The Influence of Some Bacterial Enzymes on the Heat Coagulation of Milk

Abstract

The results of the first part of the work where only milk cultures of the sweet curdling organisms were used can be summed up as follows: Small amounts of acid inhibit heat coagulation over a small range. Larger amounts of acid increase heat coagulation. Small amounts of a milk culture of a sweet curdling organism inhibit heat coagulation over a small range. Larger amounts of a milk culture of a sweet curdling organism hasten heat coagulation. In the presence of little acid the addition of portions of a milk culture of a sweet curdling organism lowers the acidity at which heat coagulation will take place. In the presence of larger amounts of acid this action of the culture of a sweet curdling organism is obscured or eliminated. The results of the second part of the work when the rennin-like enzymes of certain bacteria were isolated and purified are as follows: The rennin-like enzymes of sweet curdling organisms were isolated and purified to some extent by methods described. Bacterial rennin is destroyed by a temperature of 60°C. for fifteen minutes. The purified bacterial rennin has no effect upon heat coagulation when the milk is heated immediately upon addition of the rennin. When the rennin has been allowed to act on the milk for a time, the heat coagulability of the milk is increased. The rate of this action of the rennin is dependent on the concentration or strength of the enzyme present. The rate of action of the rennin is dependent on the temperature at which the enzyme is allowed to act. A temperature of 40° to 43°C. is the optimum for most remains. An increase in temperature up to the optimum temperature hastens rennin action. Action goes on at a slow rate at temperatures as low as 10°C. The addition of acid in small amounts accelerates the rennin action. The presence of soluble calcium salts in small amounts accelerates the rennin action. The presence of citrates in small amounts retards the rennin action. The effect of acid plus rennin upon heat coagulation is additive. The effect of soluble calcium salts plus rennin upon heat coagulation is additive. A definite amount of curdling enzyme can cause heat coagulability only in a limited amount of milk. This limit of influence is the same whether the rennin is allowed to complete its action at its optimum temperature or autoclave temperatures are applied after the action is started. Chymosin resembles the bacterial rennins in its influence upon the heat coagulation of milk. Bacterial rennin may be an important factor in the coagulation of milk upon sterilization in the condenseries. The rennin may work in conjunction with acid in the milk or an excess of soluble calcium. © 1925, American Dairy Science Association. All rights reserved.