It's the first quarter-final of the series, which will see the week's best competitors don their chef whites to compete for a place in knockout week. Two challenges stand in their way.First up, an invention test with a twist. The chefs will have to rustle up a dish using a mystery ingredient. In this round, it's stout. They will have a glorious selection of produce in the pantry, but the stout will need to shine. It's a delicate balance, as too much could kill the dish completely.The pressure then ramps up with the infamous critics' challenge. Two courses in one hour and 15 minutes is daunting by any stretch, but taking their seat in the dining room are esteemed critics Jay Rayner, William Sitwell and Leyla Kazim. If there's a time to up the ante, it's now. Every element on the plates will be scrutinised, and no error will escape unnoticed.With the critics' verdicts to hand, it's down to Marcus and Monica to decide which chefs have what it takes to cope with the pressure of knockout week, and continue in the battle to become this year's MasterChef: The Professionals champion.