value in custom field different by attribute

i wish to be able to set a custom field with a different value, depending by selected selected, or combination of attributes

my products have all expiry date, easy to set for a product with a simple custom field, but impossible to do if product has attributes.

example:

i have a whey protein..... 3 sizes (2lbs, 5lbs, 10lbs) and 3 flavors (chocolate, vanilla, strawberry) i create the choices with 2 attributes, but i can have 9 different expiry date.

maybe there is a smarter way to manage all buy i don't find it, except creating 9 different products