- Sweating is the first stage of sautéing onions or other
vegetables. As they slowly heat up, moisture from their
interior (onions are roughly 75 percent water by weight,
some other vegetables are even more watery) begins to
evaporate, forcing its way out of the cells and causing
them to rupture in the process. This breakdown of the cells
is what causes vegetables to soften.
- Enzymatic reactions take place as the contents
of the vegetable cells—a complex mix of sugars, proteins,
and aromatic compounds (in the case of onions,
mercaptans, disulfides, trisulfides, thiopenes, and other
such long, no-reason-to-memorize chemicals)—are spilled
out and begin to mix with each other.
- Caramelization begins to occur once most of the
liquid has evaporated and the temperature of the onions
starts creeping up into the 230°F-and-above zone. This
reaction involves the oxidation of sugar, which breaks
down and forms dozens of new compounds, adding color
and depth of flavor to the onions.
- Sweetening of the onions also takes place. The
large sugar molecule sucrose (aka white sugar) breaks
down into the smaller monosaccharides glucose and
fructose (the same two sugars that corn syrup is made of).
Since one glucose molecule plus one fructose molecule is
sweeter than a single sucrose molecule, the flavor of the
caramelized sugars is actually sweeter than the sugar they
started out as.
