Q: Does anybody now what is the best way to pasteurise coffee grounds?
It is certainly best to use them when fresh as they have already been pastuerised by the brewing process. However, if you are not able to inoculate them within 24 hours, your best choices are either:
1) refridgerate
2) freeze
The key thing to bear in mind is that from the moment the grounds go through the coffee machine, the door is open to growth from competing moulds, so you want to do one of these options as soon as you can. When refridgerating, spores of competing moulds can still germinate and start to grow, but their growth will be much slower than if left at room temperature. We’ve found refriderating for up to 4 or 5 days can have reasonable success. Freezing will prevent competitor organisms from growing completely, but you will need to wait for the coffee to de-frost when you take it out before inoculating (which can take some time if it freezes into a large block!)
These options are preferable to re-pastuerising at a later date – not just because of the extra effort and expense of re-pasteurising, but also because you standa greater chance of ending up with contamination when re-pastuerising coffee grounds which have already developed alot of mould growth. To avoid this, you would need to fully dry the grounds as soon as you picked them up, and then re-hydrate them, before then pasteurising. This is alot of extra work so I would choose to refridgerate if you can inoculate within 5 days of collection, or freeze if it will be for longer. Do bear in mind that both of these options are likely to increase your chance of contamination over just inoculating straight away.
Hope this helps!
Best wishes,
Adam
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