Omid, can fermentation happen aerobically in yeast and bacteria? Why if yes or no? How do you define fermentation?
Dear Pulcinella, your questions fall within the realm of microbiology, and I am not a microbiologist to be able to competently answer your questions. Nonetheless, I will try my best.
Fermentation is a broad subject, and the metabolic diversity of bacteria and yeast can make this subject complex and perplexing. Because of the metabolic diversity of bacteria and yeast, there are many different types of fermentation, each utilizing a particularized fermentation pathway. Therefore, for the purpose of answering your questions as accurately as I can, I will limit this post to only sugar-fermenting bacteria and yeast (such as Saccharomyces cerevisiae
, i.e., the baker’s yeast) that are commonly used in preparing pizza dough.
First, let me make some prefatory statements. Fermentation is a property of (but not limited to) fermentative microorganisms such as bacteria and yeast. Moreover, each species requires a particular type or particular types of substrates (i.e., saccharides or sugars) to metabolize. Which sugars are metabolized by different bacteria or yeast vary by species and strains within each species, although some may have the same substrate requirements. In understanding fermentation (and various fermentative microorganisms), it is imperative to understand:
1. What fermentable sugar or sugars (such as glucose, fructose, sucrose, maltose, pentose, etc.) the fermentative microorganisms use as sources of food;
2. What end products and byproducts the fermentable sugars are converted to in the course of fermentation by the microorganisms; and
3. The conditions (such as anaerobic, aerobic, etc.) under which the microorganisms ferment the sugars.
For the sake of brevity, I will limit this post to only one substrate, glucose (a six-carbon sugar), which is the most common fermentable sugar used by the bacteria and yeast that we are interested in. Basically, to make a long story short, the starch (i.e., amylose and amylopectin) molecules of wheat flour are enzymatically hydrolyzed in several stages to form glucose molecules before fermentation actually begins.
In the science of physics, "energy" is defined as follows: "Energy is that property something has that enables it to do work." Cellular activity or cellular work in bacteria and yeast requires energy. No energy, no cellular work. Fermentation (in conjunction with glycolysis) is one mode of production of energy in bacteria and yeast so that they can carry out cellular activities in order to survive.
Bacteria and yeast have specialized energy- and matter-producing "metabolisms"—geared toward their biological survival. Fermentation is a metabolic function
of fermentative bacteria and yeast. What is "metabolic function" or "metabolism"? Metabolism is the critical chains of biochemical reactions that occur within the lifespan of a living bacteria or yeast in order to sustain its biological life by producing energy and matter. To be more specific, metabolism is the cellular chemical reactions that extract energy
from suitable nutrients (glucose) and convert the energy to usable forms in order to carry out catabolic (destructive) and anabolic (constructive) reactions:
1. Catabolic reactions
are energy-producing reactions. Catabolism constitutes the chemical reactions that break down glucose molecules (besides the protein and lipid molecules of wheat flour) into smaller molecules in order to release and harvest the inherent potential energy of the chemical bonds (covalent bonds) of glucose. Anaerobic cellular respiration (fermentation) and aerobic cellular respiration are two examples of catabolic reactions.
2. Anabolic reactions
are energy-demanding reactions. Anabolism constitutes the chemical reactions that synthesize larger molecules from smaller molecules. The synthesis of smaller molecules to form new molecules requires energy, which is sourced from catabolic reactions.
Hence, catabolic reactions (such as aerobic respiration and fermentation) fuel anabolic reactions. Yeast cell reproduction, growth, and repair are few examples of anabolic reactions.
