Hang around any forum that deals with cooking with smoke, and you will eventually hear the advice to hold off on putting your food into the cooker until you see "good smoke" coming out the top. Usually, "good smoke" is defined to be "thin, blue smoke", while "bad smoke" is defined to be "thick white smoke" or anything else other than thin blue smoke. The explanation given for this waiting is usually something like "you have to wait for the smoking wood chunks and the charcoal to heat up and drive off the chemicals that make bad smoke" or something similar. Which then leads to questions about how can you get good smoke during a long cook because not all the smoking wood is smoking at the beginning. So don't the chunks that burn later still produce bad smoke for a while and then good smoke?
Well, let us address the "good" vs. "bad" smoke question first. Wood is composed of three primary components: cellulose, hemicellulose and lignin. These three materials burn at temperatures ranging from 390 to 750 degrees. When they burn at low temperatures, they are transformed into various molecules which produce the smoke flavor we all know and love. For example, cellulose and hemicellulose break down into molecules which produce sweet, fruity, flowery and bread-like aromas. And lignin breaks down into volatile molecules which produce the aromas of vanilla and cloves, as well as other spicy, sweet and pungent aromas. Production of these desired aromas which translate into the pleasant smokey flavor we seek takes place at low smoldering temperatures between 570 and 750 degrees. This is how you produce "good" smoke.
However if the ingredients in your wood chunks burn at higher temperatures, the molecules which produce these wonderful flavors are themselves broken down into smaller molecules which are either flavorless or harsh. This is how you produce "bad" smoke. So, the difference between good and bad smoke has little to do with driving off bad chemicals and more to do with the temperature at which the fuel is burning.
But then you might ask why is bad smoke produced early in the lighting of the fire when the cooker's temperature is low? And why isn't bad smoke produced later in the cook when the fire spreads to chunks of smoking wood that haven't started burning or smoldering yet?
Let us explain. Most people start their fire by having the vents wide open which allows a small area of charcoal burn at very high temperatures. Because the burning area is small, the cooker temperature remains low. As the fire begins to spread, however, the area of very high temperature spreads and the cooker starts to heat up. When the cooker starts to heat up, you start closing down vents which limits the amount of air to the burning charcoal and reduces the temperature of the burning charcoal. Eventually, you reach an equilibrium where the charcoal is burning at a much lower temperature than when you started, and this is enough heat to keep the cooker where you want it.
So, initially, you have some very hot charcoal burning at high temperatures that produce bad smoke, but eventually the temperature of the burning charcoal lowers to a point where it produces good smoke. And this is why later in your cook when chunks of smoking wood finally begin to smoke, those chunks produce good smoke.
The bottom line is that really, you can ignore all this information as long as you trust your senses to detect the thin blue smoke and you put your food in the cooker then. Don't worry about all those wood chunks which haven't started smoking yet. When the time comes they will produce nothing but good smoke.
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