Milk. 
287 
skimmctl milk is boiled. New milk gradually heated to nearly 
the boiling point of water throws up cream, whilst at the same 
time a skin of oxydized casein is formed on the surface. Thus 
in " Devonshire " or " clotted cream " we find more curd than in 
cream collected in the ordinary manner. 
The solubility of casein in milk is generally ascribed to the 
presence of a certain small proportion of free alkali. But though 
it is quite true that alkalies are excellent solvents for casein, and 
milk is frequently slightly alkaline, it may be questioned whether 
the solubilitv of casein is due to the presence of free alkali ; for 
even in milk which is slightly acid, and therefore does not con- 
lain any frfee alkali, all the curd occurs in a soluble form, nor 
does the addition to new milk of dilute acid in quantities which, 
though small, are sufficient in quantity to render it decidedly 
sour, cause the separation of casein. This takes place only after 
a large quantity of lactic acid has been formed spontaneously, or 
an excess of free acid has been put into the milk. 
The action of rennet on the soluble form in which casein occurs 
in milk is peculiar, and as yet unexplained. It was supposed 
for a long time that the rennet coagulated milk by converting the 
sugar of milk into lactic acid, and that the lactic acid, by neu- 
tralizing the free alkali, was in reality the agent in effecting the . 
separation of the curd in a coagulated condition. 
But this view is no longer tenable, for rennet at once coagu- 
lates new milk without turning it acid in the slightest degree. I 
have even purposely made milk alkaline, and yet separated the 
curd by rennet, and obtained a whey which had an alkaline re- 
action. (For detailed experimental evidence on this peculiar 
action of rennet on soluble casein the reader is referred to my 
paper on the Composition of Cheese, in vol. xxii., part i., of this 
Journal.) 
Curd exposed to air in a moist condition undergoes partial 
decomposition, and becomes a ferment, which rapidly decomposes 
a portion of the neutral fats of butter, separating from them 
butyric and other volatile fatty acids, which impart the bad flavour 
to rancid butter. Casein-ferment also rapidly converts milk- 
sugar into lactic acid. 
Like all albuminous substances, casein contains a large pro- 
portion of nitrogen, and is capable of producing flesh in the 
animal economy. 
Mulder does not mention phosphorus as a normal constituent 
of casein. I find, however, in it a considerable quantity of phos- 
phorus, and rather more sulphur than is given in Mulder's 
analysis. The following results were obtained by me on sub- 
mitting pure casein to an ultimate or elementary analysis : — • 
VOLi XXIV, "u 
