Yeast banking – #1 Introduction

Eureka, today is the beginning of another post-series in the yeast basic area. The topic of this one is all about yeast banking at home (or in a lab). Today is all about the basics of yeast banking, what to consider and what methods there are. The next posts will be about the different methods. Lets begin.

What is yeast banking?

Yeast banking is a technique to store your yeast strains over a longer period of time to either reuse it later on or maintain a collection of strains. Storing freshly harvested yeast for a short time is yeast storage in my opinion and not the topic of these posts. Yeast banking is a long-time storage of your yeast to have them ready again in a year or maybe more. Another difference between yeast banking and storage is the amount of yeast. In yeast banking you only store a small amount of yeast.

Reasons to consider yeast banking?

Yeast banking can have different advantages such as lower prizes for your yeasts (use for multiple times or use them again in a year or more), having different strains available or even maintain yeast strains which are not commercially available. In addition, you can trade yeast strains with others. Let me add, if you brew only with dried yeasts, yeast banking is not a technique you should consider. It is not worth the money.

What are the basics of yeast banking?

Fig 1: Banking with agar plates

The goal of yeast banking is to have the yeast in a state where it can survive for a long time. Dormant yeast cells are very suited for this job. First, the yeast cells have to be very vital before you bank them.

Viability and vitality. Viability describes the live/dead ratio of a yeast population. A very viable yeast sample has a high amount of live yeast cells and only a few dead ones. Typically, healthy yeast samples have viabilities greater than 95%. Meaning 95% of the yeast cells are viable, 5% aren’t. Vitality on the other hand describes the physiological state of a yeast cell. It more or less describes how fit the yeast cells are. Storing yeasts with a low viability will only cause troubles sooner or later. The first thing to consider in yeast banking is the cells viability, second the vitality.

To summarize, the yeast for the banking process should be very viable and vital. More about this later on.

The next thing to consider is the environment you put your yeast in for the storage. I call it media from now on. Any media which can stress the yeast cells is not suitable for the job. Factors causing stress are alcohol or the osmotic pressure for example. Storing yeast in distilled water will stress the yeast cells since the distilled water will lead to a swelling of the yeast cells (water flows into the yeast cells) and the yeast cells will probably burst. What are suitable media for your yeast cells? One way to go is use isotonic sodium chloride solutions. The sodium chloride prevents any osmotic pressure difference between the water surrounding the cell and the cell’s interior. So no swelling and bursting of the yeast cells. Further information about the media will follow in future posts about the different techniques.

Fig 2: Yeast banking with liquids

Third, depending on the period of time you want to store your yeast, a different kind of technique has to be used. Therefore consider how long you want to keep your yeasts in general and how much maintenance work you want to put into your yeast banking system.

To summarize, important things to remember before heading into yeast banking are the vitality/viability of your yeast, the media you put your yeast into and the time frame you want to store the yeasts. For example, banking yeast which was in the refrigerator for four weeks is probably not the best source of yeast since the vitality/viability will have suffered over time for sure. Do a starter first to increase the vitality/viability.

Another important thing is sterility. You have to work very cleanly to avoid any contaminations in your yeasts. The yeast cells might have suffered a bit over the period of time and any contamination may have an easy game to overgrow the yeasts.

What are the different techniques for yeast banking?

Four different techniques come into my mind when I think about yeast banking. I am sure there are different ones as well but many of them are derivatives of the following four:

– Yeast banking with agar plates
– Yeast banking with agar slants
– Yeast banking with isotonic sodium chloride solutions (or in any other liquids)
– Frozen yeast banking

The next four posts in this series will be about one of these techniques each.

Why am I yeast banking?

I am yeast banking due to several reasons. First, to have different yeast strains available and save some money because I do not want to buy yeast for every single batch. Second and most importantly store my special yeasts. These are yeast I isolated from different bottles of beer and are not commercially available. In addition some seasonal yeasts releases from the private collection from Wyeast’s. And last because it is fun and you can do yeast trades.

Fig 3: Frozen yeast library

What is the best method for yeast banking?

I can already answer this question: There is none. Every method has its advantages and disadvantages. Only you can decide which method is best for you. However, I am really fond of the isotonic chloride solutions because I have a lot of good experience with this method. Some yeasts can survive up to two years in there without doing any maintenance work. Unfortunately, not every strain behaves the same. Some yeast strains will be dead sooner than others.

What method do I use?

As already mentioned, I use(d) the sterile liquids approach. I store(d) my yeasts in isotonic sodium chloride solutions and had no troubles so far. At the moment, I am switching to a frozen library because of the amount of strains I have. However, I keep the sodium chloride liquids until they are either gone or the yeasts are dead.

Are there any other sources to get information about yeast banking?

Sure, yeast banking is not my invention. These techniques are very common in the labs to store nearly any kind of microorganisms for later uses. Therefore a lot of different protocols exist about banking. Since we are in the homebrewers area here, the protocols should be rather easy to follow. Very often in labs the microorganisms are stored in -80°C (-112°F) fridges or even colder. I wonder if anyone of you has such a fridge at home… Therefore other techniques have to be used.

Fig 4: Banking with agar slants

A book I recommend is “Yeast: The Practical Guide to Beer Fermentation” by C. White and J. Zainasheff. Everyone of you out there interested in getting into yeast banking and such should buy a copy. I am not related to any of the authors so no personal benefit for me in there. The whole chapter six is dedicated to “Your Own Yeast Lab Made Easy”. An excellent source in my opinion.

Other sources can be found with your search engine of choice.

What will the next posts be about?

The next four posts will be about each of the four techniques mentioned above. I will try to explain the method how they work, the material you need and tell you some advantages and disadvantages. In addition, my experiences with the technique if I have some. Stay tuned!