Cornflour and arrowroot need to be ‘slaked’, which means dissolved, in a little cold water or wine before being added to the hot liquid. They are then stirred in and the sauce thickens almost immediately. This method gives a more refined, clear sauce than thickening with a roux and is particularly popular in Chinese dishes and sweet sauces. As a general guideline, 1 teaspoon slaked cornflour or arrowroot will thicken 200ml sauce. I recommend cornflour for thickening milk and dairy-based sauces and arrowroot for meat sauces, as this adds a superior glossy sheen to the sauce, as well as giving better ‘mouth feel’.