The Ice Cream That Changed Physics

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Not every scoop of ice cream can be described as “fateful.” But a batch of ice cream Erasto Mpemba made as a teenager in Tanzania in 1963 made waves in physics that are still being felt nearly 60 years later. That’s because it appeared to be proof of a strange and counterintuitive idea: that a hot liquid may freeze faster than a cold one.

Homemade ice cream was a popular snack when he was a student at Magamba Secondary School, Mpemba wrote in a journal article published in 1969. “The boys at the school do this by boiling milk, mixing it with sugar and putting it into the freezing chamber in the refrigerator, after it has first cooled nearly to room temperature,” he explained. But competition for the freezer was intense. One afternoon, he and another boy took two different shortcuts as they jockeyed for space. Mpemba’s classmate mixed his milk with sugar and poured it straight into an ice tray without boiling it at all. Not to be outdone, Mpemba boiled his milk—but skipped the step of letting it cool so he could snag the last ice tray. An hour and a half later, “my tray of milk had frozen into ice-cream while his was still only a thick liquid,” Mpemba wrote.

A few years later, Mpemba asked his high school science teacher why this might be—why hot milk would freeze faster than cold milk, going against Newton’s law of cooling. The teacher’s response was, “All I can say is that that is Mpemba’s physics and not the universal physics.” The incident became a running joke in the classroom. Whenever Mpemba got a math problem wrong, the teacher and his classmates would call it “Mpemba’s mathematics.”

Determined to find an explanation, Mpemba repeated the experiment with hot and cold water. And when physicist Denis Osborne visited his high school, he asked him about the incident as well. Intrigued, Osborne invited Mpemba to visit what is now the University of Dar es Salaam and discuss the issue further, then set up the related research that was eventually published. The article helped a principle that Aristotle, René Descartes, and Sir Francis Bacon had all observed over the centuries become known as the Mpemba effect.

Mpemba and Osborne’s claims created decades of controversy within the physics world, since they challenged fundamental theories about how matter behaves. Many researchers tried to recreate their results, with limited success. In 2016, physicist Henry Burridge of Imperial College London and mathematician Paul Linden of the University of Cambridge published a sweeping review of many of the studies that had attempted to confirm the phenomenon, reporting “sadly” that they were not able to find any proof of a Mpemba effect. Worse, they concluded, all of those studies—including Mpemba’s original experiment—could have been easily skewed by tiny experimental factors such as the setup of equipment insulation or placement of thermometers.
Cooling and chaos

Starting in 2017, a new contingent of studies finally turned the corner on confirming Mpemba’s observation, suggesting that the explanation lies in the mysterious mechanics of chaos. And, it turns out, water itself may have been a major obstacle in proving the larger theory. It behaves differently than most other substances, especially as it changes states between solid, liquid, and gas, so scientists on the Mpemba effect case looked to remove water from the equation altogether.

In an abstract experiment meant to zero in on the forces at work, physicist John Bechhoefer and his colleagues heated microscopic glass beads (meant to stand in for water molecules) with lasers and looked at the speed of cooling. They found that not only did some hot beads cool more quickly than their cold counterparts, but sometimes they did so exponentially faster. “The simplicity of the study is part of its beauty,” theoretical physicist Marija Vucelja told Science News. “It’s one of these very simple setups, and it already is rich enough to show this effect.”
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Not long after that, another group of physicists published a follow-up article suggesting a more abstract framework for understanding the Mpemba effect, which involved modeling the random dynamics of particles. The results suggest that the key to the Mpemba mystery is a dose of chaos. In particular, a liquid moving quickly from hot to cold is said to be “out of equilibrium,” meaning that it is a system that does not follow the linear rules we (or Newton) might expect it to.

“We all have this naive picture that says temperature should change monotonically,” study author Oren Raz told Quanta magazine (meaning that we might assume a liquid that is cooling keeps going steadily in one direction without making significant reversals). “You start at a high temperature, then a medium temperature, and go to a low temperature.” But in a system out of equilibrium, “you can have strange shortcuts,” Raz said.

Various publications offered evocative metaphors to explain those shortcuts: Science News compared a hot liquid cooling under the Mpemba effect to “how a hiker might arrive at a destination more quickly by starting farther away, if that starting point allows the hiker to avoid an arduous climb over a mountain.” Alternatively, Physics Today suggested it is a bit like someone using stepping stones to cross a river, writing, “If you have the right starting energy, you can jump straight from the first to the third without ever landing on the second.” Since a hot liquid is more out of equilibrium than a cold one, it might have just the right energy to hop over stones.

Another word for that is kurtosis, a statistical term that refers to the deviation from an average, which appears to play an important role in Mpemba effect-related behavior. The temperature of a fluid generally refers to the average speed of its molecules—but every fluid will have outlier molecules behaving much differently than the others. In cases where the Mpemba effect occurs, these outliers seem to play an outsize role, Antonio Lasanta, a physicist who has published several papers confirming the phenomenon, told Cosmos. By taking into account kurtosis in experiments related to this kind of cooling and heating, “we can make analytical calculations to know how and when the Mpemba effect will occur,” Lasanta said. It’s certainly a step toward unraveling the Mpemba mystery, though there’s plenty left to figure out about when the effect shows up and how strong it is when it does.

