Combining heavier mixtures with beaten egg whites can knock the air out of them. To prevent this, before folding egg whites, begin by pouring the egg yolk mixture over the beaten whites, not vice versa. Then gradually combine the mixtures by folding, rather than stirring. Using a rubber spatula, start with a downward stroke into the bowl, continue across the bottom, up the side and over the top of the mixture. Come up through the center every few strokes and rotate the bowl often as you fold. Continue folding egg whites in this way just until the color of the mixture is uniform, with no streaks of white remaining. Fold gently to maintain volume.