You’ve probably been warned not to judge a book by its cover…and that beauty is only skin deep. But when it comes to food safety, it’s all on the surface.
In an attempt to better understand what contributes to food contamination, researchers recently conducted an experiment on two dozen varieties of common salad greens and tomatoes.
What they found challenged the conventional wisdom that rougher surfaces (like a kale leaf) would hide viruses and bacteria and make them harder to wash away.
Instead, the researchers discovered that vegetables that have a waxy layer—which naturally protects the plant against diseases and dehydration—had fewer viruses on their surface after washing, compared to their non-waxy counterparts.
The cleanest greens you can eat
Specifically, the researchers found a thousand-times fewer viral particles left on vegetables with a wax layer after being washed, compared to vegetables without this type of layer.
The waxiest produce (and therefore least likely to harbor viruses) included:
- Collard greens (Top Bunch variety)
- Kale (Starbor and Red Russian
- Cabbage (Alcosa and Gonzales)
- Tomatoes (Indigo Rose, Rose, and Sungold)
- Romaine lettuce (Outredgeous)
Other lettuces, endive, spinach, radicchio, arugula, and mustard greens had the lowest amounts of wax.
The science behind food safety
So why did the researchers focus on produce used in salads? Well, fruits and vegetables are exposed to viruses and other microbial contaminants in a number of ways. Among the top offenders are contaminated irrigation water, animal waste on the plants, and handling by farm workers.
When produce is cooked, it typically kills microbial contaminants. But when it’s eaten raw, like in a salad, food safety can be particularly problematic.
To conduct the experiment, the researchers swabbed 24 varieties of raw salad greens and tomatoes with a swine virus that mimics human rotavirus—a common pathogen responsible for gastrointestinal infections. (There is a vaccine for rotavirus, but safety and ethical questions have made it the subject of great controversy).
The researchers washed the contaminated greens and tomatoes twice in a standard salt solution.
Then, they evaluated the surfaces of the vegetables at different levels of magnification. Not only were they looking for viruses, but also the amount and composition of waxes on the produce.
While viruses typically adhere to waxes at the molecular level, the researchers found that when a wax completely covers the surface of a fruit or vegetable, it repels water and makes it harder for viruses to stick.
While the researchers did their best to make this sound chemically complicated (and relevant), it’s basically the same reason it’s simpler to keep floors clean when they are waxed. As we all know, waxed surfaces are easier to wash—whether they’re floors or fruits.
Hidden sources of wax on your produce
While wax may help cut contamination on produce, it’s important to note that not all wax on fruits and vegetables is natural.
Conventionally and even organically grown produce may be artificially waxed to prevent moisture loss and dehydration, protect it from bruising during shipping, and increase its shelf life.
That’s why you’ll often see wax on apples, cucumbers, eggplant, citrus fruits, peppers, and potatoes.
Some of this added wax is from natural sources like carnauba (from the carnauba palm tree), beeswax, and shellac (from the lac beetle). But some of it is petroleum-based.
To ensure you’re not eating petroleum, buy organic fruits and vegetables, which, by law, can only use natural waxes. Or you can buy directly from the grower—just visit your local farmer’s market or sign up for community-supported (CSA) deliveries.
Unfortunately, the only way to remove any type of added wax is to peel the fruit or vegetable. And that can remove the nutrients that lie right below the skin.
So what have we learned? While you should carefully wash all produce before eating, your salad may be that much safer (and nutritious) if you load it up with kale, cabbage, collards, tomatoes, and red romaine.
Watch out for pesticides too
Of course, there are other contaminants on produce besides microbes that can cause health issues. Particularly pesticides.
According to the nonprofit Environmental Working Group, nearly three-quarters of conventional produce samples tested by the USDA in 2014 contained pesticide residues. And the really disturbing thing is that the pesticides remained on fruits and vegetables even after they were washed. And in some cases, even after they were peeled!
Every year, the EWG releases its “Dirty Dozen”—the 12 types of conventional fruits and vegetables that are most contaminated with pesticides.
The 2016 Dirty Dozen includes strawberries, apples, nectarines, peaches, celery, grapes, cherries, spinach, tomatoes, sweet bell peppers, cherry tomatoes, and cucumbers.
And the EWG notes that hot peppers, kale, and collard greens can be contaminated with insecticides and pesticides that are particularly toxic.
So how can you avoid these deadly chemicals? It’s simple. Just eat organic produce—which, by law, can’t be sprayed with pesticides, insecticides, or chemical fertilizers.
 “Influence of Epicuticular Physicochemical Properties on Porcine Rotavirus Adsorption to 24 Leafy Green Vegetables and Tomatoes.” PLoS One. 2015 Jul 16;10(7):e0132841.