Contents
Gas N' Go Grapefruit
![Mexican fruit flies laying eggs in grapefruit](/is/graphics/photos/nov96/k7500-1i.jpg)
Mexican fruit flies laying eggs in grapefruit before a test of the
reducedoxygen treatment.
(K7500-1) |
How do you suffocate Mexican fruit flies that may be hitchhiking in a
shipment of grapefruit--without smothering the fruit's quality before it
reaches grocery stores?
A simple answer may be to take just enough oxygen out of the atmosphere in
the shipping container so the flies are starved of it.
The fruits should fare well. Because they would respire or
"breathe" more slowly during shipping, they would hold on longer to
their fresh-picked appearance and to their flavor, sugars, moisture, and
nutrients.
The principle works, but the answer is far from simple. For example, solid
proof is required that air with less than 1 percent oxygen instead of the
normal 20.9 percent would reliably kill all the flies. Ninety-nine percent
mortality isn't good enough.
But tough federal requirements like this have not prevented progress on a
new project led by Robert Mangan. An Agricultural Research Service entomologist,
he heads the Crops Quality
and Fruit Insects Research Unit at ARS' Subtropical Agricultural Research
Laboratoryin Weslaco, Texas.
Mangan attempts to kill the bugs with a range of gas mixtures that plant
physiologist Krista Shellie determines don't damage the fruit. Mangan, Shellie,
and colleagues Guy Hallman and Sam Ingle began the new project this year. They
aim to see if technology known as controlled atmosphere, or CA--long used
commercially to maintain quality of shipped produce during 1 to 3 weeks of
transport in sealed containers--could be adapted for double duty.
Weslaco's CA studies are part of a 3-year cooperative research and
development agreement (CRADA) with TransFRESH Corp. of Salinas, California.
"TransFRESH is the world leader in transporting fresh produce in
refrigerated containers under a CA environment," says TransFRESH
laboratory manager Susan Ajeska.
In 1995, the company supplied its patented CA system for shipping more than
400,000 tons of perishables from 16 terminals, including 6 in the United
States. Customers are mostly exporters, importers, transportation companies,
and supermarket chains. Commodities include avocados, cherries, asparagus,
lettuce, melons, peaches, nectarines, plums, and other perishables.
"Ordinary refrigeration will take shipped produce a long way,"
Ajeska notes. But "CA can take it further, and it will arrive in better
shape. If a controlled atmosphere quarantine treatment comes from the CRADA, it
could help expand U.S. export markets in the Far East and Pacific Rim
countries," she says.
![Biological technician artificially infects a mango with a fruit fly larva](/is/graphics/photos/nov96/k7500-2i.jpg)
Biological technician Eleazar Moreno artificially infects a mango with a fruit
fly larva in one of the controlled atmosphere (CA) tests.
(K7500-2) |
The Weslaco researchers want CA to provide shippers with a new, in-transit
insurance policy: a quarantine treatment for citrus when Mexican fruit flies
infest a production area. Fruit fly invasions can trigger costly eradication
programs requiring repeated aerial sprays of insecticide, and they can
devastate agricultural production and trade.
The flies, native to Mexico, triggered a quarantine in part of Texas as
recently as this year. Fortunately, the disruption was minor, since the
outbreak was near the end of the citrus harvest. But Mexican fruit fly
invasions are an annual threat in Texas and other fruit-producing states, such
as California and Florida.
A Nondamaging Alternative?
Mangan, Ingle, and Shellie have developed quarantine treatments using hot
water, air, or water vapor. But fruit quality sometimes suffers. Plus, the
Weslaco treatments--like all other approved ones, including methyl
bromide--must be done before fruit can be shipped. The CA approach could
eliminate the threat of the flies without delaying shipping or damaging the
fruit.
Another issue driving the Weslaco project is an environmental one: the
scheduled phaseout of methyl bromide by 2001. This chemical, besides its
quarantine applications for killing insects in infested fruits and vegetables,
is used to protect crops from soilborne pests and diseases. It's also used to
shield harvested commodities from pests during storage.
The phaseout results from an international accord signed by the United
States and other countries in Montreal, Canada, in 1991. This came about
because methyl bromide--along with other synthetic chemicals--contributes to
depleting the ozone layer of Earth's atmosphere. The ozone layer helps protect
the planet's inhabitants from excessive solar radiation.
"The Agricultural Research Service is a world leader in developing
methyl bromide alternatives," Mangan says. Congress increased the agency's
research funds for alternatives for methyl bromide--pre-planting and
postharvest uses--from $7 million in 1993 to $13.7 million in 1996.
[Click here to
see ARS' Methyl Bromide Alternatives newsletter.]
![A maze of hoses delivers precise CA gas mixtures to stored fruit.](/is/graphics/photos/nov96/k7500-4i.jpg)
A maze of hoses delivers precise CA gas mixtures to stored fruit. Here,
entomologist Robert Mangan and TransFRESH technician Jaya Cummaragunta examine
mangoes from test containers.
(K7500-4) |
Mangan emphasizes that the CRADA's short-term CA goal for citrus is limited
to finding an alternative to only one of methyl bromide's many uses. This goal
is to see if CA can kill any and all Mexican fruit flies--adults, larvae, and
eggs--infesting sealed containers of citrus for a shipping duration of 1 to 3
weeks.
