Web edition: August 19, 2005
Ready-to-eat fresh produce is convenient, which is why it's one of the fastest growing segments of the grocery industry. Sales this year are expected to top $15 billion, almost quintuple what they were 11 years ago. However, sales might be higher still if these foods stayed fresh longer, says Olusola Lamikanra. As anyone who has tasted these foods recognizes, they can begin losing some flavorand eye appealshortly after they've been peeled or sliced.
However, Lamikanra thinks he might have a solutionliterallyto this problem: Call it immersion therapy.
Cutting fruits and veggies essentially wounds them. Affected tissues respond with a cascade of physical and chemical changes that can ultimately diminish their flavor, texture, and shelf life. However, slicing and dicing them while they're submerged in water can trick fruits and veggies into thinking they haven't been wounded, says Lamikanra, a chemist at the Agriculture Department's Food Processing and Sensory Quality Research Unit in New Orleans.
Chemical analyses and taste tests show that such bath-cut produce remains fresh looking and tasting for 8 to 14 daysdepending on the foodwhich is typically 2 to 4 days longer than when the item is sliced in open air.
A host of electrical, chemical, and hormonal changes commence in produce within seconds of it being peeled or cut. Lamikanra says the fruit or vegetable's tissues interpret these assaults as death threats, and respond defensively with protein changeswhat plant scientists call a stress responseaimed at promoting wound healing and limiting bacterial attack. Unfortunately, he adds, the plant's internal rally to arms comes at a price: It speeds deterioration of the tissues.
However, a bath can melt away that stress, leaving the fresh-cut produce relatively calm andwell, fresh.
One reason water immersion helps, Lamikanra explains, is that if a cut occurs underwater, the plant doesn't experience one of its primary signals of wounding, the loss of turgoror hydraulic pressurecaused by the sudden release of fluids when cells are ruptured. However, he says, "cutting underwater prevents that sudden surge [of plant fluids], because the water acts somewhat as a barrier." Indeed, he's found, seeding the bath with a mineral such as calcium can create enough osmotic pressure in the water to virtually halt the oozing of plant fluids from a cut.
The soothing effect of water on the stress response seems to persist even after the newly cut food has dried. Underwater, the plant parts seem to come to a long-term accommodation with the injury, Lamikanra says. Moreover, he's found, even a little underwater surgery does the trick.
For instance, he cut up cantaloupes in air, underwater, or in a mix of conditions and found that even just a first cut underwater helped preserve the fruit.
In one cutting procedure, the researcher sliced a melon underwater and then took half out to be dried and cut into smaller pieces in air. The other half was kept underwater as it was cut into such pieces there, before being exposed to air. All the melon pieces were then placed in a refrigerator and monitored for freshness and taste.
The firmest and most flavorful pieces came from melon cut entirely underwater. But even air-cut pieces from the melon that had been halved underwater stayed fresh a few days longer than did slices from fruit that had received all of its cuts on the countertop.
An extra benefit to submerging produce during cutting: The water helps wash away plant enzymes that turn the flesh of apples and other produce brown once they're exposed to air.
Another way to keep cut produce fresh longer is to heat fruits and vegetables before they're cut to about 50°F above the temperature at which they normally grow. Lamikanra dunked whole cantaloupes in a 144°F bath for 1 hour before drying, peeling, and cutting them in air. The fruit remained fresh some 60 percent longer than did cut fruit from unwarmed melons. Work by other scientists has showed that lettuce can be protected from brown spots by an even milder 3-minute warm-up.
Such treatments cause plant tissues to generate what are known as heat-shock proteins, explains Lamikanra. The presence of these chemicals inhibits the formation of several wound-initiated proteins. Although some of those normal wound proteins play a role in repairing or protecting damaged tissue, Lamikanra notes, some also impair freshness.
The same tests revealed heat treatment brings out some other, unidentified proteins that might play some role in keeping the food looking and tasting good longer.
A hot-water bath also benefited produce that will not be cut for weeks. In one experiment, Lamikanra bathed cantaloupes in hot water and then stored the uncut melons at 40°F. After 30 days, fruit given the hot bath looked as fresh as it did just after bathing, he told Science News Online. In contrast, untreated cantaloupes "were now all moldy and shriveled," he says.
The hot bath thus appears to retard the growth of molds and other pathogens, says Lamikanra, and holds promise for improving the shelf life of imported produce that must be held in quarantine.
Cutting fruit under ultraviolet light is yet another strategy that Lamikanra is investigating to extend shelf life and tastiness. The radiation kick-starts a cascade of defense mechanisms in the fruit, he says. These include the production of enzymes that fight the oxygen-driven processes that brown and injure tissues in fresh-cut produce.
In the future, Lamikanra says, "we want to combine these processes and see if we can achieve further improvements in how long cut foods stay fresh."
None of these tactics is quite ready for prime time, Lamikanra admits, although he reports substantial commercial interest from food processors. The chemist notes that before the new techniques can be scaled up for industrial testing, the optimal treatment characteristicssuch as duration, solution chemistry, temperatures, and ultraviolet wavelengthsmust be identified for each type of fruit or vegetable.
Several other questions should also be answered, such as: What specific protein or enzyme changes are triggered by cutting and to what degree are these changes altered or inhibited by his team's novel treatments? Lamikanra would also like to quantify how long the treatments retard produce from turning brown and mushy.
However, home cooks don't have to await such details. The cutting bath is a technique anyone can employ todayespecially if preparing a week's worth of fresh fruit salad is on the agenda.
Olusola Lamikanra and Karen L. Bett-Garber
Agricultural Research Service
Southern Regional Research Center
Department of Agriculture
1100 Robert E. Lee Boulevard
New Orleans, LA 70124
Gorman, J. 2001. Tasteful new wrapping can protect produce. Science News 159(Jan. 6):5. Available at [Go to].
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