What is the best way to wash lettuce and remove germs?

Leafy greens have a lot of bacteria. Most of it is probably harmless. Rinsing in water removes a small amount of the germs but not much. Rinsing does remove insects and other critters, so I think it is worth it. Shredded Iceberg lettuce is shockingly germy. Best to buy a ball of iceberg and shred it yourself. 


What has more bacteria, a raw chicken breast or bagged lettuce?


For this experiment, I purchased bagged romaine lettuce and Tyson raw chicken breasts from Walmart. With clean gloves, I rubbed a small handful of the romaine lettuce onto a sterile agar plate. 


Then with clean gloves, I rubbed one raw chicken breast all over another sterile agar plate. Then I rubbed a second raw chicken breast all over the same agar plate to make sure it got good and dirty.


I incubated the agar plates overnight in a warm incubator (about 95 degrees F) and examined the plates in the morning. 


Much to my surprise, the bagged lettuce had much more bacteria than the raw chicken breasts! (This does not mean you should start eating raw chicken breasts. Please continue cooking chicken to 180 degrees F.)  Now nobody panic. We have all been eating bagged lettuce for years and don't usually get sick. Most of these bacteria are probably harmless. Our intestines require good bacteria to function, and our bodies need to learn to handle the regular flora in our food. That being said, bagged lettuce has been responsible for many outbreaks and recalls including the latest cyclospora outbreak. My goal is to determine if buying heads of lettuce and washing it ourselves is any better or safer than buying the bagged stuff.  If you scroll down, you will see that I have tested a few other bagged lettuces already.



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Negative Control

I wanted to make absolutely certain that my tap water and clean gloves were not contributing bacteria to my experiments. So, I put on a clean pair of gloves, got them wet under the tap water, and rubbed them on a clean agar plate. 

I incubated the plate overnight and was pleased that there was no bacterial growth at all the next day. 


Bagged Spring Mix

Brace yourselves, this isn't pretty. I wanted to find out if ready-to-eat triple washed bagged lettuce was germy and if washing it with tap water helped. First I rubbed a small handful of the bagged lettuce onto an agar plate. 


Then I washed the lettuce in tap water. I filled up the bowl with water, mixed it up with my hand and let it sit for 2 minutes. I emptied the water and filled it up again with clean tap water. I swished it around with my hand again. I emptied the water and spun the lettuce to get rid of most of the water. 


Then I rubbed a small handful of this clean lettuce onto another agar plate. I incubated the plates overnight. 


Much to my dismay, the lettuce was filthy! Washing it with water hardly improved it! 


Soaking Spring Mix in 25% white vinegar


I used that same container of organic spring mix for an experiment the following day. This time I soaked some spring mix in tap, and I soaked more spring mix in 25% white vinegar in water. (1.5 cups of vinegar and 4.5 cups of water.) I let them soak for 10 minutes.


After 10 minutes, I rinsed them under running water for 2 minutes. I moved them around with my hand to thoroughly rinse them. I rinsed the spring mix soaked with vinegar first and left the other spring mix soaking in the water. So, the spring mix that was soaked in the water really sat there for about 12 minutes instead of 10. I washed the strainer between each one. 


 
After I had rinsed both spring mixes, I set them on paper towel briefly.


Then I rubbed a small handful of each onto an agar plate.


I incubated the plates overnight. I am very happy to say, that soaking in 25% white vinegar for 10 minutes and made a huge improvement in the amount of bacteria!


Bagged Romain Compared to a Head of Romaine


For this experiment, I wanted to determine if bagged ready-to-eat romaine lettuce is dirtier than a whole head of romaine. Wearing clean gloves, I rubbed a small handful of bagged romaine onto a sterile agar plate. 


Then I took one of the inner leaves from one of the heads of romaine. This romaine was NOT WASHED and the bag says that you need to wash it before you eat it. I did NOT WASH it for this experiment. I tore up the leaf so it was similar in size to the bagged romaine and rubbed it on another agar plate.


I incubated the plates overnight and examined them in the morning. In my opinion, the bagged romaine is about as dirty as the whole unwashed romaine. 


At the same time that I was doing the previous romaine experiment, I also took one of the inner romaine leaves, tore it up, and rinsed it under tap water for 30 seconds.


I rubbed the washed romaine onto a sterile agar plate and incubated it overnight with the others.


It appears that the quick 30 second wash decreased the amount of bacteria a little bit. I will test a 10 minute soak with 25% white vinegar next time. 


Romain soaking in 25% white vinegar


I repeated the experiment with another whole head of romaine lettuce. I tore up one of the dirty leaves and rubbed it on an agar plate.  
Then, I rinsed another leaf for 30 seconds under tap water, tore it up, and rubbed it on the agar plate. Then, I took another leaf and soaked it for 10 minutes in tap water. I also soaked another leaf in tap water with 25% white vinegar for 10 minutes. 


