Total Volume of Yeast Starter in Regards to Evaporation from Boiling Wort

Question

Today I did my first yeast starter. I purchased the 2000mL flask and DME from northernbrewer, and it came with instructions for doing both 1000mL and 2000mL batches. It suggests 1000mL for 5 gallons of wort with OG of up to 1.080.

For 1000mL, it says to put in 650mL of water and 1/2 cup DME.

For 2000mL, it says to put in 1300mL of water and 1 cup of DME.

So I meant to do a 1000mL starter but ended up putting a full cup in, so I added enough water to equate 1300mL.

I did have a boil over (!) which will disrupt this question, but I thought it was relatively small.

My final liquid wort volume was 800-850mL. After pitching in the liquid yeast, the final amount rose to 1000mL.

So my "2000mL" starter is now half the volume.

It is impossible that the boil over I had took out one half the volume.

However I did a strong boil which most definitely added to the loss from the evaporation.

1) Does it matter at all if a 2000mL starter ended up as 1000mL of liquid? As long as I have the full cup of DME, the total volume shouldn't matter, right?

2) If not, Is it best to do a very light boil instead of a rigorous boil, so that the final amount of liquid will be closer to the amount you started with?

Answer 1

There are two key variables in a yeast starter - the volume of wort and the gravity of the wort.

The volume principally determines how many cells you get out of the starter. The gravity also has some affect, but most texts recommend a gravity in the range 1.030-1.040. This is to avoid too high stresses on the yeast, and also because oxygen dissolves more readily in a lower gravity wort.

When you reduced your starter from 2000ml concentration to closer to 1000ml, ignoring the small boilover, you probably almost doubled the gravity of the wort, from 1.040 to 1.080, which is much higher than the recommended 1.030-1.040. For example, Mr Malty says:

When making starter wort, make sure you keep the OG around 1.030 to 1.040. You do not want to make a high gravity starter to grow yeast.

I don't think anything bad will happen, but you may not get as many yeast cells as expected. Still, go ahead and pitch the starter when done, you'll still have more cells than when you started.

If you have two flasks, you can split the starter in two and top up each You can top up to the indended volume with sterile (e.g. boiled then cooled) water. This will bring the gravity inline with what's recommended.

EDIT: Brewkaiser has an experiment comparing yeast cell count and viability for different starter worts. The 20P (1.080) wort had ca 1/3 less growth compared to the other lower gravity worts and ca 10% less viability (90% vs 100% for the others.)

Answer 2

1. It matters a little bit. The advice I've seen is that a wort of around 1.040 is best for a yeast starter, presumably because that's the optimal level of fermentables for yeast propagation. 1/2 cup of DME in 500ml of water yields a gravity around 1.040. So the wort from 1 cup in 800ml will have a higher gravity -- somewhere around 1.065. The yeast will not propagate as effectively, and consequently your final cell count will be lower than if you'd used a 1.040 wort.

2. You're only boiling to sterilize the wort. 10 minutes at a low boil is more than enough.

Those instructions for preparing the starter wort seem strange to me. How could 1/2 cup of DME and 650ml of water produce a 1000ml starter? 1/2 cup is ~125ml, and less than that when it's in solution (Chem 12 was a long time ago, so I don't know the actual resulting volume). Palmer's How To Brew recommends 1/2 cup of DME in 1 pint (~500ml) of water. I'm guessing that when the instructions talk about a 1000ml starter, they mean "a starter than can be made in a 1000ml flask."