On Food and Cooking

for vegetables meant to hold their shape in a
long-cooked meat dish, or potatoes in a potato
salad, or for foods to be preserved by canning.
It’s also valuable for boiled whole potatoes
and beets, whose outer regions are inevitably
over-softened and may begin to disintegrate
while the centers cook through. These and
other long-cooked root vegetables are usually
started in cold water, so that the outer regions
will firm up during the slow temperature rise.
Firm-able vegetables and fruits have an
enzyme in their cell walls that becomes
activated at around 120ºF/50ºC (and
inactivated above 160ºF/70ºC), and alters the
cell-wall pectins so that they’re more easily
cross-linked by calcium ions. At the same
time, calcium ions are being released as the
cell contents leak through damaged
membranes, and they cross-link the pectin so
that it will be much more resistant to removal
or breakdown at boiling temperatures.