Your milk never sours in your body. its the prefect refrigerator for it to keep it fresh. It may be more whitish after a long period of time because of the fatty cells that accumulate. and the more hind milk you get the more fatty it becomes. this is NOT a bad thing... these are very important cells for babies to nurture their brains growth. no need to fear, you would be suprised how often this question gets asked.

just so you also know,
Breastmilk is good, freshly pumped, at room temperature for 8 hours. In the refrigerator for 8 days, and frozen for 3-6 months in the regular freezer. Once frozen, it has to be used once it melts to a completely dethawed state within 24 hours. so it you take it out at 10 pm fot the following day and its not thawed till 8 am the next morning, you have until 8 am the following day to know its still fresh. In a deep freezer milk is good for a year.

I hope this helped.