Yeast is a group of unicellular eukaryotic microorganisms, among which Saccharomyces cerevisiae is the most widely known. Humans have used this yeast for thousands of years in bread making and alcoholic fermentation. During dough fermentation, yeast metabolizes sugars and releases carbon dioxide, causing bread and steamed products to rise.
Because yeasts are easy to cultivate, grow rapidly, and possess relatively simple cellular structures, they are widely used in modern biological research. Saccharomyces cerevisiae, in particular, serves as a key model organism in genetics and molecular biology and plays a foundational role in fermentation engineering.
Classification of Yeast Products
Yeast products can be classified according to their application into edible yeast and feed yeast, serving human nutrition and animal feed, respectively.
Edible Yeast
Edible yeast is further divided into baker’s yeast, food yeast, and medicinal yeast, depending on its function and processing method.
Baker’s Yeast
Baker’s yeast is primarily used for dough fermentation and is divided into three main types.
Pressed Yeast (Fresh Yeast)
Pressed yeast appears light yellow, has a compact structure, and is easy to crumble. It exhibits strong leavening power.
- Storage: ~1 month at 4 °C; 2–3 months at 0 °C
- Usage: 1–2% of flour weight
- Dough fermentation conditions: 28–30 °C for 1–3 hours
Active Dry Yeast
Active dry yeast is a granular product with a moisture content of about 8%. It is produced by culturing fermentation-stable yeast strains, followed by extrusion and drying.
- Fermentation performance similar to pressed yeast
- Packaged under vacuum or inert gas (nitrogen or carbon dioxide)
- Shelf life: 6–12 months
- Advantages: long storage life, no refrigeration required, convenient transportation and use
Instant (Fast-Acting) Dry Yeast
Instant dry yeast is a fine granular product (particle size <1 mm) with rapid and efficient fermentation capacity.
- Moisture content: 4–6%
- Produced using highly dry-resistant yeast strains, optimized nutrient formulations, and fluidized-bed drying technology
- Shelf life: over 1 year under vacuum or inert gas packaging
- Can be mixed directly with flour without prior hydration
- Short fermentation time and high fermenting power
First introduced in the 1970s, instant dry yeast quickly gained market acceptance. Studies show that Angel yeast exhibits particularly high vitality among commercial products.
Food Yeast
Food yeast is a non-fermenting yeast product intended for direct human consumption, typically in powdered or granular form.
It can be produced by recycling brewery yeast slurry or by targeted cultivation and drying processes designed to meet nutritional requirements. In countries such as the United States, Japan, and parts of Europe, approximately 5% edible yeast is commonly added to bread, cakes, biscuits, and other baked goods to enhance nutritional value.
Functional Applications of Food Yeast
- Yeast autolysates: used as flavor enhancers in meat products, soups, jams, cheeses, vegetables, and seasonings
- Nutritional fortification: added to infant foods and health products
- Enzyme extraction:
- Invertase from yeast: used in confectionery processing
- Lactase from whey-derived yeast: used in dairy processing to improve sweetness and prevent lactose crystallization, benefiting lactose-intolerant consumers
Medicinal Yeast
Medicinal yeast shares similar production methods and properties with food yeast but is specifically formulated for therapeutic use.
Due to its high content of proteins, vitamins, enzymes, and other bioactive compounds, medicinal yeast is commonly processed into yeast tablets for treating digestive disorders caused by poor dietary habits. It can also help improve metabolic function in individuals with weakened constitutions.
Trace Element–Enriched Yeast
By adding specific elements during cultivation, yeast can be enriched with trace minerals:
- Selenium-enriched yeast: used in the treatment of Keshan disease and Kashin–Beck disease; helps delay cellular aging
- Chromium-enriched yeast: used as a supportive treatment for diabetes
Feed Yeast
Feed yeast is mainly produced from Candida or Kluyveromyces fragilis, which are cultured, dried, and processed into powdered or granular products with no fermentative activity.
Nutritional Characteristics and Benefits
- Protein content: approximately 30–40%
- Rich in B vitamins, amino acids, and other nutrients
Feed yeast is widely used as a protein supplement in animal feed. Its benefits include:
- Promoting animal growth and development
- Shortening feeding cycles
- Increasing meat and egg production
- Improving meat quality and lean meat ratio
- Enhancing fur gloss
- Strengthening disease resistance in young livestock
Application of Yeast in Fermentation Engineering
Yeasts possess relatively complete gene expression regulation systems and the ability to modify expressed proteins, making them valuable hosts in biotechnology.
Saccharomyces cerevisiae Expression System
Saccharomyces cerevisiae was the earliest yeast used in molecular genetics and the first host for exogenous gene expression. In 1981, it successfully expressed the first foreign gene—the interferon gene—followed by many others.
However, when scaling up from laboratory to industrial production, expression levels often declined due to:
- Loss of selection pressure for high-copy-number plasmids
- Plasmid instability and reduced copy number
- High cost and complexity of laboratory media unsuitable for industrial-scale fermentation
Because gene copy number is critical for efficient expression, these factors significantly limited industrial productivity.
Second-Generation Yeast Expression Systems
To overcome these limitations, Wegner et al. developed second-generation yeast expression systems in 1983, represented by methylotrophic yeasts.
Common methylotrophic yeasts include:
- Pichia
- Candida
Among them, the Pichia pastoris expression system has developed most rapidly and is now the most widely used. This system combines the advantages of traditional yeast—such as proper protein folding and post-translational modification—with higher expression levels and better suitability for industrial-scale fermentation.
What are the primary differences between the various forms of commercial yeast products?
Yeast is processed into different forms to suit various industrial and storage needs:
Instant Dry Yeast: This is processed into much smaller, porous granules. It can be mixed directly with dry ingredients without prior hydration. It has a long shelf life and is highly stable, making it the standard for modern industrial fermentation and baking.
Fresh (Compressed) Yeast: This is the “raw” form of yeast with high moisture content (about 70%). It is highly active and provides excellent flavor, but it must be refrigerated and has a very short shelf life (2–4 weeks).
Active Dry Yeast: This yeast has been dehydrated into larger granules. The outer layer of cells is dormant/dead, protecting the live core. It must be “bloomed” in warm water before use to reactivate the cells.
Beyond simple leavening, what roles does yeast play in “Fermentation Engineering”?
In the context of industrial fermentation engineering, yeast acts as a sophisticated “biological factory” used for more than just making dough rise:
Animal Nutrition: Yeast is often used as a probiotic additive in animal feed to improve gut health and protein absorption in livestock.
Biofuel Production: Specialized yeast strains are used to ferment sugars into ethanol, which is used as a renewable energy source.
Nutritional Extracts: Through a process called autolysis, yeast cells are broken down to create Yeast Extract. This is rich in B-vitamins, amino acids, and nucleotides, and is used as a natural flavor enhancer (umami) or as a nutrient medium for other microorganisms in laboratories.
Biopharmaceutical Synthesis: In bioengineering, yeast can be genetically modified to produce high-value compounds, such as insulin, vaccines, and various enzymes used in the food and medical industries.
