This Is a Certified Answer

Energy content in a food is potential energy, much like GPE and elastic energy. This means that it is a store of energy, waiting to be released by transfer into another such type (e.g. heat when the food is burnt).

The Joule (J) is the SI unit (international standard unit) which is universally used for energy equations - although calories and kilocalories are still used on a large proportion of food packaging, Joules are the more correct form because they link closely with mass, speed and temperature easily using metric units (albeit in other contexts such as energy-transfers in motion and thermodynamics). Kilojoules are more appropriate in the context of food because most foods contain huge numbers of Joules.

The energy value of food is expressed in Kilojoules per 100g because this makes values comparable. If you had two foods, one with a mass of 10g and another of 394g, and were given corresponding energy values of 24J and 4873J, it is difficult to compare which one has a higher energy density (the amount of energy in a certain amount). KJ/100g provides this energy-density value, making it easy to compare between foods; in the above example:

Food A has 100(24/10) = 240J/100g
Food B has 100(4873/394) =  1236.6J/100g

Technically speaking, this value is a ratio of energy to mass. However the standard unit for mass is Kg, so why use 100g instead? This is simply down to how much we eat - it is more useful to know how much energy is in 100g of food because that is closer to the kind of portion you will have than 1Kg (1 portion = 75g rice, 120g meat, 60g cooked meat, 150g baked potato...).

I hope this helps

1 5 1