Dishwasher Testing


I have 2 large lab puppies that like to help me do the dishes. Luke is our black pup, and his sister Leia is the white one. She actually belongs to our backyard neighbor, but the pups spend most of the day together. They are adorable trouble makers and have a great life. There is nothing they like more than licking the rest of the meatloaf out of the pan or cleaning off a peanut butter covered spoon. I let them lick the dirty dishes for 2 reasons. 

#1. They are the only ones who want to help me with the dishes ever. 

#2. I assume that all the germs from the dogs' tongues are killed in the dishwasher along with all the regular dirty dishes germs including germs from raw meat. 

Don't worry, I have them trained well so they NEVER lick the clean dishes. They seem to understand. However, I wanted to be absolutely sure dishwashers were working like I expected so I tested them. 

Experimental Procedure

To do these experiments, I tested my dishwasher and 3 of my neighbor's dishwashers. (I have the BEST neighbors.) I have a Whirlpool dishwasher that is less than a year old. 

First I swabbed several of the dirty dishes that were sitting in the dishwasher. 

Then I rubbed that swab onto an agar plate. 

Next, I ran the dishwasher on the hottest, longest cycle available. It was the heavy cycle, with "sani rinse", "heat dry", and "hi temp wash". The cycle took 4 hours. I used Palmolive Eco Dishwasher Detergent

, although I suspect that the type of dishwasher detergent is irrelevant. The heat is likely the more important factor. 

When the cycle was finished, I swabbed the dishes again and rubbed the swab onto another agar plate. The plates were incubated for 48 hours in my warm incubator (about 90 degrees F). 


If you don't have much experience looking at bacteria on agar plates, let me explain. The white/yellow dots are colonies (or piles) of millions of bacteria. I single bacterium on the plate will double and double and keep doubling while in the warm incubator. After 24-48 hours you can visually see the pile of bacteria. Not every kind of bacteria will grow on these plates, and viruses don't grow on these plates. So, just because a plate looks clean, doesn't necessarily mean that no germs were present. I'm also not determining what kinds of bacteria are present. That is too much work and requires more equipment than I have. It is likely a mix of harmless and harmful bacteria. 

As you can see, my dishwasher did a great job killing germs! There was only one colony on the plate from the clean dishes. Now, there was still dried on peanut butter and ketchup on the dishes. I don't think my dishwasher is that great at removing food. However, the clean dishes are at least pretty sterile. 

My neighbor Bethany has a GE dishwasher that is about 4 years old. I did her test using the hottest setting available as well. 

As you can see, her dishwasher did great, too. 

My neighbor Jennifer has older Whirlpool dishwasher, probably 10 years old. I tested it on the hottest, longest cycle which took about 3 hours. 

It did very well, but not quite as good as mine and Bethany's. There were just a few colonies. I think this qualifies as good enough. 

My neighbor, Kelly, has an old GE dishwasher, at least 10 years old. I ran it on its "pots and pans" cycle with "hot start" and "heated dry". It took about 2 hours. 

Kelly's dishwasher did very well. There were a few colonies. I think this is still good enough. 

Is heated dry necessary?

A lot of people want to save energy and use the shortest, coldest wash cycle with no heated dry. However, the hotter and longer it is, the better it will kill germs.  So, I wanted to test it to see what difference the longer cycle with heated dry really makes. 

I ran my dishwasher on the normal cycle with no extras and no heated dry. The cycle was still 2 hours and 36 minutes. 

It still did great. There was one colony on the "after" plate. However, the "before" dishes plate was not nearly as full as usual in this experiment, so it is possible that my dishes just weren't as dirty this time. 

I ran my neighbor, Jennifer's dishwasher on the normal cycle with no extras and no heated dry. 

It appears that the normal cycle with no heated dry only left slightly more bacteria alive that the cycle with heated dry. However, these dishes were not nearly as dirty "before" for some reason. 

My previous work has shown that the normal dishwasher cycle without heated dry is not quite as good at killing germs in sponges. If you would like to see those results and my other tests to see which disinfecting methods work best for sponges, read my sponge page. So, I still vote for using the longest hottest cycle with the heated dry if you want your dishes to be as clean as possible. 

How do hand-washed dishes compare?

I have already shown that dishwashers do a pretty darn good job killing germs. But I wanted to test to see how much bacteria is on clean hand-washed dishes. To do this, I swabbed my hand-washed dishes and some of my neighbors' hand-washed dishes. No one knew that I was going to do this. So, no one did an extra good washing or anything. For the negative control, I just swabbed some tap water and rubbed it onto an agar plate. 

Kelly's hand-washed dishes

As you can see, Kelly's hand-washed dishes were very germy. However, I also tested her kitchen sponge that day, and it was a biohazard. Her drying mat was also full of bacteria. So, this is a great example of why you should NOT be washing your dishes with a contaminated sponge. You also need to wash those drying mats on hot with chlorine bleach. 

Jennifer's hand-washed dishes

My hand-washed dishes

I apologize for forgetting to photograph my tap water negative control. 

Overall hand-washing dishes is not as good at killing germs as the dishwasher. However, hand washing could be improved by using a hot water rinse and being sure to use a clean cloth to wash and dry.

Results Summary

The dishwashers that I tested do a great job killing bacteria if you use the hottest, longest dishwasher cycle available. They even still did very well on the shorter cycle. They might leave a few germs alive, and I can't vouch for everyone's dishwasher in the world. However, I think you should absolutely use your dishwasher if you are lucky enough to have one. Even if it still leaves food stuck on, if the dishes come out super hot, it is killing germs. Hand washed dishes seem to have more bacteria on them than dishwasher dishes. However, I have not tested all methods of hand washing. If you don't have a dishwasher, you might still be doing a great job on your dishes if you use hot water and clean cloths to wash the dishes. You can even boil certain high risk dishes or silverware. In the Little House on the Prairie books, Laura said that they scalded dishes and boiled laundry.