Why do ice cream and yogurt bought in the refrigerator, not eaten immediately, become crispy and crunchy?

Never leave behind a box of ice cream or half-eaten yogurt: Because when you put them back in the freezer, you think time will reverse it?

Is not! The melting ice cream will solidify, but it will never regain its original softness and flavor.

That’s because some of the water has sneaked into the ice cream, coagulating into ice crystals. The next time you take out the ice cream to eat, you will find it becomes crispy, crunchy and pale. It’s obnoxious!


But don’t worry, a team of scientists at the University of Tennessee have now found a way to combat water crystallization in ice cream. They wondered why many fish species that live in sub-zero seawater do not freeze.

And it turns out that the secret lies in a protein that sticks to the surface of the rock crystal, preventing it from clumping. This protein is also present in some plants, so scientists have used it to create an eternal soft cream.

Here is the story of that interesting discovery.

Disruptive crystals

You may not know it, but scientists study ice cream as much as they study the properties of liquid water, solids or air – because inside ice cream has all these ingredients.

Physics students sometimes play around with ice cream in the lab, using liquid nitrogen to make them. And you can also make your own ice cream at home, it’s not that difficult. Just heat the milk, cream and sugar until the sugar dissolves; cool the mixture; and add any flavoring.

Later, Slowly stir the mixture so that it solidifies. Stirring adds more air to the mixture, increasing the volume of the cream (an effect called overrun). But for good ice cream, you have to control the amount of air, with an overrun rate lower than 25%.

In contrast, some cheap commercial ice cream can overrun up to 100% to save ingredients. Which means for every ice cream you buy, half of it is actually air.

This causes the ice cream to melt faster, and it also traps more water vapor in the air, causing the freezing process to create ice. Ice cream is cheap, so eating will be messy and less delicious.

Why do ice cream and yogurt bought in the refrigerator, not eaten immediately, become crispy and crunchy?  - Photo 2.

But it’s not that expensive ice cream can’t be frozen, the process will happen if you let the ice cream melt and put them back in the fridge. In a process known as recrystallization, if the refreezing ice crystals are larger than 50 micrometers, your ice cream will become brittle and lose its texture.

To make sure ice cream stays soft for as long as possible, manufacturers often add emulsifiers like lecithin and stabilizers like guar gum, locust bean gum, carrageenan and pectin.

These stabilizers help ice cream retain moisture during storage and slow the growth of ice crystals. However, according to Tao Wu, a food scientist specializing in carbohydrate chemistry, “these stabilizers are not very effective”.

Their performance is affected by many factors, including storage temperature and time, and the composition and concentration of other ingredients. This means they can sometimes work in one cream but not another.”

So Wu and his research team at the University of Tennessee wanted to find a better alternative.


How to prevent crystallization?

Food science is not cooking”, Wu said. “It is an interdisciplinary field of chemistry, biology and engineering that aims to solve real-world problems, specifically with regard to food production. For example, to produce high-quality ice cream, we have to use very deep chemical knowledge.”

Wu and his team initially had an idea. They noticed proteins with special structure and function that keep certain fish species from freezing in seawater below 0 degrees Celsius. These proteins are also found in insects and some plants.

They do this by sticking to the outside of the surface of the ice crystals when they first form, preventing the crystals from growing into large crystals. But the problem is that the supply of these proteins is very limited, making them very expensive and cannot be used for commercial purposes.

Previous research suggested that the freezing resistance of such proteins stems from one property: They have an amphiphilic structure, which means that they possess both hydrophilic and hydrophobic surfaces at the same time. Hydrophilic surfaces always attract H . molecules2O towards itself, while the hydrophobic surface does the opposite, repelling H2O away.


Protein has an amphiphilic structure that helps some fish species not freeze in seawater below 0 degrees Celsius.

In plants, the amphiphilic structure occurs because of cellulose nanocrystals. Cellulose is an organic compound with the formula C .6HtenO5 linked together to form chains and make up the plant cell wall.

Wu and his team think that nanocellulose might inhibit the growth of ice crystals. And unlike other commercial antifreeze and stabilizers, this is a very abundant ingredient that can be grown without being too expensive.

Min Li, a scientist in Wu’s group, said they carried out experiments with mixing nanocellulose with a cup of ice cream. Initially, the added cellulose nanocrystals had no effect on the rock crystals.

But after only a few hours, these nanocrystals completely stopped the growth of the ice crystals. Furthermore, nanocrystals of cellulose performed better than commercial stabilizers, in the context of ice cream exposure to fluctuating temperatures.

In terms of the underlying mechanism, the researchers found that surface adsorption appears to be the secret to cellulose nanocrystals that prevent ice crystallization – much like antifreeze proteins. “This completely contradicts the existing belief, that stabilizers inhibit recrystallization of ice by increasing viscosity, which is thought to slow the diffusion of water molecules.“, Li said.


The addition of nanocrystals of cellulose prevents the growth of small ice crystals (bottom left) into large crystals (top left) which can cause ice cream (right) to retain its consistency. irresistible soft and delicious.

With this new discovery, Wu said that nanocellulose could become a new stable compound to create permanent soft cream products, which are safe and approved by the Food and Drug Administration.

However, their application does not stop there. Nanocellulose would also be useful for cryopreservation of biological cells, tissues and organs, which are also vulnerable to ice crystal damage.

“For example, in biotechnology and biomedication, cells are often stored in liquid nitrogen. During storage, ice crystallization can lead to cell damage or death.“, said Wu.

Now, with new crystallization inhibitors, adding them during cryopreservation can prevent ice crystals from forming and help increase the viability of those cells.”

Their new research was presented at the American Chemical Society Conference.

Refer Arstechnica chn

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