This cannot be done accurately without a thermometer. The milk bottle should be placed upon a rack in a kettle of water, with a clean thermometer inserted through the cover of the bottle. Heat the water slowly, and watch the thermometer. When it reaches 155° F., see that the water becomes no hotter. Set the kettle on a rack on the stove top, or use a simmering burner with rack and asbestos mat. It is difficult to keep the temperature even, but it should remain at 140-155° F. half an hour. At the end of half an hour, the bottle should be removed, and cooled as rapidly as possible in running cold water. =2. Rennet custard.= 1 quart milk, two tablespoonfuls of sugar, a shake of salt, flavoring, 1 rennet tablet. The flavoring may be: 1 teaspoonful vanilla, _or_ a few tablespoonfuls of orange juice, _or_ the vanilla, plus three or four tablespoonfuls of cocoa to suit the taste. _To prepare._——This is a process without cooking. Rennet tablets are made from the stomach of the calf, and contain the digestive enzyme, rennin, which results in the solidifying of the curd of the milk. Rennet custard has passed the first stage of milk digestion. Put all the flavoring substances into the milk, and warm it slightly, not more than 100° F. The cocoa when used should be “dissolved” in a small amount of hot water. Dissolve the rennet in a tablespoonful of cold water, and stir this very thoroughly into the milk. Pour the milk into the cups in which the custard will be served, and set the cups in a warm but not hot place. A good method is to place them in a pan of warm water (100° F.). The milk becomes firm in a half hour or an hour, and as soon as it is set, should be put in the ice box, otherwise the process continues and the custard becomes watery as the curd shrinks and forces out the whey. Serve very cold with fruit on the top, or whipped cream with the cocoa flavoring; or put grated nutmeg or powdered cinnamon on the top. This is a simple and delicious dessert, and one of the most wholesome. For children it should be flavored with fruit juice or vanilla rather than with cocoa. =3. Uses of sour milk.= Do not throw away any sour milk that is clean and not stale. Milk that has soured enough to set, is a delicious dessert, with cream and sugar. The acid is very pleasant, being the same as that of buttermilk. Sour milk is better for griddlecakes and muffins than sweet milk. (See Chapter XI.) Another excellent use for sour milk is to make it into cottage cheese. (See below.) =Matzoon= and other similar preparations are essentially soured milks, prepared under controlled conditions. These preparations are the common form of milk in certain parts of the Orient, where milk is never used sweet.