France to ban culling of unwanted male chicks by end of 2021 BBC news told us yesterday, and I could barely contain my disbelief. Unwanted males are the price you pay for specialised breeds, whether dairy cows or layer hens.
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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 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 huge range of stirring speeds. As David had suspected, the viscosity declines linearly with the speed of rotation.
Like good experimentalists, we turned to a nearby, very heavily used whiteboard to try and get a grip on what was happening.
David drew a circle, a cross section through the spurtle, and an elongated ellipse, a cross section through a spoon, and flow lines as they moved through the porridge. For any given speed of movement, the energy needed to move the spurtle would be lower than the energy needed to move the spoon. Likewise, for any given amount of energy, the spurtle would move faster than the spoon. Shear stress depends on speed of movement. So, with a spurtle, it takes less energy to get a certain amount of shear stress and thus a certain amount of lump-disruption.
But there’s more. Because the viscosity of porridge goes down as you stir faster, the rapidly-moving spurtle is even better at breaking up lumps than the slower-moving spoon.
And that is why a spurtle is better for stirring porridge. It’s more efficient.
Conceptually, my real breakthrough moment came as David was drawing the flow lines. There is a dead area, just in front of the spurtle and the spoon, where the porridge is not flowing past the stirrer. The dead area impedes the forward movement of the stirrer, and is considerably bigger for the spoon. However, if you remove the large blade of the spoon, you’re left with a spurtle.
Some additional thoughts:
My overall conclusion: if you have a spurtle, use it. If you don’t, don’t worry about it too much.
Huge thanks to David Gagnon and Robert Seyfarth for making this bit of experimental fun possible. And of course, any errors in this account are mine and mine alone.
Quite by coincidence, I listened to two podcasts about coffee back to back. Well, it wasn’t truly a coincidence; I saw that there were two in my queue and so I interfered with the ordering to listen to them one after the other. Anyway, Benjamen Walker’s Wake up and smell the coffee was the kind of podcast I wish I could do more often.[1] Benjamen took his growing love for coffee on a global tour of discovery that took in Paris, Copenhagen and Nairobi. I don’t know whether he planned it that way, or simply took advantage of various opportunities (there was something in the credits about having received a grant to do it, so maybe it was planned as a whole.) The result was an entertaining, complex episode that exposed parts of the coffee chain that even coffee fans might not know about.
Perhaps the most telling point was made by one of the founders of Coffee Collective, in Denmark, who explained that because in the past most coffee had been produced by enslaved labour, the cost of coffee barely reflected the cost of production. That cheapness created a culture that happily tips down the drain coffee that has sat neglected too long. You wouldn’t do that with a glass or two of wine left in a bottle (although you might feed it to your vinegar mother). The historically low price of coffee is probably what also causes right-thinking people to blanch at the thought of a $15 shot.
Slave-grown coffee also came up towards the end of Lord Bragg’s sleepwalk through an episode of In Our Time on coffee. In fact, it may even have come up only during the podcast listeners’ special treat of bonus material from Melvin and his guests. It wasn’t the only interesting thing to arise late and, in my view, much too briefly. There was also the foundations of Italian coffee culture, which mandated a top price for a coffee but only if it came without service. Standing at the bar and gulping a tiny espresso was thus cheaper than paying a lot more to linger at a table. In Italy, and in France too, the sale of coffee went to the guilds that also sold distilled spirits, which explains a lot.[2] The stove-top Moka coffee maker, launched in the 1930s and still a classic icon of Italian coffee at home, became popular partly because it made a reasonable facsimile of an espresso at home, and also because it was it was futuristic, modern, and made of aluminium.
Then there are the huge differences between Italian coffee culture and the “Italian” coffee bars in London in the 50s and 60s, where patrons did linger and where the coffee was mostly milk, as it remains in so many places today. Class and coffee is a ripe area for discussion. Starbucks, apparently, was popular in blue states 20 years before it began to make inroads in red states. In that context, and the cappuccino-sipping Guardian reader, another guest raised the milkiness of caffé latte and its kin as infantilising, people walking down the street and “sucking on their sippy cups”.
Coffee producers don’t drink coffee, just as cacao farmers don’t eat chocolate. Preparation is too much of a fuss, for one thing, and for another some countries outright banned coffee roasting; beans had to go for export. (Benjamen Walker’s interview with the first coffee roasters in Kenya was eye-opening.) Now farmers are leaving the land, in part because they cannot earn enough growing coffee, and moving to the city, where they encounter, and drink, instant coffee. Coffee remains a mostly urban drink, and as urbanisation increases in the developing world, so does coffee consumption.
What I’m saying, I guess, is that the guests on In Our Time seemed to have much more interesting information to offer than the same old stories of frisky goats, the growth of Lloyds List and invention of The Tatler and The Spectator. Perhaps I know too much, but I do think I have learned a lesson listening to those two podcasts together.
What that is, I’m not yet ready to say.
Syndicated from the mothership
Photo by Rodrigo Flores on Unsplash
I have, in fact, made a similar episode: Pushing good coffee. ↩
Caffé corretto remains one of my favourite examples of Italian hypochondria and pragmatism. I did not, however, know that the fascists had outlawed espresso machines because coffee was luxurious and an import. ↩