Matt answers your coffee roasting questions: freezing, then roasting

This week Matt Hassell, Global Buyer, QC & Sample Management for Collaborative Coffee Source, and former roaster for George Howell Coffee, will answer your roast questions. Here’s a question from @_mr_B:

Hey @collaborativeCS, when freezing (green) coffee, how much time can/should you wait before roast? And how does it impact roast? #ccsQandA

— Bjørnar Hafslund (@_mr_B) December 7, 2017

 Matt’s response:

I’ve always found that you can roast green coffee 24-36 hours after it is pulled from the freezer. Though in that short of a time, a common issue I came across was that beans on the outer part of the packaging were (obviously) a lot more thawed out than the ones in the center. So, (and this may be easier said than done) you should make your decision based on a sample pulled from the middle to ensure proper defrosting. In my own case, that often meant breaking a vacuum sealed bag to check the middle, which starts the degradation period. It pays to wait a bit longer, and to be totally sure.

An ideal waiting period for defrosting green coffee is more like 72-96 hours. Not only will you then be sure that all the beans are properly defrosted, but that the free-flowing water in the bean has had enough time to migrate back to all portions. When beans are frozen, the water migrates to the center, thus leaving the outside a bit drier. The best way to defrost is much like a drying bed in that a thin, even mass with more surface area will produce better results.

You can run into a couple of issues roasting coffee that has been frozen. If the beans have not had enough time to return to ambient room temperature and are still a bit cold, the first thing you’ll notice in the roast is your bean temperature plummeting. Bean temperature readings typically aren’t a super reliable metric anyways, compound the problem with extra cold beans and you’ll be in a tough spot. Your reaction will be to apply more heat, which will only make it worse. Throw in that the outside of the bean already has less water, and the inner part has more, and you’re headed for an uneven roast with quite a bit of scorched flavor. It is best to wait an extra day or two.

And how does freezing coffee impact the roast? I’ve been asked that question so many times, and it took me a while to figure out the answer. I was in a unique position at George Howell Coffee where the large majority (we’re talking 99%) of all the coffee I had ever roasted had previously been frozen. It wasn’t until recently that I’d had more opportunity to roast coffee that hadn’t.

I would say the biggest difference is that it seems much easier to dry coffee that had gone through the freezing and thawing out process. I attribute this to the breaking of cellular structure during the freezing process. Water expands when frozen, cell walls break, the bean is sort of “broken” or maybe “broken in” is the better analogy. Anyways, it’s more receptive to heat. However, once you’re through the drying phase of the roast, and things have stabilized, it doesn’t appear to follow any other set of roasting principles. You can apply normal theory, and effectively nothing is different. Perhaps I will discover some other abnormalities as I get more familiar with roasting non-frozen coffee.

Matt will be answering your Twitter roast questions until Dec 12, 2017. Post your questions on Twitter to @collaborativeCS and use the hashtag #ccsQandA.