The Affects Of Different Yeast On The Rate Of Fermentation

1129 words - 5 pages


To see how different yeast affect and influence the rate of fermentation, how much alcohol is being produced and how this affects the overall quality of wine produced.


Alcohol, is mostly thought of as ethanol which is the alcohol which is found in alcoholic beverages such as wine and beer. But there are many different varieties of alcohol such as methanol, propanol and butanol.

In chemistry terms “alcohol” is a compound of a hydroxyl group which is covalently bonded to a carbon chain which can be seen in figure 1.

Throughout this experiment ethanol is the alcohol which is produced during the fermentation process of sugar which is present in the grape must or juice.

Figure 1: Chemical structure of Ethanol (Wikimedia, 19 July 2009)

Yeast is a single celled living organism which is necessary in the process of fermentation of the grape must. Invertase, an enzyme is present in yeast acts as a catalyst to speed up the chemical reaction where sucrose is converted into fructose and glucose. (Donal O’Leary, 2000)

Figure 2: The word and balanced equation of the conversion of Sucrose to simple sugars (Donal O’Leary, 2000)

Another enzyme which is found in yeast is known as Zymase. It is this enzyme present in yeast which converts the glucose and fructose produced into ethanol and carbon dioxide. (Donal O’Leary, 2000)

Figure 3: The word and chemical equation for the conversion of simple sugar to ethanol (Donal O’Leary, 2000)

In the making of wine there are 2 fermentation processes known as
• Primary Fermentation
• Secondary Fermentation

In the process of primary fermentation the sucrose in converted into glucose and fructose and the glucose and fructose start to be converted into ethanol. The carbon dioxide which is produced during this process escapes into the air. (WineDefintions Staff Writer)

In the secondary fermentation the grape must get placed into an air tight contain which is majority oxygen free (may contain some minimal oxygen). This is therefore an anaerobic process. This is the final stage and this is where the final conversion of fructose and glucose into ethanol and improvements to the aromas and final flavours of the wine are produced. (WineDefintions Staff Writer)

There is also a third process of fermentation known as malolactic fermentation. This can occur naturally in the wine if there is wild yeast remaining but is often encouraged by winemakers by adding in bacteria to cause this process to occur. In malolactic fermentation, malic acid is converted into lactic acid by a naturally occurring process in which bacteria feeds on the malic acid. This process is sometimes encouraged as it reduces the wines acidity which therefore removes or reduces the undesirable flavours which can occur due to the acidity. (WineDefintions Staff Writer)

There are many various acids in wine, the main being:
• Tartaric acid (90%)
• Malic

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