Sarge is correct, Here is some science to explain it all. (borrowed from
http://www.karubecue.com/smoke_ring.html
The reddish "smoke ring" around the perimeter of a slice of Q comes from the combination of meat and fire:
From the meat: Myoglobin - a substance found in muscle. It's a spherical protein with an iron-centered group attached. What's bound to the iron atom determines the meat's color:
Freshly killed and vacuum-packed meat has deoxymyoglobin (purple)
Exposure to air forms oxymyoglobin (cherry red)
Further air exposure forms metmyoglobin (brown)
Exposure to nitric oxide produces nitric oxide myoglobin (bright red) and/or nitric oxide metmyoglobin (crimson). These are the colors of a smoke ring
The globins denature at about 140F, fixing the colors. So, the smoke ring forms in the initial hours of a cook, before the outer layer of the meat heats up.
From the fire: NOx Emissions - these come from high temperature combustion (2O2 + N2 --> 2NO2; 2NO2 --> 2NO + O2). The NOx gases dissolve in the moist meat surfaces, forming nitric (HNO2) and nitrous (HNO3) acids that leach into the meat (3-8 mm, typically) and react with myoglobin. Wood, charcoal, natural gas, and propane combustion all produce NO2, so a smoke ring can be made by most ovens. Having no combustion, electric cookers won't make a ring by themselves.
Depth and intensity - for a bigger ring, you have 3 levers to pull:
More myoglobin - beef has more than pork, which has more than fowl. Old animals have more than young ones. Recently killed animals have more. Hardworking muscles have more than lightly-used ones.
More NOx emissions - from a hot fire. Wood combustion has the added benefits of producing lots of Fuel NOx (N2 from fuel) in addition to Thermal NOx (N2 from combustion air) and lots of water vapor for the NO2 to dissolve in.
Longer exposure below 140F - load cold meat into the smoker (warning: you need a very clean fire to get away with this, as the cold meat will condense creosote from a dirty, smoldering fire). Begin the smoke at a lower temperature (~200F) for the first hour or two, then raise it to your target cooking temperature.
Cheating - you can create or exaggerate a ring chemically by rubbing the meat with sodium nitrite curing salt (e.g., Tenderquick) before cooking. Most competition rules prohibit the appearance of the smoke ring from influencing scores, because it's so easy to cheat this way. But the smoke ring adds a lot to the presentation of the meat - authentically-prepared barbecue will have it and most connoisseurs expect to see it.
