In my desire to spend some more time doing funky cooking, I bought a little bag of xanthan gum. I’ve been thinking about making a sauce or gravy with it. Reading this little ketchup article (thanks Hairpin!) tied the ‘two-finger-tap’ to my new food additive.
One interesting fact about ketchup that everyone should know is that it’s a non-Newtonian fluid. Naturally, ketchup is rather thin and watery, because the tomato pulp that gives it consistency is sieved out. As a result, commercial ketchup makers add a small amount of xanthan gum to their ketchup recipes to thicken it. But this ingredient has another side effect: It turns ketchup into a shear thinning fluid. In other words, how quickly ketchup flows depends upon the stress that is being placed upon it.
That ketchup is non-Newtonian is the main reason why getting it out of a glass bottle is so slow. Allowed to flow naturally, ketchup only travels at a speed of 147 feet per hour. The only way to speed it up is to apply force, which through the principle of shear thinning decreases the ketchup’s viscosity, and thus increases its flow rate. This is why you have to thump a bottle of ketchup to get it flowing from the bottle. The concussive force makes it flow faster.
But despite common opinion, the bottom of a bottle of Heinz Ketchup isn’t actually the best place to thump it. If you apply force to the bottom of a bottle of Heinz, the ketchup closest to where you smacked will absorb most of the force of impact. It will flow freely, but the ketchup that is viscously clogging the neck and mouth of the bottle won’t, leaving you no better off than you were before. The solution is to trigger the shear thinning effect at the top of the bottle, not the bottom. That unclogs the mouth and lets the ketchup below to freely flow.
So while the substance of Heinz’s “57 Varieties” label may be just a fanciful whim on the part of the company’s creator, its positioning is deliberate. It’s a target. By simply tapping the label with two fingers, you create the optimal conditions for shear thinning, transforming non-Newtonian ketchup into a free-flowing liquid. Physics!