Sugar

Granulated white sucrose, at a crystal of about 0.5 to 1.0 mm. In a custard, sugar does more than sweeten. It binds free water and obstructs protein from meeting protein, which raises the temperature at which egg sets by roughly 1 degree C for every 3 percent sugar by weight. That protection is local: it only works where the sugar has dissolved. Undissolved crystals leave pockets of unprotected water that set before the rest. Under heat alone it melts near 186 degrees C and decomposes into the compounds that taste of caramel. Cooled fast it sets as an amorphous glass. That glass takes up water, so a caramel crust on a wet surface softens within the hour.