Sound advanced? It isn't. Everyone has had a dill pickled cucumber with a juicy monster burger (or, say, one of the world's great patty melts at Jeri's), tomato soup with grilled cheese, tzatziki sauce on a gyro, kimchi with any/everything. This is why Tabasco is so popular (sure it's the heat, but don't unwittingly discount the co-pilot of vinegar). Sour balances sweet wine and gives it character. It raises coffee from the over-roasted (nay, burnt!) abyss of Starbucks to the delicious, more subtle and complex heights of the smaller roasters that make thinking about coffee so interesting these days.
The world of sour is complex, too; different acids are produced/enhanced via different means, whether they are just born that way (citrus), are there waiting to be revealed (wine reduction) or are invited to set up camp (fermentation). Fermentation is one of the trickier ways to get at the acid; time, temperature, attention (but not too much attention) all matter in order to receive the desired results of good bacteria growth and not make one miserably sick.
I've been reading The Art of Fermentation by Sandor Ellix Katz. It's not so much a cookbook as an encyclopedia-like volume that explains a ton of different kind of ferments, with some theory and equipment information, but mostly a whole lot of first-hand experience (whether his own or from others who have written to him or published on the web). There's even a great little aside about a friend of his, who got an extra scoop of trouble in federal prison when she was caught trying to ferment the prison coleslaw into sauerkraut, and the mean ol' warden thought it was hooch. The best part is that he readily and humbly admits that he is not a food scientist and claims that he is not an expert--he's just a guy who loves food and embraces the ethic of "the more I learn the less I know." That said, he's had a ton of experience doing so. His earlier book, Wild Fermentation offers more recipes than this one does--this one gives the "why" and a bit of "how," leaving some variables up to you, which, if a bit more challenging that being spoon-fed amounts and ingredients, teaches one so much more. Read this F.o.t.D. post from (gulp) nearly four years ago to understand why I think that.
Anyway, back to the merits of sour. Sour refreshes your palate, opens it up to receive other tastes and flavors, splashes cool water on the face of your tongue. Often times, it is accompanied by its good friend crunchy--the sour of textures--which shakes up the round monotony of rich flavors and soft textures. Used as a seasoning agent, sour goes a long way to finish what salt starts--that is, to open your palate up to other tastes. Once you understand salt, and that it is used to season things, to open things up, and not to simply make things salty, you're on the right path to understanding the same thing about sour. If you are making, say, soup for Soup and Bread, and you've salted it well and it's just not unlocking all the other flavors you worked so hard to cultivate--give it some acid. A squeeze of lemon or a dash of good vinegar, and things will open right up.
I've currently got a big batch of kimchi fermenting on the counter (see, it's not just sour, spicy and garlicky and funky get invited, too), and some yogurt doing its thing in the closet next to the furnace, as well as Indian-style lime-fermented chiles I picked up from the book above, which Katz picked up from a guy writing as Fried Sig who in turned picked it up from Madhur Jaffrey, of course. Whew! The process is simple. Using a variation on Fried Sig's method due to what I had available, I sliced a variety of chiles--use a good range of heat levels here. Heated a bit of olive oil (original recipe used mustard oil, which has certain preserving qualities, but I had none, and, well...) gently, with some mustard seeds and about five cloves of crushed (but still whole) garlic just until the garlic started to bubble. I let it sit for a few minutes to diffuse flavors, then poured it all over the chiles and added a 2" knob of ginger, peeled and sliced into thin discs. I seasoned with salt and mixed everything, tasting until the salt level was right (the chiles should taste good, nice and salty but of course, not too salty). Then packed it all into a jar. If your friend makes pickles, he'll have one you can borrow.