Analysis of global energy savings in the frozen food industry made possible by transitioning from conventional isobaric freezing to isochoric freezing

Highlights

• Food difficult to preserve with isobaric freezing can be stored in isochoric system.

• Using isochoric food cold chain rather than isobaric can reduce global energy cost.

• A numerical model is developed to describe the thermodynamics of food isochoric storage.

• Temperature profiles of different food freezing and storage modes are obtained.

Abstract

An efficient global cold food chain is critical to the sustainability of the growing world population, and it is anticipated that the global frozen food market will reach $404.8 billion by 2027. Frozen foods are typically stored under conventional industry-standard isobaric (constant-pressure) conditions at sub-freezing temperatures, however, which can degrade the textural and nutritional quality of the food and comes at high energetic and carbon costs. While efforts to reduce this energetic toll have traditionally targeted the devices used to generate refrigeration, we herein identify that significant energy savings may be attainable by altering the fundamental thermodynamics of the freezing process itself. Here we show that preserving frozen food under isochoric (constant-volume) thermodynamic conditions, as opposed to conventional isobaric conditions, may theoretically reduce annual global energy consumption by as much as 6.49 billion kWh, with accompanying carbon emission savings of 4.59 billion kg. Importantly, these savings can be achieved rapidly and inexpensively, without any costly changes to the current global refrigeration infrastructure. Furthermore, early studies demonstrate that isochoric freezing results in substantially improved food quality, extends the preservable lifetime of fresh and otherwise delicate food products, and has cross-cutting biopreservation applications in domains as diverse as medicine, biology, and pharmaceuticals.

Introduction

The development of sustainable, equitable, and inclusive global food chains in the face of a growing population represents a landmark challenge on the path towards global sustainability in the 21st century. Meeting this challenge will require innovation in multiples domains, but perhaps most pressingly in the food cold-chain. Cold storage of food is integral to global food capacitance (i.e. the ability to store and distribute food resources as necessary). Its efficacy and availability have huge implications on global food waste (which has been estimated at greater than 30% of total post-harvest yields) [[1], [2], [3]], safety [4], and accessibility. Estimates anticipate a continuous growth in the frozen food market, from $291.8 billion in 2019 to $404.8 billion in 2027 [5].

The current global cold-chain operates at significant energetic costs; domestic food cold-storage alone was estimated to account for nearly 4% of all global electricity consumption annually [6], equivalent to $6.54\times {10}^8$ metric tons of emitted carbon dioxide [7] and costing approximately 120 billion USD. The high economic and climatological price of essential cold-storage infrastructure creates systemic inequalities in food access between countries with high- and low-income economies. Thus, in envisioning an energy efficient future food chain, capable of meeting the needs of a soaring global population, fundamental innovations in the cold-storage space are needed to reduce both the economic burden, the energetic burden and the carbon footprint of the cold-storage infrastructure.

Cold storage of food is driven by the strong temperature dependence of metabolism. Lowering the temperature reduces the metabolism, thereby slowing the deterioration of food and the growth of contaminant microorganisms. However, in an isobaric (constant-pressure) thermodynamic environment open to the atmosphere (as virtually all modern food storage presently is), food matter (being comprised largely of water), will freeze at sub 0 °C temperatures. The ice crystals that form upon freezing can substantially degrade the textural and nutritional quality of the food, and the central efforts of the modern food cold storage community seek to ameliorate the deleterious effects of these ice crystals while preserving the beneficial effects of low-temperature preservation, e.g. Refs. [8,9]. Quick freezing, which can decrease the size of ice crystals and thereby reduce morphological damage to frozen foods, revolutionized the food industry in the early 20th century [10] and is still today considered the gold standard of frozen storage [11,12]. The rapid cooling methods that are commonly used in the food industry are: a) freezing to liquid nitrogen temperatures (−196 °C), b) freezing to −100 °C, c) freezing to −40 °C and d) hyperbaric freezing in which the pressure of the system is first increased (an energy consuming process) and then suddenly released [13,14]. Maintaining these ultra-low temperatures at which food is rapidly frozen during subsequent continuous storage is extremely energy-intensive and requires specialized refrigeration systems. Therefore frozen food is typically stored at substantially higher temperatures than those at which it is initially frozen. However, this transition in storage temperatures can also lead to food damage, due to such mechanisms as ice recrystallization and fluid migration [15]. Therefore, in the cold food chain, a balance is sought between temperatures low enough to reduce recrystallization and temperatures at which the energy costs are acceptable. Most commercial cold storage systems use storage temperatures between −10 °C and −18 °C. In order to attain energy savings in the cold storage domain, many contemporary research efforts aim to re-examine the technological methods by which we generate this cold, from improving the efficiency of conventional vapor-compression freezers [[16], [17], [18]], to conceiving new refrigeration processes based on novel thermodynamic principles [18]. However, little research by comparison has questioned the fundamental thermodynamics at play within the freezing food matter itself, and the potential energy-saving benefits of extreme departures from conventional frozen storage.

