July Newsletter | Yeast Management for Craft Breweries

Yeast Management for Craft Breweries?- Part Four?

Written by Lenzie Kinyon

In the final section of this 4-part series on yeast management, we will look at what might happen when we collect and pitch the same yeast over and over again. This will include four areas of concern that craft brewers should consider as they decide on how many times, they will reuse their yeast.?

Yeast cells multiply through asexual reproduction. A single yeast cell divides into 2 yeast cells by producing a bud cell, which splits off when it is 60 to 70 percent grown. After this daughter cell is fully grown, it also begins to produce bud cells. The original mother cell and the daughter cells continue this process as long as the environment is suitable for growth. During fermentation, this is what we call the “exponential growth phase.” We can describe this yeast growth with an exponential equation where P, the final population, equals S, the starting population, raised to the power of n, where n is the number of times a mature yeast cell produces a bud cell. The equation for this growth is P = Sn. Since the yeast in most ale and lager fermentations divides 3 or 4 cell times, n will be either 3, 4, or some number in between. For example, if we pitch with 8 million cells per milliliter and the yeast divides 3 times, P would be 83, or 64 million cells per ml. Most of us would just use the much easier calculation by saying 8 doubles to 16; 16 doubles to 32; and 32 doubles to 64. J?

The key take-away from the exponential growth shown above is you can collect more yeast after fermentation than you started with. This means once you start, you should have a never-ending supply of yeast. Breweries large enough to take advantage of this can save money by reusing their yeast and making fewer yeast purchases. However, the more times we collect and re-pitch yeast, the greater chance there is for picking up some microbiological contamination. Also, as yeast cells age, they undergo undesirable changes; and they eventually die.?

Sometimes, we can see the results of these changes over time. For example, the overall rate of fermentation may slow down. Yeast flocculation may decrease. The flavor profile for some beers and ales may change. The start of fermentation may appear to be normal, but it may stop prematurely. The percentage of dead cells may increase. Certainly, there are other factors that may contribute to these changes; but the general condition of our pitching yeast is often the main cause.?

Microbiological contamination, although not related to any specific change in the yeast cells, will become a greater risk as the yeast is collected and pitched again and again. Microorganisms are everywhere, including in the cleanest brewery. Even with good sanitation and good yeast-handling procedures, bacteria and wild yeast contamination can increase from one or two organisms to enough contamination to cause serious problems as time passes. This is especially true for wild yeast, which cannot be eliminated by acid washing the pitching yeast.?

Collecting and reusing yeast for many cycles also increases the chance for mutations. Mutations, which are actual changes to the yeast cells, occur at the molecular level and can only be determined with advanced equipment and techniques. During the cell-division process of producing new daughter cells, very minor changes sometimes occur. This results in new DNA that is not an exact copy of the DNA from the mother cell. As these mutated daughter cells multiply, they may cause changes during fermentation that do not seem to have any obvious reason.??

Unfortunately for craft brewers, there is no method to differentiate changes due to mutations from changes that are caused by temporary stress on the yeast. We know that yeast cells are exposed to different stressors that may affect yeast vitality. These stressors may be pronounced, depending on the fermentation conditions. For example, high-alcohol products are more stressful to the yeast than low-alcohol products. Other stressors like osmotic pressure, pH, temperature, and poor nutrition may temporarily cause problems. Often it can be difficult to determine if a problem is a result of a stress condition, or some unknown yeast mutation.?

As with all living organisms, yeast cells grow old and eventually die. However, yeast cells are unique in that cell aging is related to the budding process. This process is called “replicative aging.” Generally, we think that aging is caused by the passing of time (chronological aging), but yeast cells age each time they produce a daughter cell. The more daughter cells produced, the older the mother cell becomes. When a mother cell produces a daughter cell, a bud scar is left where the bud was previously attached.? These scars are small, crater-like rings of chitin-like tissue (similar to cellulose) that is left on the surface of the mother cell. They are visible with an electron microscope, but not with the typical microscope found in a craft brewery lab.?

New buds do not form on old bud scar tissue, or very close to existing scars. Based on the surface area of a typical yeast cell and the size of a typical bud scar, researchers have calculated and experimentally verified that the maximum lifetime number of cell divisions (bud formations) ranges from around 25 to 35 for most yeast cells. This range is called the “replicative capacity” of each cell, which varies by yeast variety. As aging yeast cells approach their replicative capacity, budding and cell functions decrease, and the cell soon dies. Understanding this yeast cell aging process is very important for good yeast management.?

