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Cryochemistry: chemical reactions accelerated by freezing

Surprisingly, some chemical reactions can be accelerated by freezing. The possible applications of studying such cryoreactions include the food and pharmaceutical cold chain, as well as environmental remediation in cold climates. 

The rate of chemical reactions is usually reduced or even inhibited in a frozen state. Yet, studies have shown that some reactions in solution can be accelerated by freezing. Researchers consider the "freeze concentration effect" to be the main factor in the mechanism of accelerating reactions in a frozen system. [1]

 

The freeze concentration effect is the result of the conversion of water into relatively high purity ice crystals during freezing. Ice is intolerant to impurities, and foreign atoms cannot enter the ice lattice. Therefore, all non-aqueous components escape to the freeze-concentrated solution and are concentrated in a reduced liquid phase. In these liquid pockets of highly concentrated solutes, chemical reactions occur at a faster rate. The acceleration caused by the concentration effect is greater than the inhibition caused by freezing, thus the final manifestation is that the chemical reaction is accelerated by freezing. 

 

Applications of cryoreactions in the cold chain [1]

 

Studying undesirable accelerated reactions at low temperatures and the resulting changes of food, protein products and drugs is crucial for extending the storage period and maintaining the safety of food, biological reagents and drugs. 

 

Sodium nitrite is often added to processed meat and many amines are used as food colour additives. Unfortunately, recent studies have found that the reaction of dimethylamine and nitrite to produce N-nitrosodimethylamine (NDMA), which is a carcinogen, could be accelerated when it occurs in frozen concentrated solutions. Dangerous amounts of carcinogenic nitrosamines may be produced if this reaction occurs during freezing. Therefore, studying the N-nitrosation reaction in frozen systems is very important for the cryopreservation of food. 

 

In the pharmaceutical cold chain, researchers are investigating the stability of drugs or biological reagents that need to be stored under frozen conditions for extended periods of time. For example, for most protein drugs, preventing and reducing protein aggregation is the key to maintain their stability. However, when an aqueous solution containing monosodium and disodium phosphate is frozen, the selective crystallization of disodium hydrogen phosphate will cause the pH of the freeze concentrate liquid to decrease, which can lead to protein degradation. Research into pH changes by freezing can help provide a solution for stabilizing the pH and therefore prevent protein aggregation. 

 

Applications of cryoreactions for environmental remediation  [1-4]

 

There is an increasing number of studies on cryoreactions for the degradation of pollutants in cold regions, which are natural frozen systems.  For instance, recent studies have shown that in a −20 °C environment, IO4− can degrade furfural (FFA) and other organic pollutants (tryptophan, 4-chlorophenol, bisphenol A and phenol) whereas the degradation of these organic substances cannot be achieved in water at 25 °C. Therefore, compared to other methods of activating the same dose of IO4−, freezing allows more organic pollutants to be degraded. [1]

 

Among the contaminants of emerging concern, antibiotics are increasingly being detected in aquatic ecosystems worldwide, reflecting their widespread use both in animal and human health care. Antibiotics can have a direct toxic impact on aquatic organisms. Sulfonamides, especially sulfamethoxazole (SMX) and sulfamethazine (SMZ), are the antibiotics most frequently detected in contaminated ecosystems. [2-4] Recent studies have shown that the chemical reactions that may contribute to the degradation of SMX are negligible at ambient temperature but significantly accelerated under freezing conditions. [1] 

 

Freezing promotes reactions, which is rewarding for the treatment of pollutants in cold regions in a more energy-efficient and greener way. These accelerated reactions may become important weapons for protecting ecological environment in cold areas in the future. [1]

 

 

Sources 

[1] An L-Y, Dai Z, Di B, Xu L-L. Advances in Cryochemistry: Mechanisms, Reactions and Applications. Molecules. 2021; 26(3):750. https://doi.org/10.3390/molecules26030750  
[2] Pesce, S., Kergoat, L., Paris, L., Billet, L., Besse-Hoggan, P., & Bonnineau, C. (2021). Contrasting Effects of Environmental Concentrations of Sulfonamides on Microbial Heterotrophic Activities in Freshwater Sediments. Frontiers in Microbiology, 12. https://doi.org/10.3389/fmicb.2021.753647 

[3] Which factors make drugs persistent? A look at sulphonamides in Polish rivers.  “Science for Environment Policy”: European Commission DG Environment News Alert Service. Issue 480. 12 January 2017.  https://ec.europa.eu/environment/integration/research/newsalert/archive_yr/archive2017.htm 
[4] Kergoat L, Besse-Hoggan P, Leremboure M, et al. Environmental Concentrations of Sulfonamides Can Alter Bacterial Structure and Induce Diatom Deformities in Freshwater Biofilm Communities. Front Microbiol. 2021;12:643719. Published 2021 May 7. https://doi.org/10.3389/fmicb.2021.643719