Coffee Bean Roasting: I knew I could make this more complicated than it needed to be.

Mac's been roasting beans using a nifty whirly pop popcorn maker. Kudos to you Mac. I love those things. Eager to jump in and be cool too, I turned to my $1.91 generic (thanks Volunteers of America!) popocorn popper to roast green coffee beans. I got the beans from Mediterranean Food Imports on Dodridge and N High (home of the only source of Merguez in Columbus) for $4/lb. The owner thinks they're from Brazil, but is not sure.

The question I had is: While coffee's roasting, do the stages of roasting, indicated by the casually tossed around term "cracking," correspond to detectable thermal transitions? Temperature profile to the rescue.

I popped 50 grams of green beans to the hopper, dangled my thermocouple into the headspace and let the baby rip for 15 minutes (a time based on some futzing around). Here's the profile:

Coffee roasting temperature profile
Really boring temperature profile over roasting period.

The beans are shinier in the image because of my inability to use flash properly

The beans were slightly shiny, and dark roasted and are resting before I grind. I've never been able to hear that definitive cracking - all I hear is the dog whimpering, the cat attacking my feet or the kid running in the hall, so I've relied on time to provide an endpoint for roasting. According to my time profile, there appears to be no thermal events to dictate the perfect roasting endpoint. Just have to stick with plain old trial and error. A fun set of observations though.

... Honey, the popcorn is starting to taste funny.


mac said...

Dude. They got green beans on High? I'm sending my mom. I just don't know how folks (you, my brother, my sister) are getting such short roast times, are you using gas? On my K-Mart hot plate 335g of beans slightly toasted after 15 mins. On my second batch I went 35 minutes and got an espresso roast, lots of foam but not quite as dark as I like it. The third time 45 mins, French roast, but not as foamy, but my cup of coffee. I don't know about the cracking business, and I wish I had an IR probe (I may move back to CMH so that I can borrow yours), I just go by color. How about weight change? My beans gave up about 60g in roasting

After I ran out of green beans I went back to my old stuff and I have to say (and Bonne Femme agrees) the home roasted stuff is better.

Dave said...

Yeah, the green beans in Med Imports is a new one. It's what got me interested again.

I'm an amateur too and don't know when the endpoint should be. Ideally, I'll have to make a batch go too far and burn it (so it'll taste like Starbucks). But, my batch size is tiny, only 50 grams. This shit takes a long time to thoroughly heat. 335 grams is a lot.

Great idea on a critical point I missed - final weight. Thanks for the catch. The big limitation of a popcorn hot air popper is capacity. Max batch is about 50 grams or so.

I'm finally going to grind and drink some brew to see how it is. Wish me luck.

Bear said...

Sweet, coffee geekery!

Only have time for a couple of thoughts... few people know this but just about all of the independent roasters in town sell all of their beans green. Peaberries are good for home roasting because of the shape.

There are two distinct "cracking" stages. First one is H20 converted to gas reaching a high enough pressure to burst out of beans; second one is structure of cellulose starting to break down (coincides roughly with appearance of oil on surface of bean). Once you hit second crack, you're within shouting distance of actual combustion. Which does happen. The progression is French roast, Italian roast, call 911.

Anyway, there are temp jumps at first and (to a lesser degree) second crack, but the intensity varies by bean. Get a dense bean grown in high altitudes and my guess is you'll see more of a blip in your reading, esp. if the air is in an enclosed area where the steam won't just blow off.

As to general principles behind roasting and the roast curve... it's a question I asked a while back on a coffee forum. Lots of people had interesting input, I'd refer you to that discussion for more: http://coffeegeek.com/forums/coffee/homeroast/369274

gtg... look forward to reading more!

Dave said...

Thanks for stopping by Bear! Comments of an expert are appreciated. I'll be getting a fire extinguisher just in case.

Bear said...

On behalf of my wife, who had to drink my mistakes when I started roasting our coffee, and is still kind enough to choke down the occasional loser, I feel compelled to say, "Expert? HAH!"

Anonymous said...

If you can't use the popcorn popper for popcorn anymore, try the microwave. Inspired by Mark Bittman we have been making our own microwave popcorn. You use a brown paper lunch bag, put in some kernels (1-2tbsp) (you can swirl them in some oil first if you want). Start the microwave on high and when the popping slows down (couple of minutes) stop it. Works really well.

Dave said...

That's - like the most useful comment I have ever received. Thanks sooo much!

Anonymous said...

Ha - yeah, it was a life changing moment for me! The book is called 'Food Matters'
I borrowed it from the library.

Microwaving corn on the cob was a similar revelation!

Dave said...

I'm no 2 on the waitlist.
Thanks again.