Hi George,
Good question! We have experimented with spent hops in the past and had mixed results. They definitely help to introduce extra moisture to the mix and many bags we made grew brilliantly, but we found two issues with them in particular:
1) the hops tended to clump together alot and didn’t easily distribute amongst the coffee
2) contamination rates (green and blue mould) increased, even when using fresh hops straight from the brewing process. There appeared to be more mould surviving the pastuerisation than occurs with pastueriused straw or coffee.
It must be said that we only tested this in large column bags and only for a relatively short period of time, so it certainly deserves further investigation. I would have thought the fresher you can use the spent hops after brewing the better, and avoiding contact with open air by transferring straight to clean buckets, then carrying out inoculation under the cleanest conditions possible would all increase the chance of success.
Best of luck and let us know how you get on!
Cheers,
Adam
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