The next step is to add a strong, biting acid known as sulphuric acid to the test-bottle into which you have just put the milk. A glass marked to show just how much acid to use also comes with the machine. Fill this glass measure to the mark. Then pour the acid carefully into the test-bottle. Be sure not to drop any of the acid on your hands or your clothes. As the acid is heavier than the milk, it will sink to the bottom of the bottle. With a gentle whirling motion, shake the bottle until the two fluids are thoroughly mixed. The mixture will turn a dark brown and become very warm.
Now fill the other bottles in the same way with samples drawn from different cows. Treat all the samples precisely as you did the first. Do not forget to put on each sample the name of the cow giving the milk and on each test-bottle a number corresponding to the name of the cow.
You are now ready to put the test-bottles in the sockets of the machine. Arrange the bottles in the sockets so that the whirling frame of the machine will be balanced. Fit the cover on the machine and turn the handle slowly. Gradually gain in speed until the machine is whirled rapidly. Continue the turning for about seven minutes at the speed stated in the book of directions.
After this first turning is finished, pour enough hot water into each test-bottle to cause the fat to rise to the neck of the bottle. Re-cover the machine and turn for one minute. Again add hot water to each bottle until all the fat rises into the neck of the bottle and again turn one minute.
There remains now only the reading of the record. On the neck of each bottle there are marks to measure the amount of fat. If the fat inside the tube reaches only from the lowest mark to the second mark, then there is only one per cent of fat in this cow's milk. This means that the owner of the cow gets only one pound of butter-fat from each hundred pounds of her milk. Such a cow would not be at all profitable to a butter-seller. If the fat in another test-bottle reaches from the lowest mark to the fourth mark, then you put in your record-book that this cow's milk contains four per cent of butter-fat. This record shows that the second cow's milk yields four pounds of fat to every hundred pounds of milk. This cow is three times more valuable to a butter-maker than the first cow. In the same way add one more per cent for each higher mark reached by the fat. Four and one-half per cent is a good record for a cow to make. Some cows yield as high as five or six per cent but they do not generally keep up this record all the year.
Fig. 274. Babcock Tester and How To Use It
The tester, acid, acid measure, test-bottle, and thermometer at bottom; filling the pipette on right; adding the acid and measuring the fat at top
The Babcock tester shows only the amount of pure butter-fat in the milk. It does not tell the exact amount of finished butter which is made from 100 pounds of milk. This is because butter contains a few other things in addition to pure butter-fat. Finished and salted butter weighs on an average about one sixth more than the fat shown by the tester. Hence to get the exact amount of butter in every 100 pounds of milk, you will have to add one sixth to the record shown by the tester. Suppose, for example, you took one sample from 600 pounds of milk and that your test showed 4 per cent of fat in every 100 pounds of milk. Then, as you had 600 pounds of milk, you would have 24 pounds of butter-fat. This fat, after it has been salted and after it has absorbed moisture as butter does, will gain one sixth in weight. As one sixth of 24 is 4, this new 4 pounds must be added to the weight of the butter-fat. Hence the 600 pounds of milk would produce about 28 pounds of butter.