Experimental cookery

clotting is brought about in two steps, the first being the action of rennin on the casein and the second the precipitation of the changed casein. Rogers reviews the many theories of rennin coagulation. Some investigators claim the changes are purely chemical; others maintain the rennin affects only the physical state of the calcium caseinate. However, if the change can be explained on the basis of colloid chemistry, it is probable that absorption and the electric charge play an important role in the process. Rogers states that Hammarsten regards casein in milk as a calcium caseinate-calcium phosphate complex. '*As a matter of fact the compound called calcium caseinate is most probably a true calcium phosphocaseinate, if, as seems likely, the second and third hydrogens of the orthophosphoric acid esterfied with certain of the amino acids in the casein molecule react with calcium. The correct conception of the term 'calcium phosphocaseinate,' as it is now commonly employed, is that of a colloidal calcium phosphate (or phosphates) sol protected by a calcium caseinate (or caseinates) sol in a manner as yet imperfectly understood." The stabilization of sols is best explained by the theory of Helmholtz, i.e., each colloidal particle is sur- rounded by an electrical double layer. ''In the case of negatively charged sols, in which class calcium caseinate and calcium paracaseinate evidently fall, the outer layer consists of hydrogen ions. If these are replaced by a sufficient number of positively charged ions of higher charge, e.g., calcium ions carrying two positive charges" ; or, in other words, if these ions are more strongly adsorbed than the hydrogen ions, the colloid particle will readily precipitate, the rate of clotting being determined by the rate of replacement. Richardson and Palmer state that rennin itself may reduce the charge of the calcium caseinate micelle and thus reduce the stability of the casein sol. They found indications that the isoelectric point of rennin is about pH 6.9 to 7.0. Above this pH the rennin is negatively charged and below pH 6.9 it is positively charged. They found that rennin lowered the electro- phoretic velocity of calcium caseinate and calcium phosphocaseinate micelles when the casein sol was negatively charged and the rennin was positively charged, but not when the rennin was negatively charged (above its iso- electric point, pK 6.9 to 7.0). From this evidence and from the fact that paracaseinate micelles are not affected by rennin, which agrees with the fact that casein once coagulated by rennin has lost its sensitiveness to this enzyme, they suggest that rennin acts by sensitizing the casein by a pre- liminary reduction of the electric charge on the casein micelles. During the clotting of the milk, aside from the consistency of the milk, there is little change in its physical properties. The hydrogen-ion concen- tration does not change during the clotting process. Factors affecting action of rennin. Several factors influence the activity of the rennin in bringing about coagulation. These may be listed as follows: (1) temperature for rennin action; (2) heating the milk before the addition of rennin; (3) hydrogen-ion concentration; (4) concentration FACTORS AFFECTING ACTION OF RENNIN 303 of casein, calcium, and phosphate ion; (5) character of cations used for coagulation.