This isn't a guess. This is something that applies to most people I know. That means that this "myth" is true around me. I've cut steak open a lot, and I can see easily that more juices flow out when I use a dull knife compared to when I use a sharp knife. A simple thought experiment: do you think cutting open a steak with a razor blade will make the same amount of juice flow than with a butter knife? I know for a fact that the razor blade will spill almost no juice, and the butter knife will spill a lot, by pressing the steak.
The question is now: Do most people have dull knives? Or do they have sharp knives? Most people around me have dull knives, except for chefs. I've seen lots of cooking videos, and chefs often tend to have good and sharp knives. So my guess here would be that chefs have sharper knives than most people. Meaning that a chef can't properly test this myth unless they take this factor into account.
I haven't been able to find any data on knife spending, or average sharpness of knives depending on if people are chef or not. Here's an experiment we could try: have people evaluate the sharpness of a single knife, and see if non-chef tend to evaluate it as better than chefs.
I'll finish this by saying I have a huge respect for J. Kenji López-Alt. I only raise that point because here, I believe being a chef can introduce bias.
> I know for a fact that the razor blade will spill almost no juice, and the butter knife will spill a lot, by pressing the steak.
> The question is now: Do most people have dull knives?
I would say the question is: Does cutting a steak with a dull knife cause enough juice loss that it's actually going to be noticeable in the finished steak?
Even the author admits that you lose just by cutting it open. But he says that it isn't enough for anyone to notice. So the question isn't whether or not the dull knife causes more juice loss, it whether it's even perceptible. And you're going to have a hard time convincing me that you actually know the answer to that question off hand.
That's a good point, I forgot the important part: are people going to notice the difference? I think they would, for people around me, but this would be hard to extend to other places. The reason I think that is that where I live, in France, steaks tend to be much smaller than what I see when people talk about "steak" online. I'd say the most common size would be something like ~150g. They're also relatively thing, often less than an inch. At that size, cutting with a dull knife would lead to a lot of juice loss compared to all the juice in the steak.
So that's a new variable to take into account with knife sharpness: steak size.
