Tuber Physiological Disorders - The Potato: Botany, Production and Uses - page 245

14.10 Bruise

There are three types of bruising: blackspot
bruise and shatter bruise, caused by physical im-
pacts during harvest and handling, and pressure
bruise, which results from the weight and pres-
sure of other tubers or container surfaces during
storage, when tubers lose weight due to mois-
ture loss.
Blackspot (also called internal black spot or
blue spot) bruise appears as a small to large blu-
ish-gray or black discolored area just beneath
the skin of the tuber in the perimedullary tissue
( Fig. 14.4) . There are no external symptoms and
damage is visible only when tubers are peeled.
When tubers bounce against hard surfaces with
enough force, membranes rupture and allow the
enzyme, polyphenol oxidase (PPO), to come into
contact with the phenol tyrosine in the intercel-
lular space. The PPO oxidizes the phenols to
form the black pigment, melanin (Dean et al .,
1993; Dean, 1996). The damaged area turns
pink to reddish-brown first, then grayish-black
as the melanin forms. At 21°C, the discoloration
begins to appear in 6 h and is complete by 24 h
(Strand, 2006).
Cultivars vary in susceptibility to blackspot
bruise, and blackspot bruise susceptibility has
been correlated with tuber tyrosine level (Dean
et al ., 1993; McNabnay et al ., 1999) and tuber
cell size (Hudson, 1975). The stem end of the
tuber (Kunkel and Gardner, 1959), large tubers,
tubers with high specific gravity, dehydrated or
non-turgid tubers, and very mature tubers are
more susceptible to blackspot bruise (Brook,
1996; Corsini et al ., 1999; Karlsson et al ., 2006).
Inadequate potassium levels increase tuber
blackspot bruise susceptibility (McNabnay et al .,
1999). Tubers grown on sandy or loamy soils
tend to have more blackspot bruise due to the
lower water-holding capacity of the soil (Corsini
et al ., 1999). Immature tubers harvested from
green vines are more resistant to blackspot
bruise than more mature tubers (Pavek et al .,
1985). Blackspot bruise susceptibility increases
during storage (Corsini et al. , 1999).
Karlsson et al . (2006) studied tuber calcium
levels and blackspot bruise incidence. Two culti-
vars with consistently low tuber calcium con-
centrations, Atlantic and Snowden, had high
incidence of bruise, while Dark Red Norland
and Superior had low incidence of bruise and

Fig. 14.4. Blackspot bruise damage on a peeled
potato. (Photo courtesy of L. Mikitzel, New Brunswick
Department of Agriculture, Aquaculture and
Fisheries.)

consistently high tuber calcium concentrations.
They found blackspot bruise incidence was min-
imized when medullary calcium concentrations
were 200-250 ppm on a dry-weight basis. Tuber
calcium concentration can be increased by sup-
plemental calcium applications during the grow-
ing season.
Tuber temperature during harvest, grad-
ing, and transport influence blackspot develop-
ment. Tubers are more susceptible to blackspot
bruise at low temperatures (less than 12°C) than
they are at higher temperatures (Smittle et al .,
1974; van der Zaag, 1996; Corsini et al ., 1999).
In some cultivars, tuber damage from phys-
ical impacts is manifested as a brownish patch
with a hard, grayish white starch deposit in the
center (van der Zaag, 1996; Christ, 1998;
Thornton, 2001b). The starch deposit is com-
prised of an abundance of densely packed, large,
intracellular starch granules, two to four times
larger than those in normal cells (Keenan et al .,
1989). It is located several millimeters beneath
the skin, usually between the periderm and vas-
cular ring (Christ, 1998). This disorder is re-
ferred to as white knot, starch spot, or white spot
bruise, and is visible only on cutting the tuber.
The cultivar, Atlantic, is prone to white knot and
it is usually found when specific gravity is high
(Keenan et al ., 1989; Christ, 1998). White knot
forms a hard spot when tubers are processed.
Shatter bruise occurs when a significant im-
pact produces cracks in the outer skin that extend
into the tuber flesh below. The tuber has a shat-
tered appearance at the point of impact. As the
tissues dry out and separate, the cracks become