Yeast is a vital component of the microbial world, involved in diverse ecological processes. This amazing fungal organism is essential for baking and brewing, and also involved in countless biological functions affecting the environment, health and human culture.
Yeast in History
The term "yeast" originates from the Old English words gist and gyst, and from the Indo-European root yes-, which means "boil," "foam," or "bubble." Yeast microorganisms are considered to be among the first domesticated organisms.
Archaeologists excavating Egyptian ruins have discovered ancient grinding stones and baking chambers used for making yeast-leavened bread. Illustrations of bakeries and breweries are found to go back c. 4000 years.
Vessels found in archaeological sites in Israel from up to 5000 years ago, thought to once contain the alcoholic drinks beer and mead, reveal surviving yeast colonies. This is the first direct biological evidence of yeast use in ancient cultures.
Evidence of intentional fermentation is found in China c. 7000 BCE. Lactic acid bacteria are also major fermenters, associated with cheese and yogurt. Lactic acid bacteria fermented beverages are popular today especially in Japan.
In 1680, self-taught Dutch naturalist Antonie van Leeuwenhoek first observes yeast under his microscope. Although he coins the term animalcules for the various creatures he sees, he doesn't consider yeasts living organisms. Theodor Schwann identifies them as fungi in 1837.
About Yeast
Yeast is a unicellular fungus, with Saccharomyces cerevisiae being the most well-known type. Commonly used as baker's or brewer's yeast, it drives many important chemical reactions.
When added to dough, yeast ferments sugars and produces carbon dioxide, causing dough to rise and creating the airiness of bread. In brewing, yeast converts sugars from grains into alcohol, making beverages like beer and wine. Wild yeast helps create natural honey mead.
As a eukaryotic organism, a yeast cell has a defined nucleus. It also contains organelles enabling the cell to perform metabolic functions. Yeasts are considered living entities as they grow, reproduce, and respond to environmental stimuli, demonstrating characteristics of life.
Scientists believe yeasts first appear on Earth over 500 million years ago. Fossil evidence suggests yeasts evolve from a common ancestor shared with other fungi. As a result of their long evolutionary history, yeasts are diverse, with over 1500 species identified to date.
Yeast in Nature
Yeasts are found in many environments, including soil, plant surfaces, and intestinal tracts of animals. They're important to nutrient cycling, contributing to the decomposition of organic matter. Yeasts can also form symbiotic relationships with plants, aiding in nutrient absorption.
Yeast can be found in soil, on rotting fruits, and tree bark. It flourishes in sugary environments, and is important to breaking down complex carbohydrates into simpler sugars, which in turn support other microorganisms. It's found in rotten fruit releasing nutrients back into the soil.
How Yeast Lives
Yeasts primarily obtain nutrients through fermentation and respiration. They absorb sugars from their environment, metabolizing them into energy, carbon dioxide, and alcohol.
Waste products, like ethanol or carbon dioxide, are expelled into their surroundings. One gram of yeast can produce up to 2 milliliters of carbon dioxide in a few minutes.
Yeasts reproduce through asexual budding, where a new cell forms off the parent cell. Some species can also reproduce sexually through the formation of spores, primarily under adverse conditions. Spores are resilient and some can survive thousands of years before gemination.
Lacking a motility apparatus, a yeast cell cannot move on its own. Yeasts can form organized populations by means of cell division and the subsequent differentiation of dividing and non-dividing cells, or by aggregating planktonic cells.
Yeast Colonies and Biofilms
Yeast colonies are groups of connected individuals arising from a single parent cell. The colonies can be seen as visible masses on solid growth media. They're often found on fruit surfaces and can be grown in a petri dish. Certain yeast species can also form biofilms.
Biofilms are complex structures composed of clusters of yeast cells encased in a matrix of extracellular substances. Biofilms can offer protection from environmental stresses and enhance survival chances.
Certain yeast species can even develop multicellular traits. They do this by creating chains of connected budding cells called pseudohyphae or false hyphae. Hyphae are the filaments of fungi. Yeast can rapidly evolve into multicellular clusters with specialized organelle functions.
Optimal Yeast Thriving Conditions
Yeasts prefer warm, moist environments rich in sugar and other organic compounds. They thrive at temperatures between 20°C to 30°C (68°F to 86°F) and can become inactive or die in extreme conditions.
Additionally, yeast likes specific pH levels. Most of the yeasts enjoy pH between 4.5-6.5, but many species can grow in more alkaline or acidic environments. Low or high pH values can cause chemical stress on yeast cells.
Oxygen is needed for aerobic fermentation, while anaerobic conditions are ideal for other types of fermentation processes. Yeast can survive with or without oxygen. Ironically, yeast is sensitive to alcohol, which inhibits its growth. Alcohol kills the yeast during brewing.
Ideal temperature for its growth ranges from 25°C to 30°C (77°F to 86°F). Yeast can double its population in just 90 minutes under optimal conditions. High heat however is deadly. When dough temperature reaches 140 ºF or 60 ºC, the yeast is killed.
It's light-sensitive and prefers dark habitats. Too much light can break apart cell membranes and destroy the yeast. Visible light of moderate intensity inhibits growth, respiration, protein synthesis, and membrane transport and has a diminishment effect on membrane integrity.
Yeasts have natural predators in the microbial community. Bacteria, protozoa and even other fungi eat yeast cells. For example specific bacteria produce enzymes to break down yeast cell walls, allowing the bacteria to digest the yeast.
Hazards to Humans
While yeasts are generally safe and beneficial, some species can pose health risks. Candida species can lead to infections in immunocompromised individuals. Yeast overgrowth can also cause health issues, such as thrush or invasive candidiasis.
Candida albicans is a common yeast in the human digestive system, but in some individuals, it can overgrow and cause infections. Poor hygienic practices in food production promote contamination by harmful yeast strains. Spoilage and foodborne illness are ongoing concerns.
Facts About Yeast
Yeasts make up about 1% of all fungi in the world. There are over 1500 known species with more constantly being discovered.
Yeasts are capable of anaerobic respiration, allowing them to survive in environments without oxygen.
An estimated 75% of the world's beer production uses S. cerevisiae. The Spanish word cerveza is derived from the Latin "cerevisia," with both terms meaning "beer." The yeast name, Saccharomyces cerevisiae, translates to "sugar-fungus of beer."
Certain yeasts are used in the production of probiotics to treat digestive issues.
Yeast can convert up to 20% of its weight into carbon dioxide or alcohol in one fermentation cycle.
Yeasts can be genetically engineered for applications in medicine and environmental cleanup and waste treatment.
The study of yeast genetics contributes to understanding biological processes, including aging and cell division. For more than fifty years S. cerevisiae has been researched as a model organism to study the way humans get older.
Yeast is a crucial model organism for scientists, especially in genetics and cellular biology due to its simplicity and rapid reproduction.
Yeast is a remarkable organism occupying vital niches in wild and civilized life. Scientists continue to explore the capabilities and functions of this tiny but powerful life form.
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