I've just started the topic of esters and esterification with my year 12s so this is fresh in my mind. An esterification reaction is where a carbon-based acid and an alcohol react together to produce an ester (a particularly fragrant and pleasant smelling chemical). Normally this reaction needs acidic conditions and heat to work. Sam's question is, would stomach acid and body temperature be enough to get the reaction going?
The first thing to point out is that most alcohols are completely toxic to humans (technically so is the ethanol in alcoholic drinks). But the main alcohols humans can safely consume are ethanol and methyl butanol, most others would be poisonous. Acids however are pretty common in our diet and there's a wide range your mouth and stomach can take.
Stomach acid can be as concentrated as pH 1.5 which is definitely enough to catalyse the reaction, the question is really about the body temperature of a human (37 degrees Celsius normally), could this be enough to get an esterification going? The answer seems to be a definite yes. There are lots of esterification reactions known to take place between room and body temperature so it's absolutely feasible to do the reaction in your stomach.
The problem is: your stomach has a lot of water in it, and water tends to break esters down. However they wouldn't reverse the reaction completely. What you'd end up would be a dynamic system where esters are constantly being made, and constantly being un-made until we reach a balance where they're being made as fast as they're being created. We call this a dynamic equilibrium. In this equilibrium, the amount of ester would probably be quite low as there's a lot more water in your stomach than ingredients (if you've outdone your own stomach capacity with alcohol and carbon based acid you need to see a doctor...and then a psychiatrist) but it is absolutely plausible to carry out esterifications in your stomach.