January/February 2021

Keeping leafy greens fresh and healthy an exact science
By Melanie Epp, contributing writer

It might not seem that impressive today, but the fact that fresh leafy greens are available all across America at any time of year is quite remarkable.

Considering that California and Arizona produce more than 90% of U.S. lettuce, if you live anywhere east of these two states, the leafy greens on your plate have traveled a long way to get there.

So how is it that they look as if they were harvested from a nearby garden just this morning? The short answer is cooperation and the right technology: Equilibrium Modified Atmosphere Packaging, or EMAP.

Equilibrium modified atmosphere packaging is technology that slows respiration and extends the shelf life of fresh produce. Unlike most food products, fresh fruits and vegetables continue to “breathe” or respire after they have been harvested. Through this process, produce consumes oxygen, creating carbon dioxide and water vapor in the process.

The primary purpose of EMAP is to keep packaged products fresh for as long as possible without harming product quality, including taste, texture and appearance, said David Bell, president of Witt Gas Controls. Witt Gas Controls manufactures equipment for gas mixing and analysis and leak detection. 

“In general, the rate of respiration can be controlled by having lower levels of oxygen — usually less than 5% — in the packaging atmosphere, and increased levels of carbon dioxide, normally around 5-15%, with the balance gas being nitrogen,” said Bell.

This is not as simple as it sounds. To meet and continue to control the gas mix in the package, the process exchanges “air” made up of 79% nitrogen and 21% oxygen. If there’s too little oxygen in the packaging atmosphere, anaerobic respiration occurs. This process causes unwanted tastes and odors, and results in product deterioration. On the other hand, too much carbon dioxide can damage some products. Nitrogen itself has no effect on the food product.

EMAP works best when the right packaging material is used and properly sealed to prevent leaks. Permeability and breathability are important qualities in packaging, especially for leafy greens.  

“If the products are sealed in an airtight package, oxygen will soon become depleted and undesirable anaerobic conditions could develop,” Bell explained. “On the other hand, if the material is too porous, the modified atmosphere will escape and no benefit will be derived.”

The target is to reach the desired range by verifying, controlling and monitoring the environment inside the package. Correct EMAP can extend shelf life up to 100%, Bell said.

Leak-Master Pro 2

Witt Gas showcased several new pieces of equipment suitable for modified atmosphere packaging at Fruit Logistica 2020. Packagers of leafy greens and other fresh produce may want to check out Oxybeam, a new non-destructive gas analyzer that uses laser technology to determine the residual oxygen in packaged food. The company also considers Leak-Master Pro 2 an essential component for quality assurance. The package leak detector is based on CO2 technology.

But, like Bell said, the success of the technology is also dependent on packaging. Companies like Coveris produce films suitable for products that require gas flushing, as well as products that don’t.

Work has been done to better understand optimal head space for leafy greens and how fast different leafy greens respire, said Eric Duncan, Head of Food Science, Coveris.

Leafy greens respire slower than other produce, which means they require assistance in reaching the optimum headspace. This includes a fine balance of proper packaging permeability and active gas flushing. In dark leafy greens like spinach, for instance, natural respiration leads to yellowing of the leaves, which leads to food waste. In pale-colored lettuces like iceberg and Romaine, too much oxygen can cause the greens to turn pink.

“What we do is we combine the correct permeability of film with a gas flush,” said Duncan. “And that will allow us to instantly reduce the oxygen to prevent yellowing and pinking, and instantly increase the carbon dioxide.”

“It’s still just the natural respiratory gases — oxygen and carbon dioxide — that we use in the gas flush, it’s just that the gas flush gets the head space to those optimal levels very quickly,” said Duncan.

While Coveris can be contacted individually for packaging solutions, when launching a new product Duncan said it’s best if equipment suppliers work together as a project management team. Ideally, that team would include the packaging supplier, the gas flushing equipment manufacturer and sealer equipment providers. For the latter, Duncan mentioned Proseal.

“It makes communication a lot better for us to have a team of technical service engineers that work closely with packaging suppliers, like Proseal, that understand what the sealing requirements are,” said Duncan, pointing to potential compatibility issues that can arise without cooperation.

Together, that team evaluates everything from line speed to temperature requirements for sealing to product respiration rates. For the latter, it helps to have the right equipment on site for testing, he said.

“The Witt mixers are really nice because they allow you to change the composition of O2 and CO2,” said Duncan.

Proseal manufactures machinery that seals films like those supplied by Coveris to a flexible tray. It also applies the pre-mixed gas necessary to modify the atmosphere within the package before sealing.

Tray sealing technology offers advantages to retailers, especially in the current crisis. Top seal packaging is secure, food-safe and tamper-evident, and it allows for modified atmosphere packaging.

“Current packaging for leafy greens is a rigid bottom with a rigid lid,” said Anna Leigh Prochaska, sales associate at Proseal. “You can’t modify the atmosphere with a rigid lid. You have to have that film seal.”

Using film seals allows customers to see if packaging has been tampered with prior to purchase as well, she said.

“We’ve seen a lot of interest from customers because they want that food safety element,” she added.

Prochaska reiterated Duncan’s comment on the importance of working together as a product management team.

“Once our customer has decided what they want, we work with their chosen film and tray suppliers to give them exactly what they want,” she said. “It’s important that all of those parties are brought together in the initial conversations. The customer has to be very involved in the process as well, because they know their product better than anyone else.”


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