The interrelation between cellular catabolism and cellular anabolism indicates that aerobic respiration (which is an energy-producing catabolic reaction), anaerobic respiration (which is also an energy producing catabolic reaction, of which fermentation is a part), and yeast multiplication and growth (which are energy-demanding anabolic reactions) are interlinked
Catabolic metabolism in yeast and many bacteria is of two types: aerobic cellular respiration (which requires oxygen as the terminal electron acceptor
) and anaerobic cellular respiration (which requires an organic molecule, never oxygen, as the terminal electron acceptor
). Aerobic and anaerobic respiration are comprised of several stages:
1) Stages of aerobic cellular respiration
(where oxygen is required)
a. Glycolysis (conversion of glucose to pyruvate)
b. Pyruvate oxidation/carboxylation
c. Citric acid cycle
d. Electron transport chain & chemiosmosis
2) Stages of anaerobic cellular respiration
(where oxygen is not required)
a. Glycolysis (conversion of glucose to pyruvate)
1. Alcoholic fermentation (by yeast)
a. Conversion of pyruvate to carbon dioxide and acetaldehyde
b. Conversion of acetaldehyde to ethanol (alcohol)
2. Lactic acid fermentation (by bacteria)
a. Homolactic fermentation (conversion of pyruvate to lactic acid by homofermentative bacteria)
b. Heterolactic fermentation (conversion of pyruvate to lactic acid, ethanol, and carbon dioxide by heterofermentative bacteria)
c. Homo-Hetero-lactic fermentation (performed by facultative heterofermentative bacteria, which can switch between homo- & hetero-lactic fermentation)
So, you asked, "Can fermentation happen aerobically in yeast and bacteria?" According to my studies, NEVER. There is no such thing as "aerobic fermentation", although I have seen many websites and books that treat fermentation as a partly aerobic phenomenon. Here is an example. According to the website "Bake Info - Baking Industry Research Trust" of New Zealand (which is supposed to be a professional website):
Yeast uses sugars by breaking them down into carbon dioxide and water. The yeast needs lots of oxygen in order to complete this type of fermentation.
C6H12O6 + 6O2 ←→ 6CO2 + 6H2O+ Energy
Sugar yeast Water
The yeast needs lots of oxygen
in order to complete this type of fermentation
?!! With all due respect, this is NOT fermentation, not at all. I know this as an indisputable fact. The cited equation is the overall chemical equation for "aerobic cellular respiration", not "anaerobic cellular respiration"—which is comprised of glycolysis (as the first stage) and fermentation (as the second stage). Please, take notice of the "O2
" in the equation. Any professional microbiologist will tell you that the fermentation pathway is not equipped with the biochemical mechanisms to utilize oxygen in order to bring about oxidation (breakdown) of glucose (C6
) to carbon dioxide (CO2
) and water (H2
O). Indeed, we need to be more critical in understanding the concept "fermentation", for it is often confused—by both professional and nonprofessional bakers—with almost the entire catabolic metabolism of bacteria and yeast.
The overall equation for alcoholic fermentation by yeast is:
→ Glycolysis (Pyruvate) → 2 C2
OH + 2 CO2
Glucose Ethanol Carbon Dioxide
©©©©©© ©©+©© ©+©
(Each "©" stands for one carbon atom.)
The overall equation for homolactic fermentation by homolactic bacteria is:
→ Glycolysis (Pyruvate) → 2 C3
Glucose Lactic Acid
The overall equation for heterolactic fermentation by heterolactic bacteria is:
→ Glycolysis (6-Phosphogluconate) → C3
OH + CO2
Glucose Lactic Acid Ethanol Carbon Dioxide
©©©©©© ©©© ©© ©
Fermentation is always anaerobic by nature. To say that fermentation can happen aerobically is not only illogical, but also a biological absurdity. The fermentation enzymes, in conjunction with their attendant cofactors and coenzymes, can only bring about oxidation of glucose molecules by "substrate-level phosphorylation", which is purely anaerobic. When bacteria or yeast shift from anaerobic to aerobic respiration, then they need to use "oxidative phosphorylation" (which is purely aerobic), in addition to "substrate-level phosphorylation", in order to be able to cause oxidation of glucose to carbon dioxide, water, and energy in the forms of ATP and heat.
Bear in mind that fermentation is carried out with or without
the presence of oxygen. Nonetheless, fermentation is a purely anaerobic reaction. While it can take place in the presence of oxygen, oxygen is never
involved in the reaction, nor does it alter the reaction or its outcome. The technical reason that fermentation can never be aerobic is that it, unlike aerobic respiration, lacks the biochemical mechanisms of citric acid cycle, electron transport chain, and chemiosmosis—where oxygen functions as the terminal electron acceptor.
As far as the bacterium and yeast cells are concerned, the ultimate goal of fermentation, in conjunction with glycolysis, is to generate biochemical energy (about 2 ATP net per glucose molecule) by oxidation of glucose—without the involvement of oxygen
. And, As far as the bacterium and yeast cells are concerned, the ultimate goal of aerobic cellular respiration is to generate biochemical energy (about 36-38 ATP per glucose molecule) by oxidation of glucose—with the direct involvement of oxygen
. Good day!