The Chemistry of Winemaking - Fermentation
Fermentation is a chemical process that breaks down organic matter. Microbes like bacteria carry out this process. Mold and yeast act upon molasses and mineral salts to create penicillin. Yeast breaks down sugar taken from malted grain and turns it into ethyl alcohol and carbon dioxide gas to make beer. French Scientist Louis Pasteur discovered that microbes ferment beer and wine. Sugar from grape juice breaks down the same way to make wine. Fermentation is also used to make bread, cheese and yogurt. Sometimes fermentation can be unhealthy; for example milk that has been fermented turns sour.

Who first realized why alcohol forms from fruit?
In 1810 French chemist Joseph Louis Gay-Lussac (1778-1850), the same scientist whose results on combining gas volumes were explained by Avogadro, discovered the overall reaction that converts glucose to alcohol and carbon dioxide (alcoholic fermentation): C₆H₁₂O₆ -->2 CO₂+ 2C₂H₅OH.

In 1837 it was independently suggested by German physiologist Theodore Schwann, Botanist Friedrich Kutzing and French physicist Charles Cagniard de la tour that alcoholic fermentation requires yeast cells and it is a physiological function of these organisms. 

How did Loius Pasteur contribute to the chemistry of winemaking?
Pasteur realized that yeast was responsible for the conversion of glucose to alcohol. By adding yeast to a simple sugar solution, he showed that alcohol could be formed. He also revealed that the metabolism of yeast was pH -dependent; the acid-level played a role in determining wine's properties. One of grapes' two principal acids, tartaric acid (malic acid is the other), led to Pasteur's  discovery of enantiomers, molecules that are mirror images of one another.

Why is the acid-level important in winemaking?
Low acidity (high pH), common in grapes that are too sweet because they were grown in excessively warm climates, lowers the amount of subtle flavors in the grapes and consequently in the wine. As mentioned before, there is an optimum pH for fermentation as well. Prior to fermentation, pH is measured either by titration or more conveniently with a meter, and if it is too high, tartaric acid is added.

What is the basic composition of grapes?

Component	Percent Range
	70-80
	18-25
	0.3-1.5
20 amino acids	0.7
Vitamin C	11 mg per 100 g
	73 IU per 100 g
potassium, K+	185 mg per 100 g
esters	less than 0.1%
13 anthocyanins	less than 0.1%

What is the role of sulfur dioxide, SO₂ in winemaking?
After the grapes are crushed (the stems are mechanically separated), the blend of pulp, skin and seeds are transferred to a vat. (For white wine only the pulp is used). Here SO₂ is introduced to kill wild yeasts. These are too varied in composition, leading to a competition amongst themselves that causes fermentation to stop prematurely. They are replaced with a pure culture, usually of Saccharomyces cerevisiae .In addition,SO₂, inhibits enzymes that oxidize phenolic compounds responsible for discoloring wine.

What role does temperature play in winemaking?
Fermentation is an exothermic process (i.e., it releases heat). But in wine-making, the temperature cannot exceed 85 F = 29.4 C for red wines or 60 F = 15.3 C for white wines), otherwise the growth of yeast cells will stop. Moreover, a lower temperature is desirable because it increases the production of esters, other aromatic compounds and alcohol itself. This makes the wine easier to clear and less susceptible to bacterial infection. In general, temperature control during alcoholic fermentation is necessary to (1) facilitate yeast growth, (2) extract flavors and colors from the skins, (3) permit accumulation of desirable by-products, and (4) prevent undue rise in temperature, killing the yeast cells.

What positive role do bacteria play in winemaking?
To clarify the wine, the fermented juice is transferred into a settling vat, or if made on a smaller scale, into a demijohn. In these, suspended yeast cells, cream of tartar and particles of skin and pulp settle to the bottom of the container. As the yeast cells break down within the precipitate, they stimulate the growth of Lactobacillus bacteria that convert the wine's malic acid into lactic acid. This process is especially important in wines made from highly acidic grapes because lactic acid is a weaker acid than malic acid. (Bacteria decarboxylate malic acid , thus removing the acidic carboxyl group), so it mellows the wine's taste.

Is the level of oxygen important in winemaking?
After the demijohn stage, the wine is repeatedly racked to leave behind less and less precipitate. During the repeated pourings, the wine is also given a chance to rid itself of the excess carbon dioxide from fermentation. As the CO₂ escapes, oxygen enters the wine with each transfer, helping eventually to age the wine.

