Optimal roasting times

I have a question about considering optimal roast times based on charge weight and coffee bean. Obviously each bean will be different, but we might say if your total roast time is 5 minutes, that’s too fast and won’t allow for development of flavours, and if its 20 minutes its too long, and your beans are potentially baking. But when planning out a roast, how do you identify what you consider to be the best development, and total roast times?

Apart from doing a number of different tests, when considering creating a profile, what factors are you considering that are good starting points for establishing yellow (I’ve heard 4-5 minutes is good), difference between yellow and FC (I’ve heard shoot for 3 minutes) …???

I’ve been on a Behmor, but the Bullet is a different beast and need some tips! Thanks!

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I’ve been thinking about this lately too. In scaling up from 325g to 500g batch size, I originally tried to use higher preheat and more power to follow the same IBTS curve. But this hasn’t produced consistently good results, and has resulted in tipping for some soft beans. I’m now wondering if larger batches simply need more roasting time. The first few attempts have been promising using the same old trusty 210C preheat from 325g batch size, extending the early P8 power interval by a couple of minutes, and roasting for a couple of minutes longer. Curious to hear what others think.

There are a number of good books out there that dive into roasting theory and affecting the flavor profile. Scott Rao’s and Rob Hoos’ books are probably some of the most well known. Many note that these are geared towards gas roasters, but I’ve read them and look at the theory and use that with how the Bullet operates. Rob Hoos also did a great class specifically on the Bullet. Check into availability of upcoming classes or if he has a replay option.

Generally speaking, when switching between batch sizes I keep the time points the same and adjust other variables like charge temp, heat and fan application, etc…to hit those same time points. Assuming I am trying for the same flavor profile from the same bean and only changing the batch size.

Basic time parameters. Most say 8:00-12:00 min for the total roast. Maybe longer if you are going dark dark. Yellow some where in 3:00-5:00 min. Rao says FC to end of roast should be 20-25%, Hoos goes by time after FC.

I’ve hard good luck matching flavor profiles when scaling from a 450g to 800g batch. Adjusted the PH temp, power and fan temps and hit the same marks. Taste in the cup matched between the different batch sizes.


Interesting theory @stone.danielj I will need to run a few more batches to experiment. So far I find larger batches definitely need higher PH temp (I follow the guide from manual and add a bit more for denser beans) but also need gentler scaling down adjustments for power and not as many fan changes, but definitely more time to hit FC and 20% DV ratio to drop. I have been trying to carry more heat from Yellowing to Maillard phase from higher PH and less P decreases, so that I can hit 20% DV in a reasonable time after FC and not get hung up on exit temp or TSFC.

I think I will do some batches focusing on these other targets now and see where my sweet spot falls.

With higher preheat and initial power I was getting some off-putting bitterness in the cup. In his roast defects webinar, Rob Hoos mentioned that scorching can occur early in the roast, where it is easy to overlook because it is later masked by browning. I’ve started checking closely with the tryer during Yellow and Maillard too. Didn’t really understand what to look for, but his pictures are very helpful. Finally noticed the tipping: tiny dark brown spot on only one end of the bean where it would germinate. Of course this is strongly bean dependent, but something to keep in mind if you’re chasing a bitterness issue.

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@bradm do you modifying the initial power level depending on batch size to get more development? So would you start with P6-7 for 350g, but as you scale up, use a higher power level (e.g. P8)? I’m wondering to what degree you need to shift the power (and fan) depending on batch size.

For 325g I really like starting with P7 for one minute, then increase to P8. For 500g I played with hotter preheats and/or increasing early power further to P9, but that seems to be too much for some soft beans anyway. Lately I’ve tried just extending the P8 interval and accepting the fact that the roast time will be a bit longer. This one turned out to be a great tasting dark roast, right at the verge of SC:


A couple more recent roasts look promising, but bottom line is I don’t have a good formula to scale up a profile with batch size. Need to check out what @stone.danielj has been doing, especially since he’s going much larger!


I gave that profile a go, but had some facing/scorching on some beans. I’m thinking that P8 is too much heat (I was roasting some Brazil), have you had any issues with using P8 for a small charge (I had 330g).

So here’s your Brazil Roast 8 from today, overlaid on my roast from the above post:

Mine was a 500g profile, which is what I thought you wanted, whereas yours is only 330g. Sorry my above post isn’t too clear on that. It also looks like your roaster was preheated differently, judging from the BT curve comparison at time=0 which shows you started at about 140C whereas I started closer to 120C. The preheating is really important if you want to replay a profile and get the same IBTS temperature curve.

For 325g I’ve been really happy with the profile used in this Brazil:


This one was a medium roast that had a nice balance of flavors: some chocolate, some fruit, some nuttiness, and good caramel sweetness. It started out with a slightly ashy bitterness on day 2, but at day 7 it really mellowed and improved.

