Washing the kitchen sink experiments

Doesn't just washing the sink with soap, water, and a washcloth remove germs? 

Answer: Not very well.

Most of us have assumed that we don't really need cleaners to KILL germs. We can just wash them off with soap and water, right? This is a picture of my mother washing my kitchen sink. She loves to scrub out sinks and does it whenever she visits any of her children. You can always count on my mom to do the dishes, wipe up, and scrub out the sink whenever she visits. She can't cook, but she sure can clean up! She doesn't use any cleaning chemicals. She just used Dawn dish soap and a cotton washcloth. But does that work as well as my beloved 3% hydrogen peroxide? 

Experiment Details

Here I set out to determine if I could remove most of the bacteria by just washing the kitchen sink really well with soap and water. For these experiments, I tested my kitchen sink and my neighbors' sinks. My sink usually didn't have enough bacteria to start with to get good results because I clean it too often. A sink needs to not be cleaned for a week before I can do an experiment in it. Luckily, my neighbors don't seem to sanitize their kitchen sinks very often and don't mind me doing experiments in them. First, I rinse the sink with warm water to remove food debris. All of the sinks LOOKED clean to start. Then, I swabbed the dirty sink and rubbed the swab onto an agar plate. That plate would be the dirty control.

Then I put some blue Dawn dish soap onto a cotton washcloth (wet with warm water) and washed one half of the sink for 2 minutes.  

I really did a thorough job. I doubt many people would normally wash this hard for this long. 

Then I rinsed the sink with warm water for about 30 seconds. I did not use hot water because I wanted to test how well the washing removed germs, not how well the hot water would kill them. I can do that another time. 

Next I swabbed the cleaned sink and rubbed the swab onto an agar plate. (I did not dry the sink.) 

Then I sprayed 30-40 sprays of 3 percent hydrogen peroxide onto the other side of the sink, let it sit for 5 minutes, swabbed that side, and rubbed the swab onto an agar plate. I do not wipe or wash when I use hydrogen peroxide. I just spray it on, let it sit, and then swab it. I realize that is a lot of hydrogen peroxide, but it takes that many sprays to completely cover the sink since I don't wipe it or spread it around at all. If I miss a spot, that will mean live bacteria on the agar plate. However, when you clean with hydrogen peroxide, you can use less as long as you spread it around to completely cover the surface. I don't wipe it for these experiments because I don't want the "wiping action" to contribute to the germ-killing. 


Here are the results when I tested my neighbor's sinks. 

Here are the results when my mom cleaned my sink. She only scrubbed for 1 minute, though. 

As you can see, kitchen sinks are generally pretty germy and it is a lot harder than I thought to wash the germs off of the sink. I am going to continue spraying my sink with 3 percent hydrogen peroxide. If you don't want to sanitize your sink every few days, that is probably ok as long as you don't eat out of the kitchen sink or prepare fresh food in it. Don't wash lettuce directly in your kitchen sink or eat a strawberry that fell into your sink. 

Comparing washing the sink with blue Dawn to washing with just a wet washcloth. 

After the previous experiments where it appeared that washing with blue Dawn did not remove much bacteria, I wanted to compare washing with blue Dawn to washing with just a wet cotton washcloth. For these experiments, I used identical 100% cotton washcloths (Target Threshold brand which I love). The cloths were wet with tap water. One cloth had 1 teaspoon of blue Dawn on it. 

I swabbed each side of the sink separately so there were 2 dirty controls. 

Then I washed one side with the washcloth with blue Dawn for 2 minutes. (That is much longer than anyone would normally wash their sink.) 

Then I rinsed the sink for about 20 seconds with warm water. 

Then I swabbed the "clean" sink and rubbed the swab onto an agar plate. 

Then I washed the other side of the sink for 2 minutes with just the wet cloth with no soap at all. I rinsed it for about 20 seconds with warm water and swabbed that side. The plates were incubated for about 48 hours in my warm incubator.


I was stunned that sometimes my washcloth with just water sink turned out cleaner than the sink I washed with Dawn!  I repeated the experiments several times and that didn't always happen. Sometimes the cloth with water and the cloth with blue Dawn did about the same.  

How could the washcloth with just water do about as good or better than a washcloth with blue Dawn? I even swabbed my blue Dawn itself to make sure it wasn't growing bacteria. It wasn't. 

My best GUESS as to why the washcloth with water removed bacteria better than the washcloth with Dawn is FRICTION. There was a lot more friction generated when I washed the sink with just the washcloth and water. The bubbly Dawn acts as a lubricant making washing out the sink easier with less friction. I repeated the experiments pressing as hard as I possibly could with the washcloth and Dawn combination to increase the friction. 

When I pressed super hard, the washcloth with blue Dawn did slightly better or about the same as the washcloth with just water. 

I think the take-away-message here is that it is very difficult to wash all of the germs out of your sink. Pressure and friction are probably more important than detergent. However, I have only tested one kind of dish washing detergent and one kind of cloth. I have many more experiments to do! I want to test different soaps and different cloths. I scratch pad might work much better. This is a great experimental set-up for me to test other soaps and even hand soaps. Feel free to e-mail me your soap suggestions. phd.annie at gmail.com. Also, please like my facebook page if you want to be alerted when new results are posted. Thank you.

--Annie Pryor, Ph.D.