From Clarence Birdseye's invention of the quick freezing method to modern-day freezing systems, frozen food companies have continually innovated and delivered a variety of products. Frozen foods have revolutionized the way we store and consume food, offering convenience, longer shelf life, reduced food waste, and global access to year-round nutrition. With U.S. frozen food sales exceeding US$74 billion in 2023 and a global market size of nearly US$300 billion, manufacturers meet market demands from industrial customers to everyday consumers across the world for high-quality and safe frozen foods.¹ At the same time, frozen food production and logistics involve an energy-intense series of steps.

The frozen food industry is an integral part of the retail, foodservice, and industrial food system. Frozen fruits and vegetables, as well as other frozen commodities such as meats, seafood, and dairy, serve as ingredients for further manufacturing of frozen meals, entrees, side dishes, and desserts, or preparation at restaurants, cafeterias, and quick-service outlets, or simply for cooking and consumption in the home. Furthermore, frozen foods are a proven format to reduce food waste across the supply chain from foodservice to consumers.²

It is imperative for our food system to rely on freezing to preserve our foods and to incorporate frozen foods in meal and dietary plans. As these nutritious, convenient, and affordable frozen ingredients and foods are stored and distributed across the marketplace, their integrity, quality, and safety are paramount. Action to curb the overall carbon footprint of the food and agriculture sector must be done with care in the frozen category, as the quality of frozen foods reaching the consumer depends on a collaborative effort among all links in the supply chain.

Freezing Foods for Storage and Distribution: Energy Requirements and FSQ Considerations

The energy requirements involved with freezing foods are substantial. The frozen food chain comprises multiple stages: preparing commodities and ingredients, freezing, storage, transportation, etc. Whether it is energy expended to move ingredients from one point in the facility to another or across a series of unit operations, each of these steps requires energy to run mechanical operations and refrigerants to maintain necessary temperatures.

Freezing

The practice of freezing foods is complex. While it encompasses the use of low temperatures to remove heat and form ice, the freezing rate is influenced by size (dimensions and shape) of the product or package, the temperature of the product entering the freezing system, the temperature of the freezing medium, and the heat transfer properties of the product or package. In fact, frozen food processors have fine-tuned their operations to optimize the time and temperature profiles of freezing procedures to match the quality requirements of different types of frozen foods, thereby optimizing moisture, texture, color, and flavor attributes. The designs of equipment are also complex and influence energy requirements for the removal of thermal energy from the product.

The speed at which food is frozen is critical to end-product quality. Modern-day, individually quick-frozen (IQF) processes reduce the temperature of food from ambient to –20 °F (–29 °C) to –40 °F (40 °C) in a matter of a few minutes. Rapid freezing forms very small ice crystals, while slow freezing produces larger ice crystals. The larger ice crystals formed during freezing can adversely affect food microstructure; this becomes evident during thawing, such as causing the tissue to soften with accompanying loss of moisture, flavor, aroma, color, and texture. Freezing also slows down natural chemical reactions such as oxidation, moisture migration, etc., helping maintain flavor, appearance, and ultimately product shelf life.

It is critical during freezing to limit the extent of dehydration (water loss from the product when exposed to cold airflow), which is a direct result of the time the food particle spends in the freezing equipment. Different foods are susceptible to different levels of dehydration, based on their composition. Manufacturers optimize freezing to reduce temperatures rapidly, without compromising food safety and quality, and should be attentive to efficiencies in energy consumption. For example, reducing the product temperature to below its optimum storage temperature is wasted energy.

Given the delicate balance that manufacturers need to achieve to maximize the quality of frozen foods, considerations to modify freezing protocols to conserve energy must be undertaken with caution.

“A variety of refrigerants are used in the industry, with the most common being ammonia, a natural refrigerant known for its high efficiency and useful thermodynamic properties at low temperatures.”

Frozen Storage and Distribution

Frozen foods need to be stored and transported at low temperatures, and it is critical to avert disruptions across the cold chain to ensure product quality and safety. Energy demand for frozen storage is continual, as frozen storage containers must operate without interruption and ensure optimal temperature conditions. Construction of energy-efficient buildings with appropriate insulation, high-performance refrigeration systems, advanced reefer air conditioning (AC) systems, energy management, and automation to monitor and adjust temperature are examples of strategies that are being implemented.

Refrigerants play a crucial role in the freezing and preservation of food products. They are responsible for absorbing heat from the environment inside the freezer and dissipating it outside, maintaining the low temperatures necessary for food safety and quality. A variety of refrigerants are used in the industry, with the most common being ammonia, a natural refrigerant known for its high efficiency and useful thermodynamic properties at low temperatures. Ammonia systems are also energy efficient and produce limited environmental impact, as they do not contribute to global greenhouse gas (GHG) emissions.