I really like this profile for 325g batches, with some fine tuning of the duration of each power interval and the timing of the switch to F3 depending on the bean. For Colombian I’ve been increasing the preheat to 210C. To replay this roast it is essential to begin with a cold roaster and preheat for exactly 15 min. If for some reason your roast isn’t behaving the same, adjust the power settings during the roast to follow this IBTS curve. I roast indoors, under a range hood, so ambient conditions are pretty nominal. I’m also using a calibrated fan.

Sure hope this is helpful, and that your last roast is OK.


Wow, thanks for such a quick and comprehensive reply. I’ll give it a go and see how it turns out. Thanks so much.

Rob Hoos has his Bullet classes recorded and available (for a fee) on vimeo. I have his book but haven’t decided if I want to pay to watch those videos. @stone.danielj - did you take Hoos’ class and did you find it useful?

I did do the Rob Hoos class and I would say it is worthwhile, especially if you are new to roasting. I had read Rao’s book beforehand plus lots of other stuff. Always trying to understand the theory behind roasting and applying it to the Bullet. So some of Hoos’ class was review, the Bullet specific stuff was nice, some of it new some of it validation of what I already experienced with the Bullet.

Seems like a big take away from all of the stuff above is pay attention to your beans. Been said before but so easy to get hung up on the tech. I’ve had success with my roasts, but can’t say it is foolproof by any means. Different beans probably will reach differently.

So many variables. Each specific Bullet seems to have a bit of its own personality in how it roasts and measures temps. Environmental conditions, I’ve seen bigger differences with a 20% humidity change than a 20 degree ambient temp change. Etc, etc…

Thanks @stone.danielj for that input about Hoos’ class on the Bullet - I’ll have to think about it since $100 to access the video for 30 days is something I can’t seem to justify yet. I try not to get hung up on the tech either and tend to keep my profiles simple rather than trying to manage the ROR every second, if you know what I mean.

I can relate with the humidity change and have been avoiding roasting on a rainy days here in the New England recently!

@bradm, I’m going to take a peak at this profile of yours before I roast next time. I did something similar but didn’t stretch the P8 any where nearly as long. I was thinking this was something to try next to get a darker roast. I did two roasts this past Saturday and preferred the second roast. Similar to what @stone.danielj pointed out in his comment about Hoos going by “time after FC”, that’s what I have been doing while trying to set (loosely) set time targets for the yellowing phase. I think I’m going to stick with 401F for PH for now for these 500g batches. Not ready to up the ante to 750 or 800g.

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I accidentally closed out RT early on in a roast once. Had to finish it out by sight sound and smell. It was kinda liberating.

Try some time. Or even start the roast then turn your computer screen around. See what it looks like when you’re done.

I do prefer the Hoos time method as it seems to work well with the Bullet. A lot of roasts still end up falling into the 20-25% range. But others outside of either end of it with great results. Wouldn’t have been as tasty if I had stay married to the %.

There are no rules in coffee, just guidelines.

Yes give that Mexico CBV profile from 7/11 a try! Been searching for a dark roast profile for quite a while, that particular roast makes a great brew. For the larger 500g batch I increased preheat time to 20 min. Then it’s a Rao-ish curve until about 2:00 after FC, at which point I tried to just hold the RoR and add some fan to blow out any smoke. I guess this loosely follows the S-curve method that @coffee_mook recommended a while ago.

Also, BTW, that Mexico Chiapas Bella Vista bean is excellent if you like a Guatemala Huehuetenango. Great brews for both light and dark roasts. Not sure if Royal has any more, but just got 5# of the very similar sounding bean from Happy Mug that I’m eager to compare.


EXACTLY! Completely agree with that statement!

@bradm - I’ll definately will try to mimick that MX CBV profile from you. Thanks!

Actually Robs philosophy is don’t keep twiddling the knobs just because they’re there and you can…

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In my opinion larger batches will require longer times. If I’m trying to roast a larger batch to the same degree of development (same colour), I would allow for longer times to hit yellow and FC, but with roughly the same phase balance and DTR.

Consider the physics, you’re increasing the temperature of a larger mass, and if you manage to trace the IBTS curves, the only way that can happen is to increase the temperature difference between the drum and the outer bean. This increases the rate of conduction from the drum to the outer bean → which will possibly lead to scorching for lower density coffees or uneven development (assuming the smaller batch size had even development).

I’m not sure that the only way to get the same roast rate with higher bean mass is to increase ∆T across the drum-bean boundary.

Caveat: I never took thermo in school so I could be making a total fool of myself here. If I am, please call me out, my feelings won’t be hurt!

Two additional factors are significant -
1.) Direct conduction of heat from drum to bean surface is only part of the heat transfer. From what I’ve read, the split for a typical drum roaster is 30% conduction and 70% convection.
2.) The heat transfer boundary area increases with increased bean mass, so you can get the same ROR by maintaining the same heat flux. To do this you need add more energy/unit time (power) to make up for the transfer of the initial system energy across that larger boundary.

So roasting at higher power can help a larger roast track a similar curve. My experience is that larger batches tend to take longer but that can be offset (up to a point) by roasting at higher power.

The initial ∆T you get by setting charge temp is another variable but as you note, charging too hot can result in scorching.