I’m going to finally brew this imperial stout that I’ve been planning for a couple of weeks, and in the spirit of forever trying something new I’m preparing a yeast starter, using Brewfather for guidance. I’ve figured out that the quantity of yeast specified for most of the recipes on that platform actually implies creation of a starter, but it’s not immediately obvious unless you open up the yeast calculator and and hit ‘save starter’ at which point it tells you how much DME and water to add to the indicated yeast. If you calculate the amount without ‘save starter’ it’s the equivalent of pitching the yeast directly, and the amount required suddenly jumps up significantly.

Anyway, having tweaked the Yeti recipe to reflect my local fermentables’ values I turned my attention to yeast, and worked out that I’ll be needing 1.8 litres of water and 177g of DME if I want to stick with using just one sachet of M44. The calculator tells me this will result in 398 billion cells, which is 2 billion over target. My biggest Erlenmeyer flask is two litres so 1.8 doesn’t leave me with a lot of head room in the event that my yeast should go ballistic while being stirred overnight, but building for two sachets of M44 (which worked out as needed 1.3 litres and 128g DME) seemed a bit counter-productive to the whole idea of growing yeast in a starter, not to mention unnecessarily expensive: DME is cheap, yeast is not.

I decided to go for the single sachet approach, encouraged by the fact that Brewfather assumes an OG of 1.036 for DME and I have a feeling mine will be closer to 1.040, which is what some labs recommend as an ideal gravity for growing healthy yeast. Adding 100 ml to cover evaporation during the boil I filled a large saucepan to just below 2 litres and cranked up the heat, decanting exactly 177g of Muntons Light Spraymalt once we had a boil going. 10 minutes later we were done, and I’d obviously lost more than 100 ml because the pan’s internal scale read just over 1.5 litres, maybe 1.65 but certainly less than 1.75. Oh well – the mixture was transferred to my sanitised 2 litre Erlenmeyer flask and measured a smidge below 1.6 litres once I’d drawn off a sample for gravity measurement and started chilling in cool water.

OG for my sample measured 1.040 at 364 ℃ which equates to 1.045 once you correct for temperature. Taking that value back to Brewfather I unticked the ‘calculate starter size’ box in order to allow me to input not only the gravity but also the amount of water, and I saw that with my one pack of M44 and 1.5 litres at 1.045 I’d be looking at around 408 billion cells – 11 billion over my target. That’s not too shabby at all; I’m not concerned about over-pitching 3 or 4%, 5 points over the ideal gravity shouldn’t stress the yeast too much, and I’ve got some extra head room in my flask for safety.

The stir bar was already in the flask from when that was sanitised by boiling a couple of inches of water (worked just fine on the halogen hob) so it was just a question of getting it down to temperature before pitching that M44. Not wanting to bet everything on borosilicate’s supposed resilience to thermal shock, I started off by putting the flask in a saucepan ¼ filled with cool water, then added cold water and left it to drop for 10 minutes before replacing the water with fresh, ice-cold water. Even so it was only 17.5 ℃ and therefore going to take ages to reach my target of 18, so I poured in two trays of ice cubes and checked in every couple of minutes using the infrared thermometer. Didn’t take long at all before I was at 18.1 ℃ so I threw in the yeast, refitted the tin foil lid, and gave it a good swirl before placing the flask on the stir plate at 13:17.

Over the next few hours my starter lightened considerably in colour and gained a layer of froth, which had stopped spinning when I checked in at 21:00. It also seemed as though the wort immediately below the foam was slightly darker, which I took to mean that the yeast was congregating in the lower part of the flask and a few mm of darker wort (spent?) had floated to the top. On giving the flask a good shake the colour evened out again and the foam head rose up quite dramatically, but soon settled down. On increasing the rate of spin to just below 50% I’m now seeing the foam mobile again. Only one last thing to do before turning in for the night: put the flask in a thin plastic bag. I don’t know what the Krausen’s going to get up to over the next few hours and I don’t want to find out if my stir plate is waterproof any more than I want to find out if our smoke alarms are loud enough.