This week we studied the properties of foams and emulsions using egg whites and egg yokes. Foams and emulsions are different types of dispersions in liquids. Foams, like meringue, consist of air bubbles dispersed in liquid egg whites. Emulsions, like mayonnaise, consist of fat globules of oil dispersed in liquid egg yokes. We saw experimentally how foams and emulsions are thermodynamically unstable. Because they consist of two essentially immiscible phases, they will separate back into two distinct phases over time. We saw how air bubbles could pop and deflate our meringues and how adding too much oil at once could prevent the formation of our mayonnaise.
Meringue: Egg yokes contain many different types of proteins. The proteins were denatured using acid (in our recipe, in the form of vinegar) so that they would unfold and be able to incorporate air. We made two versions of our recipe, one with powdered confectioners sugar and one with regular sugar. The batch made with powdered confectioners sugar was deflated and had a chewy-er texture than the batch made with regular sugar. Powdered confectioners sugar has added cornstarch, which absorbs moisture. Perhaps the less dissolved regular sugar in the second batch was able to better stabilize the egg foam.
Mayonnaise: Egg yokes contain lecithin, a fatty acid with both a polar hydrophilic end and a non-polar hydrophobic end. The egg yoke and oil mixture had to be mixed constantly and the oil poured extremely slowly in order to emulsify only 3 egg yokes with nearly 2 ½ cups of oil. Because the mayonnaise contains so much oil, the fat droplets are constantly pressed up against each other and can easily coalesce, ruining the emulsion.