Purell Surface Disinfectant and Citrus II Independent Testing

How well does Purell Surface and Citrus II kill germs?

Answer: Purell Surface kills germs on relatively clean surfaces, claims to kill a norovirus surrogate, and is safe to use on more surface types than bleach or hydrogen peroxide.


Procedure

To test these products, I did the same countertop experiments that I usually do. I sectioned off squares on my countertop using masking tape. I made "germ water" using dirt from the backyard and water. I filtered it through a paper towel to remove chunks. Then I scraped colonies of bacteria from a previous days agar plate and mixed that in. The previous days plate was a "dirty sink" plate. So, there was lots of dirt and lots of bacteria in these experiments. I put 1mL of germ water onto each square. 


I used a gloved finger to spread the germ water around the square. Then I let the squares dry completely which took about 2 hours. If you would like to see pictures of every step in this process, please scroll to the top of this mile-long page. I go into greater detail about my experimental procedure up there. 


After the squares were dry, I put .5mL of each product onto its respective square. I spread the product completely around the square using a clean gloved finger. For the dirty positive control, I just put water on the square. For the clean negative control (which didn't have any germ water on it), I just put sterile water onto the square. I set timers and let the product sit on the square for 5 minutes. 

After 5 minutes, I swabbed the square and rubbed the swab all over a clean agar plate. 



The plates were incubated for 48 hours in my warm incubator (about 90 degrees F). 

Understanding Agar Plates

In case you are new at looking at agar plates, let me explain. Agar is a Jello-like substance that bacteria and fungus like to grow on. The whitish/yellowish dots you see are colonies (or piles) of millions of bacteria. Some types of bacteria are not able to grow on these agar plates. Viruses can not grow on these agar plates. So, just because a plate looks clean, doesn't mean that no microorganisms whatsoever were present. We assume that a clean plate means that most bacteria were killed. However, there is the tiny possibility that the cleaning product just stopped the bacteria from growing as opposed to actually killing it. Also, I can't make any determination as to the time it took the product to kill the bacteria since the product was still with the bacteria on the agar plate. (Both the bacteria and the cleaner would be picked up in the sterile swab.) So, I don't know if it took 1 minute to kill the bacteria or hours. These experiments are still very useful when used to compare cleaning products and cleaning methods. 

Results

In these experiments, I'm comparing everything to 3% hydrogen peroxide which always works really well. Most cleaning products do not do well in these experiments because they can't handle so much dirt and germs. It is surprising how many products are supposed to be used on already clean surfaces. Some products that don't do well in these experiments, might still do a good enough job on your only slightly germy countertop. 


Citrus II seemed to do a little better than Purell Surface. Purell Surface is a product advertised to be safe for food contact surface and its active ingredient is ethanol. I've learned from my experiments that most alcohol products don't do well in the presence of actual dirt. So, Purell Surface might be fine on relatively clean kitchen countertops. I will be testing it in the sink experiment. 

Sink Experiments

Many cleaning products are not strong enough to do well in my countertop experiments with dirt and lots of germs. So, I also like to test everything in a real life situation, the kitchen sink. Any cleaner that we expect to kill germs, should be able to kill germs in my kitchen sink experiments. To do these experiments, I use my neighbors' kitchen sinks. They don't disinfect their sinks very often so they are usually germy enough for me to do a good experiment. I can only do one experiment per week per kitchen sink because there needs to be time for plenty of bacteria to grow again. So, these experiments take some time. I have not done all of the products yet. Check back for more results. 



First I rinse off the sinks so there is no food debris stuck on them. Then I swab both sides of the dirty sink and rub the swabs onto agar plates. Then I spray the product all over one side of the sink. I usually use at least 20 sprays to fully cover that side of the sink. The other side is the control and I spray that side with 3% hydrogen peroxide because that always seems to generate very clean results.


Purell Surface in the sink

For these experiments, I used 25 sprays of Purell Surface disinfectant on one side of the sink and 25 sprays of 3% hydrogen peroxide on the other side of the sink. I let them sit for 5 minutes and then swabbed. 




As you can see, the Purell Surface worked well. However, in this experiment, I did not rinse the product off the sink. The the hydrogen peroxide and the Purell surface were picked up by the swabs and were incubated with the bacteria on the agar plate.  So, I repeated the experiments. This time I used 40 sprays of each cleaner. I let them sit for 10 minutes. I swabbed the sinks. Then I thoroughly rinsed off the cleaning product and swabbed AGAIN.  I also swabbed the sides and the bottom of the sinks separately to see which was dirtier or if perhaps the cleaner has a harder time killing germs on the sides of the sink because it drips off.



The results show that the bottom of the sink is usually dirtier than the sides. Also, the Purell Surface and the Hydrogen peroxide left a lot of live bacteria after they were rinsed off. So, they seem to take more than 10 minutes to kill the germs. 

There is still hope for a clean sink. When I spray my sink out with 3% hydrogen peroxide at night and leave it on all night until morning, then rinse it off, the sink is very clean. The side that had no hydrogen peroxid on it was still super dirty even after sitting dry all night. 



In conclusion, I think Purell Surface is a very good disinfectant if you use it on pretty clean surfaces. It contains a lot of ethanol and it makes me cough when I use it. However, it claims to kill norovirus surrogate so that is a bonus. Obviously, it probably doesn't kill norovirus on a dirty surface.