+199. Hot-iron test.+[89]--This test is employed to determine the amount of acid in the curd. A piece of iron, such as an iron pipe two feet long, is heated in the fire to proper temperature. If the iron is too hot it will burn the curd, and if not hot enough the curd will not stick to the iron. When hot, it is taken from the fire and wiped clean with a cloth. A handful of curd is taken from the vat and squeezed dry, either in the hand or in a cloth. This curd is carefully pressed against the hot iron and drawn away. If the iron is at the right temperature and the curd has sufficient acid development, the curd will stick to the iron and when pulled away will form fine threads. The length of these threads determines the amount of acid in the curd. The acid is usually spoken of in terms of the length of threads, as 1/8 inch of acid, ½ inch of acid and the like. The curd must have a slight development of acid before it will stick to the iron. This test takes advantage of the peculiar properties[90] of curd which are produced by the action of the acid on the casein. +200. Firmness of the curd.+--The cheese-maker must be able to judge the firmness of the curd by physical examination. The particles of curd should have shrunken to about one-half their original size and should be of uniform consistency throughout; they should not have any soft centers. The curd should be firm and springy. When a double handful is pressed and suddenly released, the curd particles should spring apart. The curd should have a "shotty" feeling when in the whey. If the curd has attained the proper firmness, and the acid has not reached the correct stage to remove the whey, it may be left in the whey until sufficient acid development has been attained. This is liable to cause the curd to become too firm and to result in a hard dry cheese. If there is no evidence of the presence of undesirable organisms, such as bad odors, or gas holes in the curd, it is better to remove the whey and develop the acid when the curd is in the "pack." The pack refers to the first piling of the curd. [Illustration: FIG. 36.--Whey siphon with strainer.] The whey may be removed either by means of a faucet or gate in the vat or by a siphon (Fig. 36). With either form of removal a whey-strainer (Fig. 37) should be used to prevent loss of curd particles. It requires considerable time for the whey to escape from a large vat. After the curd has been heated to the proper temperature, it is well to remove a portion of the whey. In doing this the surface of the whey should not be drawn down quite to the top of the curd. When ready, the remaining whey can be quickly removed. [Illustration: FIG. 37.--Whey strainer with spout to fit into the gate in the vat.]