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Fermentation
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Fermentation in progress: Impurities formed by CO2 gas bubbles and fermenting material.
Overview of ethanol fermentation. One glucose molecule breaks down into two pyruvates (1). The energy from this exothermic reaction is used to bind inorganic phosphates to ADP and convert NAD+ to NADH. The two pyruvates are then broken down into two Acetaldehyde and give off two CO2 as a waste product (2). The two Acetaldehydes are then converted to two ethanol by using the H+ ions from NADH; converting NADH back into NAD+ (3).
Fermentation is a metabolic process that converts sugar to acids, gases and/or alcohol. It occurs in yeast and bacteria, but also in oxygen-starved muscle cells, as in the case of lactic acid fermentation. Fermentation is also used more broadly to refer to the bulk growth of microorganisms on a growth medium. French microbiologist Louis Pasteur is often remembered for his insights into fermentation and its microbial causes. The science of fermentation is known as zymology.Fermentation takes place in the absence of oxygen (when the electron transport chain is unusable) and becomes the cell’s primary means of ATP (energy) production. It turns NADH and pyruvate produced in the glycolysis step into NAD+ and various small molecules (see examples below). In the presence of O2, NADH and pyruvate are used in respiration; this is oxidative phosphorylation, it generates a lot more ATP in addition to that created by glycolysis, and for that reason cells generally benefit from avoiding fermentation when oxygen is available. Exceptions include obligate anaerobes, which cannot tolerate oxygen.The first step, glycolysis, is common to all fermentation pathways:::C6H12O6 + 2 NAD+ + 2 ADP + 2 Pi ? 2 CH3COCOO- + 2 NADH + 2 ATP + 2 H2O + 2H+Pyruvate is CH3COCOO-. Pi is phosphate. Two ADP molecules and two Pi are converted to two ATP and two water molecules via substrate-level phosphorylation. Two molecules of NAD+ are also reduced to NADH.In oxidative phosphorylation the energy for ATP formation is derived from an electrochemical proton gradient generated across the inner mitochondrial membrane (or, in the case of bacteria, the plasma membrane) via the electron transport chain. Glycolysis has substrate-level phosphorylation (ATP generated directly at the point of reaction).Fermentation has been used by humans for the production of food and beverages since the Neolithic age. For example, fermentation is employed for preservation in a process that produces lactic acid as found in such sour foods as pickled cucumbers, kimchi and yogurt (see fermentation in food processing), as well as for producing alcoholic beverages such as wine (see fermentation in winemaking) and beer. Fermentation can even occur within the stomachs of animals, such as humans. Auto-brewery syndrome is a rare medical condition where the stomach contains brewers yeast that break down starches into ethanol; which enters the blood stream.
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