Wine fermentation

History

Since fruits ferment naturally, fermentation precedes human history. Since prehistoric times, however, humans have been taking control of the fermentation process. The earliest evidence of winemaking dates from 6000 BC, in Georgia, the former Soviet Republic. 7000 year old jars of wine have been excavated in the Zagros Mountains, which are now on display at the University of Pennsylvania. There is strong evidence that people were fermenting beverages in Babylon circa 5000 BC ancient Egypt circa 3000 BC, pre-Hispanic Mexico circa 2000 BC, and Sudan circa 1500 BC. There is also evidence of leavened bread in ancient Egypt circa 1500 BC and of milk fermentation in Babylon circa 3000 BC. The Chinese were probably the first to develop vegetable fermentation.

French chemist Louis Pasteur was the first zymologist, when in 1857 he connected yeast to fermentation. Pasteur originally defined fermentation as respiration without air.

Pasteur performed careful research and concluded, "I am of the opinion that alcoholic fermentation never occurs without simultaneous organization, development and multiplication of cells.... If asked, in what consists the chemical act whereby the sugar is decomposed ... I am completely ignorant of it.".

The German Eduard Buchner, winner of the 1907 Nobel Prize in chemistry, later determined that fermentation was actually caused by a yeast secretion that he termed zymase.

The research efforts undertaken by the Danish Carlsberg scientists greatly accelerated the gain of knowledge about yeast and brewing. The Carlsberg scientists are generally acknowledged with jump-starting the entire field of molecular biology.


Uses
The primary benefit of fermentation is the conversion of sugars and other carbohydrates, e.g., converting juice into wine, grains into beer, carbohydrates into carbon dioxide to leaven bread, and sugars in vegetables into preservative organic acids.

According to Steinkraus (1995), food fermentation serves five main purposes:

Enrichment of the diet through development of a diversity of flavors, aromas, and textures in food substrates
Preservation of substantial amounts of food through lactic acid, alcohol, acetic acid, and alkaline fermentations
Biological enrichment of food substrates with protein, essential amino acids, essential fatty acids, and vitamins
Detoxification during food-fermentation processing
A decrease in cooking times and fuel requirements
Fermentation has some uses exclusive to foods. Fermentation can produce important nutrients or eliminate antinutrients. Food can be preserved by fermentation, since fermentation uses up food energy and can make conditions unsuitable for undesirable microorganisms. For example, in pickling the acid produced by the dominant bacteria inhibit the growth of all other microorganisms. Depending on the type of fermentation, some products (e.g., fusel alcohol) can be harmful to people's health.

In alchemy, fermentation is often the same as putrefaction, meaning to allow the substance to naturally rot or decompose.



In its strictest sense, fermentation (formerly called zymosis) is the anaerobic metabolic breakdown of a nutrient molecule, such as glucose, without net oxidation. Fermentation does not release all the available energy in a molecule; it merely allows glycolysis (a process that yields two ATP per glucose) to continue by replenishing reduced coenzymes. Depending on which organism it is taking place in, fermentation may yield lactate, acetic acid, ethanol, or other reduced metabolites. Yeast produces ethanol and CO2; human muscle (under anaerobic conditions) produces lactic acid.

Fermentation is also used much more broadly to refer to the bulk growth of microorganisms on a growth medium. No distinction is made between aerobic and anaerobic metabolism when the word is used in this sense.

Fermentation usually implies that the action of the microorganisms is desirable. Occasionally wines are enhanced through the process of cofermentation. When fermentation stops prior to complete conversion of sugar to alcohol, a stuck fermentation is said to have occurred.





The need for Fermentation

Glucose
In aerobic respiration (the branch of respiration ‘normal’ for most organisms), a molecule of Glucose (C6H12O6, shown left) is broken down through the process of glycolysis into pyruvate (C3H3O3, shown right). In the first half of glycolysis, two Adenosine triphosphate (ATP) molecules cause glucose to break into two molecules Glyceraldehyde-3-phosphate (G3P). In the next step, a Nicotinamide adenine dinucleotide (NAD+) molecule removes a hydrogen atom from a G3P, converting G3P to 3-Biphosphoglycerate and NAD+ to NADH.

Pyruvate
When oxygen is present, NADH carries its hydrogen elsewhere. Eventually, oxygen reacts with the hydrogen carried by NADH to make water (H2O). When oxygen is not present in sufficient quantity, NADH cannot give up its hydrogen to oxygen, and so much of the cell’s supply of NAD+ is converted to NADH that G3P can no longer be converted to 3-Biphosphoglycerate, and the generation of ATP by the cell ceases, unless another substance can be used to remove the hydrogen from NADH.