Building from fundamental thermodynamic principles, we have developed a new concept for food storage at subfreezing temperatures, “isochoric cold storage” [19]. This storage modality aims to replace industry-standard cold storage, which occurs under constant atmospheric pressure (isobaric) conditions, with cold storage that instead occurs in constant volume (isochoric) conditions. Superficially, the difference between isobaric and isochoric systems appears minor: an isobaric container (volume) is open to the atmosphere (which functions as a pressure reservoir), while an isochoric container (volume) is closed to the atmosphere. However, thermodynamic theory and experiment show that the isochoric process of cold storage is profoundly different from its isobaric equivalent [[19], [20], [21], [22], [23], [24]] resulting in substantial improvements in the quality of the preserved food [25,26]. Furthermore, isochoric systems can be used for storing foods that are otherwise difficult to preserve with isobaric freezing, such as tomatoes [27], spinach [28], cut potatoes [29], sweet cherries [30], etc. For example, isochoric preservation of tomatoes for one month led to improved quality stability of tomato fruits when compared with conventional preservation techniques such as individual quick freezing (IQF) [27]. Tomatoes preserved at −2.5 °C in an isochoric system showed the most desirable sensory characteristics in terms of mass, shape, volume, color and texture retention, and retained 98% ascorbic acid, 98% lycopene, 88% phenolic compound and 94% antioxidant activity. In comparison, ice formation during IQF freezing led to 17% mass loss and 16% volume loss, which contributed changes to both overall visual quality and firmness and reduced nutrient retention, (87% ascorbic acid, 33% lycopene, 40% phenolic compounds and 36% antioxidant activity). In another recent study, cut raw potatoes preserved at −3 °C for one month under isochoric conditions had lower drip loss and volume shrinkage as well as better preserved texture and microstructure than cut raw potatoes conventionally preserved at atmospheric pressures [29]. Finally, the potential of isochoric freezing for the development of value-added products and processes has also recently been demonstrated: value-added food products with improved nutrition and functionality can be produced using the enhanced hydrostatic pressures generated during isochoric freezing, which allow the infusion of bioactive compounds such as nutritional compounds, anti-browning agents, firming agents, antimicrobials, etc. into foods [31]; and processes that combine food storage and food safety measures can be developed by leveraging the unique bacterial sterilization effects observed in isochoric systems at low temperatures [32,33].

Illustrative examples comparing isochoric and isobaric storage of sensitive foods are shown in Fig. 1. Our results in these papers show that optimal isochoric storage temperatures are between −2.5 °C and −7 °C. The ability to cold-store these difficult-to-freeze products can have a major effect on world-wide reduction of food waste efforts.

It should also be noted that the process of isochoric freezing is not limited to the food domain, but is in fact applicable to issues of biological preservation in domains as divergent as medicine, conservation biology, and space travel. Notable recent efforts have employed isochoric freezing in the global effort to preserve complex organs and tissues outside the body for transplant [34,35], demonstrating the first-ever preservation of full mammalian hearts at sub-zero centigrade temperatures without chemical cryoprotectants [36], the first-ever multiday preservation of autonomously beating genetically-human engineered cardiac tissue [37], and successful multiday preservation of mammalian pancreatic islets [38].

