Managing environmental temperature in the roaster

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

another_jim wrote: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. Starches 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 :wink:
This was a response by Jim in the recent post "Gaging Temperature from Bean Color"
I've heard many times about stalling the bean mass temp. rise especially during first crack and it's negative effect on the roast. Also we often hear about keeping the roasters environmental temps below 500f for various reasons. But I've not heard much on the negative effects of lowering the environmental temp. inside the roaster. If I'm understanding what Jim is saying, lowering the environmental temp. even if the bean mass temp. has not stalled, will effect the fruited and subtle notes in the roast especially if done around or during first crack. Many homeroasters tend to ramp the temp up approaching first crack and then lower to slow the momentum and to stretch the time between first and second. But now I'm thinking I need to look at my ramp temps rather than applying the brakes so to speak. Also the effect of thermostats in many home roasters and their often large temp. swings between on and off seems to be even more important than I had previously thought (the reason I went variac and by-passed my thermostat that often had a 30 degree swing). I hope we can have a more detailed discussion on this as it seems like an important factor many of us had not considered.
LMWDP #167 "with coffee we create with wine we celebrate"

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#2: Post by another_jim »

My source for most of this info is Carl Staub.

Ed is right that the major caveat is never letting bean temperature drop. But a probe in the bean mass measures the average, and will respond slowly. By the time it shows a drop, a substantial proportion of the beans will have lowered surface temperatures. The safest course is never to let the drum or inflowing air temperature drop.

I found this out the hard way when I replaced my roasting variac with a PIDed ramp controller. I got very inconsistent, and overall lousy roasts, even when doing back to back roasts with the identical bean temperature profile. The reason was that the inflowing temperature would start oscillating wildly to maintain the programmed bean temperature profile absolutely precisely, and this killed the taste. I had always operated the variac by jigging it up slowly over the course of the roast and never backing off. I subsquently went to controlling the inflowing air rather than the bean temp in the same way. Finally I figured out a way to tune very overdamped PID parameters on bean temp that created a "more or less right" profile, and never caused the supply temp to drop.

There's a larger lesson I learnt here. We coffee hobbyists tend to get over-focused sometimes. Getting one variable absolutely right is no achievement if it comes at the cost of screwing up the others.
Jim Schulman

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#3: Post by TimEggers »

Jim many thanks for your insight here. Your posts are always a wealth of information and very helpful. Indeed I've been making the bad move of lowering MET in my grill roaster, no doubt affecting the roast.

Now it makes sense why my beans typically look the way they do and why they look different then the commercial blends I get. And my sweet spot for extraction always came early and dissipated very rapidly. Now it makes more sense.

Many thanks Jim.
Tim Eggers
LMWDP #202


#4: Post by sully »

another_jim wrote:Finally I figured out a way to tune very overdamped PID parameters on bean temp that created a "more or less right" profile, and never caused the supply temp to drop.
So you are back to controlling on bean mass temp, just with some different params in the Fuji? If so you must not be using the auto-tune anymore - are you using Ziegler-Nichols?

Are you still playing around with insulation and blower speed to manipulate your MET? I recall a post on a while back saying you lowered your MET on one of your blends with the result that you only got caramels and all your chocolates went missing, but when you raised the MET back up about 10 degrees or so the chocolates came back. This seems to imply that the lowest MET that sustains the roast profile isn't always necessarilly the best, and that within some acceptable range MET can be yet another creative variable in the roasting process, even within the same profile. Starts to make airflow look a lot more important.

BTW, great information as usual Jim, and that Staub link rocks. Your posts are really amazingly helpful. Thanks a bunch.

- Mark


#5: Post by pauljolly65 »

Thanks indeed for the information, Jim. For years I've heard comments like "slow the ramp but keep adding heat" and "you can back off the roaster's temp but never stop adding heat" and I never knew why. Well, I should say that I knew that if I cut the heat-- if even only periodically to keep the ramp from getting too steep--the roast would end up stale, flat, sour, astringent, boring, 'baked', you name it. It's good to have a clear idea of why this happens to the beans.


