From Cooking For Engineers:The use of a beer can inserted into a chicken is an old barbeque trick to provide flavored steam to the inside of the chicken as it cooks. At the same time, the beer supposedly adds flavor to the chicken. Problem is, I don’t quite buy it. If the beer is giving off steam, then most of that steam is just going to be water… most of the beer flavor will just be concentrating in the can. However, it seems that it would be sacrilegious if I used the beer can but left out the beer.
So he used a 24 oz keg-can of Heineken.
I don’t think the term “sitting on the can” meant this. No magazine.
Alcohol has a lower boiling point than water, so the water-alcohol vapor escaping from the beer can during cooking will have a higher alcohol concentration than the original beer, whereas the water-alcohol liquid remaining in the can will have a lower one. Eventually, the boiling point of the liquid approaches that of pure water. In reality, it’s even more complicated than that because beer contains many chemicals besides water and alcohol. In particular, the starches in the liquid produce a foam that transports flavor compounds to the inside of the cavity.
The primary objective, however, is simply to steam the bird from the inside. Using a somewhat alcoholic beverage means this happens at a slightly lower temperature than would be possible with pure water, resulting in gentler cooking. Wet steam keeps inside moist, transfers heat more efficiently than air circulation alone and therefore reduces total cooking time.