Cooking at high altitudes differs from cooking at sea level. Recipes that yield reliable results in Philadelphia may not turn out properly in Denver. The reason for this has to do with the differences in atmospheric pressure between locations.
Boiling Water Above 3,000 Feet
The higher the altitude, the lower the atmospheric pressure. Lower pressure, in turn, causes water to evaporate more quickly, and water actually starts to boil at a lower temperature.
In general, each 500-foot increase in elevation translates to a decrease of 1 degree Fahrenheit in water's boiling temperature. So at 500 feet above sea level, water boils at 211 F instead of 212 F. But that small change won't be discernible to most people.
At elevations higher than 3,000 feet, you may start to notice the difference though. At sea level, water boils at 212 F but it only needs to reach 207 F to boil at 3,000 feet. At 5,000 feet, bubbles start to break the surface at around 203 F, and at 7,500 feet, at 198 F. That 14 degrees of difference significantly affects how long it takes to cook something.
At any altitude, the boiling temperature of water is as hot as that water will get. You can turn up the flame beneath the pot but the temperature will remain the same. So at 7,500 feet, you can't get water any hotter than 198 F.
Therefore, you need to cook foods a bit longer than you would at sea level. Pasta, for example, may take seven minutes to reach the al dente state at sea level, but it could take nine or 10 minutes to achieve the same result at 3,000 feet.
In addition to adjusting cooking times, you should also keep a tight-fitting lid on the pot when you cook at high altitudes. This is standard procedure when preparing braised dishes, but it's a good rule to follow at high altitudes because water evaporates so much more quickly.
Roasting and Grilling Meats at High Altitudes
Because the reduced atmospheric pressure of high altitudes affects the boiling point of water, it's moist-heat cooking techniques that are affected the most. Dry-heat cooking techniques like roasting or grilling are not affected in the same way because high altitudes don't alter the way air is heated. So a roasted chicken recipe shouldn't require any adjustment at higher elevations.
On the other hand, since water evaporates more quickly at high altitudes, meat cooked on the grill tends to dry out more quickly than when cooked at sea level. Note that the temperature isn't affected, just the moisture content of the food. So a grilled steak might be drier at high altitude than at sea level — even if it's not overcooked temperature-wise.
There's not much you can do about that, other than to make sure that you give grilled and roasted meat a chance to rest so the juices redistribute before you cut into it.
Cooking Eggs at High Altitudes
You'll also find that eggs take a bit longer to cook at high altitudes because they naturally have a lot of water in them. But since fried eggs or scrambled eggs are cooked with dry heat rather than moist, take care that you don't compensate by using a hotter pan, which will just result in burnt eggs. When it comes to eggs, cook longer, not hotter.
Baking at High Altitudes
Another difference caused by the lower atmospheric pressure is that leavening agents such as yeast, baking powder, and baking soda have more rising power. That's because the thinner air offers less resistance to the gasses created by the leavening agent. Therefore, you should use less leavening (about 20 percent less at 5,000 feet) as your elevation increases.
And because of the faster evaporation described earlier, you may need to increase the amount of liquid in a batter or a dough. You can do this by adding an extra egg or using extra large eggs instead of large.
Using Microwaves at High Altitudes
You may also notice a difference in how microwave ovens work at higher altitudes. That's because microwaves cook by exciting the water molecules in food. So you may need to allow for extra cooking time in a microwave as well.