Ice cream can develop an unpleasant, grainy texture over time. Freshly made ice cream contains tiny ice crystals, but during storage and transport, the ice can recrystallize to form larger crystals. Controlling the formation and growth of ice crystals is, thus, important for high-quality frozen foods. Some species in nature can survive in sub-zero temperatures because they produce antifreeze proteins. However, antifreeze proteins are very expensive and limited in supply, so it is not practical to add them to ice cream. Polysaccharides such as guar gum are used instead, but these stabilizers are not always effective.
Tao Wu, University of Tennessee, Knoxville, USA, and colleagues have found that cellulose nanocrystals can be used to inhibit ice recrystallization, e.g., in ice cream. Antifreeze proteins served as an inspiration for this approach: These proteins are amphiphilic, i.e., they have hydrophilic as well as hydrophobic surfaces. Nanosized crystals of cellulose are also amphiphilic. These cellulose nanocrystals (CNCs) can be extracted from plant cell walls in agricultural and forestry byproducts, so they are inexpensive, abundant, and renewable.
In a 25 % sucrose solution used as a model for ice cream, the CNCs initially had no effect: Ice crystals were the same size whether CNCs were present or not. After the model ice cream was stored for a few hours, the team found that the CNCs inhibited the growth of ice crystals, while the crystals continued to grow larger in the untreated sample. The researchers found that the cellulose nanocrystals inhibit ice recrystallization through surface adsorption. CNCs, like antifreeze proteins, appear to adhere to the surfaces of ice crystals, preventing them from fusing.
CNCs were found to be more protective than current stabilizers when the sample was exposed to fluctuating temperatures, such as when ice cream 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. The stabilizer is nontoxic at the levels needed in food, but would require regulatory review before its use as an additive. With further research, CNCs could potentially be used to protect the quality of other foods or perhaps to preserve cells, tissues, and organs in biomedicine.
- Presented at the ACS Spring Meeting 2022 of the American Chemical Society (ACS)