While these attributes of isochoric cold storage are all of value to the global food economy and health, the primary motivation for this work comes from a distinct and nuanced thermodynamic finding: In addition to improvements in the quality and safety of the cold stored food, isochoric cold storage consumes substantially less energy than isobaric cold storage [39,40].

The process of freezing in an isochoric system can be explained in thermodynamic terms by application of Le Chatelier's principle or analysis of Helmholtz free energies of water and ice [22,23]. From a high-level physical viewpoint, however, the unique behaviors experienced in isochoric systems can be thought of as a result of the expansion undergone by liquid water in transitioning to ice-Ih, which has a lower density than water. If water is confined within a constant-volume container, as freezing initiates, the lower-density ice Ih cannot freely expand, and it thus compresses the yet-unfrozen portion of the liquid, forcing the freezing process to proceed along the “liquidus line” of the Temperature-Pressure phase diagram. Fig. 2a illustrates the thermodynamic path that isochoric freezing takes (as an ice-water two-phase mixture along the liquidus line) alongside the isobaric freezing path (in which the entire system will freeze after the marked constant pressure line intersects the liquidus line in temperature). Fig. 2b shows that at −18 °C (the industry-standard frozen storage temperature), less than 50% of an isochoric volume will freeze. This of course suggests the possibility that food matter can be preserved in the remaining liquid phase of the isochoric system at subfreezing temperatures in an unfrozen state, and therefore, without concern for the effects of ice crystals on the quality of the food and without the need to resort to complex and energy-intensive cold storage protocols such as quick freezing. An illustration of the concept of isochoric freezing is shown in Fig. 2c.

It is important to emphasize in advance that refurbishing the global “isobaric” cold storage food chain with “isochoric” storage does not require any substantial changes in global refrigeration infrastructure, as it is a mode of storing food within a refrigeration system, and thus avoids the economic and logistical specter of replacing the billions of domestic and industrial refrigeration units currently in use. Isochoric cold storage simply replaces conventional storage containers with sealed, constant-volume containers; it is not dependent on any specific means of refrigeration, and thus may also be seamlessly integrated with both current conventional infrastructure and any novel refrigeration systems that may achieve widespread use in the future. For completeness, we also mention another thermodynamic food storage method that is less common, supercooling, in which the food mass is cooled only mildly past the freezing point and held in a metastable ice-free liquid state [24,34]. This form of cooling also exists in isobaric and isochoric variants, and we have recently demonstrated through thermodynamic analysis and experiments that supercooling under isochoric conditions is substantially more stable than supercooling under isobaric conditions. It is important to emphasize here, that in isochoric cold storage, either through freezing or supercooling, the processing and storage considerations which have conventionally constrained isobaric cold storage due to considerations of ice-related damage simply do not apply. Isobaric and hyperbaric freezing results in ice crystal formation, driving substantial degradation of the textural and nutritional quality of the food [[13], [14], [15]]; isochoric storage modes (freezing/supercooling) preserve food without ice damage [[27], [28], [29], [30],37], resulting in improved quality. Therefore, while for isobaric cold storage there is a strong argument for storing frozen foods at −18 °C to avoid secondary affects related to ice formation (such as recrystallization), in isochoric storage any optimal subfreezing temperature can be used.

In this work, we analyze the energy savings, reduction in global energy burden, and reduction in carbon footprint that could be accrued from transitioning the global food cold chain from isobaric cold storage to isochoric cold storage. We develop a simple two-part phase change model to describe the thermodynamics of frozen food storage in several thermodynamic storage modes: traditional isobaric freezing; hyperbaric (or pressure-shift) freezing [13,14]; isochoric freezing, and isochoric supercooling [24], which we evaluate at several storage temperatures reflecting various processes found in the industry. The Methodology section of this paper describes the mathematical model and calculations used to compare the energy expenditure for cold storage in isochoric systems and various isobaric systems.