Monday, 12 November 2012

Yeast Banking I: Managing A Large Bank

Safale S-05 from our yeast bank,
bacteria-free & ready-to-pitch!
On the web one can find - literally - thousands of articles on yeast banking; the storing of yeast for long periods of time for use in future batches of beer. Depending on exactly what the brewer is trying to do, this can involve anything from pitching a new beer on top of a the yeast cake left over from the last batch, to collecting yeast from the kraussen of a fermenting beer (top-cropping), to yeast washing, to storing yeast on agar slants or frozen in glycerol-containing liquid media.

All of these methods have their pluses and minuses – including equipment limitations, difficulty, storage life, and a variety of other factors. I am fortunate enough to run a biology lab, and thus have access to tools and methodologies not easily accessed by most home brewers. As such, I am using these resources to maintain a yeast bank for my brew club, the London Home Brewers Guild, in which we are banking as many yeast as we can manage. As part of this process I am writing a series of protocols and articles to inform my club as to how the bank works. Many of these articles I will simul-publish here on my blog, for those on the web whom are also interested.

This first article is a brief intro into our yeast bank. I will follow this article in soon (edit: here is is!) with a detailed method (with pictures!) of how we bank our yeast, ensure they are free of contaminating bacteria, and distribute the yeast to members.

More below the fold...

Commercial breweries use a variety of methods to collect, purify & store their yeasts. A description and instruction of these methods (as well as methods more applicable to home brewers) can be found in the excellent book Yeast by Jamil Zainasheff and Chris White. Often, the yeast from one batch of beer will be collected and re-pitched into new batches of beer. While this re-pitching is cost effective, every ten or so batches new cultures, grown from some sort of stock, must be used to overcome the drift in yeast characteristics that evolution inevitably creates over time. Often, these stocks are maintained as frozen liquid cultures or refrigerated slants, providing ready access to a well characterized strain.

Key to this process is maintaining pure stocks of yeast. The conventional method, used by many breweries and yeast manufacturers, is to streak out a small amount of yeast on a petri-dish containing yeast-friendly media. Each individual yeast cell will divide, after many generations forming a visible colony comprised of genetically-identical individuals*. Single colonies are collected, grown up into larger quantities of yeast, and tested for their characteristics. Good yeast can then be selected, and stored for long-term use using a range of culture and/or freezing methods.
*Every cell division, yeast accumulate a small number of mutations, so they're not perfectly identical.

On the surface, these commercial methods seem ideal for yeast-banks; indeed, they are copies of the standard culture methods used in biology labs such as my own. But there are two major drawbacks of this method – cost & loss of genetic diversity.

Cost is a limiting factor for a guild-operated yeast bank. To grow on agar, screen (involving brewing a small batch of beer for each tested colony), test for bacterial contamination, then slant or freeze the desired strain, can easily cost $15 to $20 per strain. In our first week of operation we banked 10 yeast strains, and four more are awaiting banking. Given that we hope to have over 50 strains of yeast banked, including locally harvested strains, our “bank fees” could easily grow to over a thousand dollars.

The loss of genetic diversity is also a potential issue. Although most yeast producers (and breweries) aim to limit genetic diversity, some is desired. The slight variations within a strain are often required to get the full character of the strain – especially in 'house' strains which have been evolved over repeated use in a single brewers home/brewery. Often the flavours of these house strains are actually due to a mixture of slightly different yeast, each imparting a portion of the overall character of the 'strain'. Picking single colonies destroys this diversity, formally called “polyclonality”, potentially at the detriment of future beers.

So how do we, as a brew club, overcome these issues? And how do we do it in a way where my full-time job does not become yeast wrangling? What we need is simple:
  1. A way to ensure the banked yeast has minimal/no bacterial contaminants,
  2. A long-term storage method which does not require frequent maintenance
  3. A quick and easy way of performing “withdrawals”
For ensuring bacteria-free yeast, there really are two options – the plating method described above, or growing the yeast in media containing antibiotics that'll kill common contaminants without killing the yeast. We've chosen the second option, thereby saving time, money, and preserving the polyclonal nature of many of the yeasts we are banking. This method will be described in-detail in my next post, but in brief we first check the yeast for obvious signs of contamination, then if clean, grow up 50 ml of the yeast in DME at a specific gravity 1.038 plus penicillin and streptomycin, which are sufficient to kill the common bacteria that will contaminate beer.

Freezer box for yeast storage -  81
yeast strains in a few inches space
For long-term storage there are two methods – slants and freezing. Slants consist of sterile tubes filled with an agar + wort mixture, which is solidified on an angle to exposure a larger surface area to air (hence, slant). Yeast are streaked and grown on a slant, and kept for weeks/months by refrigerating the slants. The second option is to freeze the yeast – suspend yeast in wort + 20% to 30% glycerol (AKA glycerin/glycerine). The glycerol prevents ice crystals from forming, allowing the yeast to be frozen at temperatures as low as -80C for years. We've chosen the second option, for two reasons: it requires less maintenance (slants must be passaged – the yeast transferred to new slants – every few months) and it requires less space (81 samples can be stored in a single box 13.5cm x 13.5cm x 5cm (5.25" x 5.25" x 2") in size.

For “withdrawals” we're using a simple system – Monday morning I check an email account for orders, and transfer a loop of yeast from the frozen stocks to a 7 ml of sterile wort in a small (15 ml) tube. This is cultured over night, and then given to the brewer. Withdrawal costs are less than a dollar – although we charge a little extra to cover the “deposit” costs.

These methods will be outlined in detail in my next post.

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