The Manufacturing Process
The process of wine production has remained much the same throughout the ages, but new sophisticated machinery and technology have helped streamline and increase the output of wine. Whether such advances have enhanced the quality of wine is, however, a subject of debate. These advances include a variety of mechanical harvesters, grape crushers, temperature-controlled tanks, and centrifuges.

The procedures involved in creating wine are often times dictated by the grape and the amount and type of wine being produced. Recipes for certain types of wine require the winemaker (the vintner) to monitor and regulate the amount of yeast, the fermentation process, and other steps of the process. While the manufacturing process is highly automated in medium- to large-sized wineries, small wineries still use hand operated presses and store wine in musty wine cellars.

A universal factor in the production of fine wine is timing. This includes picking grapes at the right time, removing the must at the right time, monitoring and regulating fermentation, and storing the wine long enough.

The science of winemaking is called enology. The basic processes can be divided into four distinct steps: harvesting and crushing grapes; fermenting must; ageing the wine; and packaging:

• Harvested grapes are de-stemmed and crushed, where the "must" is extracted ("must"=juice+skins+seeds±stems for red; juice only for white). Initial quality evaluations are made and the vinification process is decided upon. Adjustments of sugar level and acid can be done at this point.
• Sulfur dioxide is added to prevent oxidation and color deterioration, selectively activate certain yeasts for fermentation, and kill bacteria and other undesirable microbes.
• Must is transferred to fermentation vats for sugar alcohol conversion. Fermentation is stopped by racking or dispensing wine into containers; sometimes SO2 is used to stop the process and act as a preservative.
• Wines are filtered, then may be aged in oak barrels for some time prior to bottling.
• Bottling takes place at appropriate age, after ultra-filtration for clarity and antisepsis.

Harvesting and Crushing Grapes

• Vineyards inspect sample clusters of wine grapes with a refractometer to determine if the grapes are ready to be picked. The refractometer is a small, hand-held device (the size of a miniature telescope) that allows the vineyards to accurately check the amount of sugar in the grapes.
• If the grapes are ready for picking, a mechanical harvester (usually a suction picker) gathers and funnels the grapes into a field hopper, or mobile storage container. Some mechanical harvesters have grape crushers mounted on the machinery, allowing vineyard workers to gather grapes and press them at the same time. The result is that vineyards can deliver newly crushed grapes, called must, to wineries, eliminating the need for crushing at the winery. This also prevents oxidization of the juice through tears or splits in the grapes' skins.

  Mechanical harvesters, or, in some cases, robots, are now used in most medium to large vineyards, thereby eliminating the need for hand-picking. First used in California vineyards in 1968, mechanical harvesters have significantly decreased the time it takes to gather grapes. The harvesters have also allowed grapes to be gathered at night when they are cool, fresh, and ripe.

• The field hoppers are transported to the winery where they are unloaded into a crusher-stemmer machine. Some crusher-stemmer machines are hydraulic while others are driven by air pressure.

  The grapes are crushed and the stems are removed, leaving liquid must that flows either into a stainless steel fermentation tank or a wooden vat (for fine wines).

Fermenting the Must

• For white wine, all the grape skins are separated from the "must" by filters or centrifuges before the must undergoes fermentation. For red wine, the whole crushed grape, including the skin, goes into the fermentation tank or vat. (The pigment in the grape skins give red wine its color. The amount of time the skins are left in the tank or vat determines how dark or light the color will be. For rose, the skins only stay in the tank or vat for a short time before they are filtered out.)
  

• During the fermentation process, wild yeast are fed into the tank or vat to turn the sugar in the must into alcohol. To add strength, varying degrees of yeast may be added. In addition, cane or beet sugar may be added to increase the alcoholic content. Adding sugar is call chaptalization. Usually chaptalization is done because the grapes have not received enough sun prior to harvesting. The winemaker will use a handheld hydrometer to measure the sugar content in the tank or vat. The wine must ferments in the tank or vat for approximately seven to fourteen days, depending on the type of wine being produced.
  
  

• After crushing and fermentation, wine needs to be stored, filtered, and properly aged. In some instances, the wine must also be blended with other alcohol. Many wineries still store wine in damp, subterranean wine cellars to keep the wine cool, but larger wineries now store wine above ground in epoxylined and stainless steel tanks. The tanks are temperature-controlled by water that circulates inside the lining of the tank shell. Other similar tanks are used instead of the old redwood and concrete vats when wine is temporarily stored during the settling process.

