Firmness
Firmness (6 mm deformation) was measured at four
different parts of five potatoes (two measurements at
the midpoint and two measurements at the top and
bottom of the potatoes) with an Instron Universal Testing
Instrument (Model 1132, Instron Corp., Canton,
OH, USA) for each time of storage. A 100 Newton (N)
load cell was used for firmness determination of the
potatoes. Cross-head speed was 10 mm/min. This
speed was selected considering that potatoes would
have to be cooked before eating and therefore what was
important is that the chosen speed allowed relative
changes of firmness of raw potatoes to be measured. A
7-mm diameter convex tip Magness-Taylor type probe
was used. This test measured individual tuber firmness
based on the resistance of the potato flesh to deformation
by the probe (Kader 1982).