With fossil fuels being non-renewable and contributing to global warming, biofuels are increasingly being considered as a possible alternative for the future.
Some biofuels are produced by using microorganisms to anaerobically ferment carbohydrate in the plant material - as is the case with bioethanol and biogas production (each process uses different microorganisms).
When ethanol is made by fermentation, sugar is converted into ethanol and carbon dioxide if conditions are anaerobic. Single-celled fungi, called yeast, contain enzymes that are natural catalysts for making this process happen.
In some countries, such as Brazil, the source of sugar is sugar cane – which yeast can directly ferment into ethanol. In other countries, plants such as maize are used. As maize contains starch rather than sugar, the enzyme amylase must first break down the starch into sugar before the yeast can ferment it into ethanol.
The ethanol produced by yeast only reaches a concentration of around 15 per cent before the ethanol becomes toxic to the yeast. In order to make it sufficiently concentrated to be burnt as a fuel, the ethanol must be distilled.
There are some disadvantages to growing biofuel crops (such as sugar cane and maize) to be used as bioethanol.
When ethanol is oxidised by microbes, it forms a compound called ethanoic acid (CH3COOH), which is a form of carboxylic acid. This is the acid found in vinegar.
Other alcohols can also be oxidised in a similar way. For example, methanol can be oxidised to form methanoic acid. As well as the gain of oxygen through the oxidation process, hydrogen atoms have been reduced.
When alcohols are gently heated with a mixture of potassium dichromate(VI) and sulfuric acid, the mixture changes colour from orange to green. This chemical test was originally used to test for the presence of alcohol on people’s breath if they were suspected of drink-driving.