Mary Ronald,
The Century Cook Book
“ The secret of making a good white sauce is in cooking the flour until the starch grains have burst, which removes the raw and pasty taste one finds where this care is not used. There is no difficulty in making it smooth if the milk is turned in slowly, as directed above. A common way of making this sauce is to rub the butter and flour together, and then stir them into the boiling milk, but this does not give as good a result as when a roux is made. The intense heat of frying butter cooks the flour quickly, while milk boiled long enough to cook the flour is changed in flavor. ”
