Why is there an expiration date on sour cream? –Al Malmberg, Colorado Springs, Colorado

Al, you nut! I mean–just spelling it out for the benefit of the slow–it’s already sour, right? But tickled though I am by your appreciation of life’s little absurdities, I am obliged once again to bury you 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 at a Kiss concert–they produce the lactic acid by excreting it, which you then pay to eat. No accounting for it, though as I look back it does explain the onetime popularity of est. Chilling the sour cream after the bacteria have had 12 to 16 hours to do their thing halts the “ripening” (i.e., souring) process, resulting in a product that’s merely tangy rather than 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 but brief interval separating the hopelessly bland from the unspeakably vile. A perfect analogy to the positions on the cultural continuum occupied by Barney the Dinosaur, myself, and Howard Stern.

Two questions. (1) Why do you blow on hot coffee to cool it, but you blow on your hands in winter to warm them up? (2) How come vegetables have no fat, but vegetable oil is 100 percent fat? –Dick Wolfsie, WIBC, Indianapolis

You broadcast types are such a pain. But when you’re a guy like me, singlehandedly keeping the book industry afloat, you do what you gotta to promote the product. (1) What’s the big mystery about this? Your breath (98 degrees F or thereabouts) is cooler than the coffee, so the coffee cools. It’s warmer than your near frostbitten hands (perilously close to 32 degrees F), so your hands get warm. (2) Vegetables are low fat, but they’re not no fat. Any living cell has fat in its cell membranes and elsewhere. In vegetables the fat content typically is less than one percent, but it’s higher in the seeds, from which most vegetable oils are made. Some vegetables are pretty fatty even if we ignore the seeds–olives and avocados, for example. (Purists may consider olives and avocados fruits, but I’m using vegetable to mean any plant product–as opposed to mineral oil, derived from petroleum, or animal oil, e.g., lard. Animal, vegetable, mineral. Possibly explains certain puzzling aspects of “Twenty Questions.”) Olive oil is one of the few types of vegetable oil made from the flesh of the plant, as opposed to the seed. They’d probably make avocado oil from the flesh too if there were any market for avocado oil, which there isn’t. Be that as it may, the fat-storage bodies in avocado flesh are what make it smooth and creamy.

Let’s see, what other interesting stuff do we have in the veggie-fat file? I know, rancidity! You’re thinking, gee, my ordinary refined flour never gets rancid, but my whole wheat flour sometimes does–how so? Because refining removes the wheat germ (germ is part of the seed), which contains the wheat oil, and rancidity is a property of oil. But you think that’s amazing? Consider linseed oil. The stuff is prized as a furniture finish because it gets so rancid it turns solid! (Technical details on request.) Why don’t linseed oil marketers tell you these things? I know I’d buy a lot more.

Art accompanying story in printed newspaper (not available in this archive): illustration/Slug Signorino.

Cecil Adams is the world’s most intelligent human being. We know this because: (1) he knows everything, and (2) he is never wrong. For more, see The Straight Dope website and FAQ.