BACKGROUND INFORMATION ON THE FERMENTATION PROCESS

 

What is Wine?

Wine is an alcoholic beverage made by the fermentation of grape or fruit juice. Wines are distinguished by colour, flavour, bouquet or aroma, and alcoholic content. Wines are red, white, or rosé (depending on the grape used and the amount of time the skins have been left to ferment in the juice). Wines are also classified as dry or sweet, according to whether the grape sugar is allowed to ferment completely into alcohol (dry), or whether some residual sugar has been left (sweet).

In a natural wine, all the alcohol present has been produced by fermentation. You can also add more alcohol through other liquors as in fortified wines, such as sherry, port, Madeira, and Malaga.  These wines have brandy or other spirits added in them to increase the alcohol level, and change the flavour.  Fortified wines contain a higher alcohol content (from 16% to 35%) than normal wines (from 7% to 15%). The percentage of alcohol varies depending on how much sugar is fermented and how much additional alcohol is added by fortifying with brandy or other spirits.

 

How Does the Fermenting Process Work?

Fermentation is the chemical changes in organic substances produced by the action of enzymes (yeast). You can tell when something is fermenting when bubbles start to rise from the bottom like in carbonated drinks. When the bubbling stops it is finished fermenting. Scientists today often say that fermentation is the action of specific enzymes, called ferments, produced by minute organisms such as molds, bacteria, and yeasts. Probably the most important type of fermentation, and the type that I am focusing on, is alcoholic fermentation, in which the action of zymase secreted by yeast converts simple sugars, such as glucose and fructose, into ethyl alcohol and carbon dioxide. In other words, fermentation is the breakdown of complex organic substances into simpler ones through the action of catalysis (chemical transformation). The action of zymase causes starch to break down (hydrolyzed) into complex sugars, then simple sugars, and finally alcohol. Theoretically, the finer the sugar, the faster it should ferment and turn into alcohol. 

On the observations page, I have explained how I created an experiment demonstrating this. To estimate the amount of alcohol in your solution (wine), you need a hydrometer. This is a weighted glass tube which floats in a liquid and measures the Specific Gravity (density) of the solution. The sides of the hydrometer are lined with numbers. A hydrometer floating in water will sit at or read 1.000. As you add sugar, the water gets denser, and the hydrometer starts to rise. A sugar/water solution could start at 1.080, and when the fermenting process is finished, the reading should be close to 1.000 or lower (0.990). This is because pure alcohol is lighter than water.  As the sugar is converted to alcohol, the specific gravity (density) of the solution reduces. It is impossible to get pure alcohol by fermenting like I am - this is because after the alcohol content reaches about 18%, the yeast quits working.  Fortified wines like Port require a special fermentation process or the addition of Brandy to help reach a higher alcohol content.

 

What is Yeast?

Yeast consists of microscopic, one-celled fungi.  They are important because of their ability to ferment carbohydrates in various substances. Yeasts, in general, are found in nature, occurring in the soil and on plants.

Pure yeast cultures are grown in a medium of sugars, nitrogen sources, minerals, and water. The final product may take the form of dried yeast cells. In the making of wines, beers, spirits, and industrial alcohol, the fermented medium is the desired product, and the yeast itself is discarded or used to make animal feeds. There are many kinds of yeast such as bread yeast or wine yeast.  Bread yeast reacts in a shorter time and causes your bread to rise.  Wine yeast takes a lot longer to ferment and is what turns the sugar into alcohol.