An egg wash is used to give pastry a rich golden colour and also to seal pastry when baking it blind so that the filling doesn’t seep into the pastry and make it soggy.
How to make an egg wash
To make an egg wash, mix an egg yolk with a little milk or water to thin it. Brush on top of pastry or bread dough to ensure a rich golden colour.
When you bake a tart case ‘blind’, remove the baking beans and paper after 15 minutes in the oven, brush the base of the pastry with egg wash and put it back in the oven for 5-10 minutes to seal it so that any liquid mixture you add doesn’t seep through and give you a soggy bottom! (You can also use egg white instead of egg wash.)
You can also use the egg yolk on its own but adding the milk will enrich it further and make it easier to brush over the pastry or bread.
When you are making challah, or any shaped bread that swells and changes shape during baking, brush it with egg wash before it goes in the oven, and after 20 minutes of baking, check to see if any parts are not egg-washed and egg wash them again. That way you won’t have bald spots on your bread!