  After fermentation, certain wines (mainly red wine) will be crushed again and pumped into another fermentation tank where the wine will ferment again for approximately three to seven days. This is done not only to extend the wine's shelf life but also to ensure clarity and color stability.

  The wine is then pumped into settling ("racking") tanks or vats. The wine will remain in the tank for one to two months. Typically, racking is done at 50 to 60 degrees Fahrenheit (10 to 16 degrees Celsius) for red wine, and 32 degrees Fahrenheit (0 degrees Celsius) for white wine.

• After the initial settling (racking) process, certain wines are pumped into another settling tank or vat where the wine remains for another two to three months. During settling the weighty unwanted debris (remaining stem pieces, etc.) settle to the bottom of the tank and are eliminated when the wine is pumped into another tank. The settling process creates smoother wine. Additional settling may be necessary for certain wines.
  
  
• After the settling process, the wine passes through a number of filters or centrifuges where the wine is stored at low temperatures or where clarifying substances trickle through the wine.
  
  
• After various filtering processes, the wine is aged in stainless steel tanks or wooden vats. White and rose wines may age for a year to four years, or far less than a year. Red wines may age for seven to ten years. Most large wineries age their wine in large temperature-controlled stainless steel tanks that are above ground, while smaller wineries may still store their wine in wooden barrels in damp wine cellars.
  
• The wine is then filtered one last time to remove unwanted sediment.

  The wine is now ready to be bottled, corked, sealed, crated, labeled, and shipped to distributors.

• Most medium- to large-sized wineries I now use automated bottling machines, and most moderately priced and expensive wine bottles have corks made of a special oak. The corks are covered with a peel-off aluminum foil or plastic seal.
  
  Cheaper wines have an aluminum screw-off cap or plastic stopper. The corks and screw caps keep the air from spoiling the wine. Wine is usually shipped in wooden crates, though cheaper wines may be packaged in cardboard.
  
  

Quality Control
All facets of wine production must be carefully controlled to create a quality wine. Such variables as the speed with which harvested grapes are crushed; the temperature and timing during both fermentation and ageing; the percent of sugar and acid in the harvested grapes; and the amount of sulfur dioxide added during fermentation all have a tremendous impact on the quality of the finished wine.

Different Process for Different Wines
Winemaking is the conversion of grape juice to wine by yeast. This can be easily and inexpensively done at home. Yet, it involves many complex chemical processes and much more than the fermentation of some of the sugar in grape juice to ethanol.

a. Dry vs. sweet

• In dry wine, all the sugar has been converted to ethanol.
• Sweet wine still has a lot of the sugars from the fruit juice. It is prepared by doing only short fermentations or by using yeasts with low alcohol tolerances so that not all the sugar is consumed.

b. Champagne vs. sparkling wines

• Production of champagne involves a final in-bottle fermentation, that generates the carbon dioxide in the final solution. The yeasts that settle in the long neck of the bottle, which is inverted during this process, are frozen and removed. A small amount of sugar is added, and the bottle is tightly corked.
• In sparkling wines, carbon dioxide is injected. Consequently, sparkling wines lack the flavors from having the yeasts do the final fermentation, as in champagne.

c. White vs. red

• White wines use both red and white grapes, but the skins are removed after a short time of fermentation. White wines are usually not aged -- generally, they are sold after five months.
• Red wines are made from red grapes only, and the skin is kept in the fermentation mixture for longer times before being removed. Red wines undergo several years of aging in barrels before being bottled and sold.

d. Young vs. mature

• In mature wines, the malate in grape juice is converted to ethanol by Lactobacillus. This makes mature wines less acidic and more flavorful compared to young wines.
• In addition, many other chemical reactions occur during aging, most of which are not yet understood.

e. Bad

• Why do we sample wines in restaurants before agreeing to buy them?
• Why do wine connoisseurs lay corked wines on their sides?

If the cork is allowed to dry, air and microorganisms may enter the bottle causing the following reaction:

CH₃CH₂OH + 2 NAD⁺ -----> 2 CH₃COO^- + 2 NADH

Generation of acetic acid (CH₃COO^-) from ethanol amounts to the production of vinegar. Thus the wine will be soured.