After the 10 minute soak, I rinsed each leaf for 30 seconds under tap water, tore them up, and rubbed them each on a sterile agar plate. 


I incubated the 4 plates overnight and examined them in the morning. 


As you can see, the quick 30 second rinse under tap water did a great job cleaning the romaine. Soaking for 10 minutes in both 25% vinegar and plain water made the lettuce even better. 

Shredded Iceberg Lettuce


For this experiment, I wanted to see how dirty the ready-to-eat shredded iceberg lettuce is. I put a small handful of Dole shredded lettuce onto an agar plate and used a sterile swab to push it around. 


Then I rinsed the lettuce in a strainer under cool water for 30 seconds. I used my other hand to mix it up so it all got rinsed.


Then I put a small handful of this cleaned lettuce onto an agar plate. I used a sterile swab to push it around.


I incubated the agar plates overnight. 


Unfortunately, the shredded iceberg was filthy even after washing it! I repeated this experiment the following week with a new bag of Dole Shredded iceberg and it was still full of bacteria. 

Head of Iceberg Lettuce



After seeing how much bacteria bagged chopped iceberg lettuce has, I wanted to find out if a regular head of iceberg lettuce has that much bacteria. First I peeled off the outer 3 layers of the lettuce and threw them away.  Then I split the next leaf into 2 pieces. One piece I kept "dirty" and chopped it up. (I had previously washed my knife, sprayed it with 3% hydrogen peroxide, and rinsed it again.)


I rubbed this unwashed chopped iceberg lettuce all over a sterile agar plate using a sterile swab.


Then I took the other half of the iceberg leaf and rinsed it under tap water for 30 seconds.


I chopped this washed iceberg lettuce up. (Yes, I had washed and sterilized my knife in between.) I rubbed this washed chopped iceberg all around another sterile agar plate using a sterile swab.


I incubated the plates overnight in my warm incubator (about 95 degrees F) and examined the plates the next day. The iceberg barely had any bacteria.


I repeated this experiment several more times to compare bags of shredded iceberg to balls of iceberg. 








My results show that if you buy a ball of iceberg and discard the outer 2 layers, you are eating MUCH LESS bacteria than if you eat bagged shredded iceberg. I did not even wash the balls of iceberg in these experiments. I just compared a leaf from an unwashed ball of iceberg to the bagged shredded iceberg. 



 Kale

--soaked in tap water or tap water with white vinegar

Leafy greens like kale are very healthy but seem like they would be hard to clean. I have always just rinsed them in water,  and I want to see how well that works. For this experiment, I divided a large kale leaf into 3 similar sized sections. 


I rubbed one dirty piece of kale all over an agar plate. 

Then I soaked the 2 other pieces of kale in clean containers of tap water. The containers contained 2.5 cups of water. One container also contained 1 tablespoon of white vinegar (creating a 2.5% solution). I left the Kale sit in the water for 5 minutes. Twice during that time I used sterile swabs to mix them up a bit which would aid in the cleaning. 


When I removed them from the water, I set them on clean paper towel briefly. I did not let them dry. 


Then I rubbed the cleaned kale all around agar plates. 


I incubated the plates in my incubator which was about 95 degrees F for 15 hours. 


After 15 hours, I examined the plates.  The kale definitely improved after the soaking, but I did not see a significant difference with the white vinegar. I would like to improve my kale cleaning methods more. I will try rinsing the kale under running tap water and soaking it in a 25% vinegar and water solution in future experiments.

A few days later, I had a new head of kale and decided to try more experiments. This time, I compared dirty kale, kale that had been rinsed under tap water for 30 seconds, and kale that had been sprayed with Fit fruit and vegetable wash. First I tore up a leaf of dirty kale and rubbed it onto a sterile agar plate.


Then I rinsed another leaf of kale under tap water and rubbed it for 30 seconds. 


I tore it up and rubbed it on another sterile agar plate. 


I sprayed 2 sprays of Fit fruit and vegetable wash to a third piece of kale and rubbed it in for 30 seconds. Then I rinsed this leaf for 30 seconds under tap water. 


I tore up this leaf and rubbed it onto an agar plate.


I incubated the plates overnight and examined them the next day. 


As you can see, the kale was still pretty dirty after the 30 second rinse but the Fit spray significantly improved it. I am going to try soaking kale in 25% white vinegar to see how that does. 
                                                                                                                   

Spinach

Next, I decided to find out how much bacteria is in ready-to-eat spinach. After all of my previous experiments, I did not have high hopes.


First, I rubbed 5 spinach leaves onto a clean agar plate.


Then I rinsed a handful of spinach under warm water for 30 seconds. I mixed it around with my other hand.


I rubbed 5 of those rinsed leaves onto another agar plate.


Then I soaked a handful of spinach leaves in either warm tap water or warm tap water with 25% white vinegar for 10 minutes.


After 10 minutes, I rinsed the spinach under warm tap water for 30 seconds.


