An oil obtained from the yolk of hard-boiled eggs by pressure or by solvents. Yolks of hen’s egg contains about 30% fatty matter. On account of its oily nature, small portions of yolk of egg left among the white are sufficient to prevent the whites from forming a “snow” when whisked. Meringue goods are sometimes spoiled from this cause.
From The Baker’s ABC by John Kirkland, formerly Head Teacher of National School of Baking, published 1927 by Gresham