Overview: Sian Williams and Stefan Gates get to the heart of supermarket food - the ingredients - finding out why what appears on the front of the packet may not always be the same as what's inside. Stefan highlights the use of cheap ingredients by comparing pizzas made with real cheese and processed cheese. Can a consummate pizza chef tell the difference? Sian looks at the amount of fat in ready meals and Stefan reveals how ice-cream companies use the cheapest secret ingredient of them all - air.