From my dietetic background, I learned that free fatty acids go rancid faster
than fats packaged as triglycerides.  Triglycerides are degraded by lipase
into free fatty acids, so the lipase in HM is more likely to degrade the
triglycerides with time. Perhaps the important factor is how long the milk
was stored prior to freezing.  Unsaturated fatty acids also go rancid more
quickly than saturated fats.  That is the reason why they hydrogenate
(saturate) fats in crackers, cereals , etc. so that they will have a shelf
life of "forever" which is what the American public wants.  HM has a lot of
unsaturated fatty acids.  Probably the combination of both unsaturated fatty
acids and the increase in free fatty acids with time cause the milk to go
rancid.  I have a friend at the hospital who was unable to store her milk in
the refrigerator longer than 2-3 days--her milk went rancid.  She is a
"mostly" vegetarian, so could be the fats she had in her milk were more
unsaturated than those of meat-eaters.  --Martha Brower RD LD IBCLC