Re: During the fermentation of sugars, what reactions form acids??

hi Julia,

Thanks for your question. Fermentation is an important process for many microorganisms - it allows them to generate some energy in the absence of oxygen. When yeast ferment sugars, they are using a biochemical pathway called glycolysis. Glyco- refers to sugar and -lysis means "to split", so the name tells us that this pathway breaks down sugar.

Glycolysis is explained in this article at Wikipedia. It's quite a detailed article, so don't worry if it seems complicated. You could also search Google Images with the word "glycolysis" and you will find lots of diagrams which illustrate the chemical reactions. I like this one.

The key steps in glycolysis are:

• A phosphate group is added to a glucose molecule to make glucose-6-phosphate.
• The glucose-6-phosphate is rearranged into another sugar called fructose-6-phosphate and another phosphate is added. I mentioned that fermentation allows cells to generate energy but these first steps actually consumes energy, as ATP is used.
• The sugar-phosphate molecule, which has 6 carbon atoms, is split into two molecules with 3 carbon atoms each.
• The 3-carbon molecules are then oxidised and rearranged in several steps, ending up with a molecule of pyruvate. During the oxidation and rearrangement, ATP is generated so the cell gets some energy back.

You correctly mentioned other sugars - sucrose and fructose. Yeast occur naturally on fruit, which contains fructose and sucrose that are used when we brew alcohol. Fructose can enter the glycolysis pathway and is converted to fructose-6-phosphate, instead of going via glucose. Sucrose is a disaccharide made from one glucose and one fructose molecule. The yeast can split the sucrose and then use the glucose and the fructose in glycolysis.

OK - where are the acids? If you look at the diagram of glycolysis, you'll see some molecules (apart from "phosphate") that end with "-ate": 1,3-diphosphoglycerate, 3-phosphoglycerate, 2-phosphoglycerate, phosphoenolpyruvate and pyruvate. In these molecules, "-ate" is another way of saying "-ic acid" - so pyruvate, for example, is also called "pyruvic acid". So all of these molecules are acids - they contain a carboxyl group, HO-C=O, which forms when the aldehyde group in glyceraldehyde-3-phosphate is oxidised.

So the answer to the question "which reactions form acids" is "all the ones from glyceraldehyde-3-phosphate to pyruvate". And I hope it's clear from the pathways and diagrams that oxygen is not needed for these acids to form. When we say that the aldehyde is oxidised we don't mean that oxygen is added - we mean that electrons are removed from it. This is an alternative definition of oxidation.