fermentation

Chemical Reactions During Wine Fermentation

Guest Post By Sarah Meadows

This is the second article of a three part series exploring wine fermentation:

  • Fundamentals of Wine Fermentation
  • Chemical Reactions during Wine Fermentation
  • Unconventional Methods of Wine Fermentation

I bet no one could have guessed they would find the byproduct of another living organism so delicious. But wine is just that, a byproduct of tiny yeast cells eating from their surroundings. And who can really blame them; I am pretty positive anyone dropped into a bath of sweet chocolate could not resist a taste. Beyond just basic mastication, yeast’s metabolism of carbohydrates to produce alcohol is a fairly complex series of chemical reactions. Moreover, other compounds and chemicals within the grape juice mixture can also react chemically to create various characteristics. Our discussion will begin with basic elements and build upon them until we’ve bottled our delicious wine.

Flashback to your college organic chemistry course and lament not paying closer attention for we are about to break down carbohydrates and chemical bonds. Carbohydrates, commonly referred to as simply sugars, are essential to life. Most living organisms cannot survive without carbohydrates and glucose tends to be the most utilized. The word carbohydrate indicates its chemical structure; most are composed of carbon atoms and hydrogen atoms with accompanying oxygen. For example, a molecule of glucose has a chemical structure of 6 carbon atoms, 12 hydrogen, and 6 oxygen atoms. Carbohydrates come in different sizes. Some are monosaccharides ( literally, one sugar), polysaccharides (many sugars), and oligosaccharides (few sugars). Glucose and fructose are examples of monosaccharides because they can’t be broken down into smaller compounds.

Yeast are eukaryotic single-celled organisms that propagate by means of mitosis or cellular division. They do not need sunlight to survive and most can function with low oxygen demands. Interestingly, the word yeast means, roughly, to bubble or boil; exactly the reaction created by carbohydrate fermentation and gas production. When yeast are introduced to the grape must, they begin breaking down the monosaccharides. Phosphates, that are present in the mixture and generally carry 4 ionically bonded oxygen molecules, begin bonding to the sugars by means of the hydrogen molecules. Because of this attachment and rearranging of molecules, the fructose and glucose sugars are broken apart into 3 carbon structures.

Fermentation creates the bubbling effect when 1 carbon atom, 2 oxygen atoms, and 1 hydrogen atom, also known as a carboxylic group, is released. The hydrogen atom is re-bonded elsewhere leaving a carbon atom covalently (shared electrons) double bonded to 2 oxygen atoms; this chemical reaction creates carbon dioxide. But where do the lost hydrogen atoms go? Let’s break it down a little further.

What we ultimately want is alcohol also called ethanol. The chemical structure of ethanol is 2 carbon atoms, 1 oxygen atom, and 6 hydrogen atoms. When you release carbon dioxide, CO2, the remaining chemical compound is called acetaldehyde which has a chemical structure of 2 carbon atoms, 4 hydrogen atoms, and 1 oxygen atom double bonded to one of the carbons. This means we still need two more hydrogen atoms to make alcohol. One simple rule helps clarify this reaction, a carbon atom always has to have four bonds. So, the acetaldehyde molecule will release one bond from the oxygen and take on a hydrogen atom which allows the oxygen atom (still single bonded to the carbon) to bond with yet another hydrogen atom creating ethanol.

Ethanol has many names such as grain alcohol, drinking alcohol, pure alcohol, or spirits. But regardless of the label, alcohol is an amazingly versatile compound and one of the earliest used by humanity. Fermentation, particularly by yeast, is the most common method of alcohol production. Once the alcohol content of the wine reaches the desired level, the yeast are either inactivated or killed through heating or wine cooling and storage. Wine coolers and refrigeration offer a quick way to slow fermentation. Once the yeast have been rendered inactive, they are generally filtered out, along with any other sediment, before the wine is bottled. Realistically, the details and all the various chemical components of wine production are too numerous to elaborate on in one article but we have covered the basic function of yeast in the fermentation process and the elements of chemical reactions that produce the delicious and intoxicating delight called wine.

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