This post is going to read a little bit like an episode of MythBusters. Testing claims is not something I plan to do regularly in this blog, but this particular one concerns something I’m very interested in (knives) and something I’ve written about in the past. Yes, this is going to be my third post on what makes food turn brown. See my other two here and here. This blog is about science, and science is fundamentally about testing claims; either claims made by yourself or claims made by someone else. Science typically deals with testing a hypothesis — a formal, scientific claim made by a scientist — but the scientific method can be used just as well to test less formal claims.
On to the topic at hand…
A friend of mine gave me a ceramic paring knife for christmas (Thanks, Jack!). In researching the properties of ceramic knives, I came across a claim that seems to be made by almost everyone advocating their use: using ceramic knives prevents food from turning brown after it has been cut. See, for example, this video.
One common explanation given for this purported phenomenon is that contact with the food causes steel blades to corrode and leave brown residue on on the food. Ceramic does not corrode, so it cannot cause food to turn brown for this reason. The video I linked to mentions “ion transfer” as the mechanism of browning. I don’t know what that is, but its predictions are basically the same as the corrosion mechanism.
I have written before about one factor that makes food turn brown (here and here). You will understand this post better if you read my previous ones, but I will summarize them briefly in case you’re short on time: When a fruit or vegetable is damaged (either by cutting or bruising), it produces a brown substance that prevents bacterial infection. This process is triggered by exposure to atmospheric oxygen.
But let’s pretend for a minute that we don’t know anything about enzymatic browning. What we think will happen will only get us so far — let’s do some science and see what actually happens.
If the stories are true — that food will turn brown faster with steel blades because of the corrosion of the steel — then a blade made of non-stainless steel will cause a stronger effect of food browning than a blade made of stainless steel, and both steel blades will cause a stronger effect of food browning than a blade made of ceramic.
I will use three paring knives in this experiment:
Top: The ceramic knife that was given to me. There are no markings on the blade and I threw away the package, so I have no idea what brand it is, but I know it was made in China. I’m pretty sure it is made of zirconium dioxide.
Middle: This is a handmade knife made of 52100 steel. This steel has no corrosion resistance to speak of.
Bottom: A J.A. Henckels Zwillinge Pro S paring knife. The steel is X50CrMoV15. It is very resistant to rust and other corrosion.
I have selected three foods to test these knives on: apples, potatoes and onions. I have already discussed the process of browning in apples and potatoes, so they will be good for this experiment. The naturally-occurring chemicals in onions can be corrosive to steel, so this is another good food to use in this experiment. I cut each of these three foods with each of the three knives to see what would happen.
Here is the initial setup for this experiment. The column on the left was cut only by the ceramic knife, the column in the middle was cut only by the non-stainless steel knife, and the column on the right was cut only with the stainless steel knife:
After four hours, not much has happened. There is a little bit of browning in the apples, but nothing in the potatoes or onions:
After a total of six hours, the apples are continuing to brown, but the potatoes and onions don’t look any different:
After a total of nine hours, all of the potatoes are starting to brown very slightly:
Twenty four hours into this experiment, the apples and potatoes are quite brown and dried up. The onions have dried a little bit but haven’t changed color:
At the end of the experiment, and at each documented time, there is no discernible difference in brownness between the foods cut with the ceramic, stainless, or non-stainless blades.
The point of this is not just to test this particular claim, but that it can be very easy to test claims that people make. While this particular claim turned out to be false, other claims are true. The purpose of science is not to find a particular answer, but to find out what the correct answer is.
This experiment also highlights the importance of comparing different experimental conditions. A poor way to test this claim would have been to cut food only with a ceramic knife and to leave it out. I am used to potatoes turning brown in just a couple of hours, but in this experiment they were still white after nine hours. (It was fairly cold in my kitchen when I did this experiment which probably explains why it took so long for the potatoes to turn brown.) If I had only used potatoes and only cut them with a ceramic knife, I might have incorrectly concluded after nine hours that the ceramic knife prevented the browning. But by comparing the three knives simultaneously, I found that the potatoes all took longer than I expected to turn brown and not just the one I cut with the ceramic blade.
A scientist doesn’t have to be someone with a fancy degree working in a laboratory. A scientist is just someone who makes discoveries about the world using the scientific method. Go be a scientist.
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