As cider makers, we tend to think of yeast as our partner in producing wonderful hard ciders. We might inoculate with commercial strains or let the natural microflora go to work. Either option, if you are like me, you associate yeast with fermentation and the ethanol it produces. But, yeast don’t care about producing ethanol or any other compound. Their goal is simple, reproduction. Yeast cells are trying to create more yeast cells. The ethanol and the aromas, good or bad, are just by-products of yeast’s quest to reproduce. The compounds produced during what we call fermentation and what yeast would simply call life are directly linked to the strain of yeast and its environment. The strain or type of yeast matters because yeast have different genetic structure, which defines how it processes the sugars and nutrients available to it. Its environment includes all the compounds that are around it as well as temperature.
Why does its genetic structure and environment matter? Because they have the biggest impact to how yeast reproduces. Different types of yeast will reproduce in different ways. This is also true for the environmental conditions. Yeast lacking resources or in a stressful environment will reproduce differently than yeast in an environment with an abundance of resources. There are two basic methods by which yeast reproduce. These are asexual reproduction, which is a mitosis process and sexual reproduction, which is a meiosis process(1). I defined these in a previous Mālus Trivium article but, let’s explore these two methods of reproduction in more detail.
Asexual Reproduction of Yeast
Saccharomyces cerevisiae, the most common commercially used yeast (think wine, beer, bread, and even biofuel) is often called a budding yeast. That is because its preferred method of reproduction is through a process called budding. Budding is an asexual or mitosis type of reproduction. This means there is only one parent involved in the process. Budding is its preferred reproductive process because it generally occurs when resources, sugar and nutrients, are abundant. An abundant state of resources is what you find in fresh pressed apple juice so this is how the yeast initially reproduce and grow or bulk up in the early phases of fermentation. Budding is a process where a mother cells creates a small bulge or bud into which it deposits genetical material. This bud becomes an identical but smaller copy of the mother cell. The bud can separate from the mother forming an independent daughter cell, which leaves a bud scar on the mother cell. It can also remain attached but, form its own bud and daughter cell. In this way, a mother cell can have a string of daughter cells extending from it. Budding is common but its not the only asexual method of reproduction used by yeast.
Another type of asexual reproduction is fission. Like all asexual reproduction, cell fission occurs from a single parent and results in cells that are genetically identical. Schizosaccharomyces pombe is known as a fission yeast and like Saccharomyces cerevisiae, has had its genome fully sequenced. Fission reproduction occurs when resources are abundant but, instead of the yeast cell creating small bud or daughter cell, the yeast cell splits or fissures into two cells. The fissure occurs in the center of the cell resulting in cells that are relatively equal in size. In a fissured cell, there is not a mother and daughter. Fission reproduction starts with one cell and ends with identically sized cells that have the same genetic characteristics.
For yeast, budding and fission are the two most predominant types of asexual reproduction. Other forms may be possible, with sporulation being the most likely. Sporulation is more common as a meiosis reproduction cycle so I‘ll cover it more in the next section. However, fungi and algae are known to use an asexual sporulative process for reproduction. Again, there is only one parent as this is asexual and similar to fission. The cells form inside the original cell and are duplicates of the original cell.
When we make hard cider, the yeast we are using are generally reproducing using asexual means, usually budding. This process occurs when sugar and nutrients are plentiful. You may recall from my article on fermentation stages, that reproduction is occurring during what cider makers call fermentation. The yeast reproduced is a duplicate of the original or parent cell. This is how you get the consistent flavor profile and performance from the little bit of yeast that is in your juice. If you want to evolve your yeast and change it, you need to stress it. This is when most yeast will switch to sexual reproduction.
Sexual Reproduction of Yeast
Sexual reproduction of yeast involve two cells or parents joining to create new cells, which is different than asexual reproduction where new cells form from a single parent. Sexual reproduction is not the desired method for yeast to reproduce. In fact, some commercial strains, especially beer yeasts, are no longer capable of sexual reproduction. The common practices in reusing yeast from batch to batch in brewing beer over years has made the yeast evolve to where it only reproduces asexually(2). This is because sexual reproduction is part of reproductive process that yeast employ when they find themselves in a stressed environment. However, beer yeast found themselves constantly in sugar and nutrient rich environments resulting in them losing their sexual reproductive abilities. This would be similar to cave fish losing their eyes and pigment.
Another key aspect of sexual reproduction for yeast is ploidy. This is the same ploidy as it relates to apple tree variety ploidy and their ability to pollinate other varieties. Yeast are generally diploids or haploids. Diploid cells contain two sets of complete chromosomes. You can think of this as the cell gaining a complete set from each parent, just like humans have traits from both parents. Haploid yeast cells only have one complete set of chromosomes. Haploid cells are the cells that reproduce sexually. They are created by diploid cells when the yeast finds itself in a stressed environment, which is often nutrient deprivation or desiccation. Diploid yeast cells will sporulate, creating large numbers of haploid cells with the intent of providing better survival odds.
The last key aspect of sexual reproduction is cell gender. Yeast, just like humans, have different genders. When a diploid cell sporulates, it generally creates four haploid cells with two being one gender and two being the other. These haploid cells can reproduce through mitosis or asexual reproduction, like budding or fission, or through meiosis, which is sexual reproduction. Mitosis reproduction of haploid cells will result in the creation of more haploid cells with the same generic makeup as the mother cell. However, when haploid cells go through sexual reproduction, cells of opposite genders fuse together and create diploid cells. Diploids are considered the desired state for Saccharomyces cerevisiae but other genera prefer haploid and some are commonly found in both types(1). The creation of diploid cells from haploid cells through meiosis offers a way for genetic diversity to occur in the yeast. It also reflects how yeast in a wild or natural environment have developed unique pathways to help ensure survival.
Ultimately, reproduction is one of the key goals for yeasts and it is also how yeast evolve. Sexual reproduction, where two haploid cells mate to form a diploid represents an excellent means to develop new strains with unique genetic characteristics. Beer yeast are an excellent example of this evolution over time(2). Many beer strains have learned to process maltose in order to utilize these sugars. It’s why many beer yeast have an attenuation number associated with how well they can process malt sugars. Many have also lost their ability to sporulate because they were continually moved from one nutrient rich environment to another. The yeast has adapted to maximize its ability to constantly live in this environment.
(1) M. Knop, Yeast cell morphology and sexual reproduction, C. R. Biologies 334, 599–606, 2011
(2) B. Gallone and associates, Domestication and Divergence of Saccharomyces cerevisiae Beer Yeasts, Cell 166, 1397–1410, 2016
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