Yeast facts for kids
Quick facts for kids Yeast |
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| Saccharomyces cerevisiae, a species of yeast | |
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| Cross-sectional labelled diagram of a typical yeast cell | |
| Scientific classification | |
| Kingdom: | Fungi |
| Phyla and subphyla with yeast species | |
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Ascomycota p. p.
Basidiomycota p. p.
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Yeasts are tiny, living things called microorganisms. They are a type of fungus, but unlike mushrooms, most yeasts are made of just one cell! Yeasts have been around for hundreds of millions of years, and scientists know of at least 1,500 different kinds. Even though they are usually single cells, some yeasts can link together to form chains, or even small groups of cells working together. Yeasts come in various sizes, but they are typically very small, about 3–4 micrometers wide.
Most yeasts make copies of themselves without a partner, a process called asexual reproduction. They often do this by 'budding,' where a small new cell grows off the side of the parent cell. Yeasts are different from molds, which grow in long threads called hyphae. Some fungi can switch between being a single-celled yeast and a mold, depending on their environment.
A famous type of yeast, called Saccharomyces cerevisiae, can turn sugars into carbon dioxide gas and a substance called alcohol through a process called fermentation. People have used this process for thousands of years to make bread and certain drinks. S. cerevisiae is also very important for scientists studying how cells work, including human cells. Some other types of yeast, like Candida albicans, can sometimes cause infections in people, especially if their bodies are not strong enough to fight them off. Recently, yeasts have even been used to create electricity and make biofuel (fuel from living things). It's interesting that 'yeast' isn't a single scientific group. Instead, it's a common name for many different kinds of single-celled fungi.
Contents
History of Yeast
The word yeast comes from an old English word meaning "to boil" or "to foam," which makes sense because of the bubbles it creates! Yeasts are probably some of the first living things that humans learned to use. Archaeologists found proof in Ancient Egypt that people were using yeast to make bread and drinks over 4,000 years ago! In Israel, scientists found yeast from ancient pots (some 5,000 years old!) that had been used to make drinks. This shows how long yeast has been part of human life.
In 1680, a Dutch scientist named Anton van Leeuwenhoek was the first to see yeast under a microscope. He didn't realize they were living things at first! Later, in 1837, Theodor Schwann correctly identified them as fungi.
In 1857, French scientist Louis Pasteur discovered that yeast grows faster with oxygen, but it ferments (makes alcohol and gas) better without much oxygen. This is called the "Pasteur effect." Pasteur also proved that living yeast, not just chemicals, caused fermentation.
By the late 1700s, people had identified two main types of yeast used in making drinks. People started selling S. cerevisiae for bread-making in the Netherlands around 1780. Later, in the 1800s, methods were developed to make yeast into solid blocks, making it easier to sell and use. The invention of the refrigerator in the 1850s helped people who made drinks with yeast. They no longer had to wait for certain seasons to do their work.
How Yeasts Eat and Grow
Yeasts get their energy from organic compounds, meaning they don't need sunlight to grow. They mostly eat sugars like glucose (found in fruits) and sucrose (table sugar). Some can even use other sugars or alcohols.
Some yeasts need oxygen to live, while others can live with or without it. No known yeast can only live without oxygen. Most yeasts grow best in environments that are neutral or slightly acidic.
Yeasts can grow in different temperature ranges. For example, some prefer cold temperatures, while others like it warmer. Yeast cells can even survive freezing under certain conditions.
In labs, scientists grow yeasts on special solid foods or in liquid mixtures. Sometimes, a white, thin layer of yeast called kahm yeast can appear on pickled vegetables. It's usually harmless but can make the food taste bad, so it's often removed.
Where Yeasts Live
Yeasts are very common in nature and are often found in places rich in sugar. You can find them on the skins of fruits like grapes, apples, or peaches. They also live in plant saps. Some yeasts live in soil and with insects. Scientists are still learning a lot about where yeasts live and what they do in nature.
