As a living organism, yeast is unique among the ingredients we use for beer. And while you’ll want to ensure all of your ingredients are up to scratch, ensuring your yeast is in good health is one of the most important factors in a successful brew. In this article, we’ll take look at how to manage your yeast throughout its lifecycle to provide generations of happy fermentations.
Where your Bluestone yeast comes from
At Bluestone, we bank around 150 strains of yeast at -80°C. These are maintained in a glycerol solution to prevent the cells from lysing due to the formation of ice crystals. Under these conditions, our yeast can be stored almost indefinitely, so we always have master cultures to return to. We use these to make a three-month supply of slants, which can then be used to generate industrial-sized batches to meet our orders.
The lifecycle of your yeast
Once the yeast is out of the freezer and ready for brewing it has a finite lifespan. We measure this in terms of cell division, rather than chronological age. The lifespan of your brewing yeast will dictate the number of times it can be repitched. While some yeast strains, when managed correctly, can be repitched up to 15 times and remain relatively stable, we recommend limiting their use to between five-to-eight generations. There are a few reasons for this.
Firstly, as yeast is a living organism, slight genetic variations take place over the course of each fermentation. This means that each generation is more different from the original than the previous, which can affect the flavour of the beer.
Based on feedback from our customers, we also recommend using our yeast for five-to-eight generations before switching out with a fresh culture as this range produces the best ester profile.
Yeast is also susceptible to contamination, and the likelihood of this occurring increases the more times the yeast is used to ferment a brew.
It’s also important to note that as a yeast population nears the end of its collective lifespan the time it takes for cells to divide greatly increases. This usually results in slower or stalled fermentations, which–if your beer is taking longer to brew than expected–will affect your profits.
Should you wash your yeast?
Bluestone Yeast does not recommend washing your yeast, acid or otherwise, between generations.
Washing your pitching yeast will deplete its glycogen supply, which is needed to power sterol synthesis. Glycogen also aids in a smooth transition through ordered uptake as cells shift from glucose to maltose and maltotriose. In short, washing your yeast may handicap it right before pitching, leading to poor membrane function or issues with carbon carbolite repression.
What about trub?
One of the reasons for washing yeast is to remove trub. So, trub levels in flocculated yeast can be an issue. A pro tip to reduce the transfer of trub is to drop it from the bottom of your conical fermenters roughly 24-to-48 hours after the tank is full. During this stage of fermentation, the yeast should still be in suspension. Once this window has closed, yeast will begin to flocculate into the cone where it will be ready to harvest with much lower trub levels.
A big caveat here, however, is that we don’t recommend using this method when brewing with some of our strains. A good rule of thumb is to not use this method to remove trub when you’re brewing with a high-flocculating strain. Instead, we recommend removing trub just prior to harvesting your yeast or by visualising the trub/yeast before transfer.
How much yeast to pitch?
When brewing ales, our brewers report good results pitching between 750,000 to 1M cells per mL per degree Plato. Lagers require a higher cell count and are usually pitched at 1.5M cells per degree Plato.
Lager strains that take longer to ferment will benefit from a higher pitch rate to speed up the fermentation time. This reduces the yeast’s exposure to stressful conditions during fermentation when consumable sugar is low and ethanol concentration is high. A high pitch rate will also minimise ester production and provide your lager with a cleaner flavour, which is generally desirable in the Australian market.
Beer styles with a high specific gravity also benefit from a higher pitch rate as yeast is heavily taxed both by osmotic stress at the start of fermentation and also towards the end of fermentation when sugars are low and ethanol levels high.
To help your yeast through these conditions in generally good health, we recommend a higher pitch rate. For every degree Plato above 16°P, try adding an additional 1M cells per mL. For example, if you are brewing a 20°P ale wort, pitch 24M cells per mL (20M cells per mL to cover the sugar concentration and a further 4M cells per mL to compensate for the stressful conditions).
Ensuring you have enough yeast for high-gravity beers
To ensure you have enough yeast to ferment a high-gravity beer, we recommend brewing a low-gravity beer prior and pitching from this. For example, a moderate gravity pale ale should produce a yeast cake sufficient to adequately ferment much higher gravity beer. If you’re not brewing a high-gravity beer, the yeast cake from a moderate-gravity pale ale can be used to ferment two-to-three moderate gravity beers.
In short, we recommend first brewing a moderate-gravity beer to generate plenty of healthy yeast to tackle a high-gravity beer.
Why oxygen levels matter
To help your yeast tackle a high-gravity beer, we also recommend increasing the oxygen concentration of your wort. As wort gravity increases, oxygen solubility decreases, so more oxygen is needed to hit a target dissolved oxygen content.
Oxygenation is critical at the beginning of fermentation as yeast requires it for the biosynthesis of sterols and unsaturated fatty acids. Sterols, in particular, are critical to cell membrane formation and function and promote cell growth and cell division, while also regulating membrane fluidity and membrane permeability. There needs to be enough oxygen in the wort to synthesise sterols for all of these processes to be healthy throughout the yeast population, which is higher initially due to the increased cell count.
For the most part, when aerating in-line during chill/transfer, it’s important to add enough oxygen to promote growth and fermentation. A word of caution; however, if you add more oxygen than the wort can absorb you will create foam in your fermenter. This can potentially scrub out hop character.
When adjusting the aeration level of your wort, we recommend doing so in a stepped manner and recording any change in your procedure. Also, get your sensory team in to check the beer is not losing any hop character.
In summary
We hope this article has equipped you with a better understanding of how to best care for your yeasts and some of the unique considerations when brewing different styles of beer. As always, please feel free to reach out to us via our contact page (link here) if you have any questions about some of the techniques and strategies outlined in this article.
Happy brewing!
