The preservation of foods at low temperatures is a well-established concept. While conventional methods of food freezing rely on the isobaric (constant pressure) approach, they often result in a series of irreversible changes that can seriously hamper the quality of frozen foods.
In recent years, taking its roots from the biomedical industry, isochoric (constant volume) freezing is gaining both research and commercial interest as an effective method of food preservation.
The focus of this review is to present the state of the art of isochoric freezing of foods, highlighting the underlying mechanisms that make it unique, and understanding its impact on food quality, considering reports published in the past decade. An exclusive section is dedicated to its non-food applications, and this work also provides insights into the costs and economics of the process.
Importantly, as this is an emerging area, the review concludes by highlighting the challenges and provides perspectives on the directions for future research.
[Keywords: isochoric freezing, food preservation, food freezing, food quality, low-temperature preservation]
…Thermodynamics of Isochoric Systems: The thermodynamic principles of isochoric preservation were first studied in the year 2005 by Rubinsky and his fellow researchers.3 During isochoric freezing, the volume of the system remains constant while variables like pressure and temperature vary in tandem. The phase diagram of pure water in Figure 1 shows that isochoric freezing is followed by a liquidus path that lies between ice I, ice III, and liquid water. The system exhibits equilibrium pressure until the triple point at the given sub-zero temperature. For pure water, the triple point is a temperature of 21.985 °C and a pressure of 209.9 MPa. Importantly, unique conventional freezing process, ice growth cannot occur due to the constant volume, which in turn generates a hydrostatic pressure in the isochoric system.4 Theoretical and experimental data confirm that 45% of the volume remains unfrozen at the triple point in a constant volume freezing process3, 5. This effect takes the benefit of the Le Chatelier’s principle which explains that the high pressure developed inside a system upon freezing would restrict any further development of ice 6.
…Isochoric Freezing and System Designs: In a typical isochoric process, the food material is immersed in an isotonic solution inside a rigid container that is capable of withstanding elevated pressures. Depending on the pressure and temperature, materials such as stainless steel cylinders, carbon fiber composites, and hard phenolic thermosets with pressure transducers and rupture disks are employed for isochoric processes. Sugar or salt solutions are used for preservation. Then, ice crystals are introduced in the container as the nucleation site and the chamber is tightly packed. To preserve food materials in their aqueous phase without the formation of ice crystals, it is important to insert this nucleator. The chamber is then sealed with a metal screw to restrict any passage of air in and out of the container.3 This preserves the food material in a 2-phase thermodynamic condition, without the risk of cellular injury (Figure 2). The two-phase isochoric system is achievable only if the system is tightly packed and no air or liquid can escape out of it. A temperature bath is used to cool the system. Most systems also have pressure transducers and thermistors linked to the data acquisition card and connected to a computer for data processing.6
…The impact of high pressure in isochoric freezing seems advantageous in terms of microbial destruction. Isochoric preservation completely exterminated E. coli at −15 °C because bacterial suspension at this temperature is in a metastable and amorphous liquid state, not conducive for the bacteria to survive.26 It was observed that partial destruction of E. coli occurs at −20 °C and −30 °C in the isochoric freezing process due to the ice III and ice Ih formations where some E. coli try to shelter inside ice crystals and replicate after the freezing process.26
…Combined Techniques (Spontaneous): Isochoric cryopreservation can tolerate liquid nitrogen temperatures and pressures ranging up to 413 MPa, explaining that pressure measurement is crucial for the control of vitrification and devitrification in aqueous solutions.35 Such ideas further helped in the experimental validation of the isochoric vitrification process. Vitrification in isochoric freezing can be facilitated using additives such as propanediol and dimethyl sulfoxide (Me2SO) at concentrations ranging from 0 to 49% (w/v), and this has been proven for cryopreservation of organs and tissues.36 The concentration of cryoprotective additives for isochoric vitrification is substantially less in isochoric freezing than in the case of isobaric vitrification at 1 ATM and a hyperbaric process at 1000 ATM. Therefore, isochoric techniques promote vitrification more effectively than hyperbaric systems.36
Super-cooling in isochoric conditions improves the stability of the system when exposed to various mechanical stimuli such as drop impact, vibration, ultrasonication, and thermal fluctuations. This effect is achieved by combined thermodynamic and kinetic factors that reduce the microscopic density fluctuation, eliminate the air–water interface, and provide resistance to cavitation.37 Another combined technique is the modification of the existing isochoric system in which multiple aqueous phases are employed, separated by a membrane impermeable to mass transfer but transmit heat and pressure. This multiphase isochoric freezing model can be used for the complete removal of hypertonicity and ice crystal formation in cryopreservation protocols.38
…Energy and Cost Comparisons: Slow freezing processes employed in industries for freezing of food items involve the use of deep cryogenic temperatures to reduce the size of ice crystals and then storing foods under freezing temperature, accounting to be an energy-intensive process. The consumption of energy in an isochoric system is substantially lesser than an isobaric system of equal mass. This is because of the reduction in total frozen mass and the temperature dependence of the latent heat of fusion of water. In an isochoric system, only a portion of the mass is frozen at a given sub-freezing temperature higher than the triple point, decreasing the total energy for ice fusion. However, in an isobaric system, phase transition takes place at the atmospheric freezing point and the latent heat of fusion decreases with temperature, consequently requiring more energy to freeze. Thermodynamic analyses showed that fish or meat when stored in an isochoric system at −5 °C consume 70% less energy than the conventional freezing process. Further, more energy savings can be achieved when foods like fruits and berries with high sugar contents are preserved. Isochoric storage can reduce energy expenditure at an industrial level as no ice formation takes place inside the food.4 Such systems aim to increase efficiency and can be designed by altering existing industry-scale freezers. This can be achieved without major infrastructural alterations and appliance wastages. Further, the simple design of isochoric systems makes them convenient and relatively economic in terms of usage.6