Keeping the Fresh in Fresh-Cut: A case for improving refrigerated display units

April 17, 2014

It’s no secret – or surprise to anyone in the industry these days – that maintaining the cold chain has a big effect on the quality and shelf life of fresh-cut produce.

And that doesn’t stop at the grocery store receiving department.

Just as long rows of open freezer bins have given way to freezer aisles lined with gleaming glass doors, retail refrigeration of fresh-cut produce is evolving, too.

“When it first started, (stores) were putting out fresh-cut fruits on ice,” said John Beaulieu with USDA’s Agricultural Research Service (ARS) in New Orleans. “It was always a big issue. The bottoms of containers were cold, the tops can be many degrees higher – at least 10 degrees.

“And we know these changes in temperature negatively affect everything. Respiration increases. Shrink is going to increase.”

Of course, bins of ice have given way to refrigerated display cases. But those don’t always guarantee steady and uniform temperatures.

How cases are stocked can also have an effect.

“When you overstuff a case, it can break down air flow and the ability to keep cool is diminished,” said Keith Schneider, a food safety researcher at the University of Florida.

A Case Study

Yaguang Luo, a research food technologist in the ARS Produce Quality and Safety Laboratory in Maryland, is part of a team involved in a five-year evaluation of post-harvest production and processing and storage treatments. A key component of the study is examining the impact of traditional refrigerated display cases on fresh-cut produce.

“The highly variable temperature conditions associated with storage of fresh cuts in commercial open-refrigerated display cases dramatically affects the shelf life and quality of produce,” states an abstract for the study, which runs through 2015 and is funded with a $1.7 million grant matched with support from industry partners.

FDA’s food code calls for packaged fresh-cut leafy greens to be maintained at 41˚ F or lower at all times. Researched conducted by Luo and her team is showing that’s not necessarily what happens.

“Many of the display cases used in the retail stores are old-style display cases that were traditionally developed for deli meat,” Luo said. “No technologies were available previously to address special temperature control needs for fresh-cut produce.”

So the ARS team set up a “little supermarket” with support from Hussman Corporation, a manufacturer of refrigeration and food merchandising equipment and member of the project’s industry advisory board. Dole Fresh Vegetables also provided “large quantities” of packaged fresh-cut produce.

“We created this lab area – a little supermarket area,” said Jorge Saenz, Hussman’s director of cold chain management. “Hussman’s in-kind support for the project included equipment and technical assistance. Hussman provided medium temperature merchandisers with all the necessary refrigeration equipment to run these cases.”

Hussman also provided technical support during and after installation to make sure the refrigeration was operating under appropriate conditions, Saenz added.

Telling Temperatures

Researchers mapped out the temperature profile of each of 540 salad bags in the display units during regular refrigeration, defrost cycles and then under varying conditions.

What did they learn? That temperatures fluctuate significantly depending on the product’s location in the display case.

“Since fresh-cut products must be stored below 41 degrees Fahrenheit … and can’t be stored below 32 degrees Fahrenheit (which would cause freeze damage), there is only an estimated variance of 9 degrees Fahrenheit to play with,” Luo said. “Current retail display case temperature differentials are far larger than 9 degrees Fahrenheit.”

The variations are even greater during the defrost cycle, she added.

“No matter how you adjust the operational settings, there are always some products being stored too hot at the same time that others have freezing damage risks,” she said.

To reduce the temperature fluctuations, the scientists installed insulation materials at the front of the case to block heat penetration from ambient air. They also added insulation at the back to prevent freeze damage. Night curtains and clear glass doors were additional options. Out of all, doors are showing the best results, Luo said.

“Doors will obviously help maintain a better produce temperature because you don’t have the impact of the air,” Saenz said, noting that while a few retailers use doors with fresh cut, it’s not the norm. “It obviously will have an impact on the temperature of the product.”

As the research continues, Luo said the team is testing alternatives to doors.

“A novel idea is to use phase change materials (PCMs),” she said. “By changing from solid phase to liquid phase, PCMs can absorb heat and thus reduce the case temperature. By that same virtue, the PCM could change liquid to solid and release heat.

“This can be especially useful to maintain temperature during defrost cycles.”

From Home to Store?

Steven Britz, another ARS researcher in Maryland, said a project he’s been involved in to extend the shelf life of fresh strawberries could portend encouraging findings for fresh-cut produce. He’s been working with a company that makes ultraviolet light-emitting diode (LED) lights, built so they give off short-wave ultraviolet radiation.

“We started using them in horticultural applications as greenhouse supplements,” Britz said. “In discussions with the company, we had always talked about the possibility to use it as a way of extending shelf life for produce.”

Meeting with “a major refrigerator manufacturer” he wasn’t ready to name, researchers learned that consumers have concerns over the shelf life of strawberries – at the supermarket and once they get them home.

Britz set out to see if there was a way to increase shelf life by 50 percent.

“These are strawberries bought at the supermarket like a regular consumer would,” he said. “We took them back to the lab and set them up in our simulated refrigerator storage compartment.”

During the study, strawberries’ quality based on pigment, fresh eight, dry weight and soluble sugar content was measured. The results were promising, Britz said.

“In our research, we were able to extend (shelf life) out to nine days, whereas the controls were turning moldy between three and five days. And this refrigerator company claims to have gone much further than this.”

Britz said the company is planning to offer the application for home refrigerators, possibly as early as this summer.

“You could presumably buy a refrigerator that has ultraviolet lights and these diodes installed in a compartment designed to keep berries fresh longer,” he said.

And the possibilities for commercial refrigeration may not be far behind.

“This is not a fresh-cut application, although I don’t see any reason why it couldn’t be extended to fresh-cut,” Britz said.

Kathy Gibbons  contributiong writer