It's the quarter-final, and the four best chefs from this week's heats return to the MasterChef kitchen, where they must prove themselves to the judges in two challenges set to test even the most experienced chefs.First, the chefs are given an Invention Test with a twist. They must create a dish that showcases one of the four ingredients that are wasted in the UK more than any other – bread, milk, potatoes and bananas. Starting from scratch, they have an hour and 10 minutes to create a stunning dish with one of these ingredients as the star. Only the best will do as at the end of the round, one professional must leave the competition.Next, the remaining chefs must raise their game yet again as they face the daunting task of cooking for three of the UK's most discerning restaurant critics, Jimi Famurewa, Tom Parker Bowles and Jay Rayner. Can they make an impression for the right reasons and deliver knockout dishes in one of the most intense rounds of the competition?Only the most talented will make it through to Knockout Week.