Why a spurtle makes a superior porridge stirrer

Here, for the New Year, is a confession and apology.

I was completely wrong about porridge-stirring implements and I am here to make things right.

In the episode about Porridge, I mocked the idea that the traditional Scottish spurtle, which to all intents and purposes is just a stick, might be better for stirring than a spoon. My (faulty) reasoning was that a spoon offered a greater surface area to break up lumps. In fact, as I now know, the stirrer does not break up the lumps directly.

I noted in Eat This Newsletter 110 that a friend of a friend, who is a specialist in fluid dynamics, said that he had “a working, testable hypothesis”, which required only some worthy porridge in order to test it. So, I packed a bag of my favourite oats ready to meet up at Georgetown University in Washington DC.

I should explain that Dr David A. Gagnon, the physicist in question, is the son-in-law of one of my dearest friends, who happened to be visiting when I was editing the episode. That was how David came to know of my puzzlement – and he has access to wonderful machines that can make very accurate rheological measurements. Rheology is the study of flow, and David’s idea was that the faster the stirrer moved through the porridge, the lower the viscosity of the porridge would be. Shear stress in a material like porridge would be greater at higher flow speeds, and it is the shear stress that gets rid of lumps.

You can think of shear stress as being a difference in the speed of flow of the liquid across a small distance, multiplied by its resistance to flow. That resistance to flow is what we think of as viscosity. In a viscous liquid, like honey, the stirrer produces a lot more shear stress than it would moving through water at the same speed. But there’s a crucial difference between honey and water.

In water, the viscosity is the same no matter how fast the stirrer moves. Honey is different; the viscosity decreases the faster the stirrer moves. And that means that the stirrer produces more shear stress when it is moving quickly through the honey than when it is moving more slowly. David’s idea was that fast stirring would result in lower viscosity, and lower viscosity would result in greater shear stress.

To get back to porridge, and to lumps, consider a lump as the stirrer moves past it. Relatively speaking, the part of the lump in the slower flow near the edge of the stirrer is being held fast, while the part in the faster flow is being pulled away, and that’s what destroys the lump, pulling it apart.

The rheometer measures all that. It has two circular plates, one above the other, and the bottom plate can be rotated very accurately through a known distance at a known speed. David carefully loaded a little bit of porridge onto the bottom plate and then lowered the top plate to make contact with the porridge. The layer of porridge mechanically connects the bottom plate to the top plate, which in turn is connected to sensors that accurately measure how fast and how far it rotates. Meanwhile, David had programmed the machine to oscillate the bottom plate over a wide range of frequencies. The faster the oscillation, the greater the stress on the porridge. The machine then takes all those measurements and creates a set of graphs that describe the rheological properties of the porridge.

The viscosity graph, photographed directly from the screen.The graph appeared and I confess it was really exciting to watch the points appear. A straight line down from top left to bottom right over a h...