Melons Are on a Roll
Calcium dips give vine-ripened melons added nutrition, storability.
Thanks partly to modern international shipping and handling, fresh melons
are no longer just seasonal delights. They can now be eaten year roundand
Agricultural Research Service studies to
improve their postharvest care may increase their appeal even more.
Scientists at the ARS Subtropical Agricultural Research Center in Weslaco,
Texas, and at the Children's Nutrition Research Center in Houston, Texas, have
worked out a way to extend the market life of melons. They give them a soaking
in a special calcium solution during the time it takes to cool them right after
The dunking could allow growers to provide sweet and tasty, vine-ripened
melons in greater quantities and to more distant markets.
In laboratory and preliminary field tests, the treatment prolonged market
life by at least 2 weeks. It also increased calcium levels in the melons,
especially honeydews. Even without the treatment, honeydews and cantaloupes are
rich sources of calcium, with a 1-cup serving providing about 10 percent of an
adult's daily calcium needs.
But today's sweeter and more nutritious varieties could become even better
as shippers adopt these new postharvest handling procedures.
Could consumption of melons consequently reach new highs? In 1997, per
capita consumption of cantaloupes, or muskmelons, rose to 11.7 pounds,
eclipsing the 11.2-pound record of 1946. Together, all types of melons rank
second to bananas as the most-consumed fresh fruit in the United States.
Why'd They Do It?
This melon research started with plant physiologists Gene E. Lester at
Weslaco and Michael A. Grusak at Houston.
Previously, Grusak had developed techniques for studying movement of calcium
from roots to edible portions of vegetable plants, a first step toward
determining calcium bioavailability. Lester had just completed a study in which
he found that certain calcium solutions reduced tissue aging. So Lester, whose
primary research is aimed at extending the shelf life of fruit in export
markets, asked Grusak to help him develop a way to increase calcium in whole
honeydew and cantaloupe melons after harvest.
Lester knew that, just as people need calcium for strong bones, aging
melonsespecially the tissue associated with the rindneed calcium to
maintain a degree of firmness that protects against spoilage. But in ripe
melons, calcium steadily migrates from the rind to the seeds, depleting the
rind of calcium needed for maintaining cellular functions.
Before the scientists began work on a treatment for whole melons, they had
to get an idea of calcium concentrations that might work best. Grusak started
by first analyzing portions of melons, to learn where calcium and magnesium
Discoloration indicative of spoilage developed
in the same honeydew cultivar, when soaked in plain water without
Then, Lester grew honeydews and cantaloupes in a greenhouse and submerged
whole fruits in solutions of calcium chelated, or ringed, with amino acid
molecules, as well as in non-calcium control solutions. He then cut the fruits
into millimeter sections, from the rind to the seed cavity, and dried them for
shipment to Grusak's lab. Grusak measured how much calcium was in each section
and then profiled how much calcium had migrated through the melon layers.
"We did between 200 and 300 samples," says Grusak.
"The calcium had a much stronger effect in the honeydews than in
cantaloupes, and we think that may be because of the cantaloupe's thick,
spaghetti-like outer netting."
In cantaloupes, about 10 days after the fruit begins to form, tissue
technically known as "lenticular" starts to crack through the outer
rind to develop the irregular netting. Honeydews, on the other hand, generally
have no webbing and thus convey calcium efficiently through a normally unbroken
skin into the all-important adjacent green layer of the outer rind. To confirm
that cantaloupes' lenticular tissue was a problem, Grusak treated small areas
of whole melon surfaces with the solution, but this time he labeled the calcium
with radioisotopes. This allowed him to trace the nutrient's path through the
fruit. Using the radioisotopes, he confirmed that the calcium moved more
quickly through the honeydew than through the cantaloupe. In this experiment,
the scientists treated freshly harvested melons for 20 minutes in a
40-millimolar calcium solution (1.6 grams of calcium per liter) chelated with
amino acid. Within 24 hours, labeled calcium in honeydew rind tissue measured
about 24 percent of the labeled calcium that was applied. In cantaloupes, when
the radioisotope was applied directly onto the lenticular tissue, the amount of
labeled calcium was less and variable9 to 12 percent.
The shelf life of vine-ripened honeydew and cantaloupe is generally about 7
to 12 days. Increased calcium in treated greenhouse-grown honeydews at least
doubled their shelf life, Lester says.
The scientists found an application of amino acid-calcium chelate solution
with at least 80 millimolar calcium (3.2 grams per liter) was needed to
appreciably extend the shelf life of whole honeydews or cantaloupes grown in
the greenhouse. Up to 100-millimolar solutions are now included in experiments
begun last spring with field-grown melons, which tend to have thicker rinds.
"We want to find concentrations of calcium that work best without
harming spring- and fall-grown melons, because too much calcium can be toxic to
the melon," says Lester.
Already the research has led to industry interest. A major U.S. melon
grower-shipper, Starr Produce Co. of Rio Grande City, Texas, is providing
vine-ripened melons for the research. According to David LaGrange, manager of
the company's LaCasita Farms, vine-ripened honeydews are really delicious but
more difficult to ship than those harvested as little as 3 days sooner. He says
the calcium treatment may help pave the way for extensive marketing of
By planting recently developed hybrid honeydews and harvesting them at full
maturity, growers could easily comply with the federal law stipulating that
melons destined for interstate commerce can be harvested when their soluble
solids content reaches 9 percent. Hybrid melons detach themselves from the vine
when ripe, and as they do, the soluble solids typically range from 12 to 15
percentalmost totally in the form of sugars.
Melons treated to slow down softening could be shipped by surface
transportation, rather than flown, halfway around the world. Very sweet,
unblemished, green-fleshed melons have proven themselves popular, especially in
Japan.By Ben Hardin and
Jill Lee, Agricultural Research
Service Information Staff.
Gene E. Lester is in the USDA-ARS
Crop Quality and Fruit Insect Research Unit, Kika de la Garza Subtropical
Agricultural Research Center, 2301 S. International Blvd., Weslaco, TX 78596;
phone (956) 565-2647, fax (956) 565-6652.
Michael A. Grusak is at the
USDA-ARS Children's Nutrition Research
Center, 1100 Bates St., Houston, TX 77030; phone (713) 798-7044, fax (713)
"Melons Are on a Roll" was published in the
February 1999 issue of
Agricultural Research magazine.