When I barbecue meat, there is a layer just under the surface that doesn't look like it got cooked, even though the middle looks done.  What causes that?

-- Dean

What you are seeing is called the smoke ring.  It is an area of meat that, although it is cooked, hasn't turned the greyish brown we associate with cooked meat.  There are actually two different causes for meat to still be red, despite being cooked, both of which relate to the oxygen carrying protein myoglobin found in meat.

When you barbecue, the burning wood, charcoal or gas produces trace quantities of nitrogen dioxide (NO₂) as one of the combustion byproducts.  The nitrogen dioxide combines with water molecules to a form mild acid called nitrous acid (HNO₂).  The water molecules may be on the surface of the meat, in the air surrounding the meat, or as another byproduct from the burning of the fuel,  In any case, the nitrous acid that comes into contact with the meat slowly penetrates the surface to a depth of up to ½ inch (12 mm) or so, where it comes into contact with the myoglobin, replacing the oxygen atom that is attached to it with nitric oxide (NO).  The effect is the same as using nitrates to make sausage or other preserved meats.  The red color of the myoglobin is locked in, causing the pink color to remain.

The formation of a smoke ring can be enhanced in several ways:

1. Keep the temperature as low as is safely possible, around 200°F(93°C).  This will keep the surface of the meat from drying out, thus providing more moisture for the above reactions and keeping the outside tender so that the nitrous acid can penetrate.  It also slows down the rate at which myoglobin denatures from heat and can no longer react with the nitric oxide.
2. Add moisture to the cooking environment by placing a pan of water in the barbecue.  This has two advantages, one being more moisture for the reaction and the other that the water acts as a bit of a heat regulator, keeping the temperature close to the boiling point of water.
3. Don't dry out the surface of the meat before cooking.  Most recipes are looking to have the surface dry so as to encourage the Maillard reaction (see Browning Meat for Slow Cooker).  In this case you want to leave the moisture on.
4. Where you would usually allow meat to warm to room temperature before cooking, going straight from fridge to fire will lengthen the amount of time it takes for the myoglobin to become denatured.
5. Finally, the nitrogen dioxide is a result of poor combustion.  I don't recommend fiddling with your gas barbecue, but if you are using wood, you want lots of smoke.  Not only does this help flavor, it also increased the amount of NO₂ produced.

A similar effect to smoke ring is caused by slow cooking where the temperature is low enough that the other proteins in the meat, except for the myoglobin, denature first.  Once the myoglobin starts to denature, there is nothing left for it to react with and the color stays.  This is why confit of duck legs remain pink inside after long slow cooking, or why the meat in stews can sometimes remain red.

Finally, there is a way to fake a smoke ring, by sprinkling the surface of the meat with a small amount of a tenderizer or meat preservative that contains nitrates and leaving it for an hour or so before rinsing and cooking.  Morton's Tender Quick is frequently mentioned for this less authentic method.  Barbecue aficionados are divided on whether this constitutes cheating.