On June 13 2019, the Facebook page “Crafty Morning” shared the following meme (archived here), featuring an image of salad and lettuce in bags at a supermarket along with purported tips for selecting the freshest produce:
Above an image of Fresh Express bagged lettuce varieties such as “Veggie Lover’s” and “Field Greens,” text said:
If you buy bagged lettuce or salads, choose the bag that is the flattest. When the greens are packaged, all of the air is sucked out of the bag. Then as they age, they give off [gas], making the bag puffier. Choose the flattest bag and your salad will be fresher, and last longer in the fridge without going bad.
That advice perhaps sounded intuitive to many commenters, but Crafty Morning included no source or citation supporting the claim — simply a comment in the original post, “Good to know!”
As it turns out, the “advice” proffered originated on the Reddit subreddit r/LifeProTips in April 2018. However, the original thread was unavailable as of June 2019, although the title was intact:
A stickied comment at the top of the thread simply read:
Removed as wrong.
Beneath that, a separate user commented at length in response to both the original poster and the comment about removal. That poster claimed to be a lettuce packaging expert, and made a number of claims about the shelf life of bagged salads and how leafy greens are processed:
Hi there! I’m an engineer who designs the plastic structures used to package salads and cut veggies for a living. While the LPT is indeed wrong as many have pointed out, so are the vast majority of people in the comments saying that the bags are flushed with nitrogen. Basically no one here was 100% right.
First of all, it is actually pretty rare for produce companies in the US to gas flush their salad and veggie bags. Instead, packers and copackers rely on the product itself to modify the atmosphere inside the package. This is possible because salad and other veggies are still alive when packaged and still respire–consuming oxygen and emitting carbon dioxide (and sometimes ethylene). This is where OP got his/her theory about the bigger bags being bad. The truth is, the amount of air in a bag is more a function of how the much the packer’s VFFS (vertical form fill seal) line happens to squeeze the bags during the sealing cycle than anything to do with atmospheric composition or gas/nitrogen flushing.
The reason that these companies don’t gas flush is because the optimal gas concentration is often in the range of 3-5% oxygen in the bag (our atmosphere is roughly ~21%). At this range, the lettuce will respire at a slower rate, extending its life, without ‘suffocating’. See, if you were to gas flush the bags below about 1-2% oxygen, the lettuce would switch to anaerobic respiration and break itself down prematurely. On the other hand, if the package were left open to the atmosphere, the produce would continue to respire normally and age quickly, would potentially foster microbial growth, and the more delicate varieties could undergo enzymatic browning.
When the salad or veggies modify the internal atmosphere to a low oxygen level, it creates a concentration gradient across the plastic film, driving oxygen into the package. There are two primary methods of achieving this–through passive transmission achieved through of specific low density polymers, or through micro perforation using a laser. Thus, the respiration of the product inside the bag will cause the internal air composition to reach equilibrium at an ideal oxygen level if the plastic film has been designed properly. At the end of the day though, the ‘cold chain’, or the ability to maintain temperature below 40 F throughout the distribution chain, is the most critical part of preserving shelf life.
Here’s a fun fact. If you go into a grocery store, feel the difference between material that spinach is bagged in compared to and ice berg lettuce blend. The spinach film is going to feel stiffer and more crinkly, because it is a different structure, typically all biaxially oriented polypropylene, and it will have several micro perforations to let oxygen in. This is because spinach generally breaths too heavily for passive transmission to be sufficient. On the other hand, iceberg lettuce, romaine, and spring mixes will often be in laminations of oriented polypropylene and polyethylene where passive transmission is used to match the considerably lower oxygen demands of those leafy greens.
However, the commenter’s identity and credentials were just as opaque as those of the original poster. Other commenters also disputed the original tip, pointing out, for example, that bags of lettuce often contain small holes for breathability. A number of posters linked to a since-removed article on TampaBay.com about the processing and storing of bagged lettuce and salad mix.
Other articles about the process of bagging and preserving produce and salad kits included information about the process. In 2016, NPR reported:
The bagged greens also benefit from a crucial technological innovation, called “modified atmosphere packaging.” Essentially, the plastic packaging is engineered in such a way that it “breathes” but also maintains an atmosphere inside the package that will minimize browning and spoilage. Typically, that means a lower level of oxygen, and more carbon dioxide, than the natural atmosphere.
The same claims appeared on the University of Arizona’s College of Agriculture and Life Sciences page “Lettuce 101.” In that explainer, CALS stated:
… if oxygen can be partly excluded from a ready-to-eat lettuce bag, the reaction will run at a slower rate.
This is where modified atmosphere packaging comes into play, and it’s not just used for salads. The useful life of many food products—from potato chips to wine, or for that matter any oxygen-sensitive item (even historical documents)—can be extended in many cases when packaged in a modified atmosphere. A “normal” atmosphere contains 78% nitrogen, 21% oxygen, and 1% of other gases like carbon dioxide (CO2). So any atmosphere that has intentionally different percentages (typically with a lower oxygen percentage) can count as a modified atmosphere. The amount and type of gas used varies depending on the application. For example, nitrogen gas replaces the oxygen in potato chip bags for preservation.
Bagged salad producers don’t have worry about adding any gasses, though. The lettuce adds this gas to the package itself. When lettuce is cut, the cells rupture which induces a host of physiological changes, including an increased rate of CO2 release … Lettuce is shredded, washed, and packaged within hours of being harvested. It is then put into transparent bags that are specially designed to retain the right ratio of CO2 and O2. So, as the amount of CO2 builds up in the bag from the respiring lettuce, some of the CO2 passes right through the plastic bag and into the outside air. Heavy “breathers” such as broccoli and cauliflower are put into a different type of bag than veggies with a lower respiration, such as peppers and onions. Lettuce falls somewhere in between these two categories.
So, for bagged salads and anti-browning, the trick is to keep the right balance of gases inside the package. And, a “bag that breathes” is part of the story behind the preservation of our vegetables.
Lettuce and bagged salads are delicate, and are not likely to vary tremendously in their shelf lives once brought home. The best available advice seems to be scouring best-by dates, not looking for “puffy” bags of greens. Although bagged produce extrudes gases, many bagged vegetables are indeed packaged in “breathable” containers. Some are packed flat and others are not, but the tip did not stand up to scrutiny and was later removed for inaccuracy.