This post is a “yeast starter for dummies” article because.. well… I am one.
A dummy. Not a yeast.
The information for this write-up was taken from a paper about yeast growth which can be seen here and from an excellent (superior) article written by Gavin C on Homebrewtalk.com. I’ve adopted a number of his methods with only a couple slight differences that I feel may make a more effective starter. I have no data to back this up. This only reflects my opinion and I do not claim to be an expert in anything.
None of this is original work, I’m merely attempting to put together a synthesis of science and “tribal” knowledge to try and make the best yeast starter I can. At the end is a step by step guide to making a starter from a wyeast package (which is what I usually use). So if all you want is to make a starter and don’t want to know my reasoning, just skip to the end. Otherwise, stick around and find out if I’m crazy or not!
First admission: I am new to making starters.
I’m also a quick study and a naturally curious person. Curious like a cat. That’s why my friends call me whiskers.
Any ol’ way, if I say something glaringly wrong please correct me!
My curiosity and my romantic involvement with a microbiologist have driven me to ask a few questions.
My first question was: The wyeast packages say they are good for a 5 gallons batch, but everyone still makes starters with them. So what is the benefit of a starter?
A Wyeast package contains 100 billion yeast cell and a packet of nutrients. When the nutrient pack is busted the yeast absorb the nutrients and become more active.
The problem is, there just isn’t enough VIABLE cells in one of these packs. Viability is basically just the number of alive cells vs dead cells. So one of these wyeast packets is roughly 25% viable according to Raines-Casselman so that means only 25% of those cells are alive and functioning. This means you need to create a starter to grow more yeast cells before dumping them in your beer.
This lead to an obvious “why?”
The answer to this question isn’t simple, but it’s not terribly complex either. First, a little background about yeast is necessary.
Yeast have a lifecycle where they go through stages of growth dictated mostly by the oxygen and food supply (sugar) in their environment. The yeast go through this lifecycle as a population, and each phase is triggered by environmental changes.
There are 5 main phases of yeast growth:
1. The Lag Phase – This is the period of time where the yeast get used to their new environment. They are also absorbing all the oxygen in the wort and use it to synthesize enzymes and sterols which they will use to build cell walls and increase their alcohol tolerance. As a general rule, the lag phase after pitching to your wort should be as short as possible without over-pitching. The longer the lag phase, the higher the chance of infection in your beer.
2. The Accelerated Growth Phase – The yeast start eating the simplest sugars and dividing and multiplying, pretty self explanatory
3. The Exponential Growth Phase – The yeast use all the oxygen and sugar they absorbed in the first 2 phases and start to reproduce exponentially. This means that 1 yeast cell divides into 2, which goes into 4, then 8 and 16 and so on. The end of this phase is where a starter should be when it’s pitched to the beer.
4. The Decelerating Growth Phase – This is the phase where ACTUAL fermentation occurs. Fermentation is an anaerobic process (does not use oxygen) by which sugar is incompletely broken down into CO2 and ethanol (alcohol).
I think it’s important to note that phases 1-3 are NOT fermentation. They are yeast propagation. So when I use that term, it refers to phase 4 only.
5. The Stationary Phase – All yeast metabolism begins to cease and the yeast fall to the bottom (flocculate). Yeast have the ability to go dormant during harsh living conditions and that is what’s happening here. They aren’t actually dying (for the most part) they are just going into suspended animation until conditions improve. This is also the phase in which any compounds such as diacetyls and esthers are metabolized and cleaned up thereby improving beer flavor.
Ok so now that we know some background about our wee friends, we can answer our last question: “why do we need a certain number of cells to go into our beer?” The answer is that we want to pitch our yeast when their population is large enough that they can reach saturation (the highest possible number of yeast cells in a milliliter of beer wort is ~100 million cells.) quickly to prevent infection and to prevent the production of extra esthers that can occur during a prolonged lag phase. Raines-Casselman says it best: “we want the yeast to put their energy into making alcohol, not babies!”
So here’s a good thing to keep in mind. In a perfect world we would pitch exactly enough yeast cells to give our beer exactly 100 million viable cells/mL of our entire batch.
This is clearly impossible for a very big reason: it requires a starter the same size as the beer! So this lead to my next question
So how can we prepare the yeast to get from pitching to saturation as fast as possible?
The answer again lies in the lifecyle. Yeast multiply exponentially in phase 3, therefore they should be in phase 3 and at maximum concentration in a starter that is small enough to not cause off flavors, but large enough to give us roughly 6-10 million cells/mL in our beer when we pitch it. This means that the yeast only have to double 2 or 3 times (2-3 generations) before they reach saturation.
