Fermentation, derived from the Latin word fervere (to boil), is a metabolic process that occurs in the absence of oxygen (anaerobic conditions) in which microorganisms, typically yeasts and bacteria, break down carbohydrates to produce simpler compounds like ethanol, lactic acid, and carbon dioxide. The key to this process is the substrate: the fermentable sugar.
Understanding Fermentable Sugar
A fermentable sugar is any carbohydrate molecule that a specific microorganism can metabolize through fermentation. Yeasts, particularly the common brewer's yeast (Saccharomyces cerevisiae), are only able to directly utilize monosaccharide (single-unit) sugars such as glucose and fructose.
Larger sugars (disaccharides like sucrose and maltose, or trisaccharides like maltotriose) must first be broken down into their constituent monosaccharides by enzymes produced by the yeast or present in the environment before they can be consumed.
The Fermentation Process Overview
The fundamental reaction in alcoholic fermentation, where glucose is the sugar, is summarized by the following equation:
$$\text{C}6\text{H}{12}\text{O}_6 \text{ (glucose)} \longrightarrow 2\text{C}_2\text{H}_5\text{OH (ethanol)} + 2\text{CO}_2 \text{ (carbon dioxide)}$$
This process begins with glycolysis, where a glucose molecule is converted into two pyruvate molecules, generating a small amount of ATP. The pyruvate is then converted into metabolic waste products (ethanol and $\text{CO}_2$ in yeast) to regenerate $\text{NAD}^+$ for glycolysis to continue.
Common Examples of Fermentable Sugars
Many sugars are fermentable, but their efficiency and the method required for fermentation vary depending on the microorganism.
1. Glucose (Dextrose)
Glucose, a monosaccharide, is arguably the most readily fermentable sugar. It is often referred to as dextrose, especially in brewing. Because it is a simple sugar, yeast can consume it immediately without needing to break it down first. Glucose is derived from starches found in corn, potatoes, and grains.
2. Fructose
Fructose is another monosaccharide, commonly found in fruits and honey. While simple, yeast often utilizes glucose preferentially over fructose when both are present, though fructose is still highly fermentable. In commercial fermentation, fructose can be fermented quickly.
3. Sucrose (Table Sugar)
Sucrose is a disaccharide composed of one glucose molecule and one fructose molecule. Saccharomyces cerevisiae yeast produces an enzyme called invertase, which rapidly hydrolyzes (breaks down) sucrose into glucose and fructose, which are then fermented. Table sugar, brown sugar, and cane sugar are all rich in sucrose.
4. Maltose
Maltose is a disaccharide made of two glucose molecules linked together. It is the main fermentable sugar found in brewery wort, derived from malted grains like barley. While yeast can ferment maltose, its uptake is often repressed by high glucose concentrations until the glucose is depleted.
5. Maltotriose
Maltotriose is a trisaccharide (three glucose units). It is less fermentable than maltose and glucose. Its fermentation depends heavily on the specific yeast strain; some ale yeasts struggle with it, while many lager yeasts (like Saccharomyces pastorianus) can ferment it effectively.
Fermentability Comparison Table
| Sugar Type | Chemical Class | Found In | Fermentability (by S. cerevisiae) |
|---|---|---|---|
| Glucose | Monosaccharide | Corn syrup, fruits, hydrolyzed starch | Very High (Preferred) |
| Fructose | Monosaccharide | Fruits, honey, agave | High (Secondary preference) |
| Sucrose | Disaccharide | Table sugar, cane sugar, beets | High (After hydrolysis) |
| Maltose | Disaccharide | Malted grains, beer wort | Medium (Glucose repressed) |
| Lactose | Disaccharide | Milk, dairy products | None (Requires lactase) |
Non-Fermentable Sugars
It is also important to note sugars that are generally considered non-fermentable by common yeast. The primary example is lactose, a disaccharide found in milk. Most Saccharomyces yeast strains do not produce the enzyme lactase required to break lactose into glucose and galactose. Therefore, lactose remains in the fermented product, adding sweetness and body, a technique often used in brewing "milk stouts."
Another group includes sugar alcohols (polyols) like sorbitol, xylitol, and erythritol, and artificial sweeteners, none of which are fermentable by yeast.
Conclusion
A fermentable sugar is a carbohydrate that can be broken down by microorganisms to produce energy and metabolic byproducts, most commonly ethanol and $\text{CO}_2$. The best example of a readily fermentable sugar is glucose, a simple monosaccharide that yeast can utilize instantly. While other sugars like sucrose and maltose are also fermentable, they require preliminary enzymatic breakdown. The specific type of sugar used dictates the rate of fermentation and the final characteristics of products like bread, beer, or wine.
