Lettuce 101

Why does my lettuce turn brown?

I spent a few days last month during “Lettuce Days” fielding questions about Yuma agriculture, vegetable production and the related technologies behind them. One of the more popular topics from those who happened to wander my way was, “why does my lettuce turn brown while an unopened bagged salad looks great?” The answer is a wonderful combination of food scientists benefiting from bio- and polymer chemistry.
Much of the browning in all fruits and vegetables is the result of enzyme-catalyzed oxidation. The naturally occurring enzyme is called polyphenol oxidase which when exposed to oxygen—say, upon cutting— assists in the formation of yellow colored quinones. The quinones then further react with oxygen and polymerize to eventually form melanins which are responsible for the brown or black appearance of older lettuce. Simply put, when the lettuce is cut and is exposed to oxygen, enzymatic oxidation forms the brownish colored staining. Fortunately, the enzyme does not bind oxygen particularly well, which means that 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 The useful life of many food products—from potato chips to wine, or for that matter any oxygen-sensitive item (even historical documents)—can be extended in many cases when packaged in a modified atmosphere.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.
Like so many things in life, there can be too much of a good thing when it comes to CO2. The right amount of CO2 will keep the percentage of O2 in the package low. Too much CO2, however, will induce physiological changes in the lettuce that produce unfortunate odors and flavors. So the trick is to keep the right balance of gases inside the package.
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.