Example: 5 gallon fermentation contains approximately 20,000 ml. This means that for 1.050 gravity ale you would need to add between 120 billion (20,000 ml x 6 million cells/ml) and 200 billion cells (20,000 ml x 10 million cells/ml).
So then how do we actually make a starter that accomplishes this task?
(STARTER INSTRUCTIONS SKIP TO HERE IF YOU GOT BORED)
This is actually pretty easy and short answer: we cheat.
Remember how I said that saturation cell level in wort is ~100 million cell/mL? I lied. Sort of.
That’s the limit in healthy beer wort, but not oxygenated and nutrient enriched starter wort.
we can beat that saturation level by aerating our starter and continuously stirring it with a stir plate. This will supersaturate our starter. So on a stir plate we are able to reach 180-360 million cells/mL in a 1 liter starter of 1.040 gravity DME wort as long as we use 1mL of nutrient and oxygenate it to between 8-12ppm O2.
This would give us about 180 billion-360 billion cells which is a pretty good number to start with for an average ale. Higher gravity beers need more cells to account for the greater sugar content in the wort, so just double this amount for a high gravity beer, and triple it for a barleywine.
So without further ado, here is my method for making a ~2L starter. As always when handling yeast, sanitize everything that will touch the yeast!
1. Calculate your starter volume – This can be done with a number of online calculators and is based on the gravity of your beer, the volume of your beer, and the type of yeast you are using. I favor Mr. Malty or Homebrew Dad’s calculators, or there is an excellent one on the Wyeast website as well. I just use 2L as an example because it produces enough cells for most moderate OG beers (1.050).
2. Make your wort – boil 2L of water with 200g of dry malt extract (about 1/2 cup) for 15 minutes.
3. Cool your wort – to about 70F and dump it into your starter container.
4. Aerate and add nutrients – Aerate with oxygen and an oxygen stone, if possible, for a single 60 second burst. Add the yeast nutrients ~1/4 tsp/L. I prefer the ammonium phosphate based nutrients for the reasons discussed in the Raines-Casselman paper.
5. Add the yeast – Dump your liquid yeast into the starter wort along with your sanitized stir plate magnet. Cover with loose fitting sanitized aluminum foil or a sanitized foam stopper and put the container on your stir plate. This should reach the end of phase 3 in ~12-18 hours and is then ready to pitch directly. Just dump that sucker in your beer.
If you don’t like the idea of adding all that oxygenated nasty starter wort to the beer (as I don’t) and you’re worried about shocking the the yeast with osmotic pressure of a higher gravity wort, there’s a better option!
After the 18 hours on the stir plate put the starter in the fridge and allow it to cool at least 24 hours.
Decant off the liquid at the top and allow the yeast cake to warm to room temperature while you brew.
Make about 500mL of fresh wort of a gravity that is half-way between 1.040 and your actual beer. Brewersfriend has some good calculators for estimating the gravity with extracts.
Dump in your fresh starter wort, give it a shake and put it back on the stir plate for about 3-4 hours before pitching. This ensures that the yeast are all active and awake and is similar to the process and reasoning behind the wyeast smack packs.
Viola, let there be beer!
Obviously these numbers aren’t exact for every beer, and a pitch rate calculator will give you a more precise volume to shoot for than 2L, but this should help give you the basic idea of how to make a starter, and why to make a starter.
This is just my opinion on building the optimum starter in the shortest amount of time with the least volume of starter wort. These methods have not been rigorously tested, nor have they produced award winning beers (yet). They are based on what SHOULD work best given some research, and what HAS worked for me from a few batches in my kitchen.
Here’s some things not to do to/with your starter
Don’t drink it, it’s nasty and will give you the shits
Don’t let it ferment completely! remember, you want the yeast at the end of phase 3, and if it sits for too long the yeast will start to die and this will negatively impact the viability of your cells. I wouldn’t leave a starter out much more than 48 hours before pitching.
Don’t dump cold slurry/yeast cake into room temp wort, always change the temperature of your yeast gradually or you’ll shock it.
Don’t dump a starter made at 1.040 gravity into a beer with a 1.095 gravity. The increased osmotic pressure will burst cell walls and reduce the effectiveness of your yeast. I like to step up the gravity of my starter before I do that. So for a 1.095 beer my 500mL vitality starter gravity would be about 1.065
Don’t paint your walls with it, the slurry looks like baby shit and will repel house guests.
That’s all I’ve got. I hope this has been useful to someone, and I hope I’m not totally wrong about everything I just said!
If you feel like experimenting and reporting results I would love to hear them, especially if they prove that I’m full of it.