A lot of recipes call only for egg whites, but that doesn't mean you need to throw away all of the yolks.
When we’re whipping up egg whites for meringues or angel food cake, we hate throwing out the leftover yolks. But freezing yolks for future use is problematic. The water they contain forms ice crystals, causing their proteins to cluster together in tight groups that don’t easily come apart, even once the yolks return to room temperature. The result: yolks that remain solid even after thawing and produce baked goods with hard, gelatinous flecks.
To get around this problem, commercial bake shops often use frozen yolks treated with a sugar-based “cryoprotectant” that interferes with ice-crystal formation and prevents proteins from clumping. We wondered if adding a bit of sugar to yolks before freezing would work in the same way, but this had no effect. On the advice of our science editor (who guessed the granulated sugar had not dissolved sufficiently to protect the yolks), we made a syrup of 2 parts sugar to 1 part water and stirred it into the yolks (using ¾ teaspoon syrup per 4 yolks). After storing our syrup-treated yolks in the freezer for two weeks, we used them to make flan and madeleines, comparing them with the same items made with fresh yolks.
The batches were remarkably similar, with our tasters hard pressed to tell which flan or madeleine came from the fresh or thawed yolks. With this simple trick, we’ll never have to throw out leftover yolks.