68 MILK 



Tractive index in normal milk are small, but milk from diseased 

 cows shows considerable variation. Usually the index of refrac- 

 tion of milk from diseased cows is smaller than that of normal milk. 



Specific Heat. The specific heat of milk, as determined by 

 Fleischmann, is smaller than that of water 0.9457. Therefore, 

 it takes less ice to cool milk or less heat to warm milk than to cool 

 or heat an equal quantity of water. But since milk is a poorer 

 conductor for heat than water, it takes a longer time to reach a 

 certain temperature. The specific heat of cream is lower than 

 that of milk. 



The specific heat of milk has been exhaustively studied by 

 Hammer and Johnson. These authors think that earlier observa- 

 tions were at fault in not paying attention to the temperature 

 range over which the material is to be heated or cooled. They 

 give the following average specific heats of whole milk and cream : 



AVERAGE SPECIFIC HEATS 



OM5 0-40 0-60 15-40 40 -60' 15 J -ou 



Whole milk 0.929 0.940 0.935 0.947 0.923 0.937 



15 per cent, cream 0.837 0.900 0.889 0.940 0.899 0.925 



20 per cent, cream....- 0.831 0.894 0.890 0.936 0.881 0.916 



30 per cent, cream 0.830 0.883 0.875 0.925 0.854 0.899 



45 per cent, cream 0.832 0.866 0.843 0.901 0.786 0.858 



60 per cent, cream 0.843 0.851 0.816 0.876 0.727 0.821 



Bowen has determined the specific heat of a sample of milk 

 and of 20 and 40 per cent, cream. The composition of the samples 

 was: 



COMPOSITION OF MILK AND CREAM USED FOR DETERMINATION OF SPECIFIC HEAT 



Milk. 20 per cent. 40 per cent, 



cream. cream. 



Fat 3.5 per cent. 20 per cent. 40 per cent. 



Totalsolids 12.68 " 27.27 " 44.30 



Solids not fat. 9.18 " 7.27 " 430 



Water 87.32 72.73 55.70 " 



The specific heat of these samples is given in the following 

 table and Fig. 24. 



SPECIFIC HEAT OF MILK AND CREAM 



20 40 20 40 



Temperature. Milk. per cent, per cent. Temperature. Milk. per cent, per cent. 



. cream. cream. cream. cream. 



F. C. F. C. 



35.6 2.0 .... 0.88 .... 95.9 35.5 0.93 0.89 086 

 37.4 3.0 0.92 .... 0.83 100.4 38.0 .80 



43.7 6.5 .92 .91 .90 105.8 41.0 .92 .87 



48.2 9.0 .... .92 .... 109.4 43.0 .78 



51.8 11.0 .93 .... .96 114.8 46.0 .... .87 



' 55.4 13.0 .... .94 .... 118.4 48.0 .92 .... .78 



59.0 15.0 .94 .95 1.02 123.8 51.0 .... .86 



66.2 19.0 .95 1.01 1.07 127.4 53.0 .... .... .77 



71.6 22.0 .94 .95 .... 131.0 5o.O .93 .86 



75.2 24.0 .... .93 136.4 58.0 .... .... .76 



,78.8 26.0 .93 .... .88 141.8 61.0 .93 .87 



82.4 28.0 .... .91 .... 145.4 63.0 .... .72 



86.0 30.0 .92 .88 150.8 66 .94 



89.6 32.0 .91 



