Why does lower fat milk spoil faster?
What is it about higher fat milk products that last longer than lower fat ones? In my experience, skim milk sours much faster than whole milk and whipping cream takes a really long time to go bad. And I have not ever read why that is.
The easy answer is that fat is not susceptible to rot whereas the other components of milk very much are. If you keep butter wrapped airtight under refrigeration it's damn near immortal, and the lower the moisture content the longer it lasts.
I'm not sure why it is - perhaps they've been ultrapasteurized - but I've noticed that the all of the lactose-free milks I'm forced to use nowadays refuse to go sour, at least as long as I've kept any around. When it gets too old it tends to make disgusting loogie-like clumps if you pour it into hot coffee, but it doesn't taste "turned". It's been an awfully long time since we've had any rotten milk in this house.
Backwards. Lactose turns to lactic acid. No lactose, no lactic acid can form. Analogy is fructose fermenting to ethanol and then further reacting to form acetic acid. Grape juice to wine to vinegar.
Skim milk will have slightly more lactose than whole milk simply due to displacement of fat by skim milk. The parts continue to add to 100 percent. Some skim milk has added nonfat milk solids so it doesn't taste so thin. This would add some lactose as lactose is one the the nonfat solids in milk.
I don't know whether this is true where you live, but here in Toronto there is likely a simple explanation. All major supermarket brands of cream and lactose-free milk are ultra-Pasteurized; no brand of conventional milk (0%, 1%, 2%, or 3.25% butterfat) is ultra-Pasteurized.
The result is that the conventional milk spoils within a couple of weeks. The cream and lactose free products stay usable for ridiculous amounts of time. We recently "lost" an opened carton of lactose free milk at the back of the fridge. Four months after the expiry date, it looked, smelled, and tasted fine (though we did throw it away after tasting it).
The cream products that are not ultra-Pasteurized spoil faster than milk. Our best local brand of whipping cream, Hewitt's, goes sour within a week.
I have to say that, in my experience, UP cream products only last a day or two longer than non UP cream once opened.
And of course, UP cream is (1) much less tasty, and (2) not useable in certain forms of cooking in the same way as regular cream.
But, yes, overall, lowerfat milk tends to spoil noticeably faster than higher fat milk.
re: Karl S
We certainly agree on the taste and utility of UHT cream. Although I haven't kept a tally of spoilage rates, my experience these days is roughly:
UHT whipping cream (35% BF) keeps indefinitely after best before date unopened; keeps at least 4 weeks after best before date once opened.
Non UHT whipping cream (32%-35% BF) turns within 3-5 days of best before date, whether opened or not.
UHT half & half cream (10% BF) keeps indefinitely after best before date unopened; can be used for many weeks - usually well over a month - after best before date once opened.
non-UHT half & half cream (10% BF) keeps for about a week after best before date whether opened or not.
Milk (we use 2% and 3.25%) keeps for about a week after best before date whether opened or not.
Lactose free milk is all UHT and seems to last indefinitely, though it sometimes clumps after months of storage
UHT milk is only available in tetra paks
Well, my experience is the same as yours. Nonfat goes bad much more quickly than the higher fat milks and cream. Whipping cream, in my experience, typically lasts much, much longer than its sell date, whereas nonfat does not.