Yeast is a single-celled fungus essential for fermentation. It transforms simple sugars into alcohol and carbon dioxide, a process integral to food and drink production for thousands of years. Saccharomyces cerevisiae is sometimes called the first domesticated organism.
Yeasts breaks down sugars to produce alcohol. While Saccharomyces cerevisiae is most widely used, yeasts are plentiful and found almost everywhere. They inhabit fruits, soil, fluid, air and the acidic human gastrointestinal tract (GI).
Yeast is eukaryotic, meaning each cell has a nucleus and internal structures for complex metabolic activities. Cells survive in low-oxygen or completely anaerobic conditions by producing energy through fermentation.
They work best at a pH of 5.5 but are tolerant to different pH levels. The pH is the measure of acidity or alkalinity on a scale of 1 to 14, with less than 7 being more acidic, and more than 7 more alkaline.
Simple organisms of great complexity, yeasts are abundant, with over 1,500 known species. Cells are not self-mobile. They reproduce by budding, expanding colonies with pseudohyphae made of daughter cells. In tough times yeasts can form resistant spores to survive.
Dry yeast is a granular, pourable powder composed of billions of dehydrated single-celled organisms. Upon rehydration, they begin consume the sugars around them. Their fermentation reactions are used in such products as bread, booze, coffee and chocolate.
Yeasts are classified based on their fermentation methods. Top-fermenting yeasts such as S. cerevisiae rise to the top during fermentation, and bottom-fermenting yeasts like S. pastorianus settle at the bottom. In brewing the former is typically used for ales, the latter for lagers.
Top and bottom fermenting yeasts are originally named for the way they're harvested, either taken from the top or filtered from sediment at the bottom of the vessel. However both these methods have formed their own culture and definitions.
Fermentation Process
In fermentation yeast absorbs sugars, primarily glucose and fructose. This process involves several steps, triggered by enzymes in the yeast cells.
1. Glycolysis: The First Step in Sugar Breakdown
Fermentation begins with glycolysis, a metabolic pathway converting glucose or other sugars into pyruvate. Pyruvate is an organic compound with a 3-carbon atom structure essential in both synthesis and degradation pathways in organisms.
A metabolic source of energy, pyruvate operates in the form of ATP (adenosine triphosphate). Glycolysis occurs in the cytoplasm of yeast cells as a series of enzymatic reactions.
These convert simple sugars into energy as ATP. One molecule of glucose is converted into two molecules of ethanol and two molecules of carbon dioxide. After glycolysis, pyruvate moves to the next phase.
2. Alcoholic Fermentation: Production of Ethanol
After glycolysis, in the absence of oxygen, yeast undergoes alcoholic fermentation. Here, pyruvate is further converted into ethanol and carbon dioxide through a series of reactions.
Primary steps are:
Decarboxylation of Pyruvate: In this step, pyruvate is converted into acetaldehyde. Pyruvate loses a carbon atom, which is released as carbon dioxide. This is the source of the bubbles in beers and sparkling wines.
Reduction of Acetaldehyde to Ethanol: Acetaldehyde is then reduced to ethanol by the enzyme alcohol dehydrogenase, which requires the coenzyme NADH produced during glycolysis. This step is vital as it regenerates NAD+, allowing glycolysis to continue and produce energy even in anaerobic conditions.
Equation of Fermentation
C6H12O6 + 2 ADP + 2 Pi → 2 C2H5OH + 2 CO2 + 2 ATP
Reagents C6H12O6 (glucose), ADP (nucleotide Adenosine Diphosphate) and 2 Pi (Inorganic phosphate) are transformed into 2C2H5OH (ethanol), 2CO2 (carbon dioxide) and ATP (nucleotide Adenosine Triphosphate).
Several traits of yeast aid its fermentation proficiency:
Rapid Reproduction
Yeast can reproduce quickly through budding. They double their population every 1.5 hours in ideal conditions. This rapid growth is vital to fermentation.
Sugar Versatility
Beyond glucose, yeast can also metabolize various sugars like maltose, galactose and sucrose. This versatility expands the range of fermentable products.
Alcohol Tolerance
In brewing, alcohol content can ultimately kill yeast, but more often causes it to go dormant. Species and strains have varying tolerance. S. cerevisiae is active in alcohol concentrations of up to 14% - 20%. Alcohol tolerance of yeast determines the alcohol content of the beverage.
Flavor Compounds
During fermentation, yeast creates not only alcohol but also flavor compounds like esters and phenols. These compounds contribute to the unique tastes of beers and wines. Specific yeast strains in Belgian beers, for instance, produce fruity esters enhancing the overall flavor profile.
The choice of yeast strain also determines flavor of the final product. For instance, champagne makers often use specific strains to impart desired tastes, aromas and the bubbly effect.
Among the different champagne strains, EC-1118, commonly called Lalvin EC-1118, is highly favored for robustness and versatility. Lalvin EC-1118 is a popular strain recognized for dependability and high alcohol tolerance.
Facts About Yeast and Fermentation
Ancient Fermentation: Humans have been using yeast for thousands of years. Evidence of beer brewing goes back to c. 6000 BCE in Mesopotamia. Honey mead is one of the first intentionally fermented beverages. It develops as warming trends bring flowers and wild bees into regions previously occupied by ice, c. 8000 BCE.
Diversity and Applications: While Saccharomyces cerevisiae is commonly used in food production, other yeast species like Brettanomyces and Candida are also favored in fermentation, especially in production of certain wines and specialty beers.
Temperature Sensitivity: Yeast strains are sensitive to temperature changes. Optimal fermentation for Saccharomyces cerevisiae is between 25°C to 30°C (77°F to 86°F). Temperature swings can affect the flavor profile of the final product.
Yeast Autolysis: When fermentation is complete and the wine ages, yeast cells if not removed can die by autolysis, the destruction of cells or tissues by their own enzymes. As they perish, the cells release compounds which create particular flavors in wine and beer.
Biofuel Production: Researchers are exploring the use of engineered yeast for bioethanol production, aiming to improve efficiency and yield. This could provide a sustainable alternative to fossil fuels.
Yeast is fundamental in transforming sugars into alcohol and flavor. With its intricate biochemical pathways and ability to adapt, yeast is used in brewing, winemaking, microscopy and culinary arts.
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