Our common perception of yeast is that it’s used to make bread, sometimes beer or wine. While that’s all great and all, these are actually not the only kinds of yeast. There are many more different types of yeast that can do a lot of different things.
Today, we’ll answer all your questions about yeast, such as… What exactly is yeast? What does it do, and where is it used for? How is it used in making bread and wine? Where does yeast live, and how does it reproduce? How can I observe yeast under the microscope?
What is yeast?
Yeast are single celled eukaryotic microorganisms from the Dikarya subkingdom of the Fungus kingdom. There are over 1500 species of yeast classified under sac fungi, or phylum Ascomycota, as well as higher fungi, or phylum Basidiomycota.
These unicellular fungi are largely asexual organisms survive on organic substances such as sugars, can cause fermentation of various fruits, plants, and plant by-products, and can also be a source of certain infections.
A yeast cell can reproduce via budding or binary fission, and in some cases, through a form of sexual reproduction. These are fast occurring phenomena that can easily be observed through a high powered optical microscope.
What does yeast do?
Some of the many types of yeast are considered as useful yeast, since they can be used in making baked goods and alcoholic beverages, such as bread, beer, and wine. These include baker’s yeast, brewer’s yeast, distiller’s yeast, wine yeast, and nutritional yeast.
This usually works by fermenting the yeast through “feeding” it sugar, which in turn enables it to produce carbon dioxide and alcohol. However, not all yeast can be used in such a manner. Many types of yeast are harmful and can cause infections, such as the Candida yeast.
This is a magnified image of a type of yeast called brewer’s yeast, with the scientific name of Saccharomyces cerevisiae. At 400x magnification level, you can clearly see the details of the yeast cells, including their shape and orientation.
The fermentation process for bread and alcoholic beverages is largely similar to each other, with the main difference of what ingredients are required, and how the by-products are used.
For bread, the yeast cells ferment by using the sugars in the flour, in order to produce ethanol and carbon dioxide. The ethanol eventually evaporates, while the carbon dioxide is what makes the dough rise before and during the baking process.
When it comes to beer, cereals such as barley are used as the sugar source for fermentation, and the produced ethanol and carbon dioxide are trapped in the drink, making it fizzy and alcoholic. This is the same for wine, which uses the sugars in grapes.
What you can see here are different magnified images of foodstuff as they undergo fermentation due to the yeast consuming the sugar content of the food. In fact, you won’t only be able to see yeast, but also identify certain bacteria and molds present.
Where is the natural habitat of yeast?
Since there is a wide variety of yeast, it’s only natural that they thrive in different habitats, but the common ground is that these habitats should be rich in sugar and other soluble nutrients to support the yeast’s growth and reproduction.
This is because while yeast cells do not have chlorophyll, and thus cannot make their own food, it does not mean that yeast are parasitic organisms. Rather, they survive by ingesting other organic substances such as sugar.
Among the most common habitats of yeast are fruits, flowers, and plant leaves, as well as soil and deep sea environments. When acting as infections, yeast can be found on skin surfaces and intestinal tracts of warm blooded animals.
How does yeast reproduce?
These yeast cells undergo reproduction through a couple of asexual reproduction methods, namely, binary fission and budding. There are also dimorphic yeast cells, which are a type of fungi that can grow as either yeast or hyphae.
Here is a side by side presentation of microscope images of yeast, pseudohyphae, and hyphae. You can see how each one is different, with yeast appearing as circular or egg shaped particles, while hyphae feature elongated tails.
On some occasions, yeast cells are subjected to stressful conditions, in which case, they undergo a form of sexual reproduction, which results in spores.
The process of mitotic cell division takes place with the parent yeast cell in order to produce an outgrowth in the form of a new identical cell, called a bud, that remains connected to the parent cell until it becomes independent by splitting from the parent cell. Interestingly enough, budding yeast belongs under the phylum Ascomycota and the order Saccharomycetales.
Meanwhile, during binary fission, mitosis occurs to replicate and divide the genome, before finally forming another plasma membrane to completely create a new identical cell and separate it from the parent cell. This happens through the processes of DNA replication, chromosome segregation, and cytokinesis.
