Have you ever noticed that the bubbles in a pint of Guinness, or any stout fall, yes, they descend down the glass…
Falling stout bubbles explained
Irish mathematicians may have solved the mystery of why bubbles in stout beers such as Guinness sink: it may simply be down to the glass.
Simulations suggest an upward flow at the glass’s centre and a downward flow at its edges in which the liquid carried the bubbles down with it.
But the reasons behind this flow pattern remained a mystery.
Now a study on the Arxiv server reports simulations and experiments showing the standard glass’ shape is responsible.
Many stout beers contain nitrogen as well as the carbon dioxide that is present in all beers.
Because nitrogen is less likely to dissolve in liquid, that results in smaller and longer-lasting bubbles.
But it is the sinking bubble that has confounded physicists and mathematicians alike for decades.
Like many such “fluid dynamics” problems, getting to the heart of the matter is no easy task; only recently was it proved they actually sink rather than being the result of an optical illusion.
Now the University of Limerick’s William Lee, Eugene Benilov and Cathal Cummins have discovered the simple answer to the problem – and a test that can be carried out by consumers as well.
The team has been generally interested in the formation of bubbles in liquids.
“One of the things we found was it’s actually very easy to see bubbles forming in stout beer rather than in, say, champagne where the bubble formation process is much more violent,” Dr Lee told BBC News.
But as has happened to a generation of like-minded scientists before them, the question of falling bubbles became their focus.
The team had the idea – borne out by calculations carried out by Mr Cummins – that the relative density of bubbles and the surrounding liquid could be behind the phenomenon.
“If you imagine your pint is full of bubbles, then the bubbles will start to rise,” Dr Lee said.
But the bubbles in a standard pint glass find themselves in a different environment as they rise straight up.
“Because of the sloping wall of the pint, the bubbles are moving away from the wall, which means you’re getting a much denser region next to the wall,” Dr Lee explained.
“That is going to sink under its own gravity, because it’s less buoyant, and that sinking fluid will pull the bubbles down.”
The bubbles, that is, are “trying” to rise, but the circulation that creates drives fluid down at the wall of the glass.
“You’ll see sinking bubbles not because the bubbles themselves are sinking, but because the fluid is and it’s pulling them down with it.”
The same flow pattern occurs with other beers such as lagers, but the larger bubbles of carbon dioxide are less subject to that drag.
Mr Cummins carried out calculations using a simulated pint and “anti-pint” – that is, the upside-down version of a pint glass – showing the effect at work; in the anti-pint, the bubbles rise as expected.
For those interested in experimenting in the pub, the effect can be best seen if a pint of stout is served in a straight-sided, cylindrical glass (not quite filled up).
If the glass is tilted at an angle while the pint settles, the side in the direction of the tilt represents the normal situation of a pint glass, while the opposite side is the “anti-pint” – and bubbles can be seen to both rise and fall in the same glass.
Source: BBC News
Who would have thought that common old mashed potatoes could become a blog post?
Mashed potatoes are worthy of one.
I don’t know anyone who doesn’t like/love/enjoy mashed potatoes, do you?
The basic or traditional mashed potato is simple, take freshly boiled potatoes while hot, add a splash of milk, a dollop of butter and a healthy pinch of white pepper; mash.
The secret with mashed potatoes is to boil at a medium heat and not to over boil them to avoid them becoming gluey as a result of the break down of the natural starch. Another hint, do not use a food processor to ‘mash’ your potatoes, as this breaks down the starch mechanically with the same gluey result.
Served with roast meat and lashings of gravy, or simply a dollop of butter on top to melt and drip enticingly over the serving. You can season additionally with salt and pepper, sprinkle freshly chop parsley on top or grate a cheese of your choice over them.
But what you put on mashed potatoes is not the end of the story
There are many things that you can put in them to create variety, cheeses, herbs, chopped or grated onion, garlic, chives, spring onion, fresh cream, mashed white vegetables like cauliflower, parsnips or turnips (great way to get veges into the kids), you can replace the milk with stock or mash in a packet of Maggi soup, if you want to add colour, then use a tomato soup mix (and then you can disguise carrots too for the kids benefit). You can stir in shredded raw cabbage or raw onion rings just before serving to add a crunch.
So many variations on a theme can be made. Potatoes Royale is a favourite, where you mash in raw egg.
Brazilians like mashed potatoes. Called batata puree, they are generally runnier than the British tradition as the name suggests. They heat the milk before mashing.
The French have aligot which is a melted cheese and mashed potato dish.
Of course, we can’t exclude the Irish with their champ where spring onions (scallions) are added, or colcannon which uses cabbage or kale.
In Saharan Africa, they prefer the gluey fufu, also known by many other regional names.
The Jews have Kufte de Prassa (grilled potato patties with leeks and mincemeat, ground beef) for Passover, or a simple vegetarian version with more leaks.
Carnation mashed potatoes uses evaporated milk.
The ideas are endless; sautéed mushrooms added to the middle with the potato folded over the top. Similarly you can use a cube of mozzarella cheese and make a mashed potato ball.
Minted mashed potatoes give a fresh taste accompanying roast lamb.
Savoury mashed potatoes; meats like pre-cooked bacon can also be a treat.
Cheese Puff mashed potatoes; using parmesan cheese and sour cream. You need a recipe, try the Kitchen Project.
Of course, all the above can be done with sweet potatoes too.
Now you don’t necessarily have to put something in mashed potatoes, you can use them in other forms as well, the traditional shepherd’s or cottage pies uses mashed potatoes for the topping.
Mashed potatoes can be piped on to a baking tray and cooked off in the oven to add a crunchy surface. They can also be used in a quiche, and casseroles, as a topping for meatloaf cupcakes (recipe here) or just make potato cupcakes and cook off in the oven.
Brown Butter mashed potatoes, where you cook off the butter until brown before mashing then brush with the same butter before cooking off in the oven as cupcakes or piped onto an oven tray.
Wacky mashed potatoes, spoon or pipe your version into icecream cones. Idea from: Potato.ie
Then there are all the forms of gnocchi (or potato dumplings), where flour is mashed with the potatoes and then recooked in boiling water. More about gnocchi on The Purple Foodie.
Potato dumplings can be cooked off in boiling water as a traditional dumpling , they also be fried off in the deep fry.
Left over mashed potatoes can be made into potato pancakes and waffles with a dollop of sour cream on top sprinkled with chopped chives.
Used in cakes and breads; Both the Brazilians and the Irish have fantastic potato breads.
The ideas are endless, here I have barely scratched the surface.
Like most cooking, there is no real recipe and the varieties are only limited by your imagination.
Whatever you do, please do not use Instant Mashed Potatoes, quite frankly they are an abomination in the sight of that great chef in the sky and should be illegal.
- dollop = a vague measurement used by chefs
- squirt = unspecified measurement recognised by chefs as sufficient
- healthy squirt = more than normal, a synonyms generous squirt/pinch, etc.
- lashings = a lot of, a synonym oodles