SAN DIEGO, March 20, 2022 — Ice cream can be a culinary delight, except when it gets unpleasantly crunchy because ice crystals have grown in it. Today, scientists report that a form of cellulose obtained from plants can be added to the tasty treat to stop crystals cold — and the additive works better than currently used ice growth inhibitors in the face of temperature fluctuations. The findings could be extended to the preservation of other frozen foods and perhaps donated organs and tissues.
The researchers will present their results today at the spring meeting of the American Chemical Society (ACS). ACS Spring 2022 is a hybrid meeting being held virtually and in-person March 20-24, with on-demand access available March 21-April 8. The meeting features more than 12,000 presentations on a wide range of science topics.
Freshly made ice cream contains tiny ice crystals. But during storage and transport, the ice melts and regrows. During this recrystallization process, smaller crystals melt, and the water diffuses to join larger ones, causing them to grow, says Tao Wu, Ph.D., the project’s principal investigator. If the ice crystals become bigger than 50 micrometers — or roughly the diameter of a hair — the dessert takes on a grainy, icy texture that reduces consumer appeal, Wu says. “Controlling the formation and growth of ice crystals is thus the key to obtaining high-quality frozen foods.”
One fix would be to copy nature’s solution: “Some fish, insects and plants can survive in sub-zero temperatures because they produce antifreeze proteins that fight the growth of ice crystals,” Wu says. But antifreeze proteins are costlier than gold and limited in supply, so they’re not practical to add to ice cream. Polysaccharides such as guar gum or locust bean gum are used instead. “But these stabilizers are not very effective,” Wu notes. “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.” In addition, their mechanism of action is uncertain. Wu wanted to clarify how they work and develop better alternatives.
Although Wu didn’t use antifreeze proteins in the study, he drew inspiration from them. These proteins are amphiphilic, meaning they have a hydrophilic surface with an affinity for water, as well as a hydrophobic surface that repels water. Wu knew that nano-sized crystals of cellulose are also amphiphilic, so he figured it was worth checking if they could stop ice crystal growth in ice cream. These cellulose nanocrystals (CNCs) are extracted from the plant cell walls of agricultural and forestry byproducts, so they are inexpensive, abundant and renewable.
In a model ice cream — a 25% sucrose solution — the CNCs initially had no effect, says Min Li, a graduate student in Wu’s lab at the University of Tennessee. Though still small, ice crystals were the same size whether CNCs were present or not. But after the model ice cream was stored for a few hours, the researchers found that the CNCs completely shut down the growth of ice crystals, while the crystals continued to enlarge in the untreated model ice cream.
The team’s tests also revealed that the cellulose inhibits ice recrystallization through surface adsorption. CNCs, like antifreeze proteins, appear to stick to the surfaces of ice crystals, preventing them from drawing together and fusing. “This completely contradicted the existing belief that stabilizers inhibit ice recrystallization by increasing viscosity, which was thought to slow diffusion of water molecules,” adds Li, who will present the work at the meeting.
In their latest study, the scientists found that CNCs are more protective than current stabilizers when ice cream is exposed to fluctuating temperatures, such as when the treat is stored in the supermarket and then taken home. The team also discovered the additive can slow the melting of ice crystals, so it could be used to produce slow-melting ice cream. Other labs have shown the stabilizer is nontoxic at the levels needed in food, Wu notes, but the additive would require review by the U.S. Food and Drug Administration.
With further research, CNCs could be used to protect the quality of other foods — such as frozen dough and fish — or perhaps to preserve cells, tissues and organs in biomedicine, Wu says. “At present, a heart must be transplanted within a few hours after being removed from a donor,” he explains. “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 acknowledge support and funding from the U.S. Department of Agriculture’s National Institute of Food and Agriculture, Agriculture and Food Research Initiate (AFRI) project (2019-06761) and Hatch project (1016040).
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Inhibiting ice recrystallization by cellulose nanocrystals: Influences of sucrose concentration and storage time
In ice cream products, the presence of ice crystals with sizes over 50 µm leads to an icy texture, affecting mouthfeel and decreasing consumers’ acceptance. The increase of ice crystal size mainly results from a thermodynamically favored ice recrystallization (IR) process. To inhibit IR, polysaccharides are commonly used based on a slow-diffusion mechanism. However, their ice recrystallization inhibition (IRI) effects are dependent on measurement conditions, such as concentration and type of sweeteners, and storage temperature and time. This study elucidated the underlying mechanism of how the IRI activity of a newly identified ice recrystallization inhibitor - cellulose nanocrystals (CNCs) is affected by the sucrose concentration and storage time.
CNCs demonstrated high IRI activity in low sucrose concentrations (2% ~ 10%), became almost inactive in medium sucrose concentrations (15% ~ 35%), and regained modest activity at high sucrose concentrations (45% ~ 49%). A sufficient storage time was crucial to observe the IRI activity of CNCs in 25% sucrose solution: No IRI activity was observed in a model ice cream system within a short timescale of 0.5-2.0 hours. Over longer timescales, 1.0% CNCs and 0.5% CNCs could completely stop the growth of ice crystals with corresponding final crystal sizes of 25 and 40 μm after 5 and 68 hours, respectively.
These results demonstrated that the IRI effect of CNCs is positively related to the surface coverage on ice crystals. These research findings offer a new perspective in understanding the measurement condition-dependent IRI effect of CNCs and outline suitable experimental conditions to evaluate the IRI activity of new materials, which could potentially benefit the ice cream industry in developing new materials and recipes to control ice recrystallization.