A few weekends ago, I was at a barbecue with my family, and my cousin, Matt, (who is a barbecuing genius) allowed me to tag along and help him smoke all the meats. It was amazing! There's so much science behind smoking and slow cooking, it's too much to cover in just one post, but I'd like to get us started with a mystery Matt asked me about himself. What causes a "smoke ring" to form around smoked meats? If you're not familiar, a smoke ring is a region of pink colored meat usually seen in the outermost 8-10 millimeters of smoked meats. You can see the pink zones on the pork rib pictured above and below, as well as a distinct pink color near the edges of the pulled pork shoulder in the bottom photo.
A pink (or red) color in meat usually indicates the presence of myoglobin. Myoglobin's reddish pigment is usually lost when meat is cooked because the heat causes it to denature and turn brown. The center of a rare steak remains red because it never reaches a high enough temperature to denature the myoglobin. However, the outside of smoked meat gets extremely hot over the course of cooking, so cool temperatures can't be the cause of the pink color in smoked meats. What else might cause myoglobin to stick around, despite the heat? It must be something in contact with the surface, since that's the only place the pink color appears. That means it's either the dry rub seasoning or the smoke itself. We know the dry rub can't be the cause because when the same dry rub is used on meat that's cooked in an oven or slow cooker, the pink ring doesn't develop. So, by process of elimination, we know that the smoke itself is causing the pink ring - but how?
It turns out that burning organic fuels like wood, charcoal or gas produces a variety of chemicals, including trace amounts of nitrogen dioxide (NO2) gas. When NO2 gas meets the surface, it dissolves into the meat and picks up a hydrogen molecule, becoming nitrous acid (HNO2), which then gets converted into nitric oxide (NO). NO reacts with myoglobin, and together they form a stable pink molecule that can withstand heat.1 The thickness of the ring depends on how deep into the meat the NO is able to penetrate before reacting with myoglobin.
As you can imagine, this reaction has to occur fairly early in the cooking process, before the surface of the meat reaches temperatures that would denature myoglobin. Since smoking cooks meat with gentle temperatures, this reaction has more time to occur before myoglobin is lost. Even though gas and charcoal are commonly used for grilling, you wouldn't see the smoke ring occur in grilled meats because the heat in that application is so high that the reaction doesn't have time to occur before the myoglobin around the edges is lost.
In fact, a similar reaction occurs in nitrite-cured meats like ham, corned beef, and hot dogs. That's what gives those meats their uniquely pink color! Check out my food glossary to learn a bit more about nitrites and nitrates.
1. Harold McGee, On Food and Cooking: The Science and Lore of the Kitchen (New York: Scribner, 2004), 148-149.