THE MILK CONTRACTOR 263 



represent the difference between the test diagonally opposite, and the 

 desired test in the center. Then the figure in the upper right-hand 

 corner indicates the required amount of milk or cream of the test of that 

 in the upper left-hand corner, and the figure in the lower right-hand corner 

 the amount required, of the test in the lower left-hand corner. 



Problem. In what proportion should a 3 per cent, milk be mixed with 

 a 25 per cent, cream to get a 20 per cent, cream and how many pounds 

 of each would be necessary to make 75 Ib. of the light cream? 



Five, the difference between 20 and 25, and 17, the difference between 

 3 and 20, indicate respectively the number of parts of 3 per cent, milk and 

 the number of parts of 25 per cent, cream that should be used in blending 

 the 20 per cent, cream. Having determined this proportion, the quan- 

 tities of each required to make up 75 Ib. of the light cream is calculated 

 as follows : 



% 2 X 75 = 17.04 Ib. of 3 per cent. milk. 

 x %2 X 75 = 57.96 Ib. of 25 per cent, cream 

 17.04 + 57.96 = 75 



Various manuals on milk testing show how other problems can be solved 

 in a similar manner. 



Storage in Tanks. After standardization, the milk is ready for pas- 

 teurization or, if that process is omitted, for canning or bottling but if 

 the milk is not used at once it is stored in large brine-jacketed tanks where 

 it is held at a temperature sometimes as low as 35F. until needed. The 

 amount of milk thus held for use is sometimes very great; in one of the 

 Philadelphia milk plants there are four steel glass-lined tanks each of 

 which has a capacity of 20,000 gal. 



Preservation of Food by Heat. From primeval times, mankind has 

 applied heat to food in order to make it- palatable, digestible and safe, 

 but for only the past century and a half has he heated it with the definite 

 object of preserving it. Spallanzani in 1765 boiled meat extract for an 

 hour in hermetically sealed flasks and observed that thereafter no dis- 

 integration of the extract occurred. In 1782 Scheele advised that this 

 principle be applied to the preservation of vinegar by exposing it in 

 bottles to the temperature of boiling water. In 1804 Appert of Paris 

 discovered the process of canning and in 1811 published a treatise on 

 "The Art of Preserving Animal and Vegetable Substances." Durand, 

 in England in 1810, took out a patent for preserving certain foods in tins 

 and glass jars. In 1819-20 the pioneer canners of America, Daggett, 

 Kensett, Underwood and Mitchell made the small beginnings of what 

 has since developed into the canning industry. All of these men worked 

 empirically because the preservation of food by heat, depends on bacterio- 

 logical principles that were not understood till the researches which Louis 

 Pasteur carried on from 1860 to 1864, on the " diseases of wine" were 



