Why is there an expiration date on sour cream?

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Dear Cecil: Why is there an expiration date on sour cream? Al Malmberg, Colorado Springs, Colorado

Cecil replies:

Al, you nut! I mean, just spelling it out for the benefit of the slow, it’s already sour, right? Unfortunately, as often happens, I’m obliged to spoil your little joke with the facts. Probably you have the idea that they make sour cream by taking ordinary cream and letting it sit out on the window sill for a couple hours. By and by somebody gets a whiff, goes, “Yo, that’s sour! Ship it!” and two days later you’re spreading it on a blintz.

But that’s not how it works. (Surely you suspected this.) It’s true they start with light cream or the equivalent. Having pasteurized it, which kills most of the microorganisms that make raw milk go sour, they then dump in a special bacterial culture that produces lactic acid. If I know my bacteria — and I did stand in line once to get tickets for a Kiss concert — they produce the lactic acid by excreting it, which you then pay to eat. Chilling the sour cream after the bacteria have had 12-16 hours to do their thing halts the “ripening” (i.e., souring) process, resulting in a product that is merely tangy rather than completely rank.

But bacterial action doesn’t totally stop, and if the sour cream sits around long enough it will eventually become so sour (or moldy) that it’s inedible. The same will happen to virtually any dairy product, since some sour-inducing microorganisms invariably survive pasteurization. Thus the expiration dates. We may think of sour cream, therefore, as occupying the bracingly tart if narrow interval separating the hopelessly bland from the unspeakably vile — pretty much what we at the Straight Dope aspire to do vis-à-vis Barney the Dinosaur and Howard Stern.

Cecil Adams

Send questions to Cecil via cecil@straightdope.com.