Fermentation
Introduction to Fermentation
Fermentation is a metabolic process that converts sugar to acids, gases, or alcohol using
microorganisms—typically yeast or bacteria—under anaerobic conditions. In alcoholic
beverage production, ethanol fermentation is the central process.
◼ This is not as simple as it looks however the glucose molecule passes through 12 stable
intermediate steps requiring enzymes, co- enzymes and inorganic catalysts, before being
converted to alcohol.
The biochemical pathway of alcoholic fermentation
Types of fermentation
◼ Cheese making
◼ Soy sauce
◼ Bread making
◼ Sauerkraut
◼ Salami
◼ Pharmaceuticals
◼ Vinegar
◼ Wine and beer
◼ Fuel ethanol
Influences on Fermentation
◼ Temperature
◼ pH
◼ Sulfur dioxide
◼ Nitrogen/nutrients
◼ Sugar
◼ Alcohol
◼ By controlling these parameters you cancontrol fermentation.
Alcohol conversion
There is a formula that is used to estimate the amount of alcohol that is produced from a
given amount of sugar:
Conversion Factor = % Alcohol / Degree Brix = about 0.60
So, a 24ºBrix must will result in about 14.4% alcohol, this is if the wine is fermented dry
So, 24 ºB x 0.60 = 14.4%
Production of Wine
Wine is produced by fermenting the natural sugars in grapes using yeast.
Steps in Wine Production
a. Harvesting
• Grapes are harvested when sugar levels (°Brix) are ideal (typically 22–24 °Brix).
b. Crushing and Pressing
• Grapes are crushed to release juice (called must).
• In white wine, skins are removed before fermentation.
• In red wine, skins remain with the must for color and tannin extraction.
c. Fermentation
• Natural or added Saccharomyces cerevisiae yeast ferments sugars to ethanol.
Main Enzyme Involved:
• Zymase (a complex of enzymes in yeast)
Functions of Zymase:
• Breaks down glucose/fructose into ethanol and CO₂.
• Key enzymes in Zymase complex:
o Hexokinase (phosphorylates glucose)
o Phosphofructokinase
o Alcohol dehydrogenase
o Pyruvate decarboxylase
d. Aging and Clarification
• Wine is aged in barrels or tanks; clarified using fining agents or filtration.
e. Bottling
• After stabilization, the wine is bottled for distribution.
Production of Beer
Beer is made by fermenting malted cereal grains (usually barley), flavoured with hops.
Steps in Beer Production
a. Malting
• Barley grains are soaked, germinated, and dried.
• Enzymes such as amylase are activated during germination.
b. Mashing
• Malted barley is mixed with water and heated.
• Starches are converted to fermentable sugars (e.g., maltose).
Main Enzymes:
• α-amylase: breaks starch into dextrins
• β-amylase: cleaves maltose from dextrins
• Limit dextrinase: further breaks down branched starch molecules
Relevant Reaction:
c. Lautering
• The liquid (wort) is separated from the grain husks.
d. Boiling
• Wort is boiled with hops (for bitterness and aroma).
• Proteins coagulate and are removed.
e. Fermentation
• The cooled wort is inoculated with yeast (e.g., Saccharomyces cerevisiae or S.
pastorianus).
• Yeast ferments the sugars into ethanol and CO₂.
f. Conditioning
• Beer is stored to mature and allow unwanted flavors to settle.
g. Filtration and Packaging
• Final beer is clarified, carbonated (if necessary), and bottled or canned.
Comparison of Wine and Beer Fermentation
Feature Wine Beer
Main Sugar Grape sugar (glucose, Maltose from starch
Source fructose)
Enzymes Used Zymase (yeast enzymes) Amylases (plant enzymes), then
zymase
Typical Yeast Saccharomyces cerevisiae S. cerevisiae (ales) or S. pastorianus
(lagers)
Carbonation Usually still (flat) Naturally or artificially carbonated
Alcohol Content 9–15% 3–8%
Key Enzymes and their summaries
Enzyme Source Role
Amylase (α, β) Barley (during malting) Breaks down starch to sugars
Zymase complex Yeast Converts glucose to ethanol
Pyruvate Yeast Converts pyruvate to acetaldehyde + CO₂
decarboxylase
Alcohol Yeast Converts acetaldehyde to ethanol
dehydrogenase
