Having been asked to do a yeast propagation from a slant at work I thought it’d be perfect time to talk about yeast starters and why they’re so important.
The main premise behind propagation is that you need to pitch enough yeast into your wort to ferment that wort into beer, if you don’t have enough yeast this is for the main part where problems arise in homebrewing. Low pitching rates can lead to:
- Excess levels of diacetyl
- Increase in higher/fusel alcohol formation
- Increase in ester formation
- Increase in volatile sulphur compounds
- High terminal gravities
- Stuck fermentations
- Increased risk of infection
Having a great pitching rate is one of the ways that will ensure your beer goes from being ok to being exceptional. Always remember, a brewer makes wort, yeast makes beer so to help it make the best beer possible it needs to be clean and healthy with plenty of friends to complete the job.
The general rule of thumb is 1 million cells per ml of wort per degree of plato, George Fix expands on that and states that you need to pitch 0.75 million cells per millilitre per degree of plato for an ale and 1.5 million cells per millilitre per degree of plato for a lager.
So doing some rough sums, for a beer with a starting gravity of 1.048 with 23L of beer you’d need to pitch 276 Billion yeast cells. According to George Fix 207 Billion if it’s an ale, 414 Billion if it’s a lager.
Wyeast states that their XL smack packs contain 100 billion cells and Whitelabs state a similar number at production stage so that means for a fairly modest abv beer you’re significantly underpitching.
So how do we ensure these kinds of pitching rates? The easiest way to do it is by making a starter, essentially a starter is just a small volume of wort that you’re going to add yeast, oxygen and nutrients too so that the yeast can produce more yeast. The main limiting factors we’re trying to avoid are food and oxygen, if you take one of these away it limits reproduction. Remember, we’re not trying to make beer in a starter, we’re trying to make more healthy yeast.
There are a couple of rules of thumb that can be used
At high krausen yeast densities reach approx. 100 million cells per ml so with that number and the calculated yeast requirements we can estimate the size of the starter required to create 276 billion yeast cells = 2760ml of starter wort
The other rule of thumb is that the size of the starter should be about 1/10 the volume of the wort, approx. 2.3L
Bear in mind these are rules of thumb and don’t take into account inoculation and growth rate that’s largely dependent on the age and viability of your packet of yeast. Without doing a proper cell count it is impossible to know for sure but at least making a decent sized starter will give you a greater chance of success.
A great way to calculate the size of the starter required based on the yeast you’re using is this calculator or this one, we’ll look further into the theory behind this, how to calculate this by hand and the use of stepped starters in the next blog.
How to make a yeast starter?
Always practice the most stringent cleanliness regimes, bacteria grows at six times the rate of yeast so you don’t want to be making what’s essentially a bacteria starter. Try to do all this work out of the way of any drafts, consider the option of an open flame in the area you’re working in, wear gloves and sanitise everything liberally.
Yeast starters are easy to make, all you need is an Erlenmeyer flask (temperature resistant), some tinfoil and some DME (or LME). The wort you need to make should be between 1030-1040 so for every gram of DME you need to add 1ml of water, for a 2L starter you’ll need 200g of DME. To that you can add yeast nutrient and hops if you wish although neither is strictly required. Set your flask on the burner of your stove at home with the top covered in tinfoil and boil for 10-15mins. Once boiled, cool the wort and aerate by shaking the flask, pitch the yeast and then every couple of hours aerate by shaking the flask. If you have a stir plate then your expected growth rate of yeast is even greater but a good starter can be made without a stir plate.
Starters (depending on yeast strain) can take from as little as 12 hours to 8 days to be at maximum cell density.
Once your starter is ready (at high krausen or once it’s finished) you can either let it settle and then decant the liquid and add the yeast cake to your wort or just pour the whole starter into the wort being mindful that if you do you will be diluting your starting gravity.
And that’s all there is to it.