Today, smokers and grills use charcoal, gas, or electricity to produce heat. Wood is then added to the fuel source in the form of chips, chunks, pellets, or logs to impart flavor. Additional smoke also comes as a result of meat drippings (fat, spices, protein, and sugars).
Smoke from wood or charcoal can appear white, gray, blue, yellow, brown, and even black. However, not all smoke is considered "good" smoke. "Smoking" meat is almost sort of a misnomer as you're not after white/black clouds of billowing smoke. The desirable smoke is referred to as "thin blue smoke" or "clean smoke."
Hardwoods used for smoking meat are primarily made up of three organic compounds: cellulose, hemicellulose, and lignin. The cellulose and hemicellulose make up the structural material of the wood, where-as lignin holds them together.
Cellulose and hemicellulose are chains of glucose (sugar). When burned they effectively caramelize and produce carbonyls; These compounds are responsible for most of the color components as well as the sweet, flowery, and fruity aromas.
The breakdown of Lignin creates phenolics (aromatic compounds) that create distinct elements like smokiness and spiciness, as well as pungent compounds like syringol and guaiacol. Syringol is responsible for the smokey aroma, and guaiacol is responsible for the smokey taste.
These concepts are important to understand as they are directly related to thin blue smoke.
Thin blue smoke is essentially a thermochemical process called pyrolysis. Pyrolysis is the decomposition of volatile materials (in this case hardwood) in a low/no oxygen environment featuring high heat.
Since there are low levels of oxygen present, the material doesn't combust, however the chemical compounds (cellulose, hemicellulose, and lignin) will decompose into combustible gases and charcoal.
Thin blue smoke is the result of a equilibrium (fuel, oxygen, heat) being achieved and the burning process being deemed efficient. In this state the fuel in the woods carbonize or caramelize which results in thin blue smoke.
Note: Pyrolysis is the process used to create lump charcoal.
This section is based on personal experience and what I've found after smoking meat for roughly a decade. Keep in mind, barbecue is a centered experience and what I like, you may not.
Smoke is comprised of as many as a hundred compounds. These compounds manifest themselves as solids like char, creosote, ash, and phenols. As well as gases like carbon monoxide, carbon dioxide, nitric oxide, as well as liquids like water vapor, syringol, and guiacol.
There are three things necessary to produce smoke: oxygen, combustible fuel, and fire/heat source. When all of these elements are introduced, combustion occurs. In order for smoke to be produced the heat and oxygen can be adjusted so that the combustible material will smolder rather than burning resulting in visible smoke.
With that said, most of the flavor components come from the gases, not the smoke. The composition of the gases depend on oxygen and temperature.
As described above, we know hardwoods are comprised of cellulose, hemicellulose, and lignin. These components burn simultaneously and the compounds associated with them interact throughout the combustion process.
The following temperatures are from Dr. Greg Blonder's research on this topic:
While no fire is uniform, it's best to fall within that sweet spot of 650F-750F. We can see from the above that lower temperature smoke (450F) creates an acrid bitter taste. High temperature smoke also destroys the desirables. Meaning the sweet spot is 650-750F.
This is the biggest reason some people opt to smoke hot and fast (275+) as apposed to low and slow (225). On many smokers, smoking at 225 requires you to dampen the oxygen which starves the fire and results in incomplete combustion and the non-desirables, namely wood creosote.
With all that said, virtually all wood will emit white/grey smoke during the initial phases of combustion as moisture is being released. Wood is hygroscopic meaning it will lose/gain moisture depending on the humidity and ambient temperature. Hardwoods used for smoking typically have a 15-20% moisture content.
After 20 minutes, if your smoker still has billowing clouds of white smoke, you have a problem with the wood being used or the airflow/oxygen level.
The above information is important to understand because regardless of smoke color, both will produce particulates that will stick to the foodstuff being smoked. TBS just has more desirable components.
The reason TWS is considered "bad" or "dirty" is because it implies incomplete combustion (gas particles that are left unburned). Prolonged exposure results in an acrid taste from ash and creosote. TWS particles are also much larger and will readily adhere to wet surfaces (meat).
The keyword being prolonged. In my experience, if you're cooking hot and fast, white smoke works perfectly fine for burgers, steak, fish, or chicken. However, smoking with white smoke for 10+ hours is going to create acrid/bitter flavors.
Truth be told, the thing that beginners mess up the most is the amount of smoke they introduce to meat. Thick white smoke exposed for long periods of time results in bitter/acrid tasting meat. While "thin blue smoke" certainly has more desirables, the applications of thick white smoke shouldn't be overlooked.
As with most topics in the Barbecue world, opinions in regards to minute details like the color of smoke are heavily debated. If you're a beginner, thin blue smoke should be your goal as it's way more forgiving than white.
With that said, there are products engineered to introduce white smoke or "smoky" flavor to food. These products wouldn't exist if there wasn't a demand for them.