Hi B.,
My little ones are 4 & almost 3 yrs old, so I haven't breastfed for almost 2 years, but I remember that the milk does not last very long in the refrigerator. When I was expressing milk, I would only keep it in the fridge for 48 hours maximum. I looked up breast milk expiration, because I noticed the amount of time other moms have been recommended really varied. So here is what I found:
Expressed breast milk is highly perishable. Milk should be collected in sterilized 4-ounce plastic bottles or plastic bottle liners. Immediately store the milk in the refrigerator. If you must take breast milk with you, carry it in an insulated cooler with ice or chill-packs.
Any milk that will not be used within 24-48 hours should be frozen immediately. Leave one inch of space at the top of the milk storage container to allow for expansion during freezing. Label the milk with the date it was expressed so that older milk can be used first. Frozen breast milk will keep for 3 to 4 months.
Thaw breast milk in warm water only - not in hot water or in a microwave oven. Do not refreeze breast milk. Shake the container of expressed milk before using it to distribute the fat within the milk.
I hope this helps you! Also, I always used the Gerber bags & liked them. I don't think the spoiling has anything to do with the bags, rather with the length of time the milk is being kept. After 2 days it's no good.
Good luck!