Methyl bromide is the most common treatment now available for citrus. A
hot-air treatment developed at Weslaco is also approved. But Mangan says it is
not used yet, largely because of the cost of developing equipment at a time
when the cheaper methyl bromide is still available.
Plus, Shellie notes, "uneven heating or other problems may cause loss
of flavor in late-season citrus."
CA--For Better Ripening, Less Disease
For mangoes, Weslaco's search is not for an alternative to methyl bromide,
known to damage the fruits. Instead, shippers want an alternative that works
better than the available one, heat treatment, which accelerates ripening and
can shorten shelf life.
"The controlled atmosphere treatments can result in slower, more
uniform ripening. They may also delay certain postharvest diseases,"
Shellie says.
Weslaco's research strategy for CA includes control of temperature and
humidity, as well as of oxygen, nitrogen, and carbon dioxide gases. Specific
gas combinations tested are proprietary information of TransFRESH, but oxygen
levels are below 1 percent. By contrast, air is about 20.9 percent oxygen, 78.1
percent nitrogen, and 0.03 percent carbon dioxide.
Outside a Weslaco laboratory sit a couple of TransFRESH's 30,000 multi-ton
shipping containers. Twenty feet long, they are "the small ones,"
says TransFRESH lab technician Jaya Cummaragunta. "Most of our containers
are twice that length." Cummaragunta works in Weslaco alongside ARS
researchers.
For experiments, the containers' built-in refrigeration units keep the fruit
cool. But the scientists can run several tests at a time inside them. Gas for a
test comes through a tangle of thin plastic tubes that travel underground from
a nearby utility building and disappear into the containers. There, the tubes
deliver precise CA gas mixes under very low pressure into each of several large
plastic buckets. Each bucket holds several fruits under a tight lid, with only
a tiny escape hole to keep gas pressure steady.
Inside the utility building, the scientists have turned a walk-in chamber
into a container surrogate. It's populated with more fruit-filled buckets and
gas tubes.
![Plant physiologist takes color reading from CA-treated mangos.](/is/graphics/photos/nov96/k7500-3i.jpg)
Plant physiologist Krista Shellie takes color reading from CA-treated mangos.
Circled areas allow researchers to measure fruit color, hue and intensity at
the same spot before and after treatment.
(K7500-3) |
All the tubes run to a "mixing board." The scientists use dials
and gauges on this device to set as many as six different combinations of gas
for delivery through the tubes from supply hoses on nearby tanks.
For each test, the scientists record changes in the treated fruit's skin
color, firmness, and sugar and acid contents. A panel of tasters rates fruit
flavor, sweetness, texture, and other qualities important to consumers.
Setting up CA tests using artificially fly-infested fruit can be a
challenge, says entomologist Sam Ingle. "With grapefruit, we remove a
small core of the peel, like a plug, from the stem end. Then we place several
fly eggs or larvae inside the small, natural cavity beneath the peel and
replace the core, sealing it with hot glue."
To infest the mangoes, the scientists need more cooperation from the flies,
because taking a plug would damage the fruit's skin and flesh. So, after
mangoes begin ripening, Ingle places some atop screened cages used as
laboratory homes and nurseries for tens of thousands of fruit flies at a time.
Upside down, adult female flies manage to deposit 20 to 30 eggs through the
screen and beneath a mango's skin.
"By the end of August," Mangan says, "we'd run tests with
more than 70,000 lab-reared Mexican fruit flies, about 3,500 grapefruit, and
some 5,000 mangoes. Those tests examined a half-dozen different combinations of
gases at five different temperatures. In some tests using artificially infested
fruit, all the flies died in a matter of 1 to 3 days, without compromising the
quality of the grapefruit or mangoes."
According to Shellie, "low oxygen, rather than high carbon dioxide,
seems to be the key in fruit quality. We have found no damage at all to
grapefruit or mangoes chilled at oxygen concentrations below 1 percent. But
elevated carbon dioxide causes appearance and flavor problems."
"I'm heartened by what's happened so far," Ajeska says. "The
CRADA maximizes resources of both organizations. Bob Mangan and his team design
the experiments and cover the details of quarantine requirements. They
frequently suggest, on the basis of a test result, what a potential treatment
might look like. Then we at TransFRESH can evaluate whether it may be
economically feasible. We want to do commercial tests as soon as Bob concludes
from the lab trials that the time is right."
That time hasn't yet arrived, Mangan says.
"Results from some kinds of trials have been steady; other results have
been tantalizing but inconsistent. So we know we have more work to do. Still,
out of this may come applications for other fruits and insects, as well as
Mexican fruit flies. But each insect and fruit is different."
This fall, the researchers are beginning tests with oranges, along with a
second round of grapefruit studies. -- By Jim De Quattro, ARS.
Robert
Mangan and
Guy
Hallman are in the USDA-ARS Crop Quality and Fruit Insects Research Unit,
Subtropical Agricultural Research Laboratory, Weslaco TX 78596; phone (956)
447-6316.
TransFRESH Corp. is
located in Salinas, CA; phone (831) 424-2911.
"Gas N' Go Grapefruit" was published in the
November 1996
issue of Agricultural Research magazine.
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