Culturing Yeast

At food lunatic central, we’ve been home brewing for the past 4 months.  In that time I’ve come to truly appreciate the importance of yeast.  Yeast is a living organism, without which beer would not be possible.  That’s probably why it is so expensive.  The yeast for a 5 gallon batch of beer can cost upwards of $30; as much as the other ingredients combined.  To address this, I’ve built the tools necessary to propagate yeast.  I also want to try to build up a bank of master samples I can reduce how much I have to buy in the future (possibly to zero?).

Beer is yeast’s home.  When you brew, you are actually making a little vivarium to house your pet yeast.  If you were producing soda, you could theoretically control the ingredients and processes to the extent that every batch was the same.  Not so with beer.  No matter how much you control the process of making the raw beer (wort), you are still at the mercy of the yeast you put in.  For me, that’s what makes brewing so intriguing.

Once in the beer, yeast goes through 3 stages of development: respiration, fermentation, and sedimentation.  During respiration the yeast consumes all the dissolved oxygen in the beer and reproduces until it has reached an optimum population size.  The yeast also releases a variety of by-products during this phase, some good and some bad, which affect the flavor of the beer.

Fermentation is the famous stage.  The yeast converts sugar to ethanol and CO2.  But it can’t consume all the sugars, only simple ones.  Sugars that have been caramelized and/or undergone maillard reactions cannot be digested by the yeast.  These residual sugars remain in the beer, giving it sweetness and body.

Sedimentation is the final stage.  The yeast absorbs some of the by-products produced during respiration, then clumps together and sinks to the bottom of the beer.  At this point the yeast is exhausted from the week long drunken orgy you’ve just treated it to.  But that doesn’t mean the yeast is finished by any means.  Given time to recover and the proper motivation, it will live to party another day.  The yeast from a batch of beer can be collected, washed, and used in a later batch.  The result is enough yeast for about 3 new batches.

The presence of non-fermentable sugars stresses the yeast and leads to mutations.  Furthermore the emphasis in beer is alcohol production rather than reproduction.  Yeast that has been propagated in a batch of beer is said to have aged a generation.  Most brewers believe that a population of yeast is only reliable for 3-5 generations.

That means that with an initial population of yeast, you can make 10-30 batches of beer (1, then 3, then 3 from each of those 3, and so on) without ever going to much trouble.  But what happens when you’ve used up your 10-30 batches?  Or what if you don’t want to use the same yeast EVERY time?  Or what about the problem of not having enough yeast in the beginning without spending $30?  That’s where yeast culturing comes in.

Yeast reproduces a little in beer, but it is not the correct environment for good propagation.  For higher integrity propagation, yeast should be grown in sterile wort made from pure fermentable malt sugars.  A starter solution can be made by adding 1 g of dry malt extract to 10 mL of water.  The starter is boiled until sterile, cooled, and then the yeast is added.  Since the goal is to grow the yeast population, it should be kept in the respiration phase continuously.  This means constantly infusing oxygen into the starter with an air pump, stir plate, or simply through intermittent swirling of the container.

The yeast population should be grown in 8-10 fold steps, though the initial step can be 25-100x.  A 10 mL ‘master sample’ of yeast is grown up to 250 mL, and then to a 2 L starter.  At 1-2 L (depending on the particular beer) a new 10 mL master sample can be collected from the starter, and the rest is pitched directly into the beer.

The plan is to collect as many master samples of yeast as possible.  For each beer I will propagate the master sample up to about 1.5 L using a homemade stir plate.  The stir plate is the most efficient way to infuse oxygen into the starter, thereby reducing the amount of liquid needed for a given beer.  From the starter I will collect a new master sample of the yeast and store it in my yeast bank (10 mL vials, in a cottage cheese container, in my fridge).

The indication is that freshly propagated yeast, stored in liquid medium at fridge temperature, will last for approximately 6 months.  I may occasionally have to grow a sample up to 250 mL to refresh it and collect a new master sample.  This process will allow me to 1) save money on initial yeast investments, buying 1 pack instead of two to three, 2) always pitch the correct amount of fresh, pure strain yeast into my beers, and 3) have a variety of yeast to use, according to different moods and recipes.

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4 thoughts on “Culturing Yeast”

  1. I am enjoying your beer blogs. Keep up the good work.
    I remember back in the late 70’s when I bought my first six pack of Anchor Steam and wondering why all beer didn’t have flavor, now there are so many to choose from and so many more to be made.

  2. Matt,
    It seems you and I are currently headed down very similar parallel paths. I stick to two major yeast strains, WLP001 and WLP007 to keep it simple. I harvest, wash and store for 2 weeks or so until I am ready to brew the next batch. Also Erlenmeyer flasks are your friend!

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