L.H.
You may find this FAQ page from the LLL useful to reference: http://www.llli.org/FAQ/milkstorage.html
Many sources recommend briefly scalding the breastmilk (not in the microwave!) Another possible cause is washing the bottles, equipment, etc. in tap water. Enzymes can be activated by metallic ions like those found in tap water. So, washing all equipment/bottles/sippies in distilled water, with a phosphate-free dish soap, may reduce the problem.
Best wishes to you and your little one!