I think there's a really important differentiation to discuss here-- one that has an enormous impact on flavor and aroma.

Basically it this: you can either look at airflow as a convective force for transferring energy to and from the beans, or you can view airflow as a means to alter the atmospheric pressure in the roasting chamber (most drum roasters).

The latter view is the critical factor IMHO, where airflow not only helps maintain a certain temperature profile over time, but also determines when, as well as how forcefully the coffee releases unwanted roast byproducts and conversely how the coffee retains the desirable compounds that support flavor and aroma.

Varying negative pressure within the drum at critical points in the roast curve serves this purpose. I believe this is how a skilled roaster can tease out extraordinary results. It is also a means for creating horrible coffee if applied improperly.

Additionaly, I think this is a huge limiting factor for roasters that can't suck as well as blow.

Mile High Roaster wrote: Additionaly, I think this is a huge limiting factor for roasters that can't suck as well as blow.

Care to expound on this and cite some specifics? Preferably objective data and research.

If I'm reading you right, you've misunderstood the term "negative pressure" as it applies to classic drum roasters. It's a measure of the airflow in the drum having enough velocity towards the exhaust so that if the trier is removed the trier port the roaster's internal airflow will suck air from outside the roaster. It's not really a measure of the atmospheric pressure inside the drum.

My experience is that airflow management is important; perhaps not as important as some other things -- like keeping Development RoR from stalling -- but still important. On the other hand, some very knowledgeable people downplay its role other than for its influence on temperature as trivial.

Rich

TomC wrote:Care to expound on this and cite some specifics? Preferably objective data and research.

Heat guns, air poppers-- are some examples of the limited methods I refer to because they rely primarily on a continuous blast of hot air for roasting, but lack the drum roaster's ability to control a negative pressure environment, especially one that can facilitate temperature increase, decrease, or stabilization.

boar_d_laze wrote:If I'm reading you right, you've misunderstood the term "negative pressure" as it applies to classic drum roasters. It's a measure of the airflow in the drum having enough velocity towards the exhaust so that if the trier is removed the trier port the roaster's internal airflow will suck air from outside the roaster. It's not really a measure of the atmospheric pressure inside the drum.

My experience is that airflow management is important; perhaps not as important as some other things -- like keeping Development RoR from stalling -- but still important. On the other hand, some very knowledgeable people downplay its role other than for its influence on temperature as trivial.

Rich

I could be wrong here, but its my understanding that Deidrich drum roasters are capable of significant vacuum on the roasting chamber. So much so that the man himself has commented about how this factor's manipulation plays a critical role in creating and relieving pressure in the coffee under roast. But perhaps he's not very knowledgeable by your standards?

Which side of the process flow the pressure differential is makes no difference since absolute roast chamber pressure is negligible relative to coffee internal pressure.

Mile High Roaster wrote:I think there's a really important differentiation to discuss here-- one that has an enormous impact on flavor and aroma.

Basically it this: you can either look at airflow as a convective force for transferring energy to and from the beans, or you can view airflow as a means to alter the atmospheric pressure in the roasting chamber (most drum roasters).
Additionaly, I think this is a huge limiting factor for roasters that can't suck as well as blow.

Actually testing the theory in a small drum roaster would be pretty involved. If one could hook up a gauge to the tryer, that would be the closest location. I've tried this with a 0-.50 magnehelic, the needle didn't budge. But it might work in a 75lb drum roaster with a giant exhaust blower. The other alternative is a Phidget pressure sensor; we have a member using one for data logging.

Negative air flow measurement can be done just past the drum in the exhaust pipe, before the damper works better. After the damper can have a reverse affect on the meter readings.

Apropos of nothing other than to demonstrate my limited experience with forced draft vs induced draft, back in the early '70's I was a foreman in a steel mill boiler house. There we had 2 induced draft and 2 forced draft boilers. I think the difference was efficiency, but I know for sure, maintaining the draft was critical, else the fire would go out and an explosion would likely ensue - or at least that's what I worried about. I have always imagined that a chimney with a fan in it for my Weber grill, to create an induced draft, would improve my roasting in my RK drum. Sadly, I lack the skills to accomplish this.
DickC

Mile High Roaster wrote:I could be wrong here, but its my understanding that Deidrich drum roasters are capable of significant vacuum on the roasting chamber. So much so that the man himself has commented about how this factor's manipulation plays a critical role in creating and relieving pressure in the coffee under roast.

I don't know about Diedrichs or about many other roasters besides those I've used myself, and could be wrong as well. But a lowered drum pressure implies that the air intake is restricted relative to the exhaust, that the doors, trier port and bean chute be practically air-tight, and... well... no.

In my experience -- not just with roasters but in the world in general -- meaningful lowered pressure differentials are a product of design and come with all sorts of manifestations -- like rubber gaskets, screw tops, or drop-down oxygen masks.

You can get some interesting pressure effects with high velocity air, but home/commercial sized, drum roasters aren't what you'd call high velocity and they don't "pop" when you pull their tirers during a roast. If there is lowered pressure in the drum than the surrounding environment as a result of air velocity in the drum -- other than the usual "wind-resistance" effect you'd notice facing a fan, i.e., higher pressure in your face, lower in your back -- I'd be surprised. Of course, I'm open to being surprised.

But perhaps (Steve Diedrich's] not very knowledgeable by your standards?

No need to be a jerk about it.

As I recall, Diedrich talks about the amount of moisture in the bean at a given temperature being vital to the chemical reactions which occur during roasting, and incorporating and using some sort of vacuum management as a way of timing the drying process.

But for the reasons I've already mentioned, if there's a vacuum in the drum relative to the external envirionment, I speculate it's not much of a vacuum. And, I have to say, that I haven't noticed that coffee roasted in Diedrichs is superior to coffee roasted in machines without vacuum management.

Rich

Mile High Roaster wrote:Heat guns, air poppers-- are some examples of the limited methods I refer to because they rely primarily on a continuous blast of hot air for roasting, but lack the drum roaster's ability to control a negative pressure environment, especially one that can facilitate temperature increase, decrease, or stabilization.

Your points are broadly scattered and dont appear to connect, and then go on to contradict themselves.