The browning of bread crust and of toast involves the first three mechanisms acting simultaneously. Sugars released into the dough by the action of yeast, or painted onto the loaf, undergo caramelization. On the dry outside of the loaf starch breaks down into pyrodextrins. In the moister conditions just below the surface, sugar-amine browning also takes place. The same three, and especially the last two, occur when breadcrumbs are sprinkled on top of a dish and heated to produce a brown crust over it.