Jeanette,
Though advertised as "perfect for freezing breastmilk", polyethylene bags a la
Playtex, are not very airtight, and reduce the keeping quality of the milk to
about 2 weeks.  (This does not apply to the multilayered, nylon lined
expensive bags manufactured specifically for mother's milk). Often when milk
is frozen it "saponifies"- the fats break down into soapy compounds.  Some
babies tolerate this, some do not.  When moms want to use polyethylene bags to
store their milk, I suggest that they put the bags in an airtight freezer
container as well, and in a bowl of ice if the freezer is "frost free"- the
heating of the freezer walls to defrost it can spoil foods by partially
defrosting and refreezing them.  The ice helps keep the milk solidly frozen.
Catherine Watson Genna, IBCLC  NYC  [log in to unmask]