Experimental cookery

Denaturation. Denaturation, by which soluble proteins are rendered insoluble, of egg proteins is brought about in a variety of ways, including the action of acids, salts, heat, mechanical agitation, and radiation. Mechan- ical agitation or beating of egg white, as well as the tendency of proteins in surfaces to form films, causes partial denaturation of the egg proteins. Sugar tends to prevent this denaturation. The theories for heat coagulation have been considered in Chapter I. But, however the process of coagulation is brought about, the coagulation temperature, the time required for coagulation, and the factors that cause HEAT COAGULATION OF EGG PROTEINS 333 variation in coagulation temperature are of interest in egg cookery, because they determine the temperature to which certain dishes, such as custards and cooked salad dressings, may be heated. If heated beyond this point, separation into solid and liquid may result and curdling occurs, or the salad dressing may become thinner. Coagulation temperature of egg white. Undiluted egg white coagu- lates at about 60 °C. or becomes jelly-like at this temperature. Coagulation may start at a slightly lower temperature, but the amount coagulated at the lower temperature depends on how long the egg is held at that point. In coagulating, the egg white changes from a clear, transparent mass to a white and opaque one. If the egg white is heated slowly, a point is reached — about 62°C. — at which it will not flow in a test tube. It is still firmer at 64° to 65 °C. In cooking, the temperature is seldom held a long time at a definite degree but may rise gradually and often rapidly. Coagulation temperature o£ the egg yolk. The egg yolk requires a higher temperature for coagulation than the egg white. It begins to thicken at about 65 °C. but does not reach a stage at which it does not flow until about 70°. Since the yolk does not change color during coagulation it is more difficult to determine when it is coagulated. When the white and yolk are mixed by beating and are heated slowly the mass begins to thicken at 65°C. and becomes stiffer not far from 70°. Factors affecting heat coagulation of egg proteins. The follow- ing factors affect the coagulation temperature of the egg proteins : ( 1 ) temperature, (2) time, (3) concentration of protein, (4) salt content and its concentration, (5) reaction of the egg solution or mixture, and (6) sugar. Temperature and time. The rate of coagulation increases with increas- ing temperature. At high temperatures it is so rapid that it seems nearly instantaneous. Eggs cooked in boiling water will cook at a much faster rate or in a shorter time than those cooked in water at 70 °C. Chick and Martin have found that "heat coagulation is a reaction with a high tem- perature coefficient, the reaction velocity of which varies considerably with different proteins and according to the acidity and saline content of the solution." They have reported that the temperature coefficient for crystal- line egg albumin in water solution and 1 per cent concentration is 1.9 times for 1°C. rise in temperature. The temperature coefficient is greater than 2 for the results of Lepeschkin given below.