We wanted to know if tempering really is all about temperature.
The traditional approach to preparing custards, puddings, and sauces, which rely on eggs for thickening power, requires tempering the eggs. The technique calls for heating the dairy, whisking a portion into the eggs (to which any sugar has usually been added), and then adding this mixture back to the pan with the rest of the hot dairy before cooking the mixture to the final temperature. Conventional cooking wisdom says that tempering prevents the eggs from seizing up into tight curds by allowing them to warm up gradually. We wondered if tempering really is all about temperature, so we set up a test to find out.
EXPERIMENT: We made three batches of crème anglaise, a custard sauce calling for egg yolks, milk, and sugar. For the control batch, we tempered the eggs the traditional way. For the second batch, we warmed the eggs with the sugar before adding them to the heated milk and proceeding as before. For the third batch, we mixed all the ingredients together at the outset and cooked the mixture to the final temperature. In all three cases, once the eggs were in the pot, we stirred the entire time to ensure that the eggs heated evenly from top to bottom.
RESULTS: The second batch, where the eggs were warmed before being combined with liquid, showed signs of curdling as soon as the eggs were stirred into the hot milk. The third batch was identical to the control, with the same smooth consistency and thickness.
EXPLANATION: Tempering doesn’t work because of the gradual heating of the yolks; it works because the addition of liquid dilutes the uncooked egg proteins, making it harder for them to link up and form firm clumps when heated. This was borne out by the third batch, where the egg proteins were diluted from the outset. The second batch failed because the eggs had not been diluted before they started cooking; they were able to link up before they could be dispersed throughout the mixture.
TAKEAWAY: The real benefit of tempering is that it reduces the amount of constant stirring necessary since the dairy is already heated by the time the eggs go into the pot, which jump-starts cooking. That said, it does require some back and forth and multiple bowls. If you forgo tempering and choose to put everything in the pot at the same time, just make sure to stir constantly from the outset to prevent the eggs from forming clumps.