Alcoholic Fermentation

INTRODUCTION

Alcoholic Fermentation, also referred to as ethanol fermentation, is a biological process in

which sugars such as glucose, fructose, and sucrose are converted into cellular energy and thereby

produces ethanol and carbon dioxide as metabolic waste products. Because yeasts perform this

conversion in the absence of oxygen, ethanol fermentation is classified as anaerobic.

Alcoholic fermentation is identical to glycolysis except for the final step (Fig. 1). In alcoholic

fermentation, pyruvic acid is broken down into ethanol and carbon dioxide.

Fig. 1. A comparison of two anaerobic energy-harvesting mechanisms.

Both alcoholic fermentation and glycolysis are anaerobic fermentation processes that begin with

the sugar glucose. Glycolysis requires 11 enzymes which degrade glucose to lactic acid (Fig. 2).

Alcoholic fermentation follows the same enzymatic pathway for the first 10 steps. The last enzyme of

glycolysis, lactate dehydrogenase, is replaced by two enzymes in alcoholic fermentation. These two

enzymes, pyruvate decarboxylase and alcoholic dehydrogenase, convert pyruvic acid into carbon

dioxide and ethanol in alcoholic fermentation.

Fig. 2. The Glycolysis Process.

There are two types of anaerobic fermentation:

· lactate fermentation (e.g. in muscles when an animal exercises hard)

· alcoholic fermentation (e.g. in yeast, which can be used to make wine or beer)

PROCESS

Fig. 3. Alcoholic Fermentation process.

The pyruvic acid (pyruvate) from glycolysis loses one carbon in the form of carbon dioxide to

form acetaldehyde, which is reduced to ethyl alcohol by NADH.

When acetaldehyde is reduced to ethyl alcohol, NADH becomes NAD+ (is oxidized).

Fig. 3.1.