MET and Coffee Aging/Staling

Discuss roast levels and profiles for espresso, equipment for roasting coffee.
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another_jim
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#1: Post by another_jim »

(Definition: MET stands for maximal environmental temperature. One can roast the same bean for the same time to the same temperature, for example, for 11 minutes to 435F, but still get different tastes, since some roasters run hotter, and some run cooler. The lowest possible MET has to be a little higher than the final bean temperature, and the highest MET should be less than about 510F to prevent bean surface charring or degradation.)

I've never done side by side comparisons between the same coffee roasted with lower METs to those with higher ones. So I roasted two batches of some of my regular espresso coffees and blends in the usual manner (11 to12 minutes to 435 to 445 depending on bean). The first set I did in a heavily insulated chamber with fewer beans; the second in a lightly insulated chamber with more beans. In the first set, the METs peaked at 455F, in the second at 495F.

At first, after two and three days, the results confirmed my prejudices: the low MET roasts were soft and sweet, the high MET roasts harsh, with ashy and astringent edges. But I wanted to make sure, so I put the coffees away for the week and tried them after 9 and 10 days. The low met ones tasted a bit flat, again as expected; but the high met ones were a surprise: they tasted like biting into ripe chocolate or vanilla laced fruits, close to awesome. The pattern was repeated with all four coffees I roasted over the two days I tried.

First off, this may explain the resting times recommended by different roasters; perhaps those running a hotter drum want longer rest times than those running cooler ones.

Second, these results are unconfirmed and incomplete. I need to characterize more closely exactly how the MET changes the taste. Initially the high MET roasts taste roastier, just as expected; but the effect of aging doesn't seem as easy to characterize. Also I need to compare the high MET roasts after a week to the low MET ones after a few days to catch each at their peak.
Jim Schulman

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AndyS
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#2: Post by AndyS »

another_jim wrote:I roasted two batches of some of my regular espresso coffees and blends in the usual manner (11 to12 minutes to 435 to 445 depending on bean). The first set I did in a heavily insulated chamber with fewer beans; the second in a lightly insulated chamber with more beans. In the first set, the METs peaked at 455F, in the second at 495F.

At first, after two and three days, the results confirmed my prejudices: the low MET roasts were soft and sweet, the high MET roasts harsh, with ashy and astringent edges. But I wanted to make sure, so I put the coffees away for the week and tried them after 9 and 10 days. The low met ones tasted a bit flat, again as expected; but the high met ones were a surprise: they tasted like biting into ripe chocolate or vanilla laced fruits, close to awesome.
Jim, a fascinating post on an awesome topic! I hope there is a lot more to come on this.

Using fewer beans with the lower MET was so you got more airflow to maintain approximately the same heat transfer?
-AndyS
VST refractometer/filter basket beta tester, no financial interest in the company

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TimEggers
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#3: Post by TimEggers »

At what stage(s) of the roast is a high MET more "damaging" to the beans?

In my RK Drum grill roaster I have to start with a very high MET (550F*) and cut the heat a minute or so before the start of first crack in order to get to first crack in a decent time frame and still be able to develop the roast.

*take my numbers with a grain of salt, moving my thermocouple probe even an inch can produce different numbers, basically I start quite hot and then cut back just before the start of first crack
Tim Eggers

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another_jim (original poster)
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#4: Post by another_jim (original poster) »

It's easy to measure MET and bean temp in an air roaster, just put a TC where the air comes in, and another where it goes out. Next roast I do, I'll record some logs that show what happens when you change the bean load or insulation. In an airroaster, if you run a constant the same supply air profile for a low and high load, the low load will finish faster. If you run the same bean temp profile, the low load will have lower supply air temps. This has a caveat, you need to be able to adjust fan speed so that the bean circulation runs at the same speed for both roasts. If you have the fan at constant power, there is a very narrow range of bean loads in an airroaster.

In a drum, measuring the Met is a lot tougher, maybe a sensor touching the inside of the drum would do it. But low loads require less heat or finish faster, just as in an air roaster. There are lots of exotic control methods for changing the relation of Met to bean temps, but this is the fastest method.
Jim Schulman

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#5: Post by Frost »

Thanks, Jim. It is your posts on managing MET/ET that prompted me to (finally!) install a MET probe in the Poppery I. While I'm not quite able to claim I have actually used it to create better tasting roasts :? , It has become the primary data point I use to control the heater variac, and has enabled me to (somewhat intelligently) control the fan. It has provided a much better understanding of the roaster's thermal dynamics. I highly recommend it be installed!

I insulated the 'body' of my P1 a couple years ago when cooler ambient temps brought some unwanted changes to my roast (higher power required to get the same bean temps.... meaning higher MET and my lighter roast profiles shifted to darker. I eventually concluded not to roast below 50F ambient)
Could you provide some details on how you insulate the roaster? I have not put anything on the metal 'chimney', thinking that will be next. The roaster body I wrapped in a 'sandwich blanket' of foil-fiberglass-foil, the outer plastic case stays cool through multiple roasts.

Another interesting MET fact I discovered on the P1, A constant power level will result in an increasing MET(for all practical roast times, as it will eventually stabilize), ...so to hold a constant MET requires a constant (very gradual) decrease in power level. (I did some 'LTR' profile experiments yeasterday that showed this in the extreme, will follow up detail after I see if the roast is any good )

While knowing 'bean mass' temp is still just a bit more important, the MET temp allows you to keep one step ahead of what is happening to the bean temp. Adjustments of heater power level are much better informed.