The ultimate summer treat is, arguably, ice cream. Some 1.5 billion gallons of ice cream and other related frozen desserts are made every year in the United States, with production peaking (as one might expect) in the sultry summer months, according to the International Dairy Foods Association.
What are the tricks behind creating ice cream? From a chemistry perspective, this delectable substance is actually quite an unnatural thing. Technically, the creamy treat is a colloid, meaning it consists of fine particles dispersed in a continuous medium.
"Ice cream is basically made up of little ice crystals and air bubbles and fat droplets, all sort of glued together by a viscous sugar solution," said Chris Clarke, author of "The Science of Ice Cream" (Royal Society of Chemistry, 2005). [The Mysterious Physics of 7 Everyday Things]
On their own, these ingredients would not stay isolated and oriented in a smooth, continuous structure. So, to pull this arrangement off, ice cream essentially must be frozen and whipped at the same time, then kept cold lest its separate ingredients start glomming together, ruining the texture.
With ice cream, "you're working against thermodynamics," Clarke said, meaning the science of heat and energy in systems. "You have lots of little things that can save energy by becoming one big one and want to come together," Clarke said.
Air and ice
The key to producing ice cream, Clarke said, is to make the bubbles and ice crystals small, and the smaller they are in the first place the better the ice cream will be.
Ice cream today is made much the way it was when Philadelphian Nancy Johnson invented a hand-cranked ice cream freezer back in the 1840s.
The ice cream ingredients of milk, cream, sugar and flavoring get whipped around (and thus aerated) by a blade in a tube that is chilled from the outside. In Johnson's day, ice and salt (which lowers the freezing point of water) did the trick, and nowadays liquid ammonia is more often used outside the tube, Clarke explained.
Whenever the ice cream mixture touches the wall of the tube, it freezes. To prevent the crystals from getting too big, the blade inside the tube also scrapes the crystals off right after they form. "That clears the walls and more ice crystals form," Clarke told LiveScience. "The colder and better the scraping, the smaller the ice crystals."
From their freezer to yours
The ice cream is extruded from the ice-cream-making machine, and chips, chunks of candy, fruit and so on get added in; the freshly created ice cream goes into containers.
From there, the ice cream is then quickly super-frozen to lock in its structure in a process known as hardening. "You have to put it in a really cold environment," said Clarke, usually below zero degrees Fahrenheit.
The ice cream is kept below freezing all the way through delivery and storage at supermarkets and ice cream vendors. "The moment it's out of the freezer it starts to lose that structure."
Once that structure is lost, "you can never get it back," Clarke said, as anyone can attest who has had their ice cream purchase melt on the way home from the store or left it on the countertop too long.
Clarke has been lucky enough to taste ice cream right out of the factory freezer. "Right at the source," he said, "it's the best ice cream you ever met."
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