This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Concentration and valence of added salts. If the effect of electrolytes upon hydrophobic and hydrophilic colloids is referred to in Chapter I (The Relation Of Cookery To Colloid Chemistry), the statement is found that the precipitation of the protein is brought about by the ion having the opposite charge from that of the protein. In general, the coagulating power of the ion increases with increasing valence, but there are some exceptions to this rule, some monovalent ions being more effective than some polyvalent ions. In many cases there is a zone of maximum coagulating effect. The effect of the concentration of the salt upon the coagulation of the egg can be shown in the following way. If less than 1/16 teaspoon of ferric or aluminum chloride is added to an egg, a cup of distilled water and 2 tablespoons of sugar, the custard coagulates on heating to 84°C, though the coagulum is not so firm as when milk is used. However, if a larger quantity of ferric or aluminum chloride is added to the mixture, about 1/4 teaspoon or more, the custard does not coagulate when heated to 84°C. The reaction of the custard mixture with the small amount of aluminum or ferric chloride given above is slightly acid to litmus, and the larger quantities are decidedly acid to litmus. Hence, the larger quantity of ferric chloride must peptize the egg proteins.
Woodruff and Meyer found that sodium chloride, sodium sulfate, and calcium chloride, when used in 0.2 M concentration, produced gels of approximately equal strength and slightly weaker gels than milk. Magnesium chloride and sodium thiocyanate produced stronger gels than milk. They concluded that the difference in strength of gels formed by various electrolytes seemed to be a specific function of the cation and anion of the salt, and independent of the pH of the solution. Increasing the concentration of each salt gave gels of greater strength until a maximum strength was reached, after which further increases of the salt reduced the gel strength.
The relation of reaction to setting of custards. If hydrochloric acid is added to the egg-distilled-water custard, so that the reaction is adjusted to a pH above the isoelectric point of egg albumin, pH 4.8, coagulation occurs within the range of pH 5 to 6. The coagulum is soft and not quite so firm as when salts are added, but it shows that a definite acidity tends to aid coagulation.
The coagulation of egg-distilled-water custard with added salt is somewhat similar to the jellying of fruit jells in that it occurs at a definite range of pH.
Coagulation occurs over a wider range of pH when milk is used with the egg than when distilled water is used. If the pH is adjusted with hydrochloric acid and sodium hydroxide, it occurs from pH 0.2 to 8.6 or over even wider ranges. The reaction of a milk-egg custard mixture, with no added acid or alkali, is between pH 6 to 7 with an average of about 6.5. This custard mixture was made up in a large quantity, and divided into different portions. To these portions hydrochloric acid or sodium hydroxide was added to adjust the custard to the desired pH. As the pH was lowered the firmness of the custard increased, until at about pH 5 curdling occurred. All custards with a pH below 5 curdled, the curd becoming very fine in texture, and forming a dense layer in the bottom of the container. This layer decreased in amount as the pH was lowered below the isoelectric point of egg albumin. At a pH 0.2 the custard was badly charred, and the curd was very slight and fine, the custard quite soft. With increased alkalinity above pH 6.5 the coagulum was less firm. The custards could be placed in the order of acidity by the depth of color: the greater the acidity the lighter the color, the greater the alkalinity the deeper the color. The color ranged from light cream, through yellow, to a deep orange-yellow.
Acids. Chick and Martin state that acid solution hastens the second part of the heat-coagulation process, that is, the clotting or coagulation, but does not hasten the first part of the process, the denaturation. They have reported that acid accelerates the rate of coagulation. They state that the influence of acid in accelerating the coagulation rate of a neutral solution is at first relatively small, but with each successive addition of acid its influence becomes disproportionately greater. Loeb using isoelectric crystalline egg albumin in a 1 per cent solution at a pH 4.8 found that it coagulated at a temperature not far from 60°C. When acid was added and the pH lowered to 4.39 the coagulation did not occur until about 80°C. With pH 4.25 coagulation did not occur at 95°C.
Fruits that do not have a high acidity such as dates and figs may be used in custards and tend to give a firmer custard, because they lower the pH slightly. More acid fruits, such as lemon juice, cannot be used in very large quantities. The addition of quite acid fruit juices tends not only to coagulate the casein and albumin but also to hasten curdling. Custards that are made of milk that is slightly sour will curdle more readily during heating. If the acidity has not reached the stage at which curdling occurs, the custard is firmer.
Alkalies. Chick and Martin have found that in alkaline solution the second part of the coagulation process, the aggregation or coagulation of the protein, does not occur. If after heating the alkali is neutralized with acid, coagulation occurs. Thus, if enough of an alkali or of an alkaline salt is added to a custard to render the solution sufficiently alkaline, the custard will not coagulate on heating. But if acidified after heating the custard will "set."
Sugar. The addition of the non-electrolyte sugar to an egg mixture elevates both the setting and curdling temperatures. In large enough quantities it tends to prevent both coagulation and curdling. Its effect in preventing coagulation appears to be proportional to the amount added, the greater the amount added the greater the difficulty in bringing about coagulation. Bancroft and Rutzler state that a "15-per cent egg white sol was not prevented from coagulating by the addition of 0.25 gram of dextrose to 10 cc. of the sol. However, when the sol was saturated with respect to dextrose heating in boiling water caused no coagulation."
Woodruff and Meyer found that 10 per cent of sucrose reduced the gel strength of egg-milk custards heated to 83°C, approximately one-half. Adding 30 per cent of sugar practically prevented coagulation. Adding sugar also increased the translucency of the custard.
In the salad-dressing recipe given in Experiment 65, increasing the sugar from 1/2 to 11 tablespoons (140 grams, about 30 per cent) elevates the temperature for optimum thickness about 4° to 6°C. Here in spite of the fact that the acid tends to lower the coagulation temperature the effect of the large quantity of sugar is still greater and the mixture must be cooked to a higher temperature for optimum thickening.
Coagulation by other means. Flosdorf and Chambers found that audible sound, frequencies (1000-15,000), coagulated solutions of egg albumin and synthetic plasterin almost instantly at 30°C.
Jellinek placed a raw egg between two condenser plates connected to a short-wave radio transmitter. After power of 1000 watts had been applied for 5 minutes, the egg yolk was coagulated and hard, as if it had been cooked, but the white was scarcely affected. The temperature of the yolk at the end of this period was 140°F., that of the white was 176°F.