Elements of the theory and practice of cookery

The sour taste of the milk is caused by the presence of an acid. When milk is kept at the ordinary tem- perature more than a few hours the sugar in it changes to lactic acid (milk acid). This acid, like the acid in vinegar, coagulates casein. 175. The change from sugar to lactic acid is caused by certain bacteria (bacilli of lactic acid) that get into milk from the air. (§ 446.) Recipe for Cottage Cheese Thick sour milk, 1 quart. Salt, X teaspoonful. Butter, 2 teaspoonfuls. Cream, enough to make cheese as moist as desired. Heat the milk in a pan set on the back of the stove, or into another pan of hot water ; as soon as the curd 1 For directions for pasteurizing milk, see §§ 466 and 469. 96 THEORY AND PRACTICE OF COOKERY separates from the whey, strain the milk through a cloth. Squeeze the curd in the cloth until rather dry. Put it in a bowl, and with a spoon mix it to a smooth paste with the butter, salt, and cream. Serve lightly heaped up. 176. Action of rennet on milk. — A different kind of coagulation of casein is produced by rennet, a substance prepared from the lining of a calf's stomach. Rennet is sold for use either in an alcohol solution ("liquid rennet ") or in tablets. Recipe for Rennet Custard or Slip Milk, 1 quart. Extract of vanilla, 1 tea- Sugar, X cupful. spoonful. Liquid rennet, 1 tablespoonful, or 1 rennet tablet dissolved in 1 tablespoonful of water. Heat the milk in a double boiler until it is luke- warm. Add the sugar and stir until it is dissolved. Stir in the vanilla and rennet, and pour into a glass dish. Let it stand in a warm room until it begins to thicken ; then set it in a cool place, and leave it until it is firm. Sprinkle with one-eighth teaspoonful of cinna- mon or nutmeg, and serve with cream (or milk) and sugar. To make Coffee Rennet Custard, use two and three- fourths cupfuls of milk and add one and one-fourth cupfuls of strong, cold coffee after taking from fire. Use one-fourth to one-half cupful of sugar. TISSUE-BUILDING FOODS: MILK AND EGGS 97 177. Does the curd formed by rennet differ in any- way from that formed by an acid ? If so, how ? Does curdling make milk sour, or does souring make it curdle ? 178. Digestion of milk. — Rennin, the ferment that gives rennet its power to coagulate milk, is secreted by the human stomach as by the calf s stomach, to aid in digesting milk. By its action the nitrogenous parts of the milk are sepa- rated from the rest, that they may be acted on by pepsin, while the fat and sugar pass on to be digested in the intes- tine. (Digestion of Albumin.) BUTTER 179. Fat naturally exists in milk in little spheres or globules about Yb'^'5 ^^ '^^ inch in diameter. When fat or oil is suspended in this way in a liquid it is said to be emulsified. Experiment to illustrate emulsion. — Shake some lime-water and linseed oil together in a bottle ; hold the bottle still and observe the oil globules rise. 180. Milk under the microscope. — Look at a drop of milk through a microscope, then at a drop of cream. Which has the greater number of fat globules ? Make memory drawings of what you see.