when two molecules of ADP, adenosine diphosphate, are phosphorylated. While ATP is being created, two molecules of NAD+ are reduced to NADH. This NADH will later act as a high-energy electron carrier, which can be used to create more ATP, and will regenerate NAD+ to restart the glycolysis process in the last step of fermentation. After enough energy has been made available, glycolysis will begin.
The steps of Glycolysis are as follows:
1. Glucose is destabilized by the addition of a phosphate. The phosphate in this reaction comes from the reaction of 1 ATP supplied by the cytosol. This reaction is then catalyzed by the enzyme, hexokinase.
2. Glucose-6-phosphate is rearranged into fructose-6-phosphate, by the enzyme, phosphoglucoisomerase. This rearrangement exposes an OH group which is necessary for the next step.
3. Another phosphate is added from another ATP to from fructose-1, 6-biphosphate. This step is then catalyzed by phosphofructokinase. This step is the final one that requires energy. Both of the phosphate used will later be taken back to form more ATPs.
4. The molecule is split by aldolase to form glyceraldehyde phosphate and dihydroxyacetone phosphate. The latter compound is converted into another molecule of glyceraldehyde phosphate by isomerase. Each of the following steps should be thought of as occuring twice, once for each glyceraldehyde phosphate.
5. Glyceraldehyde phosphate is oxidized by the removal of a hydrogen atom and its two bonding electrons, which join NAD+ to form NADH. At the same time a negatively charged phosphate or Pi from the cytosol bonds with its electrons to the carbon that lost the hydrogen, forming 1, 3 biphosphoglycerate. This phosphate bond is weak and can be harvested later to form ATP. Since there are two of these molucules at this stage, this provides the two ATPs which are the energy profit of glycolysis. The whole reaction is catalyzed by triose phosphate dehydrogenase.
6. The phosphate just added is transferred to ADP to form ATP and 3-phosphoglyceric acid. It is structurally identical to glyceraldehyde phosphate from step 4, except that the terminal group is a carboxylic acid instead of an aldehyde. It is that oxidation from aldehyde to acid that supplied the energy to make the ATP and NADH.
7. 3-phosphoglyceric acid is isomerized to 2-phosphoglyceric acid by phosphoglyceromutase. This places the phosphate in a more advantageous position for the next step.
8. 2-phosphoglyceric acid is dehydrated and rearranged to from phosphoenolpyruvic acid, catalyzed by enolase.
9. The final step is removed by pyruvate kinase, forming and ATP and pyruvate.

Regeneration of NAD+
Now that the yeast has broken down the glucose to form ATP, pyruvate, and NAD+ the next three steps of alcohol fermentation can begin. First, the pyruvate, created during glycolysis, is decarboxylated to produce acetaldehyde and carbon dioxide. The carbon dioxide is released, while the acetaldehyde remains and acts as the final hydrogen acceptor. The acetaldehyde then accepts the hydrogen of NADH to form ethanol and NAD+. This regenerated NAD+ will then restart the process of glycolysis until there is no more sugar to be broken down by yeast or the fermenting enzymes become inhibited by the build-up of products.