Delta Point

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Postby AndrewT » Tue Jun 15, 2010 9:18 am

The question i have is how do you approach delta point (the point at the beginning of the roast when the coffee temp stops declining and begins increasing)? How hot do you get your roaster? Do you use partial heat at the beginning all the time or just for particular roast? At what time and temp do you look for turn around? And at what point do you think there would be a significant taste difference in the coffee? A lot of questions i know but i think it is really hard to determine the range in which there is a significant taste effect, in normal roasting conditions. This is another discussion where i would love to do a blind cupping with our baristas (which i have yet to post the results from the discussion on Blending) and post the results based on a range we agree upon.
Andrew Timko
Kaldi's Coffee Roasting Co
700 St. Bernard's Lane
St. Louis, MO 63110
(314) 727-9991
@AndrewRstStl
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Postby N3Roaster » Sat Aug 07, 2010 12:25 pm

I think that it's important to remember that while the turnaround point can be useful in gauging if you're on track for repeating your target roast, it's really an artifact of how the bean temperature is measured. The temperature of the coffee should always be increasing. Here (warning, link works in just about anything other than IE) you can see a typical profile for me. The red line in the graph is the bean temperature, the green line is the temperature of air inside the drum. The turnaround is a little after a minute and a half, but that's the case for all coffees with this particular roaster and is determined by how quickly the coffee is able to cool the thermocouple down to the temperature of the bean. Differing amounts of pre-heating, heat input during this early part of the roast, and (within useful ranges for a shop roaster) changes in batch sizes will change the temperature at which that turnaround point happens, but the time does not change significantly.

I think that it's more useful to consider the amount of time until chemical changes start to occur (in this case, 300°F, you'll note the color change). Prior to this there are physical changes which can have some effect on the coffee, but as long as about the same amount of roasting time occurs in this temperature range (from room temperature which is the true temperature of the coffee at the start of the batch to 300) where exactly the turnaround point is doesn't really matter. In the example profile, you'll see that this was probably my first batch of the day. The starting temperature is fairly low so the gas is on full. About five minutes in, well after that turnaround point, I'm backing off on the flame in preparation for the part of the roast where chemical changes are happening. That's really the important point. Now here is another roast of the same coffee, roasted to the same profile, but you'll note that the starting conditions are considerably different. This is a larger batch (about 3 times as large) and I'd already roasted a few batches that day so the starting temperature is considerably hotter and there's more heat in the drum as well. As a result, the turnaround point is about 25° hotter. Once the slope coming out of the turnaround could be determined, the heat was turned off entirely to slow things down. If you compare the two profiles you'll note that they match up very well from about 25° before the start of chemical changes to the end of the roast. I'm drinking the coffee from this batch as I type this and despite the differences at the very beginning of the roast, the flavor matches perfectly with what is intended for this coffee.
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Postby AndrewT » Tue Aug 10, 2010 7:51 am

Thank you Neal for the thoughtful response. I agree that turnaround is an artifact of how the bean temp is measured and is a relative point of equilibruim. I guess i was trying to get a step beyond, "get the roaster as hot as you can without scorching the coffee" and discuss way of improving our undestanding this preliminary stage of roasting. btw, that is what i currently do and it works, but if i would rather aceive a controled, safe range that could improve our ability to get more out of the coffee. Some considerations, would be to better understand scorching of temps, actually air and drum surface temps and under what conditions this occurs, difficult at best to get this type of information maybe it is already out there. I imagine it would be a complex formula involving roaster mass, coffee mass moisture content and density, actual air and surface temps and so on.

As for set points or physical change points. I couldn't view your graphs but i agree with you in pracitce and theory. Although i would think for a smaller batch when a faster turn around is achieved due to more air flow relative to a full batch and other variables that shift forward your set points. This is presented better via graph but a faster turn around (higher temp at delta) shifts your curve up and if the slope of your curve is the same then you would achieve yellowing, first and desired development sooner. Just a thought...

Look forward to hearing more,
Andrew Timko
Kaldi's Coffee Roasting Co
700 St. Bernard's Lane
St. Louis, MO 63110
(314) 727-9991
@AndrewRstStl
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Website: http://www.kaldiscoffee.com/
Location: St. Louis MO

Postby N3Roaster » Tue Aug 10, 2010 10:09 am

Neal
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Website: https://typica.us
Location: Racine, WI
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Postby AndrewT » Wed Aug 11, 2010 7:13 am

I like the format of your profile / data logger. Is that your own design?
Andrew Timko
Kaldi's Coffee Roasting Co
700 St. Bernard's Lane
St. Louis, MO 63110
(314) 727-9991
@AndrewRstStl
AndrewT
 
