T.U.
Found this on the internet. Hope it helps!
Per Lawrence & Lawrence (p. 158), the amount of BSSL in a particular mother's milk does not vary during a feed, and is not different at different times of day or different stages of lactation. There is evidence that there may be a decrease in lipase activity over time in mothers who are malnourished.
What can I do if my storage problem is due to excess lipase? Once the milk becomes sour or rancid smelling/tasting, there is no known way to salvage it. However, newly expressed milk can be stored by heating the milk to a scald to inactivate the lipase and stop the process of fat digestion. Scald the milk as soon after expression as possible.
To scald milk:
* Heat milk to about 180 F (82 C), or until you see little bubbles around the edge of the pan (not to a full, rolling boil).
* Quickly cool and store the milk.
Scalding the milk will destroy some of the antiinfective properties of the milk and may lower some nutrient levels, but this is not likely to be an issue unless all of the milk that baby is receiving has been heat-treated.