Lots of recipes that use wine or spirits say to cook off the alcohol.  I have heard that not all of the alcohol is cooked off.  Is this true?

-- Bobbi

That's correct.  Depending on the cooking method, between 5% and maybe 75% of the alcohol that went into the dish is left behind.  But the question really is, how much actual alcohol is that?

Suppose you are making Boeuf en Daube, a classic beef stew from southern France, for company.  You might use a 750 ml bottle of wine with a 12% alcohol content and stew it for several hours to make a dish that would serve perhaps eight people. 

So you would have 750 ml X 5% of the initial alcohol left over for a long stew, which would be equivalent to 37.5 ml of wine at full strength, divided by 8 servings means that your dinner guests would consume the alcohol equivalent of about 1 teaspoon (5 ml) of wine each.

Now suppose that for dessert you served Bananas Foster.  The classic recipe calls for 1/4 cup each of banana liqueur and dark rum to serve four people.  A flambé can leave as much as 75% of the initial alcohol behind, so each serving would have about 1/2 fl. oz. of banana liqueur (50 proof) plus 1/2 fl. oz. dark rum (80 proof) X 75% which is equivalent to 3/4 fl. oz. of 65 proof spirits each, about the same as just giving everyone a straight 1 oz. shot of the banana liqueur.

Other cooking methods, like deglazing a pan to make a sauce, may leave between 10% and 50% of the original quantity of alcohol behind, depending on temperature and time.