In Friday’s post, I talked about barbecue kettles and fuel. In today’s post, I’ll pick up where we left off and discourse further upon the topic of hardwood fuels. Then we’ll talk about the role temperature plays in barbecuing, and about temperature control.
It occurs to me that I left you hanging when I mentioned wood chunks, and didn’t say exactly what wood chunks are. Let’s remedy that right now. Wood chunks are simply chunks of wood, as opposed to logs or cord wood (fireplace wood, which are logs cut crosswise, or bucked, to a length of 12 to 18 inches, and then split). Wood chunks are roughly cubical in shape, and are typically 2 to 4 inches on a side. Wood chunks are available at most barbecue and outdoor living specialty stores, at most big-box home improvement stores, and on line.
Wood chunks, due to their size, are the ideal fuel for kettle barbecuing using the indirect heat method - in which the food is cooked/smoked indirectly by hot combustion gases, not by direct radiant heat - and a drip-pan to catch juics and fat as the food cooks. This is because fire-grate space is already at a premium in a kettle, and available grate area is further reduced by a drip-pan. You’ll be using a drip-pan large enough to catch all the drippings from the meat during cooking, so the drip-pan will occupy significant area. All you’ll have left for the fire are narrow areas on either side of the drip-pan, but that’s plenty of room for 2- to 4-inch cubical chunks. You really don’t need to stoke the kettle with that much fuel at a time, anyway. After all, pork barbecue, cooked the traditional way – slowly, at a low temperature - is a time-consuming affair, but it’s worth every minute of it. You want to cook barbecue over a low, smokey fire, and in order to get the smoke, your fire needs to smolder, rather than burn with a flame. Hardwood smolders at a lower temperature than the temperature at which it burns.
The benefit of smoldering is that it produces more smoke than does burning with a flame. This is because fuel smolders instead of burning due to oxygen starvation. There is not enough oxygen for the fuel to burn completely, and the result is adequate heat with an abundance of smoke, which is the reason you’re going to all this trouble in the first place. On the other hand, it’s possible to starve a fire of oxygen to the point that your fuel can’t even smolder, much less burn. If this happens, the fire will go out completely. Therefore, you must control the temperature carefully
Temperature inside the kettle depends, assuming there’s a fire of some sort in the kettle, upon the amount of air you allow into the kettle during cooking. The fuel burns faster if it has more oxygen available, and burns more slowly if there’s less oxygen available. The trick is allowing just enough air into the kettle so that your fuel smolders instead of flaming.
As to temperature control, I think it helps to have a basic understanding of the physics of gases, and of how gases at different temperatures interact. It’s not very difficult, so please bear with me while I try to explain it. Alternatively, if you’re really not all that interested in explanations, you can simply skip the next three paragraphs. You won't miss anything important if you do.
Okay. We all know that hot air rises, right? Well, air is a mixture of different gases (collectively, gas), and our observation is true of all gas. A gas, be it air or any other gas, that is hotter than a surrounding gas, be it air or any other gas, is displaced by the surrounding gas. The reason this happens is because the hotter gas molecules have more kinetic energy than molecules of the cooler gas. That is, while molecules of any gas are always in motion, molecules in the hotter gas fly around faster and collide with each other more often, and thus are farther apart, on average, than the molecules of the cooler gas. The result is that the hotter gas is less dense (it weighs less, or has less mass, per unit volume) than the cooler gas in the vicinity of it, and is displaced by the cooler gas. This displacement happens because the cooler gas, being more dense than the hotter gas, and therefore heavier per unit of volume than the hotter gas, is drawn under the hotter gas by the Earth’s gravity, and the less dense hotter gas is displaced upward. (This is analogous to oil floating on water because water is heavier per unit volume than oil.) The net effect is an upwardly-directed flow, or current, of gas. Moreover, in general, the greater the difference in temperature between the hotter and the cooler, the greater the current,&bnsp;or rate of flow. In a barbecue kettle, it is this gas current which drives the smoke, which is not a gas but is composed of particulate matter, up and out of the exhaust vent/damper in the lid, flavoring the meat along the way.
Incidentally, the above explanation is somewhat of an oversimplification. Firstly, combustion is a chemical reaction, and gases of combustion have compositions that are different from those of atmospheric gases. Thus individual molecules of combustion gases may (and do!) have masses that are different from those of individual molecules of the gases that make up the Earth’s atmosphere. It would follow that equal volumes of the gases of combustion and atmospheric gases may have different masses at a given density. Also, my explanation, strictly speaking, applies only to gases that are unconfined; that is, gases that are free to change volume by expanding and contracting. A kettle, however, can be reasonably regarded as a confining vessel. Nevertheless, pressure of magnitudes commonly associated with confined gases are not present in back-yard barbecue kettle. Consequently, the general idea, that a mixture of cooler gases displacing a mixture of hotter gases causes an upward gas current, still applies.
Are you still with me? All right, then. In a barbecue kettle, hot gases get that way because the fuel burns, and the amount of heat depends upon the rate at which the fuel burns, which in turn depends on the amount of air present – the more air, the hotter the fire. But by the same token, the less air admitted into the kettle, the cooler the fire burns because there is less available oxygen. Thus you control the rate of burn, and the amount of heat thus generated, by controlling air flow into the kettle. To control air flow, you adjust the dampers.
Now, what exactly is a damper, anyway? The word damper has many meanings, but for our purposes, a damper is merely a mechanical device for varying the size of a ventilation opening in the kettle. It’s a movable piece of metal attached to the kettle in such a way that is adjustable so that openings in the bottom and lid, respectively, of the kettle, openings which admit air or which exhaust combustion gases, can be fully open, fully closed, or partially open. Two dampers, one on the kettle itself and the other on its lid, are each mounted in close contact with, and snuggly attached to their respective parts. The closeness of fit is intended to prevent the leakage of intake air into, and the leakage of exhaust gases out of, the kettle.
With a little practice, the volume per unit time, or current intensity, of both intake air and exhaust gases, can be controlled fairly precisely. The trick is to balance the volume-per-unit-time of intake air and the volume-per-unit-time of exhaust gases,nbsp;which is always greater than the volume-per-unit-time of intake air because there's more matter in the exhaust gas, and have the fuel smolder instead of burn. This is not as difficult in practice as it might seem.
Now, after what I just wrote, and with me being one of those engineering types and all, you might think that I use all sorts of fancy, complicated, and expensive scientific instruments, such as digital thermometers, pressure gauges, flow meters, etc., to get my fire regulated precisely. Well, I don't. I'm not that anal-retentive. Nope. I just eye-ball it; here's how.
What I do is, after the drip-pan's in and I've arranged (as described in yesterday's post) my fat wood and hardwood chunks on either side of the drip-pan, I open the bottom (intake) damper fully and light the fat wood. Then, when the hardwood chunks are fully in flame, I open the (exhaust) damper on the kettle lid fully and place the lid on the kettle, tightly so it seals good. Next, I adjust the intake damper until the flame disappears. I determine this by looking through the exhaust openings in the lid at the fire, i.e, by eye-balling, to see if the flames have disappeared and the fuel is smoldering. When the fuel is smoldering, I remove the lid, throw my pork shoulder onto the kettle's rack over the drip-pan, replace the lid, and adjust the exhaust damper so it covers about half the total area of the openings. At this point, I'm barbecuin'!
Tomorrow, I'll get to the "meat" of the matter, and describe in detail how I barbecue a pork shoulder using a cooking kettle.
Happy cooking!
Posts
