Kitchen cross-contamination advice usually comes down to one line: don’t wash raw chicken, because the splashes will spread across the counter. That advice is sound. But when researchers filmed 300 people cooking, the bigger problem turned up somewhere else.
Among people in the control group who never washed their chicken at all, bacteria from the chicken still ended up in the salad 31 percent of the time. The route it took points to the step that actually matters.
The study that changed the advice
The study was run by North Carolina State University, RTI International and the USDA Food Safety and Inspection Service, and published in 2022 in the Journal of Food Protection. Participants cooked chicken thighs and made a salad in a test kitchen. The chicken had been inoculated in advance with a harmless, traceable lab strain of E. coli. Afterward the researchers looked for that strain in the sink, on the counter and in the salad.
Some participants, 142 of the 300, had received three emails beforehand advising them not to wash chicken. The message worked: 93 percent of that group skipped the washing, against only 39 percent of the control group. The salad came out no cleaner. Among people who washed the chicken, the lettuce was contaminated for 26 percent in the control group and 30 percent in the email group. Among those who didn’t wash it, the figures were 31 and 15 percent respectively.
The researchers found this surprising. The conventional wisdom had been that the risk came from water splashing off the chicken onto nearby surfaces. Instead, the sink itself was getting contaminated, even when nobody washed the chicken, while nearby counters picked up relatively little. The bacteria reached the sink from the chicken, from the packaging or from people’s hands.
USDA figures released in 2019 from the same line of research add two more numbers. Among participants who washed their chicken, 60 percent had bacteria in the sink afterward. And 14 percent still had bacteria there after they had tried to clean the sink.
In a real kitchen it often goes like this. The chicken gets trimmed, the sink gets a quick rinse, and the lettuce goes into that same sink to be washed. For greens you eat raw, that is the shortest route from raw meat to your plate.
Hands: 97 percent
The second suspect in that study is hands. The authors name transfer by hand as a suspected factor behind the contaminated lettuce.
An earlier observation by the same partners, in 2018, shows the scale. Participants washed their hands incorrectly 97 percent of the time. Most didn’t wash for the full 20 seconds, and many didn’t dry their hands on a clean towel. While making burgers, they contaminated spice containers 48 percent of the time, the refrigerator handle 11 percent of the time and the salad 5 percent of the time.
The spice jar shows the mechanism well. You touch it after handling raw meat, you wash your hands, and then you pick up that same jar to season the salad.
A wet surface gives up almost everything
How many bacteria move from a surface to food was measured by a team from Rutgers and two other US universities. Jensen, Schaffner and colleagues put Salmonella and E. coli O157:H7 on ceramic, stainless steel, glass and plastic, then placed cut celery, carrot, watermelon and lettuce on them.
From a freshly contaminated surface, more than 90 percent of the bacteria almost always moved onto the cut produce. When the surface had dried for an hour, carrots and watermelon still generally picked up more than 90 percent. Celery and lettuce picked up anywhere from under 1 to about 70 percent.
Transfer in the other direction is weaker. Freshly contaminated celery and lettuce passed between under 2 and about 25 percent of their bacteria to the surface. After an hour of drying, only up to about 5 percent. The authors’ conclusion: surface moisture and the direction of transfer matter most.
A dry surface is no guarantee the bacteria are dead, either. At Wageningen University, Salmonella at a high starting level survived on dry stainless steel for at least four days. At lower levels it disappeared within hours. In the same study, bacteria passed from a wet sponge to the steel and from the steel to cucumber slices, with transfer rates of 20 to 100 percent.
A rinse with water isn’t enough
The most practical test came from the University of Arizona. Ravishankar and colleagues cut Salmonella-contaminated chicken and then lettuce on the same board with the same knife, under three scenarios.
| What happened to the board and knife after the chicken | Salmonella found afterward |
|---|---|
| Nothing | lettuce about 3 log CFU/g, on the order of a thousand cells per gram; board and knife about 2 log CFU/cm² |
| Rinsed with water | lettuce, board and knife 0.5-2.4 log CFU per g or cm² |
| Washed with soap and hot water | under 1 log on the lettuce, board and knife |
Water cuts the transfer down, but it doesn’t stop it. Soap and hot water, the FDA’s recommended method, gave a result the authors called effective. No bleach solution was needed.
An order of steps that accounts for all of this
- Salad first, meat second. The USDA advises preparing vegetables and salads before you handle raw meat and poultry. That way the greens pass through the sink and your hands while both are still clean.
- Clean the sink before the greens. If raw meat has already been in it, wash it with detergent and hot water before you wash any produce. Rinse the greens under running water in a colander, and don’t set them on the bottom of the sink.
- Twenty seconds with soap. Do it right after touching raw meat or its packaging. Dry your hands on a clean towel, which rules out the one you just used to wipe the counter.
- A separate board for anything eaten raw. The FDA suggests one board for fresh produce and another for raw meat, poultry and seafood. If you only have one, wash it, the knife and the counter with soap and hot water between those stages. Whether that board should be wood or plastic is covered in which cutting board to choose.
- Keep surfaces dry. A wet counter hands cut produce almost everything on it. Wipe it dry after cleaning.
- Sponges and brushes carry bacteria too. Don’t wipe the salad board with the sponge you used after raw meat. Wash your produce brush after use: in a melon study, the brush spread Salmonella to clean parts of the rind. How to cut the melon itself is covered in washing bananas, avocados and melons.
- The fridge. The FDA advises keeping produce that will be eaten raw separate from raw meat, poultry and seafood. Store raw meat in a sealed container so its juices can’t reach the greens.
Washing chicken still isn’t a good idea, and the study authors say so. For the salad, though, what matters most happens at the sink and on your hands. Why rinsing doesn’t make raw chicken safer, and what to do if you still wash it, is covered in should you wash chicken.
Key points
- In a study that observed 300 home cooks, chicken bacteria reached the salad even for people who never washed the chicken: 31 percent of the time in the control group.
- According to the authors, the sink and hands matter more than splashes on the counter.
- Hands were washed incorrectly 97 percent of the time, usually for less than 20 seconds and often without a clean towel.
- A wet surface passes more than 90 percent of its bacteria to cut produce. At high contamination, Salmonella survived on dry steel for at least four days.
- After raw meat, wash the board and knife with soap and hot water. Water alone only reduces the transfer.
- The simplest step: make the salad before you open the meat.
Why bacteria that have already attached to a leaf are hard to rinse off is covered in our piece on biofilms on produce. Whether bagged, pre-washed greens need another wash is covered in our look at triple-washed claims. For what vinegar and baking soda actually do, see our breakdowns of vinegar and baking soda. Where to put moldy food, and why you should not sniff it, is covered in our piece on mold on food. What to do with meat that thawed sooner than planned is covered in our piece on refreezing. What happens to food that lands on the floor is covered in our look at the five-second rule. Why even thorough reheating does not rescue yesterday’s rice is covered in our piece on reheating rice. The big picture is in our food safety hub.
The other half of kitchen arithmetic is temperature rather than surfaces, and it follows the same logic: the spread inside a single refrigerator reaches two degrees Celsius.
