ok, apologies for the delayed response on these:
I had never heard of this strategy before!
I'd love to use this idea to pull out the brett/lacto/pedio from some Jester King bottles without having any carryover Sacch. Have you tried this before? If so, what was your recipe, and about how long do you think it'll take for the sacch to die out?
from memory: boil maybe a cup of oats in 3-4 cups of water for 10 mins. strain out the solids using a fine mesh strainer/filter/etc (colander probably won't work). to this goop add malto-dextrine until you hit about 1.030. use a refractometer to determine how much MD to add, be sure to mix well as you add the MD. if you want some sacch growth, do a 50/50 mix of DME and MD.
alternately, skip the oats and just use MD.
start small, like 200 ml of starter to the dregs of one bottle. freeze the rest of the starter, then quickly re-boil before adding more - if you're adding more. give the bugs several weeks between steps (brett won't need that long, but the other bugs will).
If I am using a sacc strain for primary fermentation anyway, will I lose anything by simply pitching the dregs and not making a "bug starter"?
I assume that the lager yeast will have very little to eat in an environment that another sacc strain has already fermented.
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i wouldn't bother with a bug starter, just pitch the dregs directly. a bug starter will give the critters a chance to grow before being pitched into the rough enviro of an already-fermented beer, but i overcome this problem by pitching dregs of several bottles (also a convenient excuse to drink more sours).
yes, lager yeast - in fact any yeast - thrown into an already-fermented beer will have little or nothing to eat. given how inhospitable an enviro it is for a newly-arrived yeast (alcohol, low pH) they will mostly go into hibernation.
If you're using sacch for primary then you don't need a starter for the bugs
i agree. just pitch and forget.
Correct me if I'm wrong, but I think a malto-dextrine starter might be good if/when the concentration of brett/bugs in the dregs is pretty low and/or you want to speed up the souring process. This may be particularly useful when using dregs from old bottles.
right, but there is some risk to making a dextrine/starchy starter, especially with a low cell count of bugs: bugs are slow, and aren't the only critters that think that MD is tasty. there is a risk that some other nasty will set up shop. by pitching directly into the beer, you're making things rough on the bugs but much much rougher on anything else that might get in.
For faster souring, you could use dregs that are known to be very aggressive such as jolly pumpkin, crooked stave (watch out for pedio ropes) cascade or really "fresh" bottles.
i've gotten decent souring from JP dregs in 4-6 weeks. i've never gotten pedio ropes (sickness) from CS.