The chemical process of fermentation

Acetaldehyde












Ethanol

Fermentation is a chemical pathway that provides such a substance. In the ethanol fermentation used by yeasts and other organisms, the ionised carboxyl group (COO–) is removed from the pyruvate to generate a molecule of carbon dioxide, which is released by the yeast into its surroundings. The resulting molecule, acetaldehyde (C2H4O, see left), takes the place of oxygen as the chemical that accepts hydrogen from NADH. This hydrogen, together with an H+ ion released during an earlier stage of glycolysis, is added to the acetaldehyde, making ethanol (C2H6O, see right).
Ethanol respiration can be summed up in this chemical equation: C6H12O6 → 2 C2H5OH + 2 CO2 + 2ATP

Uses of Ethanol respiration
Ethanol respiration is the form of fermentation used to make alcohol and bread. Yeast cells in the dough of a bread will be cut off from their sources of oxygen, and will generate alcohol (which is boiled away due to the heat) and carbon dioxide (which form bubbles that cause bread to rise). It is also used to mass-produce alcoholic beverages. The yeasts, cut off from oxygen, will ferment a starchy grain or vegetable (such as wheat, corn, potatoes, rye).

Reaction
The reaction differs according to the sugar being used in the process of anaerobic respiration, below, the sugar will be glucose (C6H12O6) the simplest sugar.

Chemical Equation
C6H12O6 → 2C2H5OH + 2CO2 + 2 ATP (Energy Released:118 kJ mol−1)
Word Equation
Sugar (glucose) → Alcohol (ethanol) + Carbon Dioxide + Energy (ATP)

Energy source in anaerobic conditions
Fermentation is thought to have been the primary means of energy production in earlier organisms before oxygen was at high concentration in the atmosphere and thus would represent a more ancient form of energy production in cells.
Fermentation products contain chemical energy (they are not fully oxidized) but are considered waste products since they cannot be metabolised further without the use of oxygen (or other more highly-oxidized electron acceptors). A consequence is that the production of ATP by fermentation is less efficient than oxidative phosphorylation, where pyruvate is fully oxidized to carbon dioxide. Fermentation produces two ATP molecules per molecule of glucose compared to approximately 36 by aerobic respiration.
Aerobic glycolysis is a method employed by muscle cells for the production of lower-intensity energy over a longer period of time when oxygen is plentitful. Under low-oxygen conditions, however, vertebrates use the less-efficient but faster anaerobic glycolysis to produce ATP. The speed at which ATP is produced is about 100 times that of oxidative phosphorylation.[citation needed] While fermentation is helpful during short, intense periods of exertion, it is not sustained over extended periods in complex aerobic organisms. In humans, for example, lactic acid fermentation provides energy for a period ranging from 30 seconds to 2 minutes.
The final step of fermentation, the conversion of pyruvate to fermentation end-products, does not produce energy. However, it is critical for an anaerobic cell since it regenerates nicotinamide adenine dinucleotide (NAD+), which is required for glycolysis. This is important for normal cellular function, as glycolysis is the only source of ATP in anaerobic conditions.

Products
Products produced by fermentation are actually waste products produced during the reduction of pyruvate to regenerate NAD+ in the absence of oxygen. Bacteria generally produce acids. Vinegar (acetic acid) is the direct result of bacterial metabolism (Bacteria need oxygen to convert the alcohol to acetic acid). In milk, the acid coagulates the casein, producing curds. In pickling, the acid preserves the food from pathogenic and putrefactive bacteria.
When yeast ferments, it breaks down the glucose (C6H12O6) into exactly two molecules of ethanol (C2H6O) and two molecules of carbon dioxide (CO2).
Ethanol fermentation (performed by yeast and some types of bacteria) breaks the pyruvate down into ethanol and carbon dioxide. It is important in bread-making, brewing, and wine-making. When the ferment has a high concentration of pectin, minute quantities of methanol can be produced. Usually only one of the products is desired; in bread the alcohol is baked out, and in alcohol production the carbon dioxide is released into the atmosphere.
Lactic acid fermentation breaks down the pyruvate into lactic acid. It occurs in the muscles of animals when they need energy faster than the blood can supply oxygen. It also occurs in some bacteria and some fungi. It is this type of bacteria that convert lactose into lactic acid in yogurt, giving it its sour taste.
In vertebrates, during intense exercise, cellular respiration will deplete oxygen in the muscles faster than it can be replenished. An associated burning sensation in muscles has been attributed lactic acid causing a decrease in the pH during a shift to anaerobic glycolysis. While this does partially explain acute muscle soreness, lactic acid may also help delay muscle fatigue, although, eventually the lower pH will inhibit enzymes involved in glycolysis.Contrary to currently popular belief, the lactic acid is not the primary causes for the drop in pH, but rather ATP-derived hydrogen ions.Delayed onset muscle soreness cannot be attributed to the lactic acid and other waste products as they are quickly removed after exercise. It is actually due to microtrauma of the muscle fibres. Eventually the liver metabolises the lactic acid back to pyruvate.

Zymology
Zymology is the scientific term for fermentation. It deals with the biochemical processes involved in fermentation, with yeast selection and physiology, and with the practical issues of brewing. Zymology is occasionally known as zymurgy