What you see: Brown dots on your lettuce, especially along the lower parts of the ribs
What it is: Russet spotting!
Eat or toss: Eat! This is harmless (for you!), though the lettuce may rot sooner. How about salad for dinner tonight?
Many types of produce give off invisible vapors of something called ethylene gas. Ethylene occurs naturally in plants and prompts fruits to soften and sweeten. Generally speaking, it causes aging.
The trouble is that if neighboring produce is exposed to that gas, it can hasten ripening or otherwise impact quality for that produce too. Lettuce is particularly sensitive to ethylene gas; even low concentrations can cause problems like the weird little dots you see on the lettuce pictured here.
The condition is called “russet spotting.” Ethylene causes the lettuce to produce certain compounds, which leads to those brown spots. The lettuce is still safe to eat, though make sure to give it a good rinse. Those spots signal that cells have been weakened, making the lettuce a touch friendlier to any pathogens that happen to be nearby. And prioritize eating this lettuce; the brown spots indicate an accelerated rotting schedule.
Ethylene exposure can happen at home or before the lettuce gets to the store
You can reduce the risk of such russet spotting by making sure to store produce that releases ethylene, like apples, bananas and avocados, away from your lettuce. But sometimes the exposure is out of your control. It can happen on the delivery truck, if, for example, your lettuce is packed near ethylene gas releasers.
It’s not fair to exclusively blame other foods though: lettuce produces some ethylene itself. Studies have found that iceberg lettuce produces more ethylene after it's bruised or cut, making it still more susceptible to russet spotting.
Another risk factor, oddly enough, is exhaust. Propane-powered forklifts, which you could easily find in a produce warehouse, and gasoline engines also emit ethylene. Still more risk factors for russet spotting include harvesting lettuce when it’s over mature, excess oxygen exposure, and too-cold or too-warm storage. By and large, however, ethylene is the primary culprit.
While there’s not much you can do to ensure that your lettuce didn’t share air with a propane-powered forklift or carpool with a bunch of bananas, you can follow guidelines, like those at EatRight.org, to arrange your fridge with wide berth between the produce that will release ethylene and the produce that’s most likely to react poorly to the gas.
Don’t blame your fridge!
The problem, however, is unlikely to be the fridge itself. Apparently a number of people have blamed their fridges for russet spotting, prompting GE to note that brownish reddish spots are not caused by refrigerator malfunctions.
And, while the gas has an icky consequence for lettuce, keep in mind that you can also use it to your advantage. If you ever want to accelerate ripening, just stash your fruit next to an ethylene emitter. Put a banana and an avocado together in a cotton or paper bag and odds are good that your avocado will soften faster.
Jim Monaghan. Director of the Fresh Produce Research Centre. Harper Adams University. Newport, Shropshire. United Kingdom.
Karl Matthews. Microbiologist and Chair of the Department of Food Science at Rutgers, the State University of New Jersey.
Lettuce, crisp head. Marita Cantwell and Trevor Suslow. Produce Fact Sheets. University of California Postharvest Center.
Russet Spotting in Lettuce. Inspection Section. Alaska Division of Agriculture newsletter. Feb. 27, 2009.
Post-Harvest Diseases and Disorders of Fruits and Vegetables: Volume 2. Anna L. Snowden. CRC Press. 2010.
Resistance To Physiological Disorders In Lettuce. Van Dun Cornelis Maria Petrus, Velterop Joyce Sylvia, Schut Johan , Dirks Robert Helene Ghislain. (Patent Application)
Wound‐induced ethylene production, phenolic metabolism and susceptibility to russet spotting in iceberg lettuce. Dangyang Ke, Mikal E. Saltveit Jr. Physiologia Plantarum. Volume 76, Issue 3. July 1989.
The Effect of Different Postharvest Treatments on the Longevity and Russet Spotting of Iceberg Lettuce. Razia Morad, masters thesis. Rand Afrikaans University. April, 2003
Lettuce: Shipping Point and Market Inspection Instructions. USDA. May 2004.
Quality and Preservation of Vegetables. Michael Eskin. CRC Press. 1989.