Re: Why does toast taste so different from bread?

Two things happen principally when bread is "toasted":

1. Moisture loss. This makes a drier taste and shifts the texture from soft and ductile to firm and brittle.

2. An event called the "Maillard browning reaction", where the carbohydrates present in the starch and sugar reacts with the protein in the wheat under high heat to give a characteristic brown color and "burnt" taste.

This is also why a turkey will get brown on the surface even when cooked with moist heat in the oven. All meat contains sufficient residual carbohydrates to feed a browning reaction under sufficient time and temperature.

This reaction can also take place at low temperatures under low humidity for long times, and is a common spoilage mechanism for non-fat dry milk.