Months of reading and listening to people who know more than I do has helped clear things up somewhat. First, red wine is made by churning up the red grapes and letting the juice, pulp, and skins sit together for a while. All this goo is called the must. Churning red grapes gives red wine its dark color and its tannins (the compound that seems to make your mouth shrivel a little, the way strong black tea does). White wine is made by the juice being pressed out of the skins and allowed to run off and ferment separately, free of the skin color and tannins that would overwhelm a white wine's desired delicacy.
Grape juices, either red or white, can ferment and/or age in a stainless steel tank, in an oak barrel, or both in succession. Wine makers can rely on yeasts naturally present, or they can add more. When the juices have fermented and become wine, they are still going to be full of extraneous matter, even the white wines that were never churned up with their skins. If nothing else, they too carry the dead yeast cells -- the lees -- that have finished the work of fermenting and have sunk to the bottom of the tank or barrel. Wine drinkers don't necessarily want to see this, unless they have bought a great and expensive red, like a Bordeaux or a vintage port, which will go on aging and "throwing off" yeast-cell sediment even in the bottle.
So most wine is cleared before bottling either by being pumped gently out of its sediment-filled tank or barrel into a pristine one, or by having a special filtering agent laid on the surface of the liquid. (Or both.) This agent sinks slowly down to the bottom of the tank or barrel and carries all sorts of wine-gunk with it. The "agent" might be a particular form of clay, Bentonite, or it might be isinglass, taken from the swim bladders of a fish. When the wine is "fined" like this and is crystal-clear, it is ready for bottling.
Being about as germ-phobic as anybody else, I have wondered about what is done to wine to make it safe to drink. (By the way, Hugh Johnson in Vintage: The Story of Wine relates an old legend about wine being discovered in the royal court of ancient Persia, by a desperate harem maid who drank off some forgotten and weirdly bubbling grape juice, thinking it was poison and would end her troubles. When instead it cheered her and blessed her with "refreshing sleep," she told the king, Jamshid. The rest is oenophilia.)
Apparently, not a great deal is done to wine to make it "safe," because many of its properties -- alcohol, sugars, acids, tannins -- are themselves preservatives. If it should spoil it will simply become vinegar, which is itself another preservative (albeit an unpleasant drink). Wines aren't pasteurized, except for some kosher wines which are flash-pasteurized, because heat will cook the alcohol and ruin the taste. What the winemaker must do to ensure flavor is to ensure that his equipment is scrupulously clean. Cold tanks and cold cellars help, as refrigerators help us preserve our food. And what he can do is offer his land, vines, grapes, or wine, at a variety of points in the process, a little further help so that he ends up producing what he intended, and not something you would make pickles with.
One of these little helpers, to borrow a phrase, is sulfur. It is a normal part of fermentation anyway, and is present in many other foods, like bread and dried fruits. But it has also been used as a spray to prevent mildew on the vines, and to clean barrels of any surviving yeasts and bacteria that can live in wine, multiply, and make vinegar. Sulfur also protects wine from air itself, which in large doses is not wine's friend. (A cut apple or banana turns brown in air. Wine oxidizes in the same way. Johnson explains: "It has been said that before the advent of sulphuring, the grape variety scarcely mattered, since all wines rapidly oxidized and thereafter smelt and tasted similar.")
After the wine is bottled it is shipped to your local grocery store or wine shop, from whence you buy it and take it home. You can hope that the wine wasn't parked next to furnaces and boilers, or under hot lights, along the way, but at least once you bring it home the responsibility for it is yours. Some wine writers say that more wine, nowadays, is just plain good -- well made, well cared for, affordable, reliable -- than at any other time in the planet's wine-drinking history. At any rate we are more fortunate than our remoter ancestors, who used to keep the stuff in goatskins and dilute it with warm sea-water (from Johnson's Vintage, again).
And, for the germ-phobic among us, it is good to know that with wine we have a product that, as far as my limited knowledge goes, carrries no unsuspected problems in quite the way that ground beef, raw eggs, and unwashed lettuce can. It is meant to taste good and provide pleasure with meals; if it has "gone bad," the most it will do is fail to provide those pleasures, in which case of course you won't drink it. Voila.
"Sherman's Lagoon" by Jim Toomey. (c) 2008 Jim Toomey. Used by permission of the artist.