Before we consider how many times we can reuse our yeast, we need to look at one more process that takes place during yeast cell division. During mitosis, cell components inside the mother cell are replicated to form matching pairs. When the cell splits, one half of each pair ends up in the daughter cell; and the other half remains in the mother cell. As the matched pairs separate, any dysfunctional mitochondria or other component that might have some oxidative damage stays with the mother cell. The other half of the pairs that has no damage is passed (inherited) by the daughter cell. Researchers call this process “asymmetric inheritance.”?

Because senescence (aging) is directly related to oxidative damage, the mother cell will age even more than normal by retaining any cell components that are damaged. Daughter cells that contain all new genetic material will start reproducing just like the mother cell did when she first became a daughter cell many generations ago. Yeast researchers like to say that the replicative capacity clock for daughter cells gets reset to zero, even though the mother cell may be near the end of her replicative capacity.?

As brewers, we think that advancing from one generation to the next generation means the yeast has been used for one more fermentation. However, most of the yeast cells have actually reproduced 3 or 4 times during this one fermentation. This becomes important when you consider that yeast vitality will decrease as the cells approach their replicative capacity. To better illustrate this, suppose we pitch an imaginary fermenter with 1 new yeast cell, meaning it has never reproduced and has no bud scars. Since yeast will generally multiply 3 times during fermentation, this 1 cell reproduces and becomes 8 cells during the first fermentation (1 doubles to 2; 2 doubles to 4; 4 doubles to 8). Using our normal method of indicating generations, all 8 of these cells would be considered second-generation yeast cells. However, suppose we label our first cell as cell0 with the subscript showing the number of bud scars. After this new cell divides the very first time, we have a cell1 and a cell0. This is because a daughter cell is always new, and the mother cell always gains 1 more bud scar. Put another way, after the first cell division, we have 1 slightly aged cell (1 bud scar) and 1 new cell with 0 bud scars. After these 2 cells divide again, we have a cell2, a cell1 and 2 cell0. At the end of the third division, our 8 cells now include a cell3, a cell2, 2 cell1, plus 4 cell0. These are all second generation cells, but already 50% of the cells are aging (1 or more bud scars).?

If we continue this exercise for 8 generations, assuming 3 cell divisions each time the yeast is used for fermentation, we will eventually have over 16 million cells that can be collected and re-pitched. 50% of these cells will be new daughter cells with no bud scars. Since they are new, they should ferment normally. Unfortunately, the other 50% will all have bud scars ranging from 1 to 24. This means half of the yeast that we could harvest after 8 generations is getting old and may not be as vigorous as new yeast. Even more concerning, some of these aging 8th-generation yeast cells are approaching their replicative capacity and are about to die!?

Craft brewers who already collect and reuse their yeast, or those who want to start doing this, should carefully consider one question: how many times should yeast be reused? There is no one correct answer, but we can look at the four topics we just covered to help make this decision. First, the more times yeast is handled, the greater the chance of picking up some microbiological contamination, either bacteria or wild yeast. Second, mutations could occur when the yeast is used over and over again. Third, yeast cells face various stresses during their lifetime that could affect their vitality and how they ferment. And finally, yeast cells age with each fermentation; their vitality decreases; and they eventually die.?

The main take-away for all brewers should be obvious: there are advantages and risks for pitching the same yeast over and over again. Because each brewing process is different, the number of times yeast should be reused will vary. For craft brewers, deciding this number is an important part of good yeast management.?

New Installation

Martin Schau installed a DALUM CO2 Recovery Plant for a new client this month. With this new addition, they can now capture and reuse their own CO2 from fermentation, significantly lowering their carbon footprint and moving towards CO2 independence.

Article in Brewers Journal

Brewers Journal featured an article about DALUM and our groundbreaking technology that reduces carbon footprint, cuts operations costs, and assists breweries to become CO2 independent. Our mission is to support the industry’s goal of reducing carbon emissions while maintaining financial viability. Read more in the article here: https://issuu.com/meatpacking/docs/tbj_summer_24_-digital_edition/64

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