Charging at a High ET without Tipping/Scorching/Charring?

#1: Post by **coffee.me** » August 25th, 2009, 12:35 am

How can pro roasters drop in the beans at an ET of 400F without getting the roast defects in the subject line?

I can't recall the sources, but I understand it's common for pro roasters, on pro machines, to pre-heat to an ET of 400F before starting the roast. I think I saw this info, at least, on coffeed.com and RoastersGuild.org. This basically means that the drum is at 400F, while the beans are at 80F, why wouldn't they get tipped, scorched, etc?

Is it the solid drum? The batch size? The heat source? What is it?

Why shouldn't I do it on my Hottop? Care to guess a workable max charge temp?

#2: Post by **another_jim** » August 25th, 2009, 1:50 am

Large drums rotate a lot faster than small ones, and the beans hardly ever touch down. I don't think the smaller, slower rotating sample roasters get preheated that high.

#3: Post by **another_jim** » August 25th, 2009, 3:06 pm

Oops, it occurs to me that beans would have a hard time tipping or scorching when they are still moist -- you should be able to drop in at 400F without incident. I think it's after the first crack that these occur.

But I don't know enough about drum roasting to be sure.

August 25th, 2009, 3:26 pm

Thanks for trying to help two times

, WAY better than any guess I came up with.

If I'm not mistaken, I think I can get these defects to show up as early as a minutes* post 300F BT, but not before. That's still way ahead of 1st, but yes, not during drying.

But then, my HT is no Probat!

*edit: one minute, not one minutes!

August 25th, 2009, 5:08 pm

From Boot's articles, a few pics of such defects:

Now, here's one of mine(photo'd this months ago). . . .what would this be called? To me, it looked more like the lighter colored spots are the problem. . .

#6: Post by **another_jim** » August 25th, 2009, 5:25 pm

In Boot's pictures, the splits are dark, while the uncharred portions of the beans are light. Not sure what that means. Your pic is of a dark roast.

The real question is how can green beans can char inside a minute after being dropped cold into a 400F Hottop, while roasted beans don't char at 450F to 475F ETs late in the roast. The only answer I can think of is small area away from the measuring zone, a hot spot, that overheats early in the roast, then settles in as the working roaster carries away the heat. That would mean running the fan during the preheat should solve the problem.

August 25th, 2009, 5:44 pm

No, not one minute after charging, but ~one minute after the beans get to 300F. The above photo is noteless, I don't recall what the roasting profile was(hence, maybe not even done at a high charge) but thought that was an interesting thing to share . . . . and remain nameless

.

Hot spot you say? I've always wondered about the front sight glass, but that can't get too hot, it's in the front and not too close to the element. Could be the SS drum? The HT pre-heats at full heat and no fan, with the drum rotating close to the element; maybe SS catches the heat fast and releases it fast too?

#8: Post by **gyro** » August 25th, 2009, 6:55 pm

another_jim wrote:Large drums rotate a lot faster than small ones, and the beans hardly ever touch down.

I checked the RPM of my 1kg roaster (now working, not yet PID'd, but got some parts now

) and it was about 25RPM. I believe some Probats were in the 50s, but in the last few years they slowed the RPM towards the 40s. Certainly faster than mine, and you can definitely see the beans being thrown around alot more in one. Anyone know what the Hottop runs at out of interest?

#9: Post by **another_jim** » August 25th, 2009, 7:29 pm

coffee.me wrote:The HT pre-heats at full heat and no fan, with the drum rotating close to the element; maybe SS catches the heat fast and releases it fast too?

But I thought it only preheated automatically to 325F or 350F. So if you want to preheat further, wouldn't it be possible to the fan as you drop in to get rid of any hot spots?

August 25th, 2009, 8:37 pm

FWIW, my HT automatically preheats to 167F on the LCD and ~260F on my (ET&BT) probes; you can go on preheating, using the fan, after that; but I'm not sure that would cool our assumed hot spot. Maybe that SS drum reaches much higher temps than I read on my ET (don't have a MET probe) because of its proximity to the element?

So now we're thinking the MET in a HT comes from the drum? What kind of probe would work to measure the perforated(not the edge) part of the drum? I have some real thin TC wire gathering dust, any ideas?

Oh, I'd love to get a MET meter in there, the total number of probes will be 4