100 REPORTS ON THE STATE OF SCIENCE.—1918. 
degrees until that particular moment arrives when the butter particles 
suddenly become visible. There is still room for considerable 
physico-chemical research in connection with this long-known but 
little-elucidated phenomenon. 
The butter nuclei are worked up to a homogeneous mass, which 
the microscope shows to be a solid emulsion of fat, with fat the 
continuous medium, and water the disperse phase, an exact reverse 
of the system occurring in milk. (Compare section on “ Margarine.” 
The physical conditions of churning, especially the temperature, 
exert a profound influence on the butter, particularly with regard to 
its moisture content. The main factors to be considered in this con- 
nection are the fat content, the acidity, and the viscosity of the 
cream, and the agitation employed, A cream containing 30 to 45 
per cent. of fat churned at a temperature ranging from 13°C. to 18°C. 
should give good results. As cream ripens, lactic acid is produced, 
the viscosity of the cream diminishes and churning becomes easier. 
In this connection it is interesting to note that acids tend to make 
emulsions “ break,” 7.¢e., separate into oil and water, probably by some 
action on the protective (emulsifying) agent present. Here the 
casein is coagulated and precipitated if the acidity is too pronounced, 
and casein clots may be found in the butter mass. Churning must 
not be conducted too rapidly and violently, for then the moisture 
content will be too high owing to the enclosure of buttermilk within 
the:nuclei. Also churning must be stopped as soon as the butter 
granules reach the size of small peas, otherwise the granules will 
coalesce and retain an excess of butter or which cannot be washed 
out again. 
The physical chemistry of butter as a solid emulsion has been 
little studied, as indeed have any cases of solid emulsions, a matter 
which is referred to in more detail when discussing “‘ Margarine.” 
Casein. 
Casein is a phosphoprotein occurring to the extent of about 
3 per cent. in cows’ milk and about 1 per cent. in human milk. It 
is combined with the phosphates of the alkaline earths yielding a 
pseudo-solution or very fine colloidal suspension. Some writers 
adopt Halliburton’s nomenclature and use the name “ caseinogen,” 
reserving the term “casein”? for the curd produced by the action of 
rennet on milk. In this paper the term “casein”’ is used throughout: 
(following Hammarsten). 
Casein is precipitated from milk by saturating with NaCl, 
MgsS0O4, (NH4)2SO0,, and mineral acids; also by tannin, metaphos- 
paoric acid, phosphotungstic acid, CuSO,, ZnSO, and by rennet. It 
is soluble in alkalies and in solutions of salts which hydrolyse to © 
alkaline solutions, e.g., phosphates, and it is insoluble in alcohol and 
ether. 
When pure, casein is a white amorphous, odourless, tasteless solid, 
soluble in water to about 0:1 per cent. Though precipitated by 
mineral acids, it redissolves in excess of acid. Opalescent solutions — 
are obtained when casein is dissolved in just the necessary quantity 
of sodium phosphate and a little CaCl, ; some investigators believe 
this condition prevails in milk itself. 
