A classic sign of barbecue done right is the smoke ring- an intense red band, just below the bark, encircling a hunk of meat. A proxy for deep smoke flavors and slow n' low cooking.
Now, it turns out smoke rings are a poor surrogate for flavor- in blind tastings, judges can't tell the difference between smoked meat with, and without a ring. It's also easy to "fake" a ring with a quick rub of sodium nitrite curing salts before cooking. So many barbecue contests have stopped granting points for smoke rings.
Still, we "eat' with our eyes as well as our tongue, and smoke rings speak to tradition and aesthetics. So its worth trying to understand why, and when, smoke rings appear.
Among the many hazy rumors to investigate, a common belief asserts smoke rings form only below 140F. Once meat is heated above this temperature, the story goes, no additional smoke ring growth is possible.
The theory behind the 140F threshold harkens back to a combination of nitrite curing lore, and interpreting the great food writer Harold McGee's chapter on myoglobin. Everyone knows nitrites create a pink color in meats like bacon or corned beef. So people presume a pink smoke ring must arise from nitrites in the smoke. Usually, the theory claims, nitrous oxide is released by burning wood. This compound land on a wet meat surface (the moisture drawn out by a salt rub), creating sodium nitrites from nitrous oxide (NO), salt (NaCl) and water (H2O) in real time.
Then, sodium nitrite (NaNO2) migrates through the meat, just as if salt cured.
Starting with a cold slab of meat is crucial- it provides time for the nitrites to diffuse past the wet surface layer, before higher temperatures block this nitrate pathway.
According to what people read in McGee, around 130F juices start flowing. And around 140F myoglobin is disrupted, losing an oxygen molecule which normally binds with iron to create meat's familiar purplish color. If a nitrite replaces the oxygen molecule in time, it becomes the pink nitrosomyoglobin molecule and a smoke ring is formed. If it grabs onto a water molecule first, myoglobin permanently turns into the gray hemichrome molecule. Between water pushing nitrate compounds out of the muscle, and chemical bonding, the story of why smoke rings stop forming after 140F is complete.
So let's get to the bottom of this assertion. If smoke rings stop forming above 140F, then smoking partially cooked meat should halt ring development in its tracks.
The test protocol is simple (if a bit tedious). Four, one pound samples of pork shoulder with different surface treatments were cooked in a pellet smoker for 12 hours at 225F, and removed when their internal temperatures reached 195F. An additional four test samples were first "pre-cooked" in a 225F electric kitchen oven, until their internal temperatures reached 155F, and their surfaces around 165F (3 hours baking- the electric oven, of course, does not produce a ring). These pre-cooked samples were then placed into the same pellet-grill, and also allowed to smoke until reaching 195F.
Four surface treatments were chosen to test the most popular theories of smoke ring development:
- Plain- this is our baseline test. No added salt or moisture.
- Rubbed- to check if salt and sugar are prerequisites to smoke ring development. Standard salt/sugar/paprika/spice mix rubbed on and allowed to marinate overnight. Definitely draws moisture to the surface.
- Oiled- Many recipes suggest a mustard rub. This test also explores if oil (like fat caps) act as a water-repellent layer blocking smoke rings. After drying the surface with paper towels, a combination of canola oil and yellow mustard was massaged into the the pork and rested for an hour.
- Mopped- to see if surface moisture is important. The mop was a barbecue sauce thinned with water and apple juice, sprayed or brushed onto the rubbed pork shoulder every 20 minutes (more often than 3 times an hour, and you will start cooling the meat. Too thick and sugary, and it will form an impenetrable glaze).
And here are the results:
Note the strong smoke ring on the left (despite the lack of added salt from a rub), and the almost non-existent ring on the pre-cooked sample to the right. This experiment illustrates why the theory of smoke ring development terminating after 140-150F has some credence. This ring, however, is not uniform, with a more intense layer of red on the inner part of the band (see explanation below).
Rubbed samples, with the addition of salt and sugar, pretty much behave the same as the "plain" samples. Still no ring on the pre-cooked samples, and the paprika isn't acting like a red dye, artificially coloring the meat.
The oiled/mustard glazed sample on the left forms a smoke ring, while the pre-cooked oiled and mustard glazed sample does not form a very strong ring. But, even the pre-cooked samples show evidence of a middling ring (mostly on the sides and bottom)- contrary to the idea that ring development ends at 140F.
The mopped samples, whether pre-cooked or not, developed classic, intense smoke bands of uniform color.
So what is going on here? Well, it is true smoke ring development often stops around 140F. But this observation is not the result of some weird myoglobin chemistry or flowing juices- its due to lack of surface moisture. At 140F internal temperature, the surface temperature is often above 160F. And exposed to moving air. Both of which dries out the surface. This is why the ring is often stronger on the bottom or sides- moisture drips to the bottom, and often drying air currents are restricted by other pieces of meat or the grill gratings.
Simply put, no surface moisture, no smoke ring. Add surface water at any temperature, and the ring grows. In other tests, simply scoring the meat at 175F released juices, and a smoke band grew adjacent to the cut edges. Conversely, as seen in the "mustard" or "plain" examples, these meats are moist only early in the smoking, so develop the strongest smoke layer initially. This strong chemical layer diffuses inward (like a stain on a table cloth spreading outward), leaving a weaker band in its wake. Plus, without humidity to slow evaporation, a stong current of water is pushing the nitrites back out towards the surface.
You can observe this effect very clearly by comparing the shape of a smoke ring generated in a pellet cooker (which tends to quickly dry out the meat's surface due to the rapid air-flow rates) and a specially modified smoker with very high humidity levels. Note the high humidity ring never stopped growing, and is nearly 3x deeper than the pellet smoker ring after 8 hours. It's also darkest near the surface, as you would expect if smoke chemicals are continually absorbed by the meat, and then diffused inward. So people who claim it doesn't matter if you stop smoking after 3 hours are right, unless you maintain high humidity levels..
(each circle is 1" diameter)
Why is moisture so important? Is the moisture creating nitrites, or simply grabbing the much more prevalent carbon monoxide molecules which also block myoglobin from turning gray? Water does a great job physically trapping smoke- a mechanical, not a chemical, process- but is that the whole story? What limits the thickness of the ring?
You'll have to wait until the next article is posted.
For now, the keys to producing a strong smoke ring are based on retaining a moist surface, through a combination of:
- High humidity to reduce natural moisture evaporation, e.g. smoking above a properly designed water pan
- Low temperature cooking, again to reduce surface drying
- Avoiding strong air currents which parch the surface
- Frequent wet mopping to maintain a moist surface- one of the few reasons to mop. A mop containing an acidic liquid (e.g. tomato, vinegar, mustard) and sugar stays moist between applications. If the surface ever forms a hard crust, subsequent mopping is less effective. Spraying with pure apple juice, onto a standard dry rub, tends to dry out between applications, but will somewhat increase the smoke ring.