How to observe yeast under the microscope
Yeast cells are some of the smallest eukaryotic organisms with a diameter of only 5 to 10 micrometers per cell, and thus need to be viewed under high magnification optical microscopes, set to a high numerical aperture, resolution, and brightness.
The best settings for viewing yeast on a microscope is a numerical aperture of at least 1.4, and a magnification of the objective lens at around 60x to 100x.
Yeast can be viewed under the microscope through two different microscopy techniques- bright field microscopy and fluorescence microscopy. For one thing, yeast and buds can be seen under a high magnification (1000x) bright field microscope, such as a compound microscope.
This allows us to see oval shaped microscopic bodies, which are the yeast cells’ units of protoplasm. Bright field microscopy also works for observing the fermentation process of yeast in a sugar solution, as well as its reproduction process through budding.
Meanwhile, the cell organelles inside the yeast and their intracellular distribution can be identified with a fluorescent microscope. These include the nuclei, mitochondria, vacuoles, endoplasmic reticulum, and the cell wall.
Take a look at this compilation of magnified images of budding yeast under the microscope. As you can see, while they may seem mostly the same, these yeast cells actually come in different sizes and shapes.
To prepare a sample slide of yeast, the easiest way is to use a cultivated type of yeast, such as yeast cake, which contains a type of fungus that eats sugar. This can be purchased from baking supply stores. However, active yeast and a spoonful of sugar may also be used.
Below are the ways on how to prepare a sample slide of yeast to view and study under a microscope. You’ll just need a few materials, and follow a couple of easy steps to create a viable yeast solution.
In order to prepare a specimen slide of yeast cells, you will need:
- Yeast cake, or active yeast and sugar
- One cup of warm water
- A small bowl for mixing
- Mixing sticks or a spoon
- Medicine dropper or pipette
- Glass slides and cover slips
- Artificial dyes for staining*
The following are the procedures in preparing the sample for viewing under the microscope:
Slice a small piece of the yeast cake (roughly a quarter of the cake) and mix with water until it turns into a pasty texture, then add about a pint of water to create a diluted solution. Mix in a tablespoon of sugar until it dissolves completely.
Or, mix one packet of active yeast with one tablespoon of sugar and one cup of warm water. Make sure that there are no lumps of yeast or sugar. Stir it and then let sit for almost an hour.
It’s also possible to observe the bubbling process of bread yeast. This can be done by using active dry bread yeast, mixing the yeast solution in a glass bottle, and covering the top with a balloon for about 10 minutes or until the balloon inflates due to the production of carbon dioxide.
Various staining techniques may be required to dye various cell parts of the yeast before viewing since yeast cells are small and difficult to see in contrast. Common dyes include calcofluor white, DAPI, DASPMI, FM4-64, and DIOC6.
Each dye may require a specific set of procedures for staining, so it’s best to do careful research and work with a skilled professional when staining yeast cells and other specimens.
Transfer a drop of the solution on a piece of glass slide by using a medicine dropper or a pipette, then gently place a cover slip on top of the slide, making sure that the slip is perfectly aligned with the slide. Wipe off any excess solution.
Place the prepared slide on the microscope stage, and view it using the highest power objective lens, which should be either 60x or 100x, creating a total compounded magnification of 600x to 1000x with the ocular lens.
What you can see
As we mentioned earlier, you should be able to see yeast cells, cell organelles, and certain cellular processes. This includes the fermentation process of yeast as it ingests sugar in the solution, where gas bubbles rising from the yeast solution signify the fermentation.
Another cellular process that can be observed is the budding of new yeast, wherein some yeast plants may have two unequal parts that eventually separate from each other. This multiplication of yeast cells is usually rapidly occurring, and thus easy to observe on a microscope.
Of course, the various parts of a yeast cell can also be identified by using a higher powered microscope, or a fluorescent microscope. For this purpose, the yeast almost always needs to be dyed in order for the details to become clearly visible.
There are many different types of yeast, some of which are cultivated by humans for practical purposes such as baking, while some others are wild yeast that ferment or decay various fruits, and some more are infectious organisms that are dangerous for humans and animals.
Yeast may be quite difficult to view in great detail under a microscope, but these unicellular organisms have been, for a long time, widely used by scientists and researchers to observe a variety of essential cellular processes, including reproduction.