Soups are probably the least definable, widest-open category of foods there is. They can contain pretty much any ingredient there is, be clear or murky, smooth or chunky, thick or thin, hot or cold. As long as it's fluid enough to eat with a spoon, it can be a soup.
That understood, there are a few general principles worth describing.
Flavor balance in soups is important and infinitely adjustable. When seasoning, aim for a strong taste foundation, with a balance among saltiness, tartness, and savoriness. Rich soups can benefit from a counterpoint of acidity. Vegetable purees can benefit from the background savoriness of a little bacon or tomato or Parmesan cheese, or soy sauce or Vietnamese fish sauce or Japanese miso.
Many hot soups are thickened and enriched by sensitive ingredients, as are many sauces, and they need to be made with similar care.
To thicken with egg yolk or other uncooked animal protein (crustacean livers, sea urchin, pureed liver, blood), take care not to overheat and curdle the protein. Start with the soup well below the boiling point. Add small amounts of hot soup gradually to the thickener to dilute and warm it gradually, then mix all together and heat slowly, just to the point that the soup begins to thicken. Reheat leftovers to 160°F/70°C, not to the boil.
To thicken or enrich with cream, use heavy whipping cream or high-fat crème fraîche, which contain too little milk protein to form noticeable curds even at the boil. If you use light cream, sour cream, yogurt, or butter, or a swirl of flavored or olive oil, add at the last minute and keep temperature well below the boil. Reheat leftovers to 160°F/70°C, not to the boil.
To reduce the risk of curdling, choose recipes that include starch or flour, which help protect proteins from coagulating and creams from leaking fat.
To thicken soups with flour or starch, always predisperse the thickener in a roux, beurre manié, or slurry to prevent lumpiness. Add the thickener and then simmer just until the soup develops the right consistency.
Add uncooked ingredients in stages to a simmering soup to avoid over or undercooking them. First add whole grains, or firm carrots or celery, then more tender onions or cauliflower, pieces of chicken breast, or white rice or pasta; at the last minute, delicate spinach leaves, fish or shellfish. Alternatively, cook each ingredient separately in the soup liquid, and then combine them just before serving.
While serving soup, keep the pot covered and warm, around 140°F/60°C. Refrigerate leftovers within 4 hours of turning off the heat.
To serve leftovers, reheat to at least 160°F/70°C, being especially careful with protein-thickened soups to avoid curdling.
KEYS TO GOOD COOKING provides simple statements of fact and advice, along with brief explanations that help cooks understand why, and apply that understanding to other situations. Not a cookbook, Keys to Good Cooking is, simply put, a book about how to cook well.
Excerpted from Keys to Good Cooking by Harold McGee Copyright © 2010 by Harold McGee. Excerpted by permission of Doubleday Canada, a division of Random House of Canada Limited. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.