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Airflow v Temp

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  • Airflow v Temp

    Hi Roasters,

    I was having a play around on the Corretto yesterday and I thought I would try roasting a couple of different batches of PNG Wahgi, one on fan speed III and one on II (I have a Bosch 630) to see what the difference, if any, would be. I tasted the roasts today and found that already the lower fan speed has a much less bitter taste and more sweetness than the higher fan speed. Unfortunately though, this wasn't the only difference between the two roasts, the slower fan speed roast was also about 3 minutes longer and had a much slower/gradual rise to BT I will have to do some more experimenting/limiting the variables before I can be sure it is the fan speed that is the difference between the two.

    I did a bit of a search but couldn't find anything referring to the benefits of either high airflow or high temperature...does anyone have any advice or suggestions on this ? I would have thought that a higher fan speed, allowing a lower heat would be more gentle on the beans or is it the other way around ?



  • #2
    Hi Dave,

    A quick search with the terms 'Corretto fan speed' brought up this excellent
    and very helpful thread, started by Matt aka designingbycoffee.

    Have you read it, if not, then I recommend it to you.

    It might have some discussion pertinent to your question.......


    • #3
      I have the same heat gun with an insulated modified PALSONIC PAB-3600 and for some time I only used the fan speed 3, playing around with different batch sizes, longer roast times 15 to 19mins and variations on the profiles. It did a good job, I got some memorable results.

      I started playing around with smaller batches, roast times between 12 to 15min and only using the fan speed 2, without checking maybe around 6 months ago? Lots of reading and research as well.

      Have not looked back since, roasts are much more sweeter, cleaner, better preservation of origin characteristics / aromatics. I generally only roast SO and rarely bother with blending.

      I ran my heat gun / bread machine empty at varying temps and times to work out where abouts my max environment / drum temp should be. I do not go over 450HG temp on my setup, which results in about 250 - 255 celcius stable temp after 10mins in empty running bread machine.

      So to get the roast times I want, which are generally speaking, 5.30 - 6min drying, 4 to 5min maillard and then appropriate development time bean dependent, my batches need to be 200g give or take 20g.

      Upping the batch size and using a higher temp on the heat gun to get the same time results in to much roast character and can start to char. Using the same HG temp settings will give a longer roast, which for some beans works well, so I will roast 250g instead of 200g.

      Mostly, I have found that longer roasts times just flatten things out. Especially with overly long drying times and maillard zones. Good way to make lots of different beans taste the same.
      Most of what I have read suggests the same thing and especially when roasting with high airflow / heat convection environment such as can be found in corretto, especially using the fan speed 3. Ideally it would be nice to have some kind of infinite adjustment knob for fan speed control. There are others with much greater electrical knowledge than I who have done similar things.

      High convection environment really has the ability to get the heat in to the interior of the bean very quick. So its easy to over dry the inside leaving not enough moisture for good maillard reactions followed by caramelisation to take place. Follow that up with over development later on in the roast, It will roughly follow this, flat - wooody - dry - ashy - charcoal.

      So my roasting philosophy has generally become about trying to get the beans in and out of the roaster as quick as possible within reason / limits of my setup using the lower fan speed.


      • #4
        Hey Chokkidog, thanks for that. I have actually read that thread (maybe not a thoroughly as I should have though). It seems though Matt prefers to use fan III for the entire roasts and also uses much larger batches than what I roast (300g).

        Hi Steve, that actually sounds very similar to the roast that I did with fan speed II. I had a slightly larger batch of 300g, a little longer (17min) and used a higher max temp of 480deg which would all make sense.

        Most of what I have read on both Home-Barista and Home roasters seems to be along the lines of a faster roast ie <15mins and I assumed that the easiest and most common sense way of getting there was to use high fan seed and high heat but the roasts always tasted a little ashy or dominated by the roast characteristics more than the bean. I never considered dropping the roast size as an alternative.

        I'll attach the profile of the lower fan speed batch.
        Attached Files


        • #5
          Hi Sully
          I think there are elements to be gained from both forms of heating. There must be - commercial gas and electric roasters have fans too

          As mentioned, I tend to use fan 3 for most of the roast, but drop to fan 2 during the Malliards zone to slow the roast down a little. But with my larger batch sizes (750g) the gun wouldn't have enough grunt in the colder weather out this way to keep within a reasonable time frame on fan 2 alone. In this Malliards dip part of a roast, I increase the gun temp by 60deg and drop to fan 2 - and RoR still drops from 10° to 5° - pretty significant!

          So although I use fan 3 normally, I also have some fine mesh over the entry port, and use a 'splitter' (see profile album if you're interested) which directs at least 1/3 of the airflow from the gun down under/into the bean mass through a perforated sheet. IMHO these elements that cut the airflow would greatly reduce the chance of scorching, and would give the benefits of the lower fan speed - but allow for bigger batches

          But I have been pondering whether simply dropping to fan 2 throughout a half size batch for me might give me a similar profile to my normal one - as I discovered recently a small roast can really take off in a hurry on fan 3!

          It would also depend on your gun power - I used to use a Ryobi which had a much more powerful fan that the bosch - but the bosch does give smoother roasts with its reduced blow

          But keep throwing these things around! I might give a 350g batch another try on fan 2 only and see what happens - can't hurt!

          Cheers Matt


          • #6
            Originally posted by Steve82 View Post
            So my roasting philosophy has generally become about trying to get the beans in and out of the roaster as quick as possible
            Been watching Ben Kaminsky?
            Attached Files