I have read your earlier post on high altitude cooking, but was wondering whether I should always correct the temperature for the altitude that I'm at, or just in some circumstances?  Do I cook a holiday turkey to the same temperature or correct it for altitude?

--Shoshana

As noted in the post on High Altitude Cooking, the boiling point of water drops by about 2°F (1°C) for every 1,000 feet (300 meters) increase in altitude, above sea level.  For those dishes where there is some direct relationship to the boiling point of water, for example in cooking sugar to make syrups or candies, the temperature at which events occur happen sooner because water boils at a lower temperature.  In candy making, a recipe might call for cooking to the 'hard crack' stage, which occurs at between 300°F (149°C) and 310°F (154°C).  At 2,000 feet altitude, that same stage is achieved about 4°F or 2°C lower, so the range is 296°F (147°C) and 306°F (152°C).

However, when making a custard or cooking meat, the temperatures relate more to  those at which certain reactions occur with proteins.  These temperatures are independent of the boiling point of water where you are.  When making custards and sauces, the protein molecules will start to denature and begin to bond with each other somewhere around 120°F (50°C).  This process will continue up until about 180°F (82°C), at which point the proteins will become overcooked and the dish will curdle.  These temperatures will vary depending on what other ingredients are used. 

If you use a bain-marie, it takes longer for a cheesecake to cook at higher altitude than it does at sea level because the water in the surrounding water bath evaporates at a lower temperature.  This in turn means that the cheesecake is cooked at a lower temperature and therefore takes longer to heat through enough for the eggs in it to set.

When cooking your holiday turkey, the temperature you are looking for is the balance between food safety, killing any nasty bacteria that can make you sick, and cooking the meat to the point where the connective tissue breaks down, proteins haven't started to over cook and collagen melts.  The minimum temperature for that is 165°F (74°C), regardless of altitude.

For that holiday turkey, you need to cook to the recipe directions, regardless of altitude.  The same applies to any roasted meats, whether turkey, chicken, beef, pork or anything else.

Where all of this gets really tricky, though, is in baking where some of the reactions depend on the boiling point of water and others don't.  If you look at altitude corrections for baking, they typically decrease the amount of sugar and increase the amount of water in order to allow the other reactions, gelatinization of starches and coagulation of proteins, time to occur.  Typically, they also decrease the amounts of leavening (baking powder or soda) so as not to have the product over-inflate. 

So, baking whether cakes, custards, cheesecakes, and so on, require special attention and can take longer at higher altitudes. Recipes where sugar in cooked such as in making candy, need to be corrected for the affect of altitude on temperatures, but for roasting you should still aim for the same temperature as at sea level.