Isochoric freezing is a new technology with relatively low energy requirements that preserves food products at subfreezing temperatures without damage due to ice crystal formation inside the product. I wish to thank my co-author, Cristina Bilbao-Sainz, research food technologist at the U.S. Department of Agriculture, Agricultural Research Service, for contributing to this column on the topic of isochoric freezing.
Isochoric freezing was developed by Boris Rubinsky at the University of California, Berkeley. He first published the thermodynamic principles of isochoric cryopreservation in 2005 in the journal Cryobiology. He and his group have been using isochoric freezing for cells, tissues, and organ transplantation. Transplantation requires restoring the metabolism of the organ after being halted or slowed during preservation. Ice formation during preservation is the major mechanism that prevents the revival of the metabolism and therefore limits the success of the transplantation. Isochoric freezing preserves biological materials without inducing the dangerous formation of ice crystals in the organ and therefore presents great potential as a cryopreservation method for medical transplantations.
Isochoric freezing was later examined as a preservation method in the frozen food industry. Rubinsky started a collaboration with the U.S. Department of Agriculture in 2017 to explore the potential benefits of isochoric freezing in the food industry. The team has shown that freezing under certain isochoric conditions results in food products with superior quality to those preserved under conventional freezing and can likely generate substantial energy savings as well. Additionally, the team demonstrated that isochoric freezing can lead to reduction of microorganisms during storage.
The frozen food market is one of the most dynamic food markets in the world. The global frozen food market was valued at more than $250 billion in 2015 and is projected to reach $282.5 billion by 2023. This is due to the changing and busy lifestyles that have led to increased consumption of frozen foods globally. However, many foods, especially those with delicate textures, such as fruits and vegetables, are not suitable for traditional freezing since they deteriorate significantly during freezing. Isochoric freezing offers the potential to freeze these foods and maintain their fresh-like quality, which can result in significant market growth for these products.
Freezing at atmospheric pressures is one of the main processes employed by the food industry for long-term preservation and storage. Low temperatures and crystallization of water during freezing slow down deterioration reactions and inhibit growth of spoilage microorganism and pathogens. However, ice formation during freezing degrades the quality of the thawed food and reduces the consumer approval for these frozen foods. Isochoric freezing is an innovative freezing technology that can significantly improve the quality of frozen foods. The key element is the thermodynamic conditions in which freezing occurs. Traditional freezing occurs at a constant atmospheric pressure, whereas isochoric freezing occurs at constant volume. The food product is immersed in an isotonic solution inside a closed chamber so that the volume remains constant during freezing. The chamber is then gradually cooled down to a preset freezing temperature. Once the temperature reaches the freezing point of the osmotic solution, ice starts forming and growing in size, generating hydrostatic pressure inside the closed chamber until the system reaches a new thermodynamic equilibrium at the preset freezing temperature. At this point, a twophase system exists, with an unfrozen liquid portion and a frozen solid portion. The food can be safely preserved without any ice crystals formation if it remains in the liquid portion of the system. One important aspect of this technology is that temperature and pressure are correlated. The only adjustable variable to achieve thermodynamic equilibrium is the relative percentage of ice and liquid in the system. Therefore, for a certain temperature, the percentage of ice formed will generate the pressure required for thermodynamic equilibrium between ice and liquid at that pressure.
The potential of isochoric freezing for enhancing the quality of foods stored at subfreezing temperatures has been demonstrated with studies on cherries, tomatoes, and potatoes. These studies demonstrated significant improvements in the physicochemical and nutritional quality of foods preserved in isochoric systems compared to conventional preservation methodologies. The high quality is due to the absence of ice crystal formation in the sample as well as the low pressures and low temperatures of the process.
Additionally, recent work suggests that isochoric freezing can eliminate potentially harmful bacteria. Experimental studies have demonstrated that the synergistic combination of moderate cold temperature (-15˚C) and moderate high pressure (135MPa) can inactivate bacteria.
