Vinegar, lemon juice, or a mixture of the two, is used in cooked salad dressings. The two acids do not behave alike, particularly in regard to curdling. The lemon juice contains citric acid and salts. Vinegar contains acetic acid, and cider vinegar contains salts. If mustard, sugar, and salt are kept constant in the recipe given in Experiment 65 with 72 grams of egg yolk and a total of a cup of liquid, results similar to the following may be obtained, provided the rate of heating is the same in each case. The mixture is cooked in the upper part of a double boiler, 12 to 15 minutes being required for the cooking process.

Water

about

Coats spoon

°C.

Vinegar

1/8 cup

7/8 cup

4.2

82 to 85

Vinegar

1/4 cup

3/4 cup

3.9

78 to 80

Vinegar

1/2 cup

1/2 cup

3.6

76 to 78

Lemon juice

1/8 cup

7/8 cup

3.6

82 to 85

Lemon juice

1/4 cup

3/4 cup

3.3

78 to 80

Lemon juice

1/2 cup

1/2 cup

2.8

76 to 78

One-fourth cup of either the vinegar or lemon juice gives a stiffer or thicker dressing than 1/8 cup; 1/2 cup of vinegar or lemon juice gives a thicker dressing than 1/4 cup. These results seem to agree with those of Chick and Martin that acid aids coagulation. Increasing the quantity of acid lowers the temperature at which the consistency for serving is reached. The optimum dressing for serving is obtained at a temperature slightly higher than the temperature for "coating the spoon." The salad dressings become thinner after heating above this optimum temperature for serving; the ones with lemon juice are thinner than those with the vinegar. Another peculiarity is that if a portion of these salad dressings is cooked to as high a temperature as 92°C, all those made with vinegar may or may not curdle. The ones with lemon juice may not curdle, or those with the smaller quantity of lemon juice may curdle at about 85°C, but when heated to a higher temperature the curds may partially or entirely disappear. In all these instances the reaction of the dressings is probably below the isoelectric point of ovovitellin. If heated only to the temperature at which the dressings are thickest, the thinning is not noticeable if the dressing is stored.

If 92 grams of whole egg are substituted for the egg yolk, the pH remains practically the same, but the dressing when cooked contains fine curds, those in the dressings made with lemon juice being finer than those in the ones made with vinegar. Increasing the proportion of salt produces a thicker product at a slightly lower temperature but one which curdles at a lower temperature. Increasing the sugar in the recipe slightly elevates the temperature at which the best texture for serving is obtained.

The explanation for the salad dressing's becoming thin and nearly its original consistency when heated above the temperature at which the optimum thickening occurs is that the acid and continued application of heat bring about peptization of the protein. It is probably an illustration of a partial or complete reversibility of heat coagulation and one of the most common occurring in food preparation. It is probably the reason that usually only one or no recipe for boiled salad dressing containing only egg yolk as the thickening ingredient is given in most cook books. Too many cooks have had the salad dressing return to its original consistency.

Salad dressing containing starch. If maximum thickening of both starch and egg is desired when these two ingredients are used together in a boiled salad dressing, the starch and all, or a part, of the liquid should be heated to 95°C. or boiling before the egg is added. The maximum thickening of cornstarch does not occur below 91° and of wheat starch below 95°C. Since the maximum thickening of the egg in the presence of vinegar or lemon juice occurs at temperatures of 76° to 85°C. and thinning occurs at higher temperature, this suggests that the thickened starch paste should be added to the cold beaten egg and then heated to a temperature that gives maximum thickening of the egg.

On the other hand, there is the possibility that part of the egg protein may not coagulate in the presence of starch, hence not thicken the salad dressing. Bancroft and Rutzler, quoting Berlinsson, state that "albumin is prevented from coagulating even in boiling water by the presence of starch." However, this appears contrary to observed results in food preparation. Possibly there needs to be a certain concentration of starch before coagulation is prevented or peptization of the egg protein may occur when heated above the temperature for coagulation of the egg protein. It will, at least, be an interesting point to investigate.