104 REPORTS ON THE STATE-OF SCIENCE.—1918. 
of the CaCl, with the alkali phosphate and citrate in the milk, thus 
reducing the content of these, and rendering the casein less soluble. 
During the action of rennet there is no change in the hydrogen 
ion concentration, but this is raised by addition of soluble calcium 
salts, and decreased by addition of ammonium oxalate. It is 
interesting in this connection to recall the fact that rennet acts best 
in acid solution, i.e.. ia the presence of hydrogen ions. It was 
shown by Van Dam by the determination of electrical conductivity 
that the coagulation time varied inversely as the hydrogen ion 
concentration, see Zeit. physiol. Chem., 58, 295 (1908). 
Mellanby has advanced an interesting theory for the rennet 
coagulation of milk, assuming it is due to the adsorption of the 
enzyme by the casein, and the subsequent precipitation of the 
enzyme-casein complex by the bivalent Ca ions of the milk. The 
quantity of ionised Ca salt required to effect precipitation is 
intimately related to the quantity of the enzyme adsorbed. Cf. 
Jour. of Physiol. 45, 345 (1912). 
Bang observed that by adding rennet in fractions to milk, 
summation is noted, e.g., the coagulation time on adding 0-1 ce. of 
rennet to 10 cc. milk is 8 minutes, and on adding 0:2 cc. four 
minutes. If, however, 0°1 cc. is added, and 4 minutes later 
another 0°1 cc., then clotting occurs two minutes after the second 
addition. No such summation occurs if the milk, after the first 
addition of rennet, is heated to 65° C. 
Bang’s many experiments led him to conclude that rennet is 
not a coagulating enzyme, since rennet of itself does not produce 
curdling, and the final act of curdling is not true coagulation, but 
is more akin to the precipitation of protein by neutral salts. Cf. 
Ivar Bang, Skand. Archiv. Physiol. (1911), 25, 105-144. 
Another conception of the rennet coagulation of milk is 
advanced by Schryver, Proc. Roy. Soc. (1913), B. 86, pp. 460-481. 
When solutions of calcium salts and sodium cholate are mixed, a 
clot results on heating. For salts which increase the surface 
tension of water, the greater their amount present, the shorter will 
be the time required for clotting to take place. Salts which 
decrease the surface tension decrease the time for coagulation, only 
up to a certain limit of concentration, above which the time is 
increased, or the coagulation entirely prevented. The inhibition 
of coagulation is attributed to the adsorption of simple molecules 
by the more complex colloids which are thereby prevented from 
coalescing. In milk the necessary materials exist, but the adgsorp- 
tion of simple molecules from the solution stops the coalescing. 
Schryver assumes that the enzyme clears the surface of the colloid 
from the absorbed substances thus permitting coalescence to occur. 
He asserts that in milk the’clot formation depends on the presence 
of four series of substances present, viz., simple inhibitory sub- 
stances, colloids, enzymes, and calcium salts. 
Alexander advanced a somewhat similar notion, his view being 
that the casein is “protected” or held in stable suspension by the 
hydrophile colloid, lact-albumin, which rennet destroys, thus permit- 
ting the coalescence and precipitation of the casein particles. Cf. 8th 
Int. Cong. App. Chem., 6, 12-14 (1912). 
