I had a call from a local home roasting friend asking about hard and soft bean roasting. He started by using the term "more delicate" in terms of soft beans and decaf could also be included. That with higher MET/ET scorching occurs. My thought was "more delicate" was not the right way to look at it.

My thought was that soft beans have lower heat transfer abilities. The added open cell pockets in the bean acts as an insulator. Softer beans have more pockets that slow heat conduction within the bean. When the beans are exposed to higher temps. the heat is unable to transfer quickly enough into the bean and builds and scorches the surface. My guess is it also effects the free travel of moisture out of the bean in the early stage of the roast. That a scorched surface is also a somewhat seared surface.

We talk about harder beans being able to withstand higher temps. without scorching. This makes sense because the heat conduction is able to flow more quickly with less pockets/tighter structure. I roasted a batch of very hard beans last week with about the same temp. I would use for regular hard beans and they just flew to 300f bean temp. I then tried a 2nd batch where I increased the batch size and lowered the temp. and got a more reasonable time to 300f which seemed to show a more even drying and roasting from surface to inner and a better cup.

The issue then becomes balancing ET/MET with a specific roasters heat type and replenishing for transfer.
My roaster is mostly convection. For softer beans I've been lower my temp. and lowering my batch size to add more exposure to the convection to try to maximize replenishment to avoid longer times. I would guess that more exposure would help with radiant too. For a roaster that relies more on bean to bean conduction I'm thinking the batch size factor might be a little more tricky.
Any thoughts?

Well I certainly agree with your thoughts on a bean's specific ability to transfer heat. I have recently been referring to this as a bean's "specific heat capacity" as I have been trying to describe this very idea to my roasting friends as well. I'm not sure soft beans are resisting heat transfer due to lower densities though. I'd be interested to hear more thoughts on this. I wrote a couple pro roasters to hear their thoughts, but haven't heard back from them yet.

I think adding in bean geometry is another factor to consider. An extreme case is a peaberry type which I believe physically allows for a great absorbtion of heat. The geometry portion has me paying closer attention to the beans physical structure before roasting. A tight seem on the bean vs. open etc..

You also raise another valid point about type of heat transfer mechanism your roaster relies upon. I think this plays a large role in the balancing act of roasting. The kit I am currently roasting on (HG/BM) uses a lot of convection as well, and it can seriously put heat to the beans in a hurry. This is a blessing for hard beans, but can be a handful for more "delicate" beans such as DP's or Ethiopians. The part that intrigues me, is MET vs. heat transfer type. Obviously high convection outfits can transfer more efficiently so therefore they require lower MET's. My other roaster, a Hottop, has very little convection. Even though I can manipulate both roasters to produce similar curves on paper - how do the different METs affect the roast and resulting cup?

Great topic Ed - I have been mulling this over myself for quite some time. Can't wait to hear what others have to say.

~j

Lower density / softer means more space between cells which results in better insulated beans. The issue become that the outside surface overheats instead of transferring the heat energy inwards. It's a lot easier to scorch a softer bean, so its important to move along "easy does it".

Thank you for actually pointing out the logic behind softer vs. harder beans. It is really strange that I have read hundreds of articles about roasting, and none of them has been able to give away the actual reason for this logic. I have read a lot of times that hard beans like to be roasted with high met/et, but understanding this really changes the way I approach roasting. I have always been afraid to roast soft beans really slow (long roast times) because I thought they would dry out, or become flat in the cup. Beans I have thought to be hard, may actually have been soft.

Gismar wrote: I have always been afraid to roast soft beans really slow (long roast times) because I thought they would dry out, or become flat in the cup. Beans I have thought to be hard, may actually have been soft.

I still have concerns with long roast times with some softer beans depending what I'm trying to get out of them. But when I am, other then avoiding scorching with too high temps. I try to think about what I can do with the roaster and/or batch size to maximize heat transfer efficiency. This will vary with different roasters.