These cells are held together with pectin, a sort of natural
plant glue, and the walls of the cells are where the starch is
concentrated.
The starch molecules—a type of carbohydrate—come
bundled up in tight granules. As potatoes cook, the pectin
breaks down and individual cells expand and separate,
releasing starch granules into the outside environment.
These starch granules absorb water like little balloons,
eventually popping and releasing sticky starch molecules.
The concentration of released starch that makes its way into
the final mashed potatoes to a large degree determines their
consistency.
To put it simply: for lighter, fluffier potatoes, the goal is to
incorporate as little starch as possible in the final product.
- Potato type (see here–here) plays a huge role in this.
Mealy russet potatoes have cells that readily fall apart
from each other, meaning you don’t have to cook them or
work them too hard to get them to a relatively smooth
consistency. Less working means fewer burst starch
granules, which means fluffier mashed potatoes. Waxier
Yukon Golds or Red Bliss require longer cooking, and
they must be worked fairly hard to separate their cells,
making for creamier mashed potatoes. - The mashing method can drastically alter your end
results. Carefully pressing the potatoes through a tamis
(drum sieve), ricer, or food mill will separate the cells with
minimal shearing action to break up the starch. Throw
potatoes in a food processor, and an avalanche of starch
gets released, turning your potatoes into the consistency of