Posts: 47
Joined: Tue May 25, 2010 8:13 am
First Name: Andrew
Last Name: Timko
City: St. Louis
State: MO
Zip Code: 63110
Company: Kaldi's Coffee Roasting Co.
Occupation: Coffee Roaster
Roasting Since (Year): 2004
Website: http://www.kaldiscoffee.com/
Location: St. Louis MO

Postby N3Roaster » Wed Aug 11, 2010 7:54 am

Yes, some years back the dual digital count up timer that I had been using at the roaster started to become unreliable in a way that a replacement battery didn't fix. After going to every place in town that I thought might be able to sell an appropriate replacement and finding nothing, I decided it would be less hassle to just write my own timer application. Once I already had a computer next to the roaster, it didn't take long to write this data logging application. Prior to that, I would transcribe my target roast profiles into a spreadsheet and print the pages to go into a binder. I thought that arrangement was useful, so when I added an export XHTML+SVG option to the program, I modeled the output on those sheets.
Neal
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Posts: 66
Joined: Tue Mar 16, 2010 4:30 pm
First Name: Neal
Last Name: Wilson
City: Racine
State: WI
Zip Code: 53405
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Occupation: Roastmaster
Roasting Since (Year): 2000
Website: https://typica.us
Location: Racine, WI
Twitter: N3Roaster
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Postby mopheos » Tue May 03, 2011 6:43 pm

Neal,

Having read this thread and your profiles, I am a bit perplexed by the temps you reach in your profile. I have never (and I do mean never) been able to roast past 350 or so degrees F. If I try, I will surely way over roast my beans. I am roasting on a Geometrico 2.2 kg natural gas drum roaster. I typically reach 1st crack in the 305 to 320 range and then dump beans prior to 2nd crack (340-350), as my 2nd crack usually seems so hot, and well into 2nd, I pop alot of those little divits off the bean. I am fascinated to know how these 400-425 temps are reached without charring the beans and what aspects of the profile I am missing by never be able to achieve those temps.

Tim
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Postby ChrisSchooley » Tue May 03, 2011 7:20 pm

Many and most thermocouples read differently. Most should read within a range, but it is very common that any particular thermocouple and/or the system that is translating its signal is way off. Also, poor airflow can cause an off reading as well as probe placement.
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Postby N3Roaster » Tue May 03, 2011 7:56 pm

First and second cracks should be at about the same temperature pretty much regardless of what's going on in the way of a profile (physical condition of the green coffee can shift it a bit, but the numbers should be consistent between batches of the same coffee). My numbers for the temperature at easily observed points (green to yellow at 300, yellow to brown at 330, first crack at 380, second crack at 430) match up well with values obtained from an experiment (sorry I don't have the citation conveniently available, maybe someone else can provide it) that measured bean temperature by embedding tiny thermocouples inside some coffee beans and roasting them with a batch in a fluid bed roaster. That said, getting a probe placement that reproduces those values isn't always easy and, depending on the design of the roaster in question, may not even be possible (for example, there's a consistent but non-linear error due to probe placement on my sample roaster which results in a measurement of 400 degrees F when bean temperature is really 415, but there just isn't a good place to stick a thermocouple on that particular machine). I'm not familiar with that particular model (searching online I can find references to a 1Kg/2.2Lb and a 2Kg, but not to a 2.2Kg model), but with numbers that far off of expected values my first guess would be that you're not getting a bean temperature measurement.

As for if you're missing something because of this, if your measurements are consistent with your results (that is, if you can record the measurements from a batch, duplicate them in another batch, and end up with the coffee from each batch tasting the same) then this is just a quirk of your machine and not necessarily a real limitation. If your beans are cracking, they are hitting those temperatures regardless of what your indicators read.
Neal
N3Roaster
 
Posts: 66
Joined: Tue Mar 16, 2010 4:30 pm
First Name: Neal
Last Name: Wilson
City: Racine
State: WI
Zip Code: 53405
Company: Wilson's Coffee & Tea
Occupation: Roastmaster
Roasting Since (Year): 2000
Website: https://typica.us
Location: Racine, WI
Twitter: N3Roaster
YouTube: N3Roaster

Postby mopheos » Tue May 03, 2011 8:12 pm

Thanks for the quick reply, Neal - it was helpful. I guess I'm not getting accurate reading on the actual bean temp. I had thought that 1st and 2nd crack would more or less consistently happen in a standard temp range, regardless of the numbers on the temp readout. So if my readout was something weird like, say, 260 F on the gauge, but the beans were actually physically entering 1st crack, the actual bean temp would be the temp that all beans generally crack at, not necessarily what my reading on the display was - (BTW, I think my roaster is the 2kg/5lb).

One other question - what degree of intensity do you allow first and second crack to develop? Is it standard practice to develop a vigorous crack and let it run its course, or anticipate its start and pull the heat down to develop a slow, subdued crack? Thanks again for the help.
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