Vegetables – Why roast beets in vinegar

Roasting caramelizes the natural sugars in vegetables and, eventually, burns them. Roasted anything that turns out bitter is almost always the symptom of overcooking. Sometimes it also means that your ingredients were spoiled or "off" in the first place.

As for saving them - there's not much you can do in either case, other than trying to mask it with some other flavours. Beetroot pairs well with cranberry and other tart fruits, which would probably mask the bitter taste; cream, sweetened yogurt or sour cream would also be a good bet. Otherwise, I can't really think of anything else to do with them; overcooked food usually ends up in the trash for me.

I don't remember the exact science behind the potatoes not turning to mush ... I know it was discussed on an episode of America's Test Kitchen, and they even discussed the amount of vinegar and how it affected the window for cooking times. (I want to say it was on an episode where they were making an Austrian potato salad)

... and it's not specifically vinegar, and not solely potatoes. Acids will stop quite a few items from softening when cooking. I know the list inclues onions (pre-cook onions before adding acids if you want them to disolve into a sauce), potatoes, apples ... I'm guessing there's others.

For potatoes and apples, most recipes claim it's to slow down browning. You see it in potato salad, but not mashed potatoes. You'll see it in apple pies, but not in applesauce ... because they'll come out lumpy.

Matt already mentioned three uses for vingear:

• as half of a chemical leavener
• tenderizing / marinades
• preservation / picking
• coagulation (eg, when poaching eggs)

But also :

• stabalizing eggwhite foams (might just be a variation of coagulation?)
• denaturing / "cooking" (eg. in the case of ceviche; a variation of marinating?)
• flavor (might not be a 'science case' ... but sour is one of the primary tastes; especially helps to balance out fatty dishes)
• cleaning / disinfection (might not be a 'cooking' use)