Yeasts like Candida albicans can even live on our skin, for example, between our toes. They are also found in the digestive systems of animals and insects. Even deep-sea environments are home to many different yeasts!
A study in India found many different kinds of yeast living in the flowers and honey stomachs of honey bees. Some yeasts in flowers, like the stinking hellebore, can make the flower warmer. This might help attract insects to pollinate the flower. One type of black yeast is involved in a complicated relationship with ants, their special fungus, a parasite, and bacteria. The yeast can affect the ants' health by changing how the bacteria fight the parasite.
Some yeasts make special proteins called 'killer toxins' that help them get rid of other competing yeast types. This can be useful in making wine or even in medicine to fight yeast infections.
There are even 'marine yeasts' that live in the ocean! They were first found in the Atlantic Ocean in 1894. These yeasts can produce useful substances and might even help make bioethanol using seawater.
Yeast Life Cycle
Yeasts can reproduce in two main ways: without a partner (asexual) or with a partner (sexual). The most common way yeasts make more of themselves is by 'budding.' A small bump, or 'bud,' grows on the parent cell. The parent cell's control center (nucleus) splits, and one part moves into the bud. The bud then grows and breaks off, becoming a new, smaller yeast cell.
Other yeasts, like Schizosaccharomyces pombe, reproduce by 'fission.' This is like splitting in half to make two identical new cells.
When food is scarce, some yeasts can switch to sexual reproduction. They create special spores that can then join together to form new cells. Scientists have found that in nature, S. cerevisiae yeasts often reproduce with cells from their own family, rather than mixing with very different yeasts. Some yeasts can even shoot out tiny spores into the air to spread!
Uses of Yeast
Yeasts have many useful properties, which is why people use them in many ways, especially in biotechnology. Turning sugars into other substances through fermentation is one of the oldest and biggest uses.
Baking with Yeast
The most common yeast used in baking is S. cerevisiae. It acts as a leavening agent, which means it makes dough rise. The yeast eats the sugars in the dough and produces carbon dioxide gas. This gas forms tiny bubbles, making the dough expand. When the bread bakes, the yeast dies, and the air pockets stay, giving the bread a soft, spongy texture. Adding things like potatoes, eggs, or sugar can help the yeast grow faster.
No one knows exactly when yeast was first used to bake bread, but records from Ancient Egypt show its early use. People probably noticed that if a mixture of flour and water was left out, it would ferment and make lighter, tastier bread.
Today, baker's yeast is sold in different forms. One common type is active dry yeast, which is granulated and doesn't need refrigeration. It also makes dough rise quickly. Fresh yeast is sold in compressed blocks, but it doesn't last as long. You can test if yeast is still active by mixing it with warm water and sugar; if it foams, it's ready to use!
When making bread, yeast is mixed with flour, salt, and warm water or milk. The dough is kneaded and then left to rise until it doubles in size. A longer rising time can give the bread a better flavor.
Making Biofuel
Yeasts can turn sugar into ethanol, which is used to make ethanol fuel. This process starts by breaking down plants like sugar cane or corn into simple sugars. Then, yeasts are added to change these sugars into ethanol. This ethanol can then be purified to be used as fuel. Scientists have even changed Saccharomyces yeasts to ferment other types of sugars found in plant waste, making biofuel production more efficient and affordable.
Nonalcoholic Drinks
Yeasts are used to make several fizzy, sweet drinks. The fermentation is stopped early, so they produce carbon dioxide gas for bubbles, but only tiny amounts of alcohol.
- Root beer: This drink was first made by Native Americans and became very popular in the United States.
- Kvass: A fermented drink made from rye, popular in Eastern Europe. It has a very low alcohol content.
- Kombucha: A fermented sweetened tea. Yeast works with bacteria to make it. Many different types of yeast can be found in kombucha.
- Kefir and kumis: These are made by fermenting milk with yeast and bacteria.
- Mauby: A drink made by fermenting sugar with wild yeasts found on tree bark, popular in the Caribbean.
