Watermelon, Cantaloupe and Honeydew - Tropical and Subtropical Fruits: Postharvest Physiology, Processing and Packaging - page 533

reached before full color development in modern varieties.
More than likely, this has been a conscious or subconscious
selection by breeders to meet both shipping and consumer
criteria.
Ripe watermelon fruit has almost no acidity ( < 0.01%).
The primary organic nonvolatile acids are citric and malic,
and titratable acidity is usually less than 1 mg/100 ml
(Chisholm and Picha, 1986). Fruit flesh pH is high, rang-
ing from 4.5 to 5.0 in unripe to 5.5 in ripe and 6 to 6.5 in
overripe fruit (Picha, 1986; Corey and Schlimme, 1988).
The actual pH values appear to vary with both the culti-
var and the production environment (personal observation).
The higher pH of the intact or cut watermelon is of concern
as yeasts, molds, and various bacteria prefer the higher pH
of juice for growth.
Various production practices have been tested with wa-
termelon. Although fruit sugars were slightly increased,
lycopene content was not affected by application of water
stress up to 0.5 evapotranspiration (Leskovar et al., 2004). In
watermelons raised under an organic system, where weeds
became a large problem, fruit size was reduced but the
amount of lycopene increased (Davis et al., 2007). These
studies suggested that lycopene in watermelon can be max-
imized to only a certain extent through production prac-
tices and that genetics is the primary control of carotenoid
content.

cut to determine how many are fully ripe. If the percentage
of ripe fruit in the sample is high, then the thumpers go
through the field, followed by the harvest team (Rushing
et al., 2001). Ripe fruits are cut from the vine, leaving a
1-2 in stem. A field is harvested 2 to 6 times by hand at
weekly intervals, depending on watermelon price, density
of plants, and quality of plants (Rushing et al., 2001).
A number of automated techniques to detect watermelon
ripeness have been developed and tested over the years
(Sun et al., 2010). The most promising of these appear to
be use of density and mass, or near-infrared spectroscopy
at the stem end of the fruit, but these also require sorting at
the packinghouse level rather than the field and parameters
have to be set and developed for each variety (Sun et al.,
2010; Tsuta et al., 2011).
Harvested fruits are lifted from the field by hand. They
maybetossedfrompersontopersondownarowuntil
placed in a truck bed or onto a conveyor belt system and
conveyed to trucks (Rushing et al., 2001). Fruit are taken to
a pack shed and line and are re-sorted by hand to eliminate
off-shaped, sunburned, bruised, or damaged fruit. Seedless
melons are packed by number into boxes, usually 6-9 per
box, with dividers between fruit. Seeded melons are stacked
into semi trucks, sometimes in bins, or on straw. A long,
oblong shape is desired for seeded melons to minimize fruit
dislodgement and minimize weight per unit area in stacked
watermelons (Rushing et al., 2001).
The US grading standards provide guidelines for fruit
shape, sunburn, injury, and so on (USDA-AMS, 2006). In-
spectors may also cut the fruit and inspect for appearance
of hard seed coats in seedless types, degree of seed black-
ness in seeded types, internal signs of disease (internal flesh
breakdown, rind necrosis), and presence of hollow heart or
cracks. Hollow heart causes the fruit locules to draw away
from each other and in severe cases will make the outside
of the fruit appear triangular. Some varieties, especially
heirloom types, have very turgid and easily ruptured rinds.
These are especially noticeable when hot days are followed
by cool nights. Picking fruit when too cool can cause rind
splitting, either from touch or from weight from other fruit
stacked on top of them (Rushing et al., 2001).
Watermelon fruit may be held at a distribution point for
7-10 days before shipment to retail outlets. Watermelons
shipped by truck are usually held between 10 ◦ and 15 ◦ C
to avoid heat buildup for long-distance shipments (Rushing
et al., 2001).

Harvesting, packing, sorting, and transit
In the United States, watermelon harvesting remains one of
the most primitive systems used for fruits and vegetables.
Fruit can ripen from 24 to 42 days after pollination or
bloom, but tagging fruit on bloom date and following
calendar date will not guarantee ripeness. Unlike other
fruits, watermelon do not change color externally with
ripeness. Distance from the crown, daily heat units, and
light amount and quality will affect rate of ripening.
Markers of ripeness are subtle and vary with the variety.
They include tendril death (leaf and/or tendril nearest the
fruit), death of two leaves nearest fruit (Vinson et al., 2010),
amount of yellow development on the fruit side in contact
with soil (groundspot), change in the sheen (waxes) on
the fruit (Corey and Schlimme, 1988), and a hollow sound
when thumped with the finger and thumb (the air spaces in
the cells fill with liquid as fruit ripen, altering the sound)
(Rushing et al., 2001; Sun et al., 2010). Some seeded
varieties will make a crunching noise if pushed down near
the blossom end (personal observation). Ripe seeded water-
melons will have fully colored seeds. In the United States,
the harvest team leaders will go through a field, thump the
fruit, and select a sample that appears to be ripe. These are

Postharvest physiology and storage technologies
Watermelon contains 91% water. While tomatoes actually
contain more (94%), watermelon flesh is contained inside