Studies look at lengthening shelf life of fresh cut produce

Maintaining consumer acceptance of fresh-cut fruits and vegetables after processing and throughout the distribution chain is a challenge to the fresh-cut sector.

The use of special films combined with modified atmosphere packaging (MAP ) method and cleaning chemicals has long been known to improve shelf life.

However the study by the USDA’s research service provides smaller processors access to publicly-funded research that can help them compete with the big players in the market.

Fresh-cut fruit and vegetable varieties are still alive, and each respires at its own unique rate. Therefore, a film's permeability and the amount of oxygen initially infused into a package are key to extending its shelf life.

Hundreds of different types of films for packaging fruit and vegtables currently exist on the market. Each type has its own oxygen transmission rate, which allows sliced produce to continue breathing throughout storage and distribution.

If a film's oxygen transmission rate is too high for the variety it's wrapping, the product inside will brown. If it's too low, the product will prematurely decay, the USDA stated in a press release.

MAP is a method of balancing oxygen and carbon dioxide inside a package. It allows fresh-cut produce to respire slowly and stay fresh during the longest possible storage time.

With lettuce, for example, if the oxygen transmission rate of the package film is too high, it will cause the product to brown. If it is too low, it will cause decay.

“Luo's research findings led to developing a balance of oxygen and carbon dioxide inside select packages that permits a particular fresh-cut produce variety to respire slowly and stay fresh for the longest possible time,”​ the department stated.

Cilantro, a fresh-cut culinary herb known as a key flavor component of salsa, has a high respiration rate, making shelf life a challenge.

Leaf yellowing, dehydration, and loss of aroma can set in quickly after cutting. Luo has identified a packaging film for cilantro that supports a 14-day shelf life.

Using advanced packaging and other technologies, Luo has prolonged quality and shelf life of romaine lettuce, iceberg lettuce, carrots, and Salad Savoy, a close relative of kale and cabbage.

Luo also teamed up with plant pathologist James McEvoy to successfully use acidified sodium chlorite, or ASC, as a sanitizing agent on cut carrots.

The chemical had previously been found to degrade produce. But by working with specific concentrations and dipping times, they developed an ASC treatment that reduced the human pathogen E. coli O157:H7 on cut carrots by almost 100 per cent. ASC was also found to remove the bacteria that cause spoilage on carrots.

The specific formula and methodology holds promise as an alternative to the chlorine rinses many processors use to control microorganism growth on fresh-cut produce, they stated in a press release.

Luo has also received industry funding to work on maintaining produce freshness by slowing down the ripening process with a gas treatment, 1-methylcyclopropene (1-MCP), alone or in combination with ASC.

She has found that exposing cilantro to 1-MCP before cutting and then dipping it in ASC after cutting prolongs quality and slows decay when matched with the right film.

Luo, McEvoy, and plant physiologist Robert Saftner also used 1-MCP to keep watermelon fresh. 1-MCP does this by penetrates the rind of watermelon and other produce to prevent ethylene from degrading the watermelon.

Ethylene is a hormone that is produced by plants, but it degrades quality in watermelon. Ethylene performs this function by binding to chemical receptors within the plant.

A molecule of 1-MCP is so small that it enters the fruit’s flesh and competes with ethylene by binding to the same receptors. Ethylene is blocked from binding to the receptors, they discovered.

This mechanism slows down degradation in quality, which can buy extra time for shipping and prolong a product’s selling season.

Saftner also worked with retired horticulturist Judith Abbott to identify melon varieties with improved flavor and texture. They collaborated with ARS plant physiologist Gene Lester and found that orange-fleshed honeydew melons can provide an appealing alternative to cantaloupes for fresh cutting.

The scientists compared the quality, shelf life, and consumer acceptance of fresh-cut cantaloupe with green- and orange-fleshed honeydews.

“We found that consumers liked the flavor, texture, sweetness, and overall quality of the orange-fleshed honeydew chunks as well as, or better than, chunks of cantaloupe and green-fleshed honeydews,”​ says Saftner.

Among the smooth-rinded, orange-fleshed honeydews tested, Orange Dew scored highest in appearance and nutritional value. It had the highest beta-carotene concentration, color saturation, and orange hue.

Another smooth-rinded variety, Temptation, scored highest in flavor and overall eating quality. It also scored highest in melon aroma of all those tested.

“Because these orange-fleshed honeydew varieties are generally sweeter than cantaloupe and have a higher nutrient content than traditional green-fleshed varieties after being cut, they are a promising new melon type for fresh-cut processing,”​ Saftner says.

The USDA’s scientists also studied a variety of dips, coatings, and sanitizers to maintain the quality of fresh-cut produce.

The optimal concentrations of these solutions vary, depending on the product for which they are used.

Several such treatment solutions developed at at the USDA’s research service provide both antibrowning and antimicrobial benefits to apple slices and perhaps other produce.

“We also found that the wash solutions lost their antimicrobial activity over time,”​ says Saftner. “Wash solutions should not be reused on multiple batches of sliced apples.”​

The researchers said ongoing work was needed to find ways of maintaining the antimicrobial properties of wash treatments, such as adjusting the acid level and switching to one-use sprays.

More recent studies have shown that a newer wash treatment eliminated two pathogens—Listeria and Salmonella—on apple slices.

“We purposely infected apple slices with low doses of these two pathogens and found that after 1, 2, and 3 weeks, they’d been completely eliminated on the slices that had been treated with the new formula,”​ Saftner stated in a USDA release. “These results are a significant development in keeping sliced apples both fresh and safe.”​

Gross, a plant physiologist who heads the PQSL. “We’re conducting research to help find ways to widen the variety of offerings that stay fresh to the last bite.”

Packaged fresh-cut fruits and vegetables are showing up in more and more consumer markets, according to USDA research. Ready-to-eat sliced apples, for example, are now being offered by fast-food chains and school cafeterias as customers like the convenience.

“With high fiber and low calorie content, precut produce provides busy consumers with healthful options while on the go,”​ the department stated.