Foods and Nutritional Boosts
Yeast is used in foods for its savory, umami flavor.
- Yeast extract: This is made from the inside of yeast cells and used as a food additive or flavor. Products like Vegemite and Marmite are made this way.
- Nutritional yeast: These are whole, dried, inactive yeast cells, usually S. cerevisiae. They come as yellow flakes or powder and have a nutty, savory flavor. Many people use them as a vegan substitute for cheese powder or as a topping for popcorn.
Both yeast extract and nutritional yeast are rich in B-complex vitamins, making them a good nutritional supplement, especially for vegans. Many brands also add [[vitamin B12]].
Probiotics
Some probiotic supplements use a yeast called S. boulardii. This yeast helps keep the natural good bacteria in your gastrointestinal tract healthy. It has been shown to help with certain types of diarrhea and other digestive issues.
Aquarium Hobby
Many people who have aquariums use yeast to create carbon dioxide (CO2). This CO2 helps plants in the aquarium grow. Yeast systems are less expensive than other CO2 methods, making them popular.
Scientific Research
Yeasts, especially S. cerevisiae and Schizosaccharomyces pombe, are widely used in science. They are simple eukaryotic cells, which means they have a nucleus, just like human cells. This makes them great models for studying basic cell processes like how cells divide, how DNA copies itself, and how cells get energy. Scientists can easily grow and experiment with yeasts in the lab.
In 1996, S. cerevisiae was the first eukaryotic organism to have its entire genome (all its DNA) mapped out. This was a huge step in understanding how complex cells work. The genome of Schizosaccharomyces pombe was sequenced in 2002. Today, the genomes of over 50 yeast species have been mapped.
Making Useful Substances
Scientists can change yeast cells using genetic engineering to make them produce various useful substances, like medicines. This is called metabolic engineering. S. cerevisiae is easy to genetically modify, and its biology is well understood. About 20% of all biopharmaceuticals (medicines made from living things) are produced using S. cerevisiae, including insulin and vaccines for hepatitis.
Yeasts and Health
Some yeasts can cause infections, especially in people whose bodies are not strong enough to fight off germs. For example, Cryptococcus neoformans can cause serious infections, especially in people with weakened immune systems. These yeasts have a protective coating that makes it harder for the body's defense cells to fight them.
Yeasts from the Candida group can also cause infections in humans. Candida yeasts often live harmlessly on our body's surfaces. But sometimes, they can grow too much and cause irritation or infection.
Food Spoilage by Yeast
Yeasts can grow in foods that are a bit acidic and have sugars or other easy-to-eat carbon sources. As they grow, yeasts change the food, making it look, smell, or taste different, which means the food is spoiled. You might see yeast growing on the surface of cheeses or meats, or causing fermentation in juices and jams.
Some yeasts, like those from the Zygosaccharomyces group, are known for spoiling food. They can even grow in foods with lots of sugar, acid, or preservatives. In winemaking, a yeast called Brettanomyces bruxellensis can spoil the wine.
Yeast in Nature's Partnerships
Yeasts often form interesting partnerships with other living things. For example, a study found many yeast species living in the flowers and honey stomachs of honey bees. One type of yeast, Candida blankii, can even help certain flowers bloom more!
Another yeast, Spathaspora passalidarum, lives in the gut of bess beetles. It helps the beetles digest plant material by fermenting a sugar called xylose.
Many fruits produce sugars that attract yeasts. The yeasts ferment the sugar into alcohol. Animals that eat fruit are attracted to the smell of alcohol because it tells them the fruit is ripe and full of nutrients. In return, these animals help spread both the fruit's seeds and the yeast's spores.
See also
In Spanish: Levadura para niños
- Baker's yeast
- Bioaerosol
- Ethanol fermentation
- Evolution of aerobic fermentation
- Kazachstania yasuniensis – a yeast isolated in 2015
- Mycosis (fungal infection in animals)
- Start point (yeast)
- WHI3
- Yeast plasmids
- Zymology
