Before I fire off a query to the braintrust at Brewfather I just want to ask if anyone here has a different workaround than the one I am using. I know Beersmith has a checkbox to semi resolve this but I am not using Beersmith. I also know some folks are going to chime in with just decant the starter but that negates all the wisdom of throwing it in at high krausen (especially for lagers). In Brewfather, I set my batch volume to 18.9L (so it fits in the keg). I prepare the recipe then create the starter for the yeast based on the software recommendations. If, for example, it tells me to prepare a 1.9L starter I then go back and edit the equipment profile to create a batch volume of only 17L with a 1.9L top off. Hit scale and hope for the best. I have tried setting up a recipe geared at 17L and a fermentor top off of 1.9 but the problem is that the software does not know the gravity/abv of the starter only the volume and number of cells. If the starter was the same gravity as the wort that would be fine but everything (OG/FG/IBU/SRM) seems to be out by several points otherwise. For years I did all the math on paper but I have been using software for 10 years now and it has serious advantages for everything except starters. Hopefully I am just missing something simple.
Does the gravity of the starter really matter? Why not decant your starter before you pitch?
Here is a suggested workaround that may be OK. Measure the OG of your starter (or just calculate it if you use DME). Measure the OG of your wort before you add the starter, then pretend you are mixing the unfermented worts and ignore that the starter has partly fermented already. I think that should be accurate enough. For example, if you have 17 liters at 1.060 and a 1.9 liter starter at 1.035, then you act as if you have 18 liters of wort at 1.057: ((1.060x17) + (1.035x1.9))/18.9 = 1.057. Assuming that the fermentability of the wort and the starter are similar, this should work well enough.
This is what I had/have been doing. When I switched to using software I still have to do this bit by hand since the software does not do it.
The gravity does matter in that it throws the OG off by around 5 points. I don’t mind decanting on ales but with lagers I find the yeast takes a long time to leave the lag phase with just the slurry. The only lager yeast slurry I found that gets moving right away is Budvar. The wisdom has always been to throw the entire starter in at high krausen for proper yeast health and performance.
Assuming a general OG of starter medium being 1.040; if the starter fully fermented out most of the 1.040 should be gone. Normal yeast attenuation is around 75%, so you should have 1L of 1.010 beer in the starter vessel. That is not going to make a 5 point difference in a 5 gal/19L batch if wort. More like 3. 1L of 1.010 beer into 19L of 1.076 OG wort would reduce the OG down to about 1.073. (76 points * 19L + 10 points * 1L) / 20L = 72.7 points = 1.073.
Decanting off most of the starter beer only and pitching the yeast slurry should make no difference when pitched into an ale vs. a lager. Yes, lagers often have a longer lag time but that has more to do with cooler temperatures used when fermenting lagers and nothing to do with whether the entire volume of starter was pitched or just the slurry. As brewers we like to see a quick beginning to fermentation but a lag time of any reasonable length is not a bad thing. I’ve had lagers that lagged for 48+ hours and turned out great.