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#6: Post by farmroast »

Been mulling through the Carl Staub info. Answers a lot of my questions but also creates many new ones. Bear with me I'm an aging dog trying to learn new tricks. I designed my hybrid roaster(check my blog site for pictures) to have the maximum amount of adjustments and adequate abilities to monitor the adjustments. Adjustable STE (system transfer efficiency) was one element that seemed to be very important when I looked at the present and past roaster designs. It seemed that the main improvements with roasters over time has been in the STE and the control and applied evenness of such to each bean. Once one can control the STE one must determine the BRR (best reaction rate) combined with an ideal ET(environmental temp.) and energy supply(btu) for various stages in the roast process. Monitoring my bean mass temp. rise/min. will give me my reaction rate. Monitoring my air temp. as it hits my bean mass will give me a general ET. I then adjust my variac on the heating elements and my rheostats on my bean bats that whirl the beans around and the variable speed convection fan to achieve the desired ET and STE resulting a desired BRR. It then becomes a matter of what the BRR and ET should be during various stages. This depending partly on the bean type and roast characteristics desired. This also depending on the individual beans makeup and potentials, qualities and defects. When considering a profile my ET should not drop anytime during my roast, my bean mass rise/min should never stall and I must be concerned with my MET(maximum environmental temp.) and consider a BRR for the various stages of the roast. Do I have this kinda rightish so far before I present any more questions? :? Ed B.
LMWDP #167 "with coffee we create with wine we celebrate"

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#7: Post by another_jim »

Ed, you seem to have something I've been mulling on doing:

-- Gave the heater on an environmental temperature PID
-- Put the airflow on a bean temp PID
-- Run a dual profile.

The reason I haven't tried it is that the require SCR for the fan is expensive, and I wasn't sure how finicky it would be to tune (too much or too little airflow on an airroaster, and the roast breaks down). I've built a few roast control systems for friends who want high quality set it, forget it on P1s, and for them I set up a PID ramp on the ET and an auto-stop for the desired bean temp. This works fairly well, but ends up being plus/minus 90 secionds on roast length depending on charge, bean and finishing temeprature. It also requires a different ET ramp for espresso and for brewing.

I'd be interested to hear how yours turns out, and if it show distinct improvement over the simpler control schemes I'm using now.
Jim Schulman

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#8: Post by farmroast »

I don't know much about silicon controlled rectifiers? or even much about PIDs. I'm just shooting for manual accurate control and metering for now. I've been using speed control motors made by Oriental motor ... /index.htm. They are AC and have various gearhead (if needed) ratios. They also have a digital speed meter that can be added. They are much more quiet, accurate, quality with good torque. Much better than the average universal motor. They are expensive MSRP(approx $280.a set) but for the past couple years I been collecting off ebay and now have quite a few sets(motor, gearhead and control module) for cheap about $50/set The speed control meters ... 6?&seo=110 are about $199.msrp and pretty rare for a cheap price but just got one NIB for $29.
LMWDP #167 "with coffee we create with wine we celebrate"

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#9: Post by TimEggers »

I'd just like to chime in and share an experience I had yesterday and how this discussion has helped me.

I loaded my RK Drum with a pound (green) Espresso Monkey Blend from Sweet Marias and installed it into my preheated grill. I preheat with my 3 burners on low (for about 10 minutes) and the MET will raise to and stabilize at 508F*.

At which point I add the cold drum with beans in it. Of coarse the temperature drops about 50F or so and slowly climbs back up (I leave the burners alone) as the drum absorbs heat.

At around 7 minutes the temperature returned to 508F* and stabilized there. Around 11 minutes the first pops of First Crack were heard and the temperature fluttered up about 8F-10F to this point.

As FC concluded the grill had warmed up about 10F to 519F* and stabilized there. Interestingly the temperature began to drop at the conclusion of FC (I assume this is the point where the beans go from exothermic to endothermic) so I had to bump up the burners to maintain the 519F* for the rest of the roast.

I concluded the roast mid-Second Crack which was about 3.5 minutes after the conclusion of FC.

The beans look great but I also took them far enough to cover up the mottling I used to get. I wanted to share beacuse I've never seen the phases of the roast so well as I did yesterday. And really not lowering the temp didn't affect my times that much (the roast didn't race out of control for me). For me it was a lower temp pre-first crack and a smaller amount of greens that balanced out the need for a hot pre-first. Now I can start lower and still get to first in a good length of time without having to reduce the temps to stretch the roast. My results seem far superior this way and I've finally pinned down what was affecting my roasts. The more I learn the more I need to learn. Thanks again Jim and to the others, heed the advice and shed the conventions commonly passed among Internet chatter. Stop lowering your temps, it makes a huge difference.

*take my numbers with a grain of salt, after all they represent my equipment in my location yours will be different.
Tim Eggers
LMWDP #202

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#10: Post by farmroast »

Good to hear! I too tweaked my profile with improved results. Lowered and balanced my ET.
1.5lb preheat 350f Costa Rica El Puente pulp natural screen dried
Min.....BT......ET......Degree rise/min....Variac voltage
still will do some more adjusting, but think I'm getting closer. Suggestions welcome.
Ed B.
LMWDP #167 "with coffee we create with wine we celebrate"