The entire universe exists in an ice cream cone?

The process of making ice cream is full of incredible physical and chemical principles.

Wayne Thiebaud paintings

Author: Miao Yuanke, Master of Science History

Editor｜Chen Tianzhen

Ice cream is a wonderful combination of ice crystals, cream and air, which is sweet and refreshing, soft and smooth. Although your tongue may not feel it, water occupies 60%-72% of the total mass of ice cream, which exists in the form of small ice crystals. In addition, only 8%-10% is delicious fat, and 25%-50% is empty air. Therefore, ice cream is actually a very simple mixture.

But why can a small ice cream easily capture our taste buds? Because it contains incredible physical and chemical principles! Can you imagine that the production of ice cream is even inextricably linked to the formation process of minerals, the principles of cold resistance of animals and plants, and the laws of forest regeneration?

01. Ice crystals and ore formation

The size of ice crystals is the key factor in determining the smooth taste of ice cream. If the ice crystals are too large, it will be like chewing ice chips. If they are larger, the ice cream will directly turn into slush. Only ice crystals as small as a few microns, the size of blood cells, can produce a silky taste.
So how do you make ice crystals as small as possible? What conditions are needed to make water freeze into small ice crystals?

We know that rocks are mainly composed of minerals such as quartz (silicon dioxide), and ice, like quartz, is a crystal, so if you observe the microstructure of ice cream under a microscope, you will find that it is not much different from a piece of granite formed by the cooling of the earth's magma.

Under a microscope, a thin slice of rock can be seen with mineral crystals of several different colors (left), and ice crystals in ice cream sparkle under polarized light (right). | Left: Flickr user Leo-setä, right: Maxim Bilovitskiy

The faster the hot magma cools when it reaches the surface, the smaller the size of the mineral crystals in the rock. For example, lava directly erupted from a volcano will quickly form igneous rocks, which often contain very small mineral crystals. Rapid cooling can also be used to obtain tiny ice crystals.

The principle of the popular liquid nitrogen ice cream is to speed up the freezing process and make the ice crystals as small as possible. The ice crystals of ice cream made by traditional methods can only reach the level of a few microns, while the ice cream made by liquid nitrogen at minus 196 degrees can reach the level of a few nanometers. No wonder ice cream is getting more and more delicious now! It is also no wonder that the refrigerator at home can never make ice cream, but can only freeze large popsicles.

02. Ice crystals and forest regeneration

Is it impossible to make good ice cream without liquid nitrogen? Of course not, there is more than one way to get tiny ice crystals.

In the production process of ice cream, the first step is called dynamic freezing, which means freezing while stirring. In this process, ice crystals will constantly gather on the inner wall of the stirrer. The stirrer must quickly scrape off the ice crystals on the inner wall and stir them with other ice cream - the time interval between the two scrapings is very short, only 0.1 seconds. Such a quick operation can prevent the ice crystals from growing and eventually turning into ice chips.

Moreover, the scraped ice crystals can also become new crystal nuclei after being broken up, for other water molecules to attach to and grow more ice crystals. In this way, the number of crystal nuclei in the ice cream continues to increase, while the number of water molecules continues to decrease. A large number of crystal nuclei compete with limited water molecules, and as a result, each crystal nucleus has no chance to grow into ice chips.

This process is actually very similar to how forests regenerate. When a forest is attacked by external forces, such as human logging, a large wildfire, or a natural disaster like a hurricane, many trees disappear, leaving empty spaces.

In these places, densely packed saplings pop up like bamboo shoots after a spring rain. Due to the great competitive pressure and the limited resources that a small piece of land can provide, the growth rate of the second crop of trees will be very slow, just like the ice crystals that are scraped off have difficulty growing.

It takes several decades for the weak saplings to eventually die due to competition pressure, and the strong saplings can grow into big trees again. For forests, slow growth and uneven trees often create a healthy ecosystem; and for ice cream, competition is the key to a silky taste.

(Left) Ancient forests are often a mixture of large trees and saplings. (Right) Secondary forests, formed after logging or natural disasters, often have short trees of similar thickness. | TJ Watt

03. Ice cream and cold-resistant plants and animals

We all had this experience when we were young. We bought ice cream and popsicles in the dog days of summer, then ran all the way home to put them in the refrigerator. When we took them out to eat, we found that the ice cream had melted and solidified, and the taste was far worse than when we just bought them from the supermarket.

Like all delicious foods, ice cream is also very fresh, and it tastes best when it is freshly made. In the process of transporting ice cream from the supermarket to your home, the ice crystals in it will melt due to the heat, and then solidify into larger ice crystals, resulting in a hard ice lolly. In fact, even if the refrigerator door is opened to allow the temperature to fluctuate slightly, the ice cream will melt a little bit, and the taste will deteriorate after a few times.

Therefore, in every link of production, transportation, storage and sales, the ice crystals in ice cream may melt and recrystallize due to temperature changes. People add stabilizers to ice cream to slow down the movement of liquid water molecules in the ice cream mixture, so that the ice cream can remain stable for a long time. However, there is another solution to this problem, which has been found by wild animals and plants living in cold areas.

