MILK. 



359 



chemical properties of the constituents of the 

 fluid. 



The fatty matter of milk obtained by churn- 

 ing cream, and which is known by the name 

 of butter, differs from the other forms of animal 

 fat in several particulars. It yields about 88.5 

 per cent, of fixed acids on being saponified, for 

 which purpose it requires no more than four- 

 tenths of its weight of caustic potassa ; it there- 

 fore unites with alkali very easily. Of these 

 acids the margaric and oleic are in large propor- 

 tion, the stearic existing as a mere trace. Glyce- 

 rine, as is the case with other fats, is a constant 

 prod uct of the saponification of butler. The great 

 distinguishing peculiarity of this form of fatty 

 matter consists in the production of three volatile 

 acids as results of saponification; these have 

 been carefully examined and distinguished by 

 Chevreul in his admirable work, " Sur les 

 Corps gras." He has named them the butyric, 

 caproic, and capric acids. The production of 

 these acids by the action of alkali has been 

 traced by Chevreul to the existence of a new 

 form of fat which he detected in butter mixed 

 with the stearine and elain, and to which he 

 gave the name of butyrine : thus butter may 

 be regarded as composed of three different 

 kinds of fatty matter stearine, elain, and 

 butyrine the two former yielding by saponifi- 

 cation the margaric, oleic, and stearic acids, 

 and the latter the three volatile acids above 

 mentioned. The proportions of the three kinds 

 of fat vary considerably in different specimens 

 of butter. The solubility of butter in alcohol 

 is stated by Chevreul to be 3.46 parls in 100 at 

 a boiling temperature, the specific gravity of 

 the menstruum being 0.822. The stearine 

 obtained from the alcohol by cooling is more 

 crystalline and of a more brilliant white than 

 that obtained from common fat, and 1.45 parts 

 require 100 parts of alcohol of specific gravity 

 0.822 for its solution. The elain obtained from 

 butter possesses no peculiar characteristics. 

 The butyrine when separated from it, which is 

 only to be accomplished with difficulty, pos- 

 sesses the following qualities : it is an oil 

 generally of a yellow colour, but some speci- 

 mens of butter yield it perfectly white; it con- 

 cretes at 32 Fahrenheit, and possesses the 

 smell and taste of butter; it mixes with boiling 

 alcohol in all proportions; it is soluble in 

 anhydrous alcohol. Potassa and the other 

 alkalies are not the only substances capable of 

 producing the volatile acids by acting on buty- 

 rine. Alcohol if long digested produces a 

 similar effect, as does strong sulphuric acid, 

 and if bulyrine be allowed to putrify these 

 acids are developed. 



The casein or cheesy matter of milk which is 

 obtained with some slight admixture of the 

 fatty matter in the production of cheese from 

 the skimmed milk has the following chemical 

 properties. It is soluble in water after long 

 digestion; but this is most likely owing to 

 some decomposition which occurs in it, and it 

 is certain that casein in its pure, undecomposed, 

 and dry state is quite insoluble in water. 

 Casein, as it exists dissolved in skim milk, is 

 precipitable by the mineral acids and also by 



the acetic acid. The process which Berzelius 

 recommends in order to obtain this substance 

 is as follows : Skim milk is to be mixed with 

 a small proportion of dilute sulphuric acid, 

 which unites with the casein and precipitates 

 it in the form of a white clot. This is to be 

 well washed with distilled water on a filter in 

 order to separate the whey which it contains. 

 After this carbonate of baryta and water are to 

 be mixed up with the mass, by which means 

 the acid is separated and the casein remains 

 dissolved in the water, and may be separated 

 from the carbonate and sulphate of baryta by 

 filtration. Casein obtained by this process is 

 more or less soluble in water, and is precipi- 

 tated from its aqueous solution by the acids. 

 It rapidly undergoes the putrefactive fer- 

 mentation. It is soluble in the alkalies and 

 in alcohol both boiling and cold, but far more 

 so in the former, from which it rapidly deposits 

 on cooling. Casein, when dissolved by the 

 assistance of the acids, is precipitated by the 

 ferro-cyanide of potassium. It is distinguished 

 from albumen, with which it possesses many 

 physical and chemical properties in common, 

 by being precipitated from solution on the ad- 

 dition of acetic acid, and the precipitate so 

 formed being with difficulty soluble in an 

 excess of the precipitant. It must not be 

 imagined, however, that albumen under all 

 circumstances cannot be precipitated from so- 

 lution by the addition of acetic acid, for when 

 dissolved in an alkaline solution, that proxi- 

 mate principle is immediately thrown down on 

 the addition of the acid. Casein, like albu- 

 men, always contains some sulphur as a neces- 

 sary element in its composition ; the presence 

 of this body may be easily shown by boiling 

 casein in a concentrated solution of potassa, 

 when the liquor rapidly assumes a brown 

 colour, and gives out ammonia, an alkaline 

 hydro- sulphuret remaining dissolved, which 

 may be proved by the solution becoming of a 

 deep black colour on the addition of a salt of 

 lead. The aqueous solution of casein is pre- 

 cipitated by all the earthy and metallic salts 

 which precipitate albumen in the dissolved 

 state. Tannin precipitates it even from its 

 solution in alcohol, notwithstanding that men- 

 struum protects it from the precipitating action 

 of the acids. The ultimate analysis of casein 

 is, according to Thenard and Gay Lussac, 

 carbon 59.781, nitrogen 21.381, hydrogen 

 7.429, and oxygen 11.409. Caseous matter, 

 as precipitated by rennet in making cheese, is 

 liable to a peculiar kind of putrefaction, which 

 has been investigated by Proust and Braconnot; 

 the latter obtained as a product of putrefaction 

 a peculiar crystalline substance, to which he 

 gave the name of aposepedine, from O.TTO and 

 o-ri'TriSuii, indicative of its origin. Proust had 

 before noticed this substance and called it 

 caseous oxide. It may be prepared very easily 

 by allowing cheese to putrify under water and 

 evaporating the solution so obtained to dry- 

 ness ; the dried mass is then to be treated with 

 alcohol until that menstruum exerts no further 

 solvent action ; the portion insoluble in alcohol, 

 on being dissolved in water, and digested with 



