Fermentation in beer (reaction schemes)?

Question

I've been struggling with this for a while, and I can't seem to solve the problem. Alcohol is made in the fermentation process in beer:

Sugars formed during mashing (maltose and maltotriose) will undergo hydrolysis and form multiple molecules of the simple sugar glucose. This glucose will then be fermented by the yeast under anaerobic conditions into ethyl alcohol and carbon dioxide

I understand that perfectly. My problem is to figure out the reaction schemes for the reactions, for each step. Can anyone help me with that?

Answer 1

We can break the entire process into three parts:

1. breakdown of maltose and maltotriose into glucose
2. glycolysis to give pyruvate
3. reductive decarboxylation to give the alcohol.

Writing out full equations for each actual step should hopefully not be required; especially glycolysis is explained in detail in many places across the internet and practically every metabolism textbook. Hence I will restrict myself to generic equations for each part of the process. Furthermore, each set of reactions is catalysed by a specific set of enzymes; I will be ignoring these.

1. Maltose breakdown

   $$\ce{\underset{maltose}{C12H22O11} + H2O -> 2 \underset{glucose}{C6H12O6}}\tag{1}$$

2. Glycolysis

   $$\begin{multline}\ce{\underset{glucose}{C6H12O6} + 2 ADP^3- + 2 HPO4^2- + 2NAD+ -> } \\\ce{2 \underset{pyruvate}{C3H3O3-} + 2 ATP^4- + 2 H2O + 2 NADH + 2 H+}\end{multline}\tag{2}$$

3. Alcoholic fermentation

   $$\begin{align}\ce{\underset{pyruvate}{C3H3O3-} + 2 H+ &-> CO2 + \underset{acetaldehyde}{C2H4O}}\tag{3}\\ \ce{\underset{acetaldehyde}{C2H4O} + NADH + H+ &-> \underset{ethanol}{C2H6O} + 2 NAD+}\tag{4}\end{align}$$