Science of the Slushie, Discover What Goes Into Making the Famous Frozen Drink

The convenience store chain 7-Eleven, celebrates their day, July 11th, by doling out millions of free small cups of Slurpee® to their customers. However, while guzzling down your free frozen slush, did you think about the complicated chemistry that goes into making the famous cold brews? The signature frozen beverage may seem like a simple concoction of just ice and flavored syrup, but there is some serious science that goes into creating the nine million gallons of flavored slushie that are sold each year.

The brain freeze inducing drink was discovered by accident in the 1950s when Dairy Queen owner, Omar Knedlik’s, soda fountain broke. He improvised by plunging sodas into the frozen tundra of his freezer to chill for his thirsty customers. When he popped the lids, the sodas had instantly turned slushie, leaving everyone delighted by the new frothy consistency. However, the slush that the world has come to love, has evolved drastically from simply freezing a can of soda. The perfect balance of temperature, carbonation, and sweetness is required to achieve the fluffy-yet-frozen slushie texture every time.

This happy accident actually took some crazy chemistry to accomplish. There are several factors that play into the creation of the slushie soda. One in particular is the formation of what is known as a supercooled liquid. This means that the drink is actually colder than the point at which the solution transforms into ice, but is not yet frozen.

With his soda slush a hit, Omar continued to play with the idea of creating the perfect frosty drink. After tinkering with an old Taylor ice cream maker, he was able to recreate the frozen soda slushie that later was deemed “ICEE®” with the first flavor being cola.

Omar had to know some sneaky science to be able to achieve what he did. The solution of flavor syrup, water, and carbon dioxide starts out in a barrel and is chilled under pressure. The solution is then churned to keep it moving, scraping away any ice that forms on the container’s sides.

The constant movement and syrupy sugars keep the solution from freezing into a solid log. Then when you go to pour out your delicious drink to escape the summer heat, you pull down on the handle, and out comes the semi-frozen foam, where appears to puff up and solidify as it fills the cup.

Scott Rankin, a food scientist at University of Wisconsin-Madison, spoke to, and explained that an ICEE® is like an avalanche. “If you are in an avalanche, it’s sort of like you’re swimming around in snow. As soon as the avalanche stops, it becomes very rigid, very cement like.”

However, another food scientist at the University of Wisconsin-Madison has different theory. Richard Hartel believes that when the semi-frozen solution appears to puff up and form more ice crystals when poured, it is actually creating the Joules-Thomson effect. The effect is described by the temperature change of a real gas or liquid when it is forced through a valve, while kept insulated so that no heat is exchanged with the environment. Much like when the frozen slush is poured out through the barrel’s valve, the dissolved carbon dioxide starts to escape, which puffs up the ICEE® and freezes the drink even more.

As Dairy Queen’s lines for the ICEE® machine grew, so did 7-Eleven’s notice of the popular summer heat beater. In 1965, the convenience store chain bought some of the machines, renaming it Slurpee, after the director of 7-Eleven’s in-house ad agency heart the sound drinking the frozen concoction made through a straw in the 1967 meeting.

However, for every good thing, there seems to be a dark cloud that follows.

Brain freezes. We’ve all had them. In your anticipation to beat the summer heat, you end up drinking it just a little too fast, and bam! It feels like the North Pole in your brain.

The intense pain the head is scientifically referred to as sphenopalatine ganglioneuralgia (try saying that with a brain freeze). It occurs when a cold substance touching the back of the throat, lowering the temperature around the blood vessels close to the back of the throat, including the internal carotid artery and the anterior cerebral artery that supply a good amount of blood to the brain. The human body isn’t one for swift changes in temperature, and will do its best to maintain the same temperature it always has.

So when there is a sudden drop in temperature to the back of your throat, the body reacts by constricting blood vessels to conserve the heat in the warm circulating blood. Then the opposite happens when the body is exposed to sudden heats, by having blood vessels dilate so that more heat from the blood is released through the skin.

Thus, the occurrence of brain freeze. The blood vessels near the back of the through first rapidly constrict rom the coldness, then dilate when the blood vessels become warmer again. The dilating blood vessels pull the tissues around the vessels and the brain, triggering the trigeminal nerve. This nerve, also referred to as the fifth nerve, affects much of the face, including the forehead.

But why do you feel the pain in your forehead and not the back of your throat? Well, nerves cannot always differentiate the location of the original pain trigger, a phenomenon called “referred pain.” This is also why people feel pain in their heart, feel it in their shoulders or back instead. It also explains why your forehead feels like it is in a vice from sphenopalatine ganglioneuralgia, instead of the back of the throat.

So when beating the heat on a sweltering day, and you go for a refreshing Slurpee®, just remember good ol’ Omar Knedlik and his complicated chemistry and keep in mind the dangers of a brain freeze.

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