Gregg Wallace is in Italy, at an enormous pizza factory where they produce 400,000 frozen pizzas each day. He mixes up a 450 kilo batch of dough for the bases; enough for 180 000 pizzas. He watches as each one is stretched to exactly 26 centimetres in diameter, then pricked with 522 holes, each 4mm deep, before they're topped with tomato and disappear into the wood fired oven. It's 25 metres long and the floor is made of rotating panels of volcanic rock heated to 450 degrees Celsius. Gregg's pizzas emerge fully cooked after just 80 seconds. They're topped with cheese, pepperoni, chillies and onions, then frozen and dispatched on their 1000 mile journey to the freezer compartments of British supermarkets.Meanwhile Cherry Healey is asking if mozzarella – the traditional choice for pizzas - is also the scientific best bet. She finds that not all cheeses are equal, and that to melt well they must sit in a pH zone between 5 and 5.9. This explains why blue cheese and feta don't work on pizza but mozzarella and gruyere do. In Austria, she transforms 400 kilos of pork into pepperoni. She learns that this preserved sausage is fermented and salted to give it a long shelf life. A production process that takes more than 2 weeks.Historian Ruth Goodman is investigating the technology that allows frozen foods like pizza to be transported across the globe. 150 years ago we were all 'locavores', eating locally sourced food. But in the 1880s the game-changing invention of freezer ships meant that lamb and beef could be shipped from New Zealand and Australia. Combined with the 1938 arrival of the freezer truck, this created the worldwide cold chain that we rely on today. She also meets the man who popularised pizza in the UK back in 1965 when he opened his first restaurant in London's Soho.