448 PHYSIOLOGICAL CHEMISTRY. 



the action of the liberated acid on the casein by forming a lactate of 

 calcium or sodium. 



Of antiseptics, salicylic acid has been used with good results (2 

 grains to a pint), but it should be remembered that salicylic acid is 

 not absolutely harmless. 



Of all preservatives, cold is the most efficient and least objection- 

 able, and milk when cooled to within a few degrees of the freezing- 

 point may be kept for eight to twelve days sweet and without change. 



The condensation of milk is effected either simply by evaporating 

 (generally in vacuum pahs) a portion of the water, or by first dis- 

 solving in it a certain quantity of sugar (generally cane-sugar) and 

 then evaporating to the consistence of a thick syrup, which is placed 

 in suitable air-tight jars. The sugar which is added serves as an 

 additional preventive of decomposition. 



The following gives the constituents of milk which may be obtained 

 from it by mechanical processes after it has been changed as described 

 above : 



Cream, 20 j Butter 3.5 parts. 



I Buttermilk 16.5 " 



Skimmed milk, 80 { Curd 8 ' " 



I Whey 72.0 " 



Butter. Even in the thickest varieties of cream there is no 

 cohesion between the fat globules, whilst in butter the fat has actually 

 cohered. This change is accomplished by violently agitating (churn- 

 ing) the cream, when the fat particles gradually combine with each 

 other, whilst the liquid (buttermilk) separates. 



Chemically, butter is a milk-fat, .a mixture of different fats or 

 glycerides of the fatty acids, chiefly palmitic and oleic acids, with 

 small quantities of butyric, caprionic, caprylic, and stearic acids ; it 

 always contains a certain proportion, 15 or 16 per cent., of water, 

 besides traces of casein, salts, coloring matter, etc. 



For curing butter, common salt is often used ; the quantity added 

 should not exceed 5 per cent. 



The composition of buttermilk has been given above; when freshly 

 obtained from sweet cream it is a pleasant drink and a wholesome food. 



Cheese consists mainly of casein, milk-fat, water, and inorganic 

 salts ; these constituents vary as follows : 



Water 61 to 28 parts. 



Casein . . . 15 to 35 " 



Fat 20 to 30 " 



Salts. 4 to 7 " 



