While the colorful flavors and silky texture of ice cream are intoxicating, the physical and chemical processes behind its unique, delicious texture are fascinating -- if you look at it scientifically, you'll see that ice cream has similar physical and chemical properties to forest restoration, rock formation, or animal survival at low temperatures.
The faster they freeze, the smaller the crystals, the smoother they feel
Water is one of the main ingredients of ice cream, mostly in the form of microscopic ice crystals. The size of these crystals has an important effect on the quality of the ice cream. Large crystals produce a grainy texture, while smaller crystals (the size of blood cells) produce a velvety texture.
So how do ice cream makers prevent small ice crystals from growing into something larger than a dozen microns in diameter?
Ice is a mineral, like quartz and graphite, so in some ways it behaves like a mineral. Looking at ice cream under a microscope is not that different from looking at granite or other rocks cooled by magma. When magma solidifies into minerals, the crystalline parts of the minerals provide clues about the conditions under which they formed -- thick, sticky magma cools slowly underground, allowing the crystals to fully grow; At the surface, thin lava cools and hardens much faster, eventually forming rocks with smaller crystals.
Sweeteners and stabilizers thicken the ice cream, slowing the growth of the crystals, and speeding up the freezing process keeps the crystals small.
Over the years, more and more people have opted to freeze ice cream by adding liquid nitrogen, which with its extremely low temperature can produce a silky ice cream in just a few minutes.
Poison for afforestation, honey for ice cream
Another way to slow the growth of crystals is to chop them up as they begin to form in a mixing container. The first stage of ice cream making is called dynamic freezing, during which a blender continuously scrapes the newly formed crystals from the walls of the tub and stiles them into the mixture. This not only prevents the crystals on the inner wall from thickening, but also produces more crystal nuclei, which in turn attract liquid water molecules to freeze onto them and form ice.
And the way you can think of it is, all of these smaller crystals are now competing with each other for the remaining water molecules, so they're having a hard time growing very large.
This process is very similar to the forest restoration process. If a large area of trees is cut down, burned or blown down, new seedlings will grow closely in place at an even rate. It can take decades for the weaker ones to die out and make room and resources for the stronger ones. At the same time, the "secondary growth" of forests is hindered by over-crowding of trees competing for limited resources.
In the case of forests, slower-growing and variable-sized ecosystems tend to be healthier. But for ice cream, even, competitive growth is the key to creating a silky texture.
Finding 'antifreeze Proteins' in Animals
Once the ice cream is made, it's best to eat it all in one sitting. If they cannot all be eaten, they must be stored, sometimes for weeks or months. During this time, the ice cream's temperature may fluctuate as the freezer door opens and closes -- if it melts a little, the ice recrystallizes and develops larger crystals over time, with the result that it feels extremely icy. This is a tear-jerking experience for those who suffer from fridge protests.
Thickeners and stabilizers slow down the movement of liquid water molecules in the ice cream mixture, keeping it running smoothly for a long time. And if that's not enough, ice cream makers may turn to wildlife for help.
Some species of frogs, insects and plants have antifreeze proteins that help them survive in cold conditions: once formed, these proteins surround and bind to ice crystals, thus preventing liquid water molecules from bonding with the nascent crystal. Antifreeze proteins help organisms avoid cell damage and even death.
Antifreeze proteins were first found in cold-water fish. The researchers then synthesized it in the lab using genetically modified yeast. Now, ice cream makers are turning antifreeze proteins into gastronomic applications to inhibit ice recrystallization.
Ice and milk blend without stratification, emulsifying protein makes great contribution
Oil and water repel each other, so why doesn't ice cream (which is mostly a mixture of ice and cream) split into two layers? The answer can be found in its microscopic structure.
When you shake a bottle of oil and vinegar, the oil inside breaks down into small spherical droplets, which, if left alone, eventually coalescate into a layer on the surface. But when the two liquids are quickly mixed by violent shaking, they seem to merge into an emulsion, a dispersion of two insoluble liquids -- one in the form of small droplets of the other.
Most insoluble mixtures are thermodynamically unstable, which means they eventually revert to a simpler, more organized structure: one liquid sits on top of another. But a stable emulsion (like stable coconut water or homogeneous milk) is different. No matter how long you wait, the oil won't rise to the top.
These oil-in-water substances remain evenly dispersed in part because they contain natural emulsifying proteins, which act like antifreeze proteins. Instead of binding to the ice, the emulsifying proteins bind to the fat droplets and reduce the tension between the two liquids, preventing the fat from forming layers.
Milk protein keeps the ice cream system relatively stable. Usually, though, ice cream needs an extra emulsifier, such as lecithin or casein, to help the other main ingredient stay in the mixture: air.
Tiny bubbles make ice cream easier to scoop and hold (soft-serve ice cream), as long as they remain small and evenly distributed between fat and ice.
Wet the big snowflakes and dry the small ice crystals
Naturally formed ice comes in a wide range of shapes and sizes, from hollow columns or needles to thin sheets and "bullet rosettes" -- regardless of shape, the main determinant is the humidity and temperature surrounding the crystals as they form, with higher humidity leading to larger, finer flakes.
Most of these types of crystals require plenty of time, space and moist air to grow and form, which an ice cream blender doesn't provide, so ice cream crystals are more like simple prisms or sheets that form under very cold, dry conditions. In addition, the constant motion of the agitator wears away the crystals in the same way that the ocean wears away sand grains, creating tiny irregular particles.
Whether it's the making of ice cream, the formation of rocks in the earth's interior or changes in the weather, the essence of all natural processes on Earth are controlled by the same physical and chemical processes. If we understand physics and chemistry, we can understand the world, and we can make better ice cream.
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