Basic rule of thumb is to add just a little bit, and increase “to taste”.
Although this information is primarily intended for home-made soups, you can also "spice up" commercial heat-and-eat soups. (With the exception of the parmesan rind. That is intended for long, slow cooking.)
HORSERADISH. Add near the end of the cooking process. Commercial creamed horseradish seems to get the best results. Add just a half teaspoon to a large pot of soup at first, and increase the amount VERY gradually. You are after that subtle hint of a “bite”, not the horseradish flavor itself.
LEMON JUICE. Add near the end of the cooking process. Works fine in combination with horseradish. Again, add small amounts and increment gradually to get the “bite”, but avoid a lemon flavor, unless your soup recipe calls for a citrus flavor.
PARMESAN CHEESE RIND. Add EARLY in the cooking process. Any size rind is usually fine. The flavor will be noticeable, and is quite pleasant. Use the rind left over after you’ve grated off the rest of the cheese for other purposes, or some stores actually sell rinds for this purpose. You could add a chunk of the cheese itself, but that’s rather wasteful when the rind works just as well. THE RIND USUALLY WILL NOT MELT COMPLETELY, so beware of serving up the soft, gooey rind in someone’s bowl.
WINE. Can add any time during the cooking process. Add early if you wish to boil off any vestiges of alcohol. Don’t use in your soup for the first time when you are expecting company. Wine will usually alter the taste of your soup, and you should probably experiment to find a combination that pleases you. But this can result in an elegant, sophisticated flavor.
Wine serves another valuable purpose – it deglazes the bits that stick to the sides and bottom of your pot, bringing their flavor back into your gravy or sauce. Wine is even better in spaghetti sauce than it is in soups and stews.
VINEGAR. Use similarly to wine, with the exception that there’s no concern about alcoholic content, and if you add too much, your soup will acquire a sour taste. (Which some soups actually call for!) If you accidentally do “over-vinegar” a soup, try adding some sugar to counter-act. You’ll probably end up with a “sweet-and-sour” concoction.
Paul Hughes is one of the founding members of the Kingston Farm and Garden Co-op, and the co-proprietor of Farrago Farm and Vineyard.