Other refrigerants, such as carbon dioxide and new-generation hydrofluorolefins (HFOs), also offer excellent heat transfer properties and low environmental impact, but require specialized equipment for operation. Synthetic refrigerants are being phased out due to environmental concerns.

Distribution of frozen foods involves moving products from one cold storage warehouse to another, such as a distribution center or retail outlet. At the retail outlet, for example, frozen foods are subsequently transferred to freezer cabinets for consumer access. The logistics associated with cold transport and handling frozen foods in a global supply chain are complex, with different technologies, modes of travel, geographies, regulatory standards, etc.—all of which present challenges to temperature integrity.

Each entity must manage its own energy needs and secure seamless transition of products from one facility to another. The carbon footprint of frozen food distribution depends on the distances traveled and time involved. Distribution is likely the most energy-intensive step of the supply chain, as total energy demand is directly proportional to the length of time between food freezing and consumption. This includes home refrigerators and frozen product storage chests that may not be energy efficient.

Driving Energy Efficiency: Ensuring Food Safety and Quality of Frozen Foods

Cold chain monitoring and logistics significantly impact the frozen food industry's overall carbon footprint, as these are stages in the supply chain with temperature control requirements. Reducing the environmental impact while maintaining high standards of food safety and quality is essential as temperature abuses are most likely to occur in and between storage and transport activities.

One intuitive but potentially disruptive strategy that has been proposed involves adjusting frozen storage temperature across the cold chain to reduce energy consumption and energy costs. Any effort to achieve energy efficiency must be examined from the perspective of food safety implications and impact on product quality.

Frozen Storage Temperature

First, it is important to recognize that the typical temperature of frozen storage, transport, and distribution currently is –18 °C or 0 °F, a setpoint that is globally applied and is the result of research developed by the U.S. Department of Agriculture (USDA) in the period between 1948 and 1965.^3,4,5 A series of these USDA studies ultimately defined this temperature setpoint across a variety of commodities, and over the decades, 0 °F has become the standard operating temperature across the global cold chain.

This temperature setpoint is also highly relevant from a food safety perspective, as foodborne bacterial pathogens do not grow at 0 °F. The water in food stored at 0 °F is mostly frozen, reducing the water activity to levels that are too low to support microbial growth. At 0 °F, scarce amounts of available water limit cellular processes such as metabolic and enzymatic reactions.

Formation of ice crystals results in osmotic stress that inhibits microbial growth, and these crystals can physically damage cell membranes, disrupting cellular integrity and function. These ice crystals can also lead to desiccation and drying out of the food. On the other hand, bacteria have evolved to develop mechanisms to endure this harsh 0 °F environment, a temperature setpoint at which bacterial pathogens survive but are not metabolically active and do not grow and multiply.

Another important consideration in normalizing the 0 °F standard was to accommodate the technological and logistical limits of the time, which in the 1960s necessitated a shelf life of 24 months for all frozen foods. Since then, innovations have led to giant leaps in freezing technology, advancing cold chain capabilities. Today, frozen foods are typically moved across a more efficient supply chain. The most significant change since the USDA studies were originally published is a reduced time between freezing and consumption. As sustainability efforts continue, efficiencies like these also entreat further reductions in energy use and GHG emissions.

In the quest to reduce energy usage, a relatively new opportunity has challenged the foundation of the 0 °F
(–18 °C) frozen storage setpoint. Raising the standard temperature of frozen storage across the cold chain would reduce both energy costs and GHG emissions. Indeed, modeling has demonstrated significant reductions in energy use and concomitant GHG emissions associated with moving the temperature dial up to 5 °F (–15 °C).⁶

“When frozen storage temperature is altered, these reactions stimulate changes in frozen foods that impact product quality, such as color, texture, nutrition, drip loss, rancidity, and other sensory attributes.”

Food Safety and Quality

Notwithstanding the scale and complexity of changing a common practice across a broad distribution supply chain, uncertainty also looms about the food safety and product quality implications of frozen food stored, transported, and distributed across the cold chain at temperatures warmer than 0 °F. Listeria monocytogenes, a psychrophilic foodborne bacterial pathogen, is capable of growth at temperatures as low as 32 °F (0 °C), but growth is significantly hindered. From a food safety perspective, the relied-upon scientific doctrine indicates the lowest temperature at which pathogens can grow is generally around 29.3 °F (–1.5 °C).^7,8,9

In addition to temperature, other parameters such as water activity (a_w) can help illustrate the growth potential of microorganisms in frozen foods. The term a_w is used to convey the water-binding capacity of foods and represents the ratio of the water vapor pressure of a food to that of pure water at the same temperature. The a_w of pure water is 1, while most foods are between 0.95 and 1, providing sufficient moisture for microbial growth. Lowering the a_w of foods is, therefore, one strategy to limit growth of pathogens. Freezing of food is essentially the freezing of water in food, a process that occurs through ice crystal formation. The water activity of the unfrozen water phase decreases because of an increase in the concentration of hydrophilic solutes, which in turn also generates osmotic pressure in the system. When water crystallizes as pure ice in frozen systems, the water activity is directly impacted by the temperature without a bias to the nature of dissolved solutes (food nutrients and components) in a frozen food.¹⁰ In essence, temperature and water activity as parameters that determine growth may be considered synonymous.

