II. Read the text

Fermentation is a metabolic process converting sugar to acids, gases and/or alcohol using yeast or bacteria. In its strictest sense, fermentation is the absence of the electron transport chain and takes a reduced carbon source, such as glucose, and makes products like lactic acid or acetate. No oxidative phosphorylation is used, only substrate level phosphorylation, which yields a much lower amount of ATP. Fermentation is also used much more broadly to refer to the bulk growth of microorganisms on a growth medium.

The science of fermentation is known as zymology. The process is often used to produce wine and beer, but fermentation is also employed in preservation to create lactic acid in sour foods such as pickled cucumbers, kimchi and yogurt. Fermentation is a form of anaerobic digestion that generates adenosine triphosphate (ATP) by the process of substrate-level phosphorylation.

The energy for generating ATP comes from the oxidation of organic compounds, such as carbohydrates. In contrast, during respiration is where electrons are donated to an exogenous electron acceptor, such as oxygen, via an electron transport chain. Fermentation is important in anaerobic conditions when there is no oxidative phosphorylation to maintain the production of ATP (adenosine triphosphate).

Lactic acid fermentation. Lactic acid fermentation is the simplest type of fermentation. In essence, it is a redox reaction. In anaerobic conditions, the cell’s primary mechanism of ATP production is glycolysis. Glycolysis reduces (i.e. transfers electrons to) nicotinamide adenine dinucleotide (NAD+), forming NADH. However there is a limited supply of NAD+ available in any given cell. For glycolysis to continue, NADH must be oxidized (i.e. have electrons taken away) to regenerate the NAD+ that is used in glycolysis. In an aerobic environment, where oxygen is available, oxidation of NADH is usually done through an electron transport chain in a process called oxidative phosphorylation, but oxidative phosphorylation cannot occur in anaerobic environments because oxygen is absent due to the pathway's dependence on the terminal electron acceptor of oxygen. Instead, the NADH donates its extra electrons to the pyruvate molecules formed during glycolysis. Since the NADH has lost electrons, NAD+ regenerates and is again available for glycolysis. Lactic acid, for which this process is named, is formed by the reduction of pyruvate.

Spirit fermentation. All spirits go through at least two procedures, the fermentation process and the distillation process. Fermentation is where all alcohol is created; distillation is where the alcohol is separated and removed. In order for fermentation to occur, two things are needed: a raw material (in liquid form) that contains sugar, followed by the addition of yeast. Yeast is a living organism that feeds on sugar; the bi-product of this consumption is alcohol and carbon dioxide (Co2). A simple formula for fermentation is: YEAST + SUGAR = ALCOHOL + Co2. Once all the sugar is consumed, the yeast dies off and in most cases the Co2 is freely dissipated into the air while the alcohol, of course, remains in the liquid. Until it has been distilled, a fermented product is not considered a spirit, even though it contains alcohol. Distillation is the removal of the alcohol from what has been fermented. To distill the fermented product, a STILL is needed. Whether a pot still or a column still is used depends solely on what type of spirit is being produced.

Butyric acid (from Greek βούτυρο, meaning "butter"), also known under the systematic name butanoic acid, is a carboxylic acid with the structural formula CH3CH2CH2-COOH. Salts and esters of butyric acid are known as butyrates or butanoates. Butyric acid is found in milk, especially goat, sheep and buffalo's milk, butter, Parmesan cheese, and as a product of anaerobic fermentation. It has an unpleasant smell and acrid taste, with a sweetish aftertaste (similar to ether).

III. Answer the questions:

1. What is fermentation?

2. The fermentation process is often used to produce… .

3. What does the abbreviation “ATP”mean?

4. What does the abbreviation “NAD”mean?

5. What is zymology?

6. What kinds of fermentation do you know?

7. What is lactic fermentation?

8. What is spirit fermentation?

9. What is butyric acid fermentation?

10. Can you explain the process of spirit fermentation?

11. Can you explain the process of lactic fermentation?