Hello Nigella and team! I recently moved from Virginia to Denver, Colorado. The Denver (American) football stadium is called the Mile-High Stadium because of Denver's high altitude, a much higher altitude than Virginia. I have heard it said that the thinner air affects baking, but have not heard a good authority on how to balance this thin air in baking. Do you have any suggestions? I have heard a rumour that more flour will balance the difference in air quality but don't have that on authority and certainly don't have any sort of ratio for determining how much, if any, more flour. Please help!
There are two principles to remember for baking at high altitude (normally considered to be above 3,000 feet/915 metres). The first is that the air is dryer, so you need more moisture. The second is that as the pressure is reduced the air bubbles created by the leavening ingredients will tend to expand more quickly, meaning the structure of the cake can be stretched too much and increasing the risk of cakes collapsing.
The easiest way to adjust recipes is to slightly increase any liquids added - by around 2 tablespoons per cup (15-30mls per 240mls). Note that butter and eggs should be included in the liquid calcualtion The leavening (usually baking powder) should also be reduced slightly, by 1/4 to 1/2 teaspoon per teaspoonful used (ie if the recipe is for 1 teaspoon baking powder reduce it to 3/4 or 1/2). You may need to experiment slightly to get the correct level for your altitude.
Some people also like to reduce the sugar content, by 1-2 tablespoons per cup (15-30g per 200g). If the coake conatins whisked egg whites you may want to slightly underwhisk them as they are also a leavening agent.