\"If you usually breastfeed your baby from a bottle of expressed breast milk, glass bottle is the best choice. Glass bottle makes live antibodies in breast milk tend to stick LESS to the sides of glass then plastic bottle. But if your baby always gets breast milk fresh from your breast directly then plastic storage bag is an excellent choice. Many people say that breast milk storage bag has tendency to be cumbersome to use. So the best method is to put the bag into a glass bottle and then pour the breast milk in.\"
From http://www.streetdirectory.com/etoday/breast-milk-and-storage-ejpae.html