Most brewers understand that yeast starters are important for making your beer.
If you pitch the proper quantity of yeast, your beer will ferment fully and give you a clean finish.
Using too little yeast (under-pitching) will result in a diaceytl flavor (butterscotch) in your finished beer as well as high finishing gravities.
While far less common, over-pitching (too much yeast) can also result in off flavors as the yeast will run out of sugar before it completes a full fermentation cycle.
The amount of yeast you need (called the pitching rate) varies depending on the type of yeast you are using.
Most sources quote 1 million yeast cells per milliliter per degree plato for an average beer.
(The refractometer will show SG and degree BRIX; however, the formula to convert is
degree Plato = 259 - 259/SG meassured at 15.5 °C
the calculator page will do the calculation for you)
A more accurate figure from Dave Miller is 0.75 million/ml times P for ales, 1.5 million/ml-P for lager and 1.0 milion/ml-P for hybrid yeasts.
To calculate the number of yeast cells you need overall, you simply multiply the pitching rate by the volume of the beer (in ml) and gravity of the beer (in plato) to get the number of live cells you need to pitch.
So for a sample ale of 5.25 gallons ( 19.87 litres) and 1.048 gravity – the number (if you do the math converting to ml and plato) is 177 billion cells.
So if you pitch a starter with 177 billion cells, you will have a proper amount of yeast for fermenting the beer.
Knowing how many yeast cells you need for a given batch provides a starting point, but next you need to figure out how to meet that need.
Most home brewers use commercial liquid or dry yeast packets to prime their starter.
The two primary liquid yeast providers in the US are White labs and Wyeast.
White labs yeast comes in vials that contain from 80-120 billion cells each, with an average of about 100 billion cells for a fresh vial.
Wyeast labs come in large and small smack packs.
The large pack is comparable to the vials, with about 100 billion cells per smack pack.
The small smack pack has considerably less – about 18-20 billion cells per pack.
Since even the 100 billion packs/vials are less than the 177 billion cells we calculated for a moderate ale, this means that most 5 gallon batches would benefit from a starter.
Dry yeast packets (Danstar, DCL SafeAle and others), which are considerably denser, contain about 18 billion yeast cells per gram.
Dry yeast packets come in small and large packet sizes of 5 grams and 11.5 grams.
Running the numbers, the 5 gram packet contains about 90 billion yeast cells and the 11.5 gram packet contains 207 billion yeast cells.
The figures above are for fresh liquid or dry yeast packets.
Unfortunately both dry and liquid yeast cells do die off as they are stored, making older yeast less effective.
The percentage of live yeast in a sample is called its viability – a brand new packet is 100% viable, but loses viability over time.
The effect is much more pronounced for liquid yeast than dry yeast.
Dry yeast has a long shelf life.
If stored at room temperature it loses only about 20% of its viability per year (<2% per month), and if refrigerated it only loses 4% per year.
So if you refrigerate your dry yeast it will last many years.
Liquid yeast, which must be refrigerated, has a much shorter shelf life.
Wyeast lists their shelf life at 5-6 months while White labs recommends 4 months.
White labs on their web site says that after 30 days, their vials have 75-85% viability, which is a loss of about 20% of viability in the first month.
If we compound this loss (20% per month), this means that the viability of liquid yeast follows this progression:
Now even at 6 months, with 26% viability you can make a suitable starter, but you need to take into account the viability of liquid yeast when calculating the starter size.
Dry yeast does not by itself need a starter, as long as you pitch enough packets of yeast.
Generally all that is needed is that you hydrate the yeast with warm water for about 20 minutes before pitching.
Use lukewarm water at 105F (41C) in the amount of 10 ml per gram of yeast.
This works out to 50 ml (1.7 oz) of water per 5 gram packet or 115 ml (3.9 oz) per large dry packet.
If you are using dry yeast as the seed for a starter to step up for a larger starter, hydrate it as usual and then add the yeast to the starter.
As above, the 5 gram packet contains about 90 billion yeast cells and the 11.5 gram packet contains 207 billion yeast cells.
Age is seldom a significant factor unless the yeast is over a year old or has not been stored properly.
Liquid yeast, due to both the cell count and viability lost as it ages, often does require a starter.
To figure out how large the starter needs to be, you first want to calculate the number of packets needed.
Generally the way to start is by calculating how many viable yeast cells you have in your vials or packets.
This is done by multiplying the starting yeast cells for a packet by the viability (use the table above).
So if you have a White labs vial that was manufactured 2 months ago, you will have 100 billion x 64% which is 64 billion cells per vial.
Next calculate the growth in cells needed.
The beer in the earlier example (5.25 gallons of ale wort at 1.048) requires 177 billion cells.
If we were to use 1 vial of 2 month old ale yeast at 64 billion cells, we would calculate the growth at 177 billion divided by 64 billion = 2.77 — meaning that we need to expand the yeast 2.77 times to get to our target population.
This means our starter needs to grow 2.77 times, from about 64 billion cells to about 177 billion cells in order to create the proper pitching rate for our finished beer.
The next step is to figure out how large a starter we need to create to achieve this growth.
One might think this is a straightforward calculation, but it turns out that the growth of yeast is not linear – it depends on how many yeast cells you have to start with.
The graph to the right, extracted from a table in Chris White’s yeast book
Obviously if you start with a very small starter, and a lot of yeast there is not much sugar to support growth and the growth rate remains low.
At the other end of the spectrum, if you pitch a relatively small amount of yeast into a large starter (approaching 20 liters) you get high growth.
However, growth rate peaks out at around 6.0, so pitching 100 billion cells is not going to get you much more than 600 billion cells total (6x growth rate), no matter how large the starter is.
This week I covered how to calculate the number of yeast cells for a given batch as well as the viability of liquid and dry yeasts. I also explained how to calculate the number of dry yeast packs needed and how to hydrate those. We started to look at growth rates for liquid yeast starters, a topic which I’ll continue in part two. I’ll also take a closer look at the above graph and how it helps us calculating the actual starter size for a liquid yeast sample in part two.