Precautions.-Two points must be carefully observed in carrying out this test: (1) The temperature must be kept as near 98° F. as possible, in order that the bacteria may develop as desired. This can be done by keeping the temperature of the water surrounding the jars at 100° to 102° F. The temperature must be watched. (2) The thermometer and the knife used should be made not only clean but sterile each time after using in one sample before placing them in another. GERBER'S FERMENTATION TEST This test consists in heating milk in tubes 6 hours. at 104° to 106° F. and then observing the odor, appearance, taste, etc., for abnormal qualities. The milk is heated a second time for 6 hours at 104° to 106° F. Any abnormal coagulation of the milk is noticed, such as holes due to gas. Gerber states that milk coagulating in less than 12 hours is abnormal, due either to the abnormal character of the milk itself when drawn or to improper care after being drawn. Milk that does not curdle within 24 to 48 hours is open to the suspicion of containing preservatives and should be examined for such substances. METHOD OF TESTING MILK FOR SUSPENDED DIRT The amount of dirt in suspension in milk can be estimated without serious difficulty. The best way is to provide a small centrifugal machine that is made to run at higher speed than the Babcock tester. A form of hand-centrifuge is shown in Fig. 41. Special graduated tubes (Fig. 42) are made to use in this. The milk to be tested is stirred thoroughly, the tube is filled to the highest mark, placed in the pocket of the centrifuge and whirled several minutes. The sediment collects at the bottom and can be easily measured by reading the amount on the scale. In Fig. 43 is shown a Bausch and Lomb electric centrifuge. This company also furnishes hand-centrifuges capable of 3,000 to 8,000 revolutions per FIG. 42-TUBE FOR SEDIMENTATION WORK FIG. 43-BAUSCH & LOMB ELECTRIC CENTRIFUGE Speed 1,000 revolutions per minute 119 FIG. 44-INTERNATIONAL INSTRUMENT COMPANIES ELECTRIC CENTRIFUGE FOR SEDIMENTATION WORK Capable of 3.000 revolutions per minute FIG. 45 GLASS FOR COLLECTING minute. Their centrifuges and tubes can be used also in testing for fat in milk by the Babcock method. In Fig. 44 is shown another form of electrical centrifuge which is very satisfactory for collecting sediments. A method less accurate, but fairly satisfactory in the absence of any better means, is to place about 4 ounces of milk in a test-glass (Fig. 45) and let it stand for one or two hours. The dirt collects in the bottom and its amount can be roughly estimated by the eye. CHAPTER IX Methods of Testing Milk by Rennet-Extract and Pepsin In cheese-making it is necessary to have some means of finding out when the rennet-extract should be added to milk in order to secure the best results in the process. This is usually known as "testing the ripeness of milk." Two methods are in common use for this purpose: (1) The Monrad test and (2) the Marschall test. THE MONRAD TEST This test is based upon the amount of time required for a definite quantity of milk at a given tem FIG. 46 MONRAD RENNET-TEST perature to become coagulated by a fixed quantity of rennet. The pieces of apparatus (Fig. 46) required are the following: (1) A tin cylinder for measuring milk, holding, when full, 160 cc., (2) a 5 cc. pipette, (3) a |