When potatoes are fried, the oil temperatures reached are so high (more precisely, so much higher
than water boiling temperature) that a significant amount of the potatoes water will evaporate. At the
same time, starch granules in potato cells swell in the internal water of the cells, forming a puree
inside the cells.
For the external part of the samples being fried, water completely evaporates, forming the crust.
Finally fried potato samples have a soft, tender, puree inside, and a crackling crust.
As the chips are removed from the deep fat fryer, the water vapour will rapidly condense, and this will
reduce the chips internal pressure to such an extent that the oil that is stuck to the surface of the oil is
sucked into the chip.
When oil is added to boiled potatoes, it penetrates the potato tissue for very different reasons.
Significant water evaporation does not occur during boiling a potato, but when a potato is left to cool in
the presence of oil, the outer layers of the potato will cool and contract much more quickly than the
potatoes centre, so cracks will form in the outside of the potato. Oil can enter the potato tissue by
these cracks, and this also explains why potatoes to which oil is added when they are still hot will
absorb more oil than if the oil is added once they are cold, and the cracks have closed.
When frying or roasting potatoes it is often recommended that they are cooked in water first. This
allows them to form a superficial gelified layer and during subsequent frying this gel layer prevents the
starch granules from absorbing too much oil.
If they are not, the starch will not gelatinize and much more oil can enter. The oil will be sucked in due
to the reduced internal pressure in the fried potato as a result of the majority of the water evaporating.
III/VI - 2 (of 2)
Chips are often fried in two steps. This is because when lots of chips are placed in the same fryer of
oil, the hot oil is greatly cooled, especially if the fryer contains many chips and not much oil. Chips will