The potato samples (Marbelle) were heated in hot water at 90 C
in a cooking vessel for intervals of 60 s from 60 to 720 s. At each
minute a potato was taken out, cut in half and immediately cooled
in ice water bath containing 4% citric acid (the citric acid prevents
later enzymatic browning in the raw part of the potato). After cooling,
the potato samples were dried for half an hour at room temperature.
One of the two halves of each potato sample was sliced
in approximately 2 mm thickness parallel to the cut surface; this
slice was used for measuring the progression of the gelatinization
front as follows: the sample was photographed using a Canon 500D
camera with Macro lens EF-S 60 mm 1:2.8 USM. The gelatinization
front was clearly visible (see Fig. 2 later), and its distance from the
surface, dfs, was determined from the images using Adobe
Photoshop CS4. The distance was measured at 30 intervals around
the periphery of the potato, and the distance was reported as the
average of these 12 measurements. The length and width
(2 major axis and 2 minor axis) of the approximately elliptic
shape of the sample, were also measured.
Iodine staining: To demonstrate that the visible front resulted
from gelatinization, another half of a potato sample was also sliced
and stained by placing it in Lugol’s iodine solution for one minute,
followed by rinsing in distilled water for one minute. The composition
of the Lugol’s iodine solution was 2 g KI in 50 ml distilled
water and adding 0.2 ml iodine stock solution (Lamberg and
Olsson, 1989). The stained sample was photographed as described
above, and its image was compared with the image of the other,
unstained sample of the same potato.
3.