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Unlock This CourseYeast is a fascinating microorganism that can survive and thrive even when oxygen isn't available. This amazing ability makes yeast incredibly useful for humans - from baking bread to brewing beer! When yeast can't get oxygen, it switches to a different type of respiration called anaerobic respiration or fermentation.
Key Definitions:
Yeast are tiny single-celled organisms belonging to the fungi kingdom. The most common type used in food production is Saccharomyces cerevisiae. These microscopic powerhouses can switch between two types of respiration depending on whether oxygen is present or not.
When yeast finds itself in an environment without oxygen, it doesn't just give up and die. Instead, it cleverly switches to anaerobic respiration to keep producing the energy it needs to survive.
During anaerobic respiration, yeast breaks down glucose (sugar) in a completely different way than it would with oxygen present. The process is much simpler but produces far less energy.
The fermentation equation:
Glucose → Ethanol + Carbon dioxide + Energy (ATP)
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + 2ATP
Yeast uses glucose as its fuel source. This can come from added sugar, or from starches that have been broken down into simple sugars.
Ethanol (alcohol) is one of the main products. This is what makes alcoholic drinks alcoholic and also helps preserve food.
CO₂ gas is released, which makes bread rise and creates the bubbles in sparkling drinks.
Anaerobic respiration in yeast produces only 2 ATP molecules per glucose molecule, compared to 38 ATP molecules produced during aerobic respiration. This is why yeast prefers oxygen when it's available - it's much more efficient!
The products of yeast fermentation - ethanol and carbon dioxide - are incredibly useful to humans. This is why we've been using yeast for thousands of years, even before we understood exactly how it worked.
In bread making, it's the carbon dioxide that's most important. When yeast ferments the sugars in flour, the CO₂ gas gets trapped in the dough, creating bubbles that make the bread light and fluffy.
Yeast is mixed with flour, water and often a small amount of sugar. As the yeast ferments, CO₂ bubbles form throughout the dough. The heat from baking kills the yeast and sets the structure, leaving behind the familiar spongy texture of bread.
For alcoholic drinks, it's the ethanol that's the star of the show. Different types of alcohol are made by fermenting different sugar sources.
Made by fermenting malted barley. The starches in barley are first converted to sugars, then yeast ferments these sugars into alcohol.
Made by fermenting grape juice. Grapes naturally contain sugars that yeast can directly ferment into alcohol.
Made by fermenting various plant materials, then distilling to concentrate the alcohol content.
Several environmental factors can speed up or slow down the fermentation process. Understanding these helps us control fermentation for different purposes.
Temperature has a huge impact on how fast yeast works. Like most biological processes, fermentation is controlled by enzymes and enzymes are very sensitive to temperature.
Most yeast works best at around 25-30°C. At this temperature, the enzymes involved in fermentation work at their maximum rate. Too cold and the process slows down dramatically. Too hot and the enzymes denature, stopping fermentation completely.
The amount of sugar available affects how much fermentation can occur. More sugar generally means more fermentation, but there are limits.
Yeast prefers slightly acidic conditions (pH 4-6). If the environment becomes too acidic or too alkaline, fermentation slows down or stops.
Large bakeries carefully control temperature, humidity and timing to ensure consistent bread quality. They often use special strains of yeast that work faster and produce more CO₂. The dough is kept at exactly the right temperature to optimise fermentation speed while maintaining flavour development.
Understanding the differences between aerobic and anaerobic respiration helps explain why yeast behaves differently in different conditions.
Requires: Oxygen
Products: CO₂ + Water + 38 ATP
Efficiency: Very high
Uses: Normal yeast growth and reproduction
Requires: No oxygen
Products: Ethanol + CO₂ + 2 ATP
Efficiency: Low
Uses: Survival when oxygen is limited
In industrial food production, controlling fermentation is crucial for getting consistent, high-quality products.
To ensure fermentation occurs, oxygen must be excluded. This can be done by:
Sometimes we need to stop fermentation at the right moment:
Yeast fermentation is considered environmentally friendly because it's a natural process that doesn't require harsh chemicals. The CO₂ produced is part of the natural carbon cycle and the process can use renewable plant materials as raw ingredients.
Today, scientists are finding new ways to use yeast fermentation beyond traditional food and drink production.
Yeast can ferment plant materials to produce ethanol for use as a renewable fuel. This biofuel can power cars and reduce dependence on fossil fuels.
Genetically modified yeast can be used to produce medicines and vaccines through fermentation processes, making important drugs more affordable and accessible.