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The frozen food sector is now a major part of the food supply in most countries yet less than 125 years ago it did not exist. This paper summarizes the development of this sector, highlighting key technological, product and food sciencedriven advances. The existence of frozen foods in prehistoric times is known but relied on the cold climate and so were limited in impact. More than 150 years ago, unfrozen refrigerated preservation using natural ice was most common. While sorbets were documented as early as 3000 BC in China and salted ice allowed small scale creation of frozen products in the 17th century, it was only the development of mechanical refrigeration systems in the mid 19th century that allowed largescale production and transport of frozen foods. The early frozen food trade was limited to freezing, transport and storage in bulk, and these foods were usually sold unfrozen into traditional fresh markets. However, the trade contributed to economic development of many European colonies, assisted urbanization, improved nutrition via lower cost protein, reduced seasonality of food supply and was an attractive alternative to drying, salting, smoking and canning. Between the World Wars saw the development of frozen consumer products led by Clarence Birdseye, who identified the benefits of fast freezing and raised the profile of frozen food with a focus on quality, packaging and convenience. There was commensurate development of blast freezing technology and retail displays, but the trade was constrained by the lack of freezers in the home. The expansion of domestic refrigeration after WW2 saw rapid growth and diversification of the frozen food sector and further changes from bulk ingredients to complete meals and ready-to-eat products by the 1970s as domestic microwave ovens also became more common. During recent years, with the cold chain becoming ubiquitous in many countries, the increased sophistication and differentiation of frozen food products with an emphasis on healthy, convenient, addedvalue products that are as close to fresh in quality as possible has occurred. The future of the frozen food sector promises further growth and improvement in food quality and environmental impact, particularly as the sector integrates digital technologies with traditional systems. [ABSTRACT FROM AUTHOR]

Simple analysis of the thermodynamics of the refrigeration cycle suggests that lowering the condenser pressure should result in decreased compressor energy and that the lowest possible pressure is constrained by the temperature of the available heat sink (usually the ambient condition). Thus optimization of the condenser pressure seems straight-forward. However, in practice there is a more complex trade-off, as lowering condenser pressure increases the condenser fan energy. The trade-off is particularly acute when variable speed drives are used to control the condenser fans. In addition, there can be other constraints such as expansion valve performance. This paper demonstrates how the theoretical optimal condensing temperature can be defined using a mathematical model that takes into account the characteristics of the condenser, the condenser controls, the refrigeration load, and the ambient temperature. The model is used to explore practical condenser pressure control strategies for a year of operation of a case study refrigerated warehouse facility. The strategies were the optimum defined by the model and alternative control strategies including fixed head pressure set-point, minimum approach to the ambient temperature, and fixed condenser fan speed. The alternative controls are much simpler to implement and often the penalty relative to the optimum strategy is small. [ABSTRACT FROM AUTHOR]

Air cooling coils operating with a refrigerant temperature below 0oC require regular defrosting to restore cooling performance. Defrosting creates parasitic load on the refrigeration system and can result in short term temperature fluctuations in the refrigerated space. The relationships between defrost heat load, latent loads in the refrigerated facility, defrost melt water flows, and defrost efficiency are quantified. Practical suggestions to minimize the impact of defrost include: minimizing latent heat loads on the facility, establishing a defrost termination method that avoids unproductive heating to the space, varying the frequency of defrost to match the latent load the facility experiences, and monitoring defrost by measuring the melt water flow. While on-demand defrost systems should vary both the frequency and duration of defrost near optimally, they are not always reliable and can schedule defrost at inopportune times. Timed (but manually adjusted occasionally) initiation and temperature termination often provide the best combination of cost, effectiveness and robustness for industrial systems. [ABSTRACT FROM AUTHOR]

The rate of moisture sorption was measured by weighing packaging over time after a step-change in RH and/or temperature. Trials were undertaken for a range of packaging types and configurations including single sheets of cardboard, single cartons of product with waxed or unwaxed cardboard, and complete pallets of product cartons both with and without shrink-wrap. The trials were conducted at both ambient and refrigerated temperatures and under both adsorption or desorption conditions. Rates of sorption were found to be accurately represented by first order kinetic approach to equilibrium conditions defined by the moisture sorption isotherms for the wood and cardboard separately. Packaging thermal equilibration was rapid compared with the moisture sorption rates so heat transfer effects can be ignored. The moisture sorption was found to give significant packaging temperature rise (adsorption) or decline (desorption) due to the heat of sorption. There was a net conversion of latent heat to sensible for adsorption and vice versa for desorption. The sorption rate constant reduced as the product item got larger (pallet < cartons

Air relative humidity (RH) in a refrigerated facility is determined by the balance between moisture entry (principally through doors) and moisture removal (principally as frost on the air-cooling coil). It has been shown that coil frosting is "unfavorable," leading to very rapid decline in performance, if the air becomes super-saturated as it is cooled. The conditions for the transition to unfavorable frost formation can be predicted from coil design information including air-on temperature and RH, evaporation temperature, and heat load sensible heat ratio (SHR). Using advanced models for air infiltration through doors plus standard models for other heat loads, and assuming that air cooling follows a straight line approach from the air-on condition to the saturation at the coil surface temperature, the load SHR. cod SHR, and. hence, the balanced air RH were estimated for a typical refrigerated facility with a warehouse, environmental loading area (ELA), and blast freezer It was shown that unfavorable frost formation is likely to occur in low-temperature facilities even if ambient conditions are not that extreme and moisture entry is reduced by door-way protection. Unfavorable frosting can be best avoided by a combination of improved coil design (e.g., smaller air to refrigerant temperature difference. TD) and enhanced door protection (especially doors opening to the ambient), but occasionally active dehumidification or special coil design (e.g., staggered fin spacing) must be used to avoid frosting problems. [ABSTRACT FROM AUTHOR]

This paper describes a modification of the model in the ASHRAE Handbook to predict the rate of air infiltration through doors into refrigerated facilities. The model sums the contributions of air tightness (infiltration when the door is closed), air infiltration due to door openings, and the additional air exchange caused by forklift traffic through the doors. The modified model was developed based on measured air infiltration rates through rapid-roll and sliding doors (with and without strip-curtain protection) into seven refrigerated warehouses with sizes ranging from 740 to 12,000 m³ (26,000 to 424,000 ft³) operating at temperatures ranging from 3 to -22 °C (38 to -7°F) for a range of forklift movement rates. [ABSTRACT FROM AUTHOR]