What is the formula used to calculate energy content of food?

You need energy to heat the food, say, from 20 degrees to 100 degrees (the boiling point of water, under normal pressure). In some cases you can assume the food is mainly water, so use the heat capacity of water for your calculations.

Once the water has reached the temperature of 100 degrees, in theory no further energy is required to keep it at 100 degrees; if you manage to reduce heat losses to zero. For example, if you wrap the food in a blanket, it may keep hot for quite a while.