Yeast is a cultured fungus with many hidden talents. A single-celled organism, it transforms simple ingredients into delicious foods and beverages. Here's how live yeast is grown, processed and packaged as compressed, fresh or dried yeast for consumers.
Baker's or brewers yeast (Saccharomyces cerevisiae strains) are essentially the same with different purposes. Yeast loves anaerobic, slightly acidic and sugar-rich environments, making it ideal for fermentation in both baking and brewing.
Wild yeast is found in nature anywhere something sweet needs fermenting. It appears on skins of blueberries, grapes plums and other fruits as a whitish film. Citrus fruits are an exception. Yeast enjoys citrus but grows inside the fruit rather than on the skin.
The blue or whitish growth on oranges is mold. Unlike molds wild yeast is usually harmless, and people eat it all the time without thinking about it. Artisans often cultivate strains of wild yeast for brews and breads.
Commercial cultivation is done in controlled settings. These environments ensure the yeast strain is pure and vigorous. A high-quality yeast strain can improve fermentation efficiency by up to 30%, resulting in better flavor and texture in bread and beer.
Cultivation of Yeast
1. Isolation and Propagation
Yeast cultivation starts with isolating a pure strain of S. cerevisiae. This is done in a laboratory where scientists ensure the yeast is free of contaminants. Once a pure strain is isolated, it is propagated in a nutrient-rich medium.
The medium often contains molasses, which provides necessary sugars. Other media include a "broth" or mix of sugars, amino acids, vitamins, and minerals for optimal yeast growth.
This process occurs in large fermentation tanks. Conditions such as temperature and pH are carefully controlled to optimize yeast growth.
While yeast can grow in various conditions it's most prolific at temperatures of 32˚C-35˚C (90˚F-95˚F) with a pH of 5.5, slightly acidic. Higher temperatures inhibit yeast growth. The thermal death point for yeast is 55° - 60° C (130° -140° F).
If cooled, it stops activity at 40° F (4° C). Yeast can be frozen and will revive. Frozen yeast should be put in the fridge for 24 hours to let it thaw comfortably before use.
2. Fermentation Process
When the yeast has multiplied sufficiently, it undergoes fermentation. During fermentation, yeast consumes sugars and converts them into carbon dioxide and ethanol. In baking, carbon dioxide gas causes bread to rise and forms bread bubbles. The yeast dies during baking.
In brewing, the alcohol is the desired product. This fermentation can last from several hours to a few days, depending on the specific requirements of the booze. Careful monitoring of temperature and pH is crucial.
Yeast influences beer flavors through ester production. By varying fermentation temperatures and yeast strains, brewers can craft a range of beer styles. Higher fermentation temperatures can lead to fruity flavors in pale ales. Lower temperatures create the smoothness of stouts.
3. Separation and Washing
After fermentation, it's time to harvest the yeast. Commercial operations separate yeast from the liquid substrate using centrifugation or filtration. The harvested yeast may then be washed or dried, depending on its intended use.
For baker's yeast, washing involves rinsing the yeast to remove residues. Once cleaned, it can be processed into active dry yeast or kept moist as compressed yeast. Some sourdough bakers prefer fresh yeast for its rich flavor profile.
Others use active dry yeast. There are about 20 billion viable yeast cells per gram of dried yeast. A yeast package is about 7 grams total. Granule size varies from brand to brand.
For brewing, yeast can be further processed into various products, like liquid yeast for home brewers or dried yeast for commercial operations. Many brewers repitch yeast from one batch to another, recycling valuable yeast cells and ensuring consistency in their brews.
When there's an unlimited supply of sugar, alcohol content rises during yeast fermentation to between 10% and 18%. Alcohol levels exceeding 18% or 19% usually kill the yeast, which is why wines, beer and mead are lower in alcohol content than distilled drinks like whisky.
Packaging
After washing, the yeast is concentrated to a certain level, and additional stabilization processes may be used. After processing, the yeast can be packaged in several forms: fresh, active dry, or instant.
Fresh Yeast: Often used by professional bakers, it has a high moisture content and a short shelf life.
Active Dry Yeast: This culture form is dehydrated, allowing for longer shelf life and easier storage. It must be rehydrated before use.
Instant Yeast: Similar to active dry but with finer granules and no need for rehydration, making it convenient for home bakers.
The packaging of yeast is important for maintaining its viability and ensuring it reaches consumers in optimal condition.
Sealed Containers: Yeast is typically packaged in airtight bags or jars to prevent moisture and preserve its potency.
Labeling: Proper labeling includes usage instructions, expiration dates, and storage recommendations to ensure consumers are happy with the results.
Dry yeast is sealed in airtight sachets to keep it fresh and enhance its shelf life, often up to two years. From personal experience, even ten-year-old dry yeast bursts into prolific activity when re-hydrated and given sweets such as glucose and fructose.
Fresh yeast is sold in small blocks and needs refrigeration due to its higher moisture and shorter shelf life. It lasts about two weeks. Packaging should inform consumers about the product and help choose the right yeast for baking and/or brewing.
Yeast Use & Facts
1. Baking: In the baking industry, yeast is the key ingredient in bread, rolls, pastries, and pizza dough. It can also be used in sweet baked goods like cinnamon rolls and brioche.
2. Brewing: In brewing, S. cerevisiae is used to produce various types of beer, including ales, lagers, and stouts. Different strains of brewer's yeast can impart unique flavors and aromas, adding complexity to the brew.
3. Nutritional Value: Beyond fermentation, yeast is a powerhouse of nutrients. Brewer's yeast, in particular, is high in B vitamins, protein, and minerals, making it a popular dietary supplement.
4. Historical Significance: Yeast has been used for thousands of years, with evidence of its use in ancient Egyptian beer and bread production c. 1500 BCE. Use in beer brewing and wine making in Mesopotamia, and present-day Georgia go back to c. 6000 BCE.
5. Carbon Dioxide Output: In baking, for every 10 grams of sugar consumed by yeast, about 5 grams of carbon dioxide is produced. Plants use carbon dioxide to make oxygen for humans.
6. Health Benefits: Some yeast strains are considered to have probiotic properties, promoting digestive health.
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