I have been looking at recipes for creme patissiere (to make a traditional tarte aux fraises) but would very much like to know why some recipes use flour & cornflour whereas others use one or the other. As a cook and as a scientist can you tell me why this is. Thanks very much for your help. Joalex
Cornflour (or cornstarch) is made from maize and is pure starch. As the starch molecules cook they swell up and this causes a sauce to thicken. Flour contains starch but it also contains a protien called gluten and this means that it doesn not thicken quite as well as cornflour. Cornflour has twice the thickening quality of flour, so you need double the weight of flour vs cornflour to get the same thickening effect.
Usually flour is also mixed with melted fat first (a roux) to coat the flour molecules in fat and help them to blend with a liquid without going lumpy, whilst cornflour will blend smoothly with cold liquid, though in creme patissiere recipes usually the flours whisked with egg yolks before being added to the liquid. Somtimes cornflour-thickened sauces can thin slightly when they are cool, if the sauce is stirred to vigorously when it is being cooked.
We suspect that the flours used are more an issue of personal preference than exact science, though adding cornflour into the mixture would likely result in a thicker creme patissiere and using a combination of the two may result in a more stable mixture.