Freezing in an isochoric system also consumes less energy than freezing in a conventional freezer. Fundamental thermodynamic analyses have demonstrated that freezing in an isochoric system can reduce energy consumption by up to 70% compared to a traditional freezing process. This is due to the reduction in total frozen mass and the decrease in latent heat of fusion with temperature. Also, food products stored in isochoric systems will experience fewer temperature fluctuations during transport and storage because temperature fluctuations cause phase changes rather than sensible temperature changes. Consequently, an increase in temperature results in the melting of ice inside the chamber rather than in the heating of the food product.
The main disadvantage of isochoric freezing is the generation of hydrostatic pressures inside the processing chamber. Hydrostatic pressure can break the cellular tissues in biological food matters and deteriorate the overall quality. However, the maximum processing pressure can be controlled by adjusting the processing temperature since they are interrelated.
Roberto Avena-Bustillos (left) and Cristina Bilbao-Sainz demonstrate the use of isochoric freezing chambers. Photo courtesy of the U.S. Department of Agriculture
Roberto Avena-Bustillos (left) and Cristina Bilbao-Sainz demonstrate the use of isochoric freezing chambers. Photo courtesy of the U.S. Department of Agriculture
Isochoric freezing systems are simple and very easily constructed. They require only a chamber with rigid walls so that the volume remains constant during processing. The design of the chambers could be tailored to optimize product quality and energy consumption for a given industrial application. For the construction of the chamber, the designer should consider the following factors: 1) The chamber should be designed to locate the food matter far from the nucleating site of ice formation and to avoid the possibility that the ice will extend into the space occupied by the food matter and 2) The material and dimensions of the chamber should be selected to withstand the generated pressures at the desired freezing temperature. The isochoric chambers used in research have been typically built from thick-walled stainless steel cylinders. However, chambers can potentially be made from lighter materials such as carbon fiber composites or hard phenolic thermosets, depending on the target temperature and pressure.
For industrial applications, a battery of chambers can be placed in any existing conventional freezer so the food industry can incorporate isochoric freezing without committing to major infrastructural changes.
Isochoric freezing can be used in many potential applications in the frozen food industry since it can be used to preserve food products without inducing the formation of ice. This allows an increase in the marketability of food products that are currently not suitable for traditional freezing due to deterioration after freezing and thawing, such as tomatoes, berries, and leafy greens.
This novel technology could also find commercial applications in the sterilization and preservation of beverages such as milk, tea, and juices. Isochoric freezing can dramatically extend the shelf life of such products by diminishing the microbial load while preserving most of the sensory, nutritional, and functional properties of the treated products.
Isochoric freezing also offers a novel approach to introduce solutes into the food product by immersing the food in a hypertonic solution. The solution can be fortified with micronutrients and bioactive components to produce novel products with innovative sensorial properties and nutritional profiles.
Isochoric freezing shows significant promise for use in higher-quality food manufacturing and preservation. Future efforts will likely focus on the design and manufacture of industrial-scale isochoric freezing chambers to match industrial needs in a cost-effective manner. Also, research and development efforts will continue to explore the potential benefits of isochoric freezing to the food industry and ultimately to consumers.
REFerences
Bilbao-Sainz, C., A. Sinrod, M. J. Powell-Palm, et al. 2018. “Preservation of sweet cherry by isochoric (constant volume) freezing.” Innovat. Food Sci. Emerg. Technol. 52: 108–115.
Powell-Palm, M. J. & B. Rubinsky. 2019. “A shift from the isobaric to the isochoric thermodynamic state can reduce energy consumption and augment temperature stability in frozen food storage.” Food Eng. 251: 1–10.
Powell-Palm, M. J., J. Preciado, C. Lyu, and B. Rubinsky, B. 2018. “Escherichia coli viability in an isochoric system at subfreezing temperatures.” Cryobiol. 85:17–24.
