Soups are thickened with flour, cornstarch, or rice flour: one tablespoonful for a quart of soup, - heaping, if flour; scant, if rice flour or cornstarch. Flour is the cheapest, but cornstarch gives a smoother consistency. Mix the flour with a very little cold water or milk until it is a smooth paste; then add more liquid, until it can be poured easily into the boiling soup. Remember to boil the soup fifteen or twenty minutes after the thickening is added, that there may be no raw taste of the flour. Where butter and flour are used, the butter is rubbed to a cream, mixed or braided with the flour, and then made into a paste with a little of the soup.
A better way is to put the butter in a small saucepan, and when melted and bubbling stir in the flour quickly, until smooth (be careful not to brown butter for any white soup); then add gradually about a cup of the hot soup, letting it boil and thicken as you add the soup. It should be thin enough to pour. In vegetable soups or purées, as soon as the hot butter and flour are blended, they may be stirred at once into the soup. This is what is meant in many of the receipts by thickening with butter and flour which have been cooked together. The hot butter cooks the flour more thoroughly than it can be cooked in any other way When a brown thickening is desired, as in Mock Turtle Soup, melt the butter and let it become as brown as it will without burning; then add all the flour at once and stir quickly, that every particle of it may be moistened in the hot butter; add the water or soup gradually.
Flour that is browned while dry, either in the oven or over the fire, colors, but does not thicken. A certain amount of moisture, of either fat or water, is necessary with the heat to thoroughly swell the grains of starch in the flour. Thickened soups should be about the consistency of good cream. Purees are thicker.