One method of making ethanol is to use plant material which contains starch. The reaction is called fermentation where the sugar is converted into an alcohol. Fermentation is an enzyme-controlled reaction that takes place inside living yeast cells in which glucose is broken down into alcohol and carbon dioxide with the release of energy.
The carbohydrates in the food must be broken up first to form sugars. This is done by hydrolysing the carbohydrate. Hydrolysis is a reaction in which water molecules (‘hydro’=water) react with larger molecules to split them (‘lysis’= split apart) into two or more smaller molecules.
part of a starch molecule individual glucose molecules H H O H H O H H O water molecules
The sugar is then converted into alcohol. The equation for the reaction is : glucose → ethanol + carbon dioxide C 6 H 12 O 6 → 2C 2 H 5 OH + 2CO 2 Fermentation is an enzyme-controlled reaction that takes place inside living yeast cells in which glucose is broken down into alcohol and carbon dioxide with the release of energy.
Reactions that are catalysed by enzymes normally produce the best results at temperatures around 40 O C. The reactions will be slower at lower temperatures and at higher temperatures, over 70 O C, the enzyme will stop working because the high temperature denatures the enzyme.
The fermentation process takes place inside living cells therefore if the yeast dies then the reaction will stop. When the alcohol concentration reaches about 14% the it begins to kill the yeast. Fermentation reactions therefore have a limit to the percentage of alcohol they can produce.
Biological Action Yeast: Living organism Microscopic fungus Feeds on sugars Is a biological catalyst Therefore classed as an enzyme
Conditions for Fermentation pH: prefer a neutral pH (or round about) if it is too acidic or alkaline the yeast will die. Temperature: Room temperature is ideal; too cold and the yeast stops working (goes into a “sleep” state) and too warm and it dies. Alcohol concentration: yeast does not like alcohol, about 14% alcohol the yeast dies
Distilling The Alcohol The alcohol concentration can be increased by separating the alcohol from the water, this is carried out via a process called distillation. The separation by distillation is an easy process as ethanol boils at 78 O C and water boils at 100 O C although both liquids will evaporate to some extent at any temperature.