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