Many fish, insects and plants living in the poles and high altitudes can live in extremely cold environments with temperatures below zero degrees Celsius and keep their body fluids from freezing. How do they do this? It turns out that they contain an antifreeze protein that can adsorb to the surface of ice nuclei and prevent water molecules from gathering. This way, ice crystals cannot continue to grow, and organisms can avoid cell damage or even death in low temperatures.

Antifreeze proteins were first discovered in fish in polar ice waters, and were later synthesized in the laboratory using gene-edited yeast. Today, antifreeze proteins are added to ice cream as food additives to inhibit the recrystallization of ice cream, allowing us to buy smooth and soft ice cream in supermarket freezers.

Ocean cod live in the cold waters of the northwest Atlantic Ocean and have antifreeze proteins in their tissues that help them withstand temperatures near or below freezing. | Vejlenser

04. Physical Chemistry in Ice Cream

In addition to water, the main ingredients of ice cream also include fats from cream and milk. The general fat content is 8%-10%. In some high-end (very expensive) ice creams, the fat content can even reach 15%-20%.

We know that oil and water cannot dissolve in each other. Even if they are mixed together, they will separate quickly. Therefore, there is always a thick layer of oil floating on the surface of hot pot. But why do the water and oil in ice cream mix so perfectly? The answer lies in the microscopic structure of ice cream.

Everyone may be familiar with the vinaigrette commonly used for salads. Vinaigrette is generally composed of three parts oil and one part vinegar. Oil and vinegar are originally immiscible, but when they are stirred together madly, the oil will eventually break down into tiny spherical droplets and evenly disperse into the vinegar to form an emulsion.

An emulsion is a homogeneous mixture of two immiscible liquids. Most of them are unstable. After a long time, the two liquids will separate and return to a simpler, more organized structure. However, there are also stable emulsions, such as milk and coconut milk, which can always remain mixed no matter how long you wait. This is because milk contains natural emulsifying proteins. The molecular structure of these proteins is hydrophilic at one end and lipophilic at the other end. They can reduce the surface tension between oil and water, wrap small oil droplets inside, and make them difficult to aggregate. As a result, it looks like oil dissolved in water.

However, the natural emulsifying protein in milk is not enough to keep the ice cream stable for a long time. Generally speaking, emulsifiers such as lecithin and casein are added when making ice cream to help the water and fat in the ice cream maintain a more stable emulsion state.

If oil and water are mixed evenly and an emulsifier is added, a stable emulsion can be formed, like in milk or ice cream. | Pixabay

Lecithin is not only an excellent emulsifier, but also a foaming agent. Speaking of this, we have to mention another main ingredient in ice cream - air. The volume of air in ice cream can usually reach 25%-50%, so that it will have a fluffy taste when eaten.

Similar to the principle of emulsifiers, foaming agents can also reduce the surface tension of liquids, making it easier for air to be wrapped by liquids, just like adding soapy water to blow out soap bubbles. Therefore, the bubbles in ice cream are actually like a large group of frozen tiny soap bubbles.

This also leads to the maximum altitude that ice cream can exist is 3,000 meters. Above this altitude, due to the low atmospheric pressure, the bubbles in the ice cream will expand and burst, and finally the entire ice cream will collapse to half its volume, becoming a hard frozen cream and ice mixture.

Wayne Thiebaud paintings

Feynman once wrote in the Feynman Lectures on Physics that the entire universe exists in a glass of wine. He said:

"If we look closely enough at a glass of wine, we can really see the entire universe. There are physical phenomena at work here: the curvature of the liquid surface, its evaporation depending on the weather and the wind; the reflections on the glass; and the addition of atoms to our imagination. The glass is a purified product of the rocks of the Earth, and in its composition we can discover the age of the Earth and the secrets of the evolution of the stars.

…

If our tiny, limited intelligence has for some convenience divided this glass of wine - this universe - into several parts: physics, biology, geology, astronomy, psychology, etc., then remember that nature does not know all this. So let us put it all together again and not forget what this glass of wine is ultimately for. Let it give us one last pleasure! Drink it and then forget it completely! "

Perhaps, we can also say that the entire universe exists in an ice cream cone.

References

[1]https://www.smithsonianmag.com/blogs/national-museum-of-natural-history/2021/07/15/strangely-scientific-endeavor-making-ice-cream/

[2] Chinese Academy of Sciences Popular Science Cloud Platform - Mineral Museum http://www.kepu.net.cn/vmuseum/earth/mineral/index.html

[3] Institute of Earth Environment, Chinese Academy of Sciences http://www.ieexa.cas.cn/kxcb/kpwz/201801/t20180119_4936168.html

[4]M. Gail Jones, Denise L. Krebs & Alton J. Banks (2011) We Scream for Nano Ice Cream, Science Activities, 48:4,107-110, DOI: 10.1080/00368121.2010.535223

[5]Clarke, C. (2015). The science of ice cream. Royal Society of Chemistry.

[6] Wang Shaoyun, Zhao Jun, Wu Jinhong, & Chen Lin. (2011). Research progress of antifreeze proteins and their applications in food industry. Journal of Beijing Technology and Business University: Natural Science Edition, 29(4), 50-57. [7] Feynman Lectures on Physics, Vol. 1.