Then I rubbed 5 spinach leaves onto clean agar plates.


I incubated the agar plates overnight and examined them in the morning. As you can see, the ready-to-eat spinach (like any bagged lettuce) has tons of bacteria. Rinsing it in water for 30 seconds does almost nothing. Soaking in 25% white vinegar for 10 minutes helps a little. I am going to do another experiment soaking the spinach in hydrogen peroxide. 


I repeated this experiment using food grade hydrogen peroxide. I purchased the food grade hydrogen peroxide at a local healthy food store. It is 35% hydrogen peroxide and supposedly contains less stabilizing chemicals. It needs to be kept refrigerated and is harmful if you get it on your skin. 


First, I rubbed 5 unwashed leaves of spinach onto a clean agar plate. Then, I soaked a handful of spinach in 25% white vinegar for 10 minutes. I soaked another handful of spinach in a 3% solution of hydrogen peroxide in water for 10 minutes. To make the 3% solution, I put about 1/6 cup of 35% hydrogen peroxide (which is half of 1/3 cup) into a large measuring cup and then added tap water until it came to 2 cups total. 


Then I rinsed the spinach in warm tap water for 30 seconds.


I rubbed 5 leaves of the spinach onto clean agar plates. 


I incubated the plates overnight and examined the results in the morning. The 10 minute soak in hydrogen peroxide cleaned about as well as the soak in white vinegar. I repeated this experiment again and got similar results. However, it seems that bagged lettuce and spinach is always going to have a lot of bacteria. Please remember that most of the bacteria is harmless. Consider it a "probiotic". I think it is important for our bodies to learn to deal with the regular bacteria in our food. In addition, it has been shown that bacteria and viruses can enter lettuce through the roots and be INSIDE the leaves1. So, no manner of cleaning is going to get rid of all of the germs. Moderation is the key here. 


Rinsing spinach in hot water


I repeated the experiment with organic spinach. This time I rinsed the spinach in a strainer under either cold water or hot water for 1 minute. My hot water at the time was about 115 degrees F. I moved the leaves around with my hand while I rinsed. The water was so hot that I wouldn't have been able to touch it if I didn't have the gloves on. 


Then I rubbed 5 spinach leaves on to each agar plate and incubated the plates overnight. As you can see, rinsing in hot water killed a significant amount of bacteria. The heat was killing it. This isn't really cleaning it so much as it is "cooking" it. 


I repeated the experiment rinsing spinach in hot water for 1 minute again. As you can see, the hot water does kill a lot of the germs.

Hydroponic Lettuce

I had a request to test hydroponic lettuce. This lettuce is grown without soil so one would think it would be pretty clean. 


I tested 2 of the inner leaves. I tore them up and rubbed them on agar plates without washing them at all. After incubating the plates overnight, almost no bacteria grew. The hydroponic lettuce was very clean. I repeated this experiment with another bunch of the same brand of lettuce and it was also very clean.


How do I wash my spinach and lettuce?


To be 
completely honest, I scared myself out of eating salad for about 6 months after I did these experiments. Then I calmed down and realized that people are eating dirty salad every day and are usually fine. Most of this bacteria is harmless. I believe it is good and important for our bodies to learn to deal with the bacteria naturally found in our food. In fact, our intestines require a lot of good bacteria to function properly. So now I eat bagged spring mix salads a few times a week, and I use fresh bagged spinach every day in my morning blueberry-spinach-banana smoothie. I DO wash them, of course. I don't spend a long time washing them because I don't feel that it made that big of a difference. Basically, I put the spinach or lettuce in a salad spinner, fill it up with cool water, swish it around for a minute, drain it, and repeat. Then I spin it and eat it. This cleaning method won't get rid of much of the bacteria, of course. However, it will get rid of some other yucky stuff.


In just about every box of triple washed ready-to-eat spinach that I search through, I find little critters of some sort. Everyone has their own opinions but I would prefer NOT to eat these.


These critters come off with a quick rinse with cool water in the salad spinner.



So, rinsing the spinach and lettuce in cool water in the salad spinner is worth it to me. It at least removes visible bugs. I fill the salad spinner up with cool water, swish it around for 20 seconds, drain it, fill it up again, swish it around again, drain it and spin it. 






1. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3416640/




VIEWER COMMENTS

Have you ever seen reports on the levels of salmonella on chicken raised in hardcore factory  facilities (like Tyson) v. organic/ free range chicken? The organic versions are usually far worse in detectable levels of disease-causing bacteria like listeria, E. coli, salmonella, etc. So yes it is still very important to cook your chicken! ;) That said, factory farms also douse their products with chemical baths during the packaging process. They are so filthy that if they didn't they would sicken people right and left... so I was not surprised at your "clean" result with the chicken. I doubt you have the bandwidth to key out your colonies to species but it would be interesting to see if any of the bacterial growth you are seeing on the agar plates is disease causing as opposed to "benign".
--Michelle