Cold war: solving ice cream’s ice problem

What’s worse to eat than a crunchy, icy bite of ice cream? Chemists have figured out how to ensure the ice cream stays soft – using a type of cellulose taken from plants. They say their discovery could also be used to preserve other frozen foods as well, and perhaps even organ and tissue donations.

The reason ice cream sometimes picks up its criminal crunch is the ‘ice’ part. All ice cream has tiny ice crystals in it. If these crystals are very small – less than roughly 50 micrometres, or the width of a human hair – they’re not noticeable. But the larger they get, the grainier and crunchier the food becomes.

Manufacturers avoid big crystals by freezing their ice cream very quickly at very low temperatures (often with liquid nitrogen), and with lots of mechanical agitation. But if the ice cream melts and re-freezes while being transported or stored, this good work is lost.

Read more: Science of ice cream: chemistry made delicious

“Controlling the formation and growth of ice crystals is thus the key to obtaining high-quality frozen foods,” says Dr Tao Wu, a researcher in the Department of Food Science at the University of Tennessee, US.

Wu and colleagues looked to nature for an additive that prevents these large crystals, regardless of how the ice cream is frozen.

“Some fish, insects and plants can survive in sub-zero temperatures because they produce antifreeze proteins that fight the growth of ice crystals,” says Wu. These proteins are expensive and difficult to make or extract, so not a practical additive for ice cream.

Commercial manufacturers do use another type of antifreeze in ice cream: some polysaccharides gums can reduce the big crystals.

“But these stabilisers are not very effective,” says Wu.

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

Wu noted that many effective antifreezes are amphiphilic: they’re made of molecules that repel water in one part and attract it at another. He decided to investigate another, well-known polysaccharide: cellulose, formed into nanometre-sized crystals.

In addition to being made of the most common polymer in nature, cellulose nanocrystals are also amphiphilic, so they could potentially do similar antifreeze work.

The researchers tested these cellulose nanocrystals by adding them to “model ice cream”: solutions with different concentrations of sucrose. They found that the freezing time and concentration played a big role in how well the nanocrystals did their job.

One panel showing two microscope pictures, the higher one has larger blobs than the lower one, with a scale bar, and a second panel showing a scoop of ice cream
A close-up of ice cream with cellulose nanocrystals (bottom left), versus without (top left). The black bar represents 100 micrometres. Credit: Tao Wu

At a concentration of 25% sucrose, similar to commercial ice creams, the cellulose nanocrystals initially didn’t make any difference at all to the formation of ice. But after a bit of time, they kicked in: a concentration of 1% cellulose completely stopped ice crystals from getting any bigger after five hours. The ice crystals in this mixture couldn’t get larger than 25 micrometres, well below both the crunch limit and the crystals in the control solution.

Halving the concentration of cellulose, to 0.5%, meant that ice growth halted at 68 hours and 40 micrometres.

The cellulose nanocrystals were even more effective at lower and higher concentrations of sucrose.

The researchers also looked at the mechanisms for these cellulose nanocrystals, finding something that surprised them. The cellulose nanocrystals seem to work by sticking to the surface of ice crystals, preventing them from getting bigger.

“This completely contradicted the existing belief that stabilisers inhibit ice recrystallisation by increasing viscosity, which was thought to slow diffusion of water molecules,” says Min Li, a graduate student in Wu’s lab.

Cellulose nanocrystals aren’t toxic, but they’d need to be reviewed by food safety authorities before being added to ice cream. Nevertheless, Wu is excited by their potential: as well as ice cream, the crystals could be used to preserve other frozen foods. And they might even have medical applications.

“At present, a heart must be transplanted within a few hours after being removed from a donor,” says Wu.

“But this time limit could be eliminated if we could inhibit the growth of ice crystals when the heart is kept at low temperatures.”

The researchers have presented their findings at a meeting of the American Chemical Society.

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