Looking at microbial growth from the standpoint of water activity and osmotic pressure in a frozen food can also reveal food safety ramifications from –4 °F to 20 °F (both extremes reflect the frozen state). The solute concentration in the unfrozen water phase in food stored at 14 °F (–10 °C) is also considered to inhibit the growth of microorganisms.^11,12 Based on studies conducted with different solutes at different concentrations, decreases in water activity and increases in osmotic pressure that occur during freezing can be modeled. For example, –4 °F
(–20 °C) corresponds to water activity of 0.82 and osmotic pressure of 26.8 MPa¹³ (based on studies with different solutes at different concentrations), conditions that would be very effective in preservation of a variety of foods. When moving from 0 °F to higher temperatures of 5 °F and 10 °F, water activity [0.9 at 10 °F (approximately 12 °C)] and osmotic pressure [13 MPa at 10 °F (approximately 12 °C)] shift, yet are still insufficient to support microbial growth.

According to Geiges,¹⁴ the lower limit of growth of bacteria in food is anywhere between 23 °F (–5 °C) and 17.6 °F (–8 °C). In his article,¹⁴ Geiges also provides a caveat that storage temperatures above 14 °F (–10 °C) should not be used. Frozen foods as ice-water-solute systems are in a chemically, physically, and biologically dynamic state where reactions continue to occur through frozen storage. The composition (types of solutes in the unfrozen water phase) of frozen foods has a considerable impact on these reactions. When frozen storage temperature is altered, these reactions stimulate changes in frozen foods that impact product quality, such as color, texture, nutrition, drip loss, rancidity, and other sensory attributes. Based on available scientific literature, frozen storage temperatures lower than 14 °F (–10 °C) do not support the growth of pathogens; however, the impact on characteristics relevant to consumer appeal and experience need to be evaluated.

Energy Efficiency Studies

Nomad Foods, one of Europe's leading frozen foods companies, recently released results from its study confirming the potential to increase frozen food storage temperature to reduce carbon emissions. The 12-month pilot study provides empirical evidence that a 5 °F increase in frozen food storage temperatures could reduce energy consumption by 10 percent or more, with no noticeable impact on food safety, texture, taste, or nutritional value, and no need to reformulate products.¹⁵ These studies need to be extended to real-world situations, where the benefits of sensor technologies can substantiate the integrity of the cold chain in ensuring frozen food quality and safety.

In 2023, pilot studies at Unilever's ice cream manufacturing unit¹⁶ confirmed energy consumption reductions of around 25 percent per freezer cabinet at the warmer temperature of 10.4 °F, but also required ice cream reformulation to increase the temperature of Unilever's last-mile freezer cabinets to ensure the retention of ice cream quality and consumer experience. To its credit, Unilever announced it will grant a free, non-exclusive license to the ice cream industry for these reformulation patents.

Global supply chain and logistics organizations are also reassessing the long-standing temperature standards of
–18 °C (0 °F).¹⁷ These are promising initiatives, and they provide impetus for industry to transition frozen food storage temperatures to more energy-efficient levels. Equipped with the technical capabilities to undertake what amounts to an interdisciplinary and cross-functional program, the American Frozen Food Institute (AFFI),¹⁸ in partnership with the Global Cold Chain Alliance,¹⁹ is leading a U.S. marketplace effort to assess the feasibility of transitioning to a warmer frozen storage temperature through a systematic and science-based approach.

Given the above discussion, a priority will be to understand the integrity of existing systems and ultimately to assure temperature maintenance. The work has the potential to develop and utilize models and templates to not only validate a temperature transition, but also to outline an implementation plan for the broader global cold chain to ensure frozen food safety and quality. AFFI is instituting a task force of appropriate experts to delineate a potential work plan that brings together a host of stakeholders from across the frozen food industry, government, and academia.

Conclusion

The frozen food industry finds itself at an efficiency precipice, providing exciting opportunities for addressing sustainability goals. Collective action is imperative if the industry is to operationalize considerable changes such as transitioning to a warmer storage temperature for frozen foods.

Supply chain engagement plays a crucial role in fostering collaboration among stakeholders to identify and implement energy-saving initiatives throughout the production and distribution process. By prioritizing energy efficiency, the frozen food industry can simultaneously drive profitability, satisfy consumer preferences, and build a more sustainable future.

References

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