Erasto Mpemba grew up to work as a game officer in Tanzania’s Ministry of Natural Resources and Tourism and died around 2020, his ice cream having been vindicated. Though there’s still a lot we don’t know about the effect that bears his name, it seems that it is “Mpemba’s physics” after all.

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How To Make The Best Homemade Ice Cream – With Or Without A Machine

If there’s one treat that many people find impossible to resist, it’s ice cream. It’s sweet, creamy, and irresistibly satisfying. Ice cream is also surprisingly easy to make at home, so long as you have the right approach. Read on to learn how to make your favourite style — plus, get pro tips for whipping up your best batch of homemade ice cream yet.
Different styles of ice cream

There are four main types of ice cream says Michael McKinnell, executive chef at Mordecai Chicago: custard-based, Philadelphia-style, egg-free, and no-churn. Custard-based ice cream is made with cooked egg yolks, resulting in an ultra-rich dessert. The eggs also act as a natural stabiliser, so it doesn’t require other ingredients to achieve a silky texture, says McKinnell. (Store-bought ice creams often contain stabilising ingredients like guar gum and carrageenan.) Philadelphia-style ice cream, sometimes called American or New York ice cream, is an eggless variety made with milk, cream, and sugar.

Image Credit Brooke Lark/Unsplash

Since it contains no eggs, it’s less rich than custard-based ice cream and doesn’t require any cooking. Similarly, egg-free ice cream contains no eggs, but it uses cream cheese and cornstarch in the base. This type of ice cream is delightfully thick and smooth. No-churn ice cream also doesn’t require eggs or cooking—but as the name suggests, it’s made without an ice cream machine. This style of ice cream, which features a base of whipped heavy cream, is particularly airy and light. In fact, the consistency is more like a chocolate mousse, according to McKinnell.
How to make homemade ice cream

The exact process for making homemade ice cream depends on the type. But generally, it involves making a base, churning it in an ice cream machine, and freezing it until firm.

To make a classic egg-based ice cream, the first step is to make a custard. Whisk egg yolks and sugar in a saucepan, then slowly whisk in milk. Cook the mixture over medium heat while stirring constantly. The custard is ready once it evenly coats the back of a spoon, which can take between five to 10 minutes. (Not sure if your custard is ready? Use your finger to draw a line across the back of your spoon. If it retains the line, it’s good to go.)

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Next, pour the custard through a sieve into a large bowl over ice. This will stop the custard from cooking.

Once chilled, you can add ingredients like alcohol-based flavourings such as vanilla extract, says Stephen Chavez, chef-instructor of pastry and baking arts at the Institute of Culinary Education. Avoid adding these flavourings during the cooking process or while the custard is still hot, as the alcohol might cause the mix to “break,” and you’ll need to start over, he adds.

The next step is to churn the mixture in your ice cream machine. Chavez recommends churning it only to “soft serve” consistency, even if you’re not actually making a soft serve.

“Remember, ice cream is still made of cream. If it’s over-churned, you begin to create butter, which affects the final texture,” he says.

Specifically, it will become hard and chunky, rather than smooth and creamy. After churning the ice cream, you can fold in “floating garnishes” like chocolate chips, frozen strawberry bits, or nuts. It’s best to do this after churning (and before storage) as most machines don’t work well with such ingredients, says Chavez.

Finally, transfer the ice cream to an airtight plastic container, then place it in the freezer until frozen, about four hours.
Ice cream without a machine

Image Credit: American Heritage Chocolate/Unsplash

No machine? No problem. Make no-churn ice cream, which doubles as a no-cook option. Simply whip heavy cream and confectioner’s sugar until it develops a whipped cream consistency, then fold it into sweetened condensed milk.

Add flavourings, if using, and pour the mixture into an air-tight plastic container and freeze it for one hour, folding it occasionally. Continue to mix the ice cream every hour for about four hours, then let it sit in the freezer for another two hours (untouched) before serving, says McKinnell.
How to store homemade ice cream

According to McKinnell, homemade ice creams typically have short shelf lives. That’s because DIY varieties are made without chemical stabilisers, which are responsible for the longevity of store-bought versions.

Still, most homemade ice creams will last about one month in the freezer, as long as you store them properly, says McKinnell. To start, always “pack” the ice cream into its container, recommends Chavez. This involves pressing down on the ice cream to remove air bubbles. “Air bubbles cause ice crystals, which damage the final consistency of the product once it’s frozen,” explains Chavez.

Also, protect the “skin” of the ice cream by placing plastic wrap directly on top. According to Chavez, this ensures no excess air or moisture accumulates on the ice cream, helping it stay smooth and creamy. Add the plastic wrap right after you have “packed” the ice cream, then again, every time you scoop some out.

This story first appeared on www.Marthastewart.Com

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We All Scream For Homemade Ice Cream

You can never get enough strawberries, and they are good for so many things. So before summer says goodbye, Anna Rossi is taking full advantage of making some homemade ice cream while the sweet berries are still in season.

Make it at home and your family will lick it up before it even has a chance to melt.