Can sugar really “cook” egg yolks?

Reader Lily writes:
READ ONJoe, the other day I left some egg yolks that I was about to whip into pastry cream sitting in a bowl with some sugar for too long. The yolk sides that were touching the sugar turned pale and hard. My instructor said that it was caused by sugar cooking the eggs because sugar and yolks together create heat. I’m skeptical, but what do you think?

