In a drum roaster, on factor thought to influence the flavor is the drum material. Another factors are the mass of the beans and also the mass of the drum. What about the internal volume of the drum compared to the volume of beans? If this ratio is too low or too high, it could affect the air moving around the beans and the heat transfer properties (ie. less beans touching the drum).

What do you think? Could there be a golden ratio (or a max limit)? What is more important, the mass of the beans or their volume?

Cheers

Could there be a golden ratio

Perhaps, but a very tricky question I think due to the sheer number of influencing factors. For example do you want the ratio to be golden when the beans are small, green and hard or later in the roast when they are bigger and less dense. The bean expansion paradox. A bit like the problem we have trying to extract meaning from other folks TC numbers, the size of drum and drum contact time would only be comparable if all other factors were the same.

My feeling is that the 'best' performing roaster will always be the one that reacts most quickly to user input. I do not think the drum material(and other factors) affect flavour, it affects the type of profiles available, which affect the flavour. Could be wrong, but once seasoned it seems we are all roasting on a coating of polymerized oil.

Regarding character, again I think this is a flexibility issue, if your roaster forces you to roast in a certain way due to excess mass, inadequate heat, poor airflow etc indeed it will have certain characteristics- that could be easily emulated on a roaster capable of hitting a variety of profiles.

That said I would enjoy seeing the data, and for my part I shall measure the drum on my 1kg next roast.

My IR-1 can produce the same results using 1/2lb of beans or 1 kilo which I feel has much to do with the IR burner it uses. It seems to me that by changing drum rpm is a way to influence bean contact with the drum on larger roasters. My Hottop on the other hand had a sweet spot of 175g to get the best roast out of it.

Maybe there are golden limits, instead of an exact golden number. We are use to see recommended numbers but since the beans are inside the drum (out of sight), we base our intuition on the imagination. In this pic, there are two cups (~300 grams) inside my small drum roaster. Just by looking, we could maybe find good common sense limits. For example, in the pic bellow, one cup would look better to me (~150 grams). Two cups look a little over the max limit. Three cups would be too much. It would be like stuffing a frying pan with too much and expect good results. For larger batches, it would need a bigger "frying pan" and higher power, not just higher power.

Two cups of green beans


Cheers

There is no desirable heat transfer to the beans by conduction from the drum surface. According to the equations published in the literature reviews, heat gain is by convection and radiation, with conduction doing nothing but scorching beans.

Therefore, the drum acts as an automatic stirring device and a heat radiating surface. A design with lots of hot drum surface area per bean and low airflow would create a roast with a higher proportion of radiation heating than a cool drum with little surface area per bean and lots of airflow.

I have no clue how identically profiled high radiation and high convection roasts would taste. Pure airroasters are said to to produce boring malt/caramel roast flavors compared to drums (I still haven't done or heard of a convincing test of this, though), and several radiation heavy quartz lamp roasters from about ten years ago went belly up the moment people started tasting their roasts.

So my guess is that there are extremes of conduction and radiation that should be avoided in roaster design, and that this, along with efficient bean agitation, might also constrain the way classical drum roasters are designed.

Now that I somewhat understand my M3, comparing its taste to the popper, ranging over different profiles, roast levels, and environmental temperatures, is my long term project for this year.

farmroast wrote:What is the diameter and length of the Quest drum and what is the rpm?

In the Quest, the drum is ~ 6 inches long by about 4 inches diameter ~ 75 cubic inches. A cup is ~ 15 cubic inches (rounding everything). ~ 5 to 6 cups would fill the drum completely.

So for one cup of green beans in the Quest there is an approx. volume ratio of

one cup ratio = 75/15 = 5

two cups ratio = 75/30 = 2.5

three cups ratio = 75/45 = 1.66

The rpms are 50.

Maybe for commercial roasters the ratio does not change as much (drum volume much larger than bean volume). If the heat radiates from the drum surface, then a small drum would be more limited (slower to transfer heat).

Cheers

There was an article in the March/April 2007 issue of Roast magazine covering heat transfer to the bean. It was called "Heat Wave—The way heat transfers your beans from green to brown". I don't have it handy here but I remember that the larger the roaster the more heat will be transferred by convection. Typically 80% of the heat or more in large commercial roaster. I don't think you can easily conjure up a golden "ratio rule".