I bake a lot and make my own short pastry, sweet and savoury. I love new recipes and try everything I like at least once, if I'm successful I make pastries and pies over and over. That is why I'm surprised that I have never thought about the difference between using whole egg in a pastry against just an egg yolk. Recently I made a Tarte au Citroen the recipe called for just an egg yolk in the pastry, but a Key lime pie I made called for a whole egg in the pastry. Are there any chefs out there who can explain the science of using one against the other?