Why does every juice concentrate, except for grape juice, stay frozen until you get home from the store?

Dear Matt:

Grape juice. When you buy frozen juice concentrates at a store they generally stay frozen until you get them home -- except for grape juice. Frozen grape juice concentrate frequently starts getting sloshy before you even exit the store, if the checkout line is long enough. Why is this? Does grape juice contain some sort of natural antifreeze? If so, could I use it in my car's engine in an emergency?

-- Moby Grape, the net

Matt:

I've wondered about this since I was a kid, but it's so ridiculous that I've never had the nerve to ask anyone. Why is my feces green after drinking purple grape juice? Never noticed it with wine. Am I a freak of nature? Hope the elves don't have to do any hands-on analysis for this one.

-- No name, no place, pleeeeez

Heck, no. They smelled the question coming in over the transom, grabbed their Razors, and jetted out the back door. Haven't seen them in three or four days. So now it's just you and me, mister. No. Don't be shy. We're all just poopin' humans in this crazy outhouse we call life. Of course our staff quack, Dr. Doctor, is doubled over in a giggling fit, but as usual, we've sought a second opinion. The second opinion sounds a little nuts to us, but it's from our usual highly informed sources, so I can't argue. Especially without the elves to catch my back. You seem to be among a select few individuals, but no one wanted to commit to "freak of nature." Grapes and your digestive system just have an interesting biochemical relationship.

A poop analysis would reveal that it's made up of indigestible (mostly cellulose) food shreds, dead gut cells and intestinal bacteria (and lots of 'em), digestive secretions and enzymes, fats, and water. It comes in a variety of decorator shades, depending mostly on what you ate 24 to 36 hours ago. But you seem to be one of the people who doesn't absorb the sugars in grape juice very well, and in your intestines they act like a laxative. This is a common condition in children, not so common in adults. This rapid transit of not-entirely-digested food can tinge the resulting product a greenish shade. A festive touch around St. Patrick's Day. And of course if you mistake the foregoing as medical advice, well, then we'll all be doubled over in giggling fits.

And blame the nature of grape-sugar solids for that fast-thaw juice concentrate. According to the juicers at Welch's, it must be kept at 0 degrees to overcome the sugar's urge to return the brick to a liquid. It's a little like an ice cube with crud in it melting faster in the crud-filled areas than does a pristine cube. All juices are susceptible to this to one degree or another, but grape juice is the quick-melt champ.