5 Emerging and Novel Freezing Processes Kostadin Fikiin 5.1 NEED FOR PROCESS INNOVATIONS IN THE FOOD FREEZING INDUSTRY TO IMPROVE THE HUMAN WELL-BEING AND QUALITY OF LIFE As is known, heat and cold are of the same physical nature. In spite of this, they have played different roles in the development of human civilisation. Prometheus, the mythological hero who bestowed the divine fire of Olympus to mankind, is glorified in immortal poetical and musical works. However, the pioneers who created artificial refrigeration and gave it to humanity have not yet been praised in a work of art as a token of gratitude. For millennia, cold has primarily been associated with winter, diseases and people’s misery, rather than with its proven capability to preserve biological materials. More recently, in the industrialised world, food refrigeration has become a powerful instrument for improving the quality of life. Refrigeration does not have a competitive alternative to maintain the nutritional resources of humankind. The worldwide food output amounts nearly 5 billion tonnes per year, some 2 billion of which need refrigerated processing, but only 400 million are effectively refrigerated. Chilling is an indispensable element of almost all post-harvest or post-mortem techniques for handling food commodities of plant or animal origin, while freezing has been established and recognised as the paramount commercial method for long-term preservation of the natural quality attributes of perishable foods, thereby forming a substantial part of the global economy and the well-being of citizens. In terms of money, every year the global investment in refrigerating equipment is over US$ 170 billion, while all refrigerated foodstuffs cost US$ 1200 billion (which exceeds 3.5 times the US military budget). Some 700–1000 million household refrigerators and 300,000,000 m3 of cold-storage facilities are available over the world. The annual global production of various frozen foods is about 50 million tonnes (plus 20 million tonnes of ice creams and 30 million tonnes of fish), with a remarkable growth of 10% every year. Refrigeration thus accounts for about 15% of the worldwide electricity consumption, thereby determining to a large extent the global economic sustainability (in terms of energy efficiency and environmental friendliness). Food refrigeration stakeholders and cold chain professionals are part of the FAO/WHO Codex Alimentarius Commissions and played an important role in the historic world leaders’ summits in Montreal, Kyoto and Johannesburg (devoted to ozone-depletion, global warming and sustainable development). Refrigeration and the cold chain are among the top priorities of the US Presidential Council for Food Safety. While the economies of industrialised nations waste 25–30% of their perishable food production because of imperfect or lacking cold chain, the disastrous dimension of such agricultural losses in developing countries is a major contributor to malnutrition. Frozen Food Science and Technology. Edited by Judith A. Evans © 2008 Blackwell Publishing Ltd, ISBN: 978-1-4051-5478-9 102 Frozen Food Science and Technology The twenty-first century poses new challenges for the global frozen food sector, which can be summarised as follows: r r r r Although the promising capabilities of several emerging freezing technologies (detailed in this chapter) attract the attention of researchers and industrialists, many such innovations remain unimplemented in the common industrial practice. Both conventional and novel freezing techniques have a substantial potential for further optimisation by involving advanced modelling and experimental tools and enriching theoretical understanding of underlying phenomena (e.g. heat transfer, fluid flow and biochemical processes). The manufacture of frozen commodities and related scientific research are still insufficiently attractive for high-skilled experts and young specialists as compared with hi-tech branches (e.g. information technologies, electronics, communications, biotechnologies, etc.). Developing countries around the world need more refrigeration capacities and inexpensive food freezing equipment to make their economies more competitive on the global markets. Simultaneously, the high reputation of freezing as one of the safest and most nutritionally valuable preservation techniques should not create a false sense of total security and should not defeat the care and diligence when managing the frozen supply chain (IIR, 1986, 1999; Kennedy, 2000). While freezing drastically reduces detrimental phenomena in foods, a number of physical and biochemical reactions still occur and are accentuated if proper processing and handling conditions are not maintained throughout the entire chain for production, storage, transport, distribution, retail and household handling of various frozen commodities of plant or animal origin by using an integral farm-to-table approach. Contemporary food industry promotes research and innovations, which primarily deal with the sustainable processing, preservation and supply of safe, high-quality, healthier foods and beverages for the consumers. In that context, food-freezing technologies are a crucial and underexploited element of the sustainable food production and preservation and related food chain management. This importance perceptibly increases as a result of the ongoing extension of traditional food chains into new markets and emerging economies around the world. Essential measures should, therefore, be undertaken to raise professional competence and encourage a stronger public commitment to food freezing investigations. It is vital to make public authorities and food policy makers more aware of the topical professional endeavours of refrigeration scientists and industrialists around the world. Relevant funding agencies (such as the European Commission) supported, therefore, a number of successful freezingrelated research projects whose deliverables and industrial implementation result in reduced post-harvest losses, extended shelf