Agriculture Reference
In-Depth Information
a highly turgid shell; flesh cells are very large, with less
celluar protection from structural pectins and proteins. This
plus the rounded shape of the fruit makes transportation and
handling challenging in order to avoid bruising and even
breakage of watermelon. Often the fruit are harvested just
before full ripeness to minimize flesh break down. Fruit
color continues to develop after harvest, especially in fruit
held above 60 F (Showalter et al., 1955; Showalter, 1960;
Perkins-Veazie and Collins, 2006).
Watermelon varieties are separated by seeded and seed-
less types, then by rind color, shape, size, and flesh color.
There are at least 1000 named watermelon varieties, with
some of these originating 50-80 years ago still planted
(NWPB, 2011; personal observation). In the United States,
50% of acreage was in seedless types in 1985 and has in-
creased to 95% of the acreage. Watermelon rind pattern
ranges from dark green/black ('Black Diamond') to light
gray green ('Charleston Gray') and any type of stripe (nar-
row to wide) ('Sangria') or spots ('Moon and Stars').
Watermelons can be round (especially palm type) to ta-
pered (football shaped) to long and blocky (Rushing et al.,
2001). Seedless melons are often more tapered than seeded
types. For the fresh market, hand-sized watermelons (palm
type) weigh less than 5 kg. Mini watermelons weigh be-
tween 5 and 7 kg ('Mickilee,' 'Minilee') and regular-sized
fruits are between 8 and 10 kg. Very large watermelons,
including many of the older heirloom varieties, can achieve
weights from 12 to 40 kg (Rushing et al., 2001). Large size
was selected by breeders for many years, but smaller fam-
ily units and increased average consumer age have made
small sized fruit much more desirable. Large seedless wa-
termelon is used in the fresh-cut industry and for the limited
processing for juices and concentrates. Flesh color is most
often red in commercial varieties, with a few yellow or or-
ange varieties. Seedless varieties often are more consistent
in lycopene content and SSC (sweetness) and are firmer in
texture (Perkins-Veazie et al., 2001; Leskovar et al., 2004).
Over extended storage times (16-21 days), fruit color can
fade or become orange-red, and beta-carotene content in-
creases (Perkins-Veazie and Collins, 2006). In contrast, cut
fruits (flesh) do not exhibit chilling injury and in fact must
be stored at 2 -5 C to avoid microbial growth (Perkins-
Veazie and Collins, 2006; Fonseca and Rushing, 2006).
Watermelon produce little ethylene but can benefit from
the postharvest application of 1-methylcyclopropene (1-
MCP), an ethylene inhibitor. Watermelon is extremely sen-
sitive to ethylene, which causes rapid water soaking and
loss of texture of the placental tissue, due to activation of
polygalacturonase (Elkashif and Huber, 1988). The 1-MCP
blocks ethylene receptors, keeping watermelon firmer dur-
ing storage and maintaining fresh-cut products prepared
from treated watermelons (Karakurt and Huber, 2002;
Saftner et al., 2007). The 1-MCP treatment also appears
to inhibit microbial growth in fresh-cut watermelon (Zhou
et al., 2006).
Postharvest issues
As with most fruit and vegetables, decay is the major quality
concern in watermelons. The most common postharvest de-
cays arise from preharvest infections of didyomella, fusar-
ium, or anthracnose (Snowdon, 1992). Bacteria-induced
breakdown can occur as a secondary infection that can
cause fruit to explode, foam, or melt down. The simple
act of cutting watermelons with a long stem greatly re-
duces didyomella infection into the watermelon, as an open
wound on the fruit itself is avoided. Anthracnose will ap-
pear as small dimples on the rind, and the disease develops
faster in fruit held above 13 C (Rushing et al., 2001).
Loss of texture is the second most common loss of qual-
ity. Watermelon texture is described as crisp, with a burst
of juice. Mealy texture can result from sunburned or over-
ripe fruit. Slimy texture happens with overripe or ethylene
treated watermelons. Several of the new seedless varieties
have a different type of texture, much tougher and with less
juice release. If fruit of these types lose too much weight
(over 5%), then texture can become almost rubbery and
lacks juice release.
Lack of sweetness, off flavor, and poor color of water-
melon affect consumer satisfaction. Watermelons of 8%
SSC are viewed as having no flavor by most consumers,
while the ones with 12-14% SSC are considered of excel-
lent sweetness quality. The “green” flavor of unripe wa-
termelons is noticed by some consumers, and about 50%
of consumers can detect/sense an off flavor (“squash”)
in overripe watermelons (Beaulieu, 2006a; Saftner et al.,
2007). Another flavor, described as burned sugar, is de-
tected by some people in certain varieties. This flavor seems
Postharvest storage
The recommended storage temperature for watermelon is
13 C, with a range of 10 -15 C possible (Rushing et al.,
2001). At lower temperatures, many varieties develop chill-
ing injury, seen externally as brown or sunken spots on the
rind (Picha, 1986; Risse et al., 1990) and internally as fad-
ing of the red color (Showalter, 1960). A “conditioning”
period of up to 4 days at 20 C is recommended to prevent
chilling injury if fruits are subsequently stored below 10 C
(Picha, 1986). At higher storage temperatures, fruits un-
dergo weight loss and become soft. Lycopene production
is enhanced at 20 C (Perkins-Veazie and Collins, 2006).
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