Here's a better tip. Go to your local grocery store. Look at the plastic containers of milk and check the expiration date. Then look at paper cartons and their expiration date. It's a whole lot longer, because the cardboard blocks light. I bought a carton of milk, and the expiration date is sometime in early September! That's a big difference. It may be more expensive to buy milk in paper quarts rather than plastic gallons or half-gallons, but if you are slow in consuming milk, you will save money by not ever wasting milk again.