Today something odd happened with my roast. I wanted to roast some Ethiopian Yrgacheffe to City for drip coffee for a friend. I typically roast to vienna for espresso, so I started the same way using my GeneCafe. 350F for 5 minutes to warm/dry the beans, then 482F to 1st crack, then back to 460F between 1st and 2nd crack to desired roast level.

The beans achieved the color of Full City without cracking. Some beans had light splats of color and there still was a lot of chaff stuck to the beans. I could clearly see the beans had not gone through the expansion typical of 1st crack.

My topic for discussion: Is there a way to gage the temp setting during the roast and see that the beans are roasting too fast on the outside vs. internal bean temp?

I did a 2nd roast: 350F for 5 min, then 460F to completion but still I noticed some splotchy beans and the roast is a little darker than I was hoping for.

I have two points in this reply, since I don't think your problem is with interior versus exterior bean temperature. So I'll cover that first, and what I think went wrong second.

You can measure the exterior temperature of the bean by measuring the temperature of the air around the beans, In a drum roast this means a probe stuck in the bean mass, in an air or convection roaster it means a probe measuring the air as it departs from the bean mass. You cannot measure the interior bean temperature; but it has been measured in labs. It runs around 5 to 10F lower in slow roasts, around 10 to 20F in fast roasts. The slow profile of the Gene makes this a non-problem.

You went wrong by lowering the environmental temperature during the roast. This happens automatically in a lot of home and self-built roasters with poor on/off temperature controls, but you've been doing it deliberately. It is one of the major roast control errors.

When the temperature around the beans falls, the amino acids and sugars at the surface of the bean, that have developed earlier in the roast by the breakdown of starches and proteins, repolymerize. Starchs and proteins have no taste and no aroma, while sugars, amino acids and other small compounds do. So when the temperature drops in the roaster, the coffee flavors die.

Moreover, if you drop the environmental temperature at just the right time, from half way through the 1st crack to just after its end, then you will end up with ugly as well as flat tasting beans. At this stage of the roast, the beans are in their "glass phase," that is their skin is expanding smoothly. The temperature drop is like dropping a hot glass into cold water. The beans stay small, and show cracks, wrinkles and mottling.

These problems are covered up in roasts that go into the 2nd crack or beyond, since the breakdown of the cellulose then expands the beans some more. Moreover, until the end of the 2nd crack, some remaining polymers break down into roasty tasting smaller compounds. So the only remaining hint at what went wrong is that instead of a little fruit and acidity to go with the roast flavors, there'll be none at all.

I think some people get misled by larger drum roasters or unventilated sample roasters needing to cut the heat source at around the first crack. This is not to reduce the temperature inside the drum, but to prevent it from overheating. There is already so much heat stored in them, and so little opportunity for it to escape, that the drum temperature continues rising even when the heat source is off. This is never true for a convection roaster like the Gene, or even most well ventilated drums.

In summary: love the one you're with -- it's better to operate the roaster you actually have than the imaginary Probat you really desire

Jim
Interesting. I'm direct wired and controlled with a variac so I have no on/off thermostat issues. I generally ramp my temps up between about 290-370f bean mass temp. when it seems the beans are requiring more energy. I have been cutting back my temp. around 380f bean mass temp. where my bean mass degree rise/min is between 13-17degrees (usually around 9-10mins). to slow my rate of degree rise/min post first crack. Been going with about a bean mass temp. 5-8 degree rise/min during first. Thinking as long as the roast wasn't stalled the slowing down would give me a little more time and control between first and second crack.
Should I lessen my ramp and keep my temps. lower and more constant instead of lowering at 380 bean temp.?
Ed

I'm not sure what you are talking about. Are you cutting back the temperature or the heat? A varaic cuts energy, e.g. heat, not temperature.

The temperature of the air blowing into the roast chamber, before it gets to the beans, should never drop. This is the constraint limiting the range of bean temperature roast profiles you can use. In practical terms, this means

-- If the airflow in the Gene is constant, and the air is not recirculated, you must never lower the voltage on the variac. Have a low setting for the bean drying, and ramp it up to a high setting for the roast finish. Use the length and level of the drying voltage, the ramp up length, and the level of the final voltage to control, your profile. Then end the roast at your desired bean temperature.

-- If the fan speed changes, or the air recirculates, you'll need to measure the temperature of the air blowing in (i.e. as it comes off the heating element) and control your variac so that never drops.

Jim
Thanks, I hear what your saying. Will have to tweak a profile and try that approach. With my homebuilt I measure the convection fan flowing air temp. just before it hits the bean mass and control it with my variac. Then I also measure bean mass temp.. So this can be easily tried.
Ed

Thanks Jim - great advice! I was mislead by some article that I read somewhere...

But it seems I still have a struggle between getting the beans to 1st crack within a reasonable time vice ramping up the temperature more slowly to keep from blasting too quickly to 2nd crack. I'll try a more moderate setting - but my guess is that I won't reach 1st crack until 15-20 min into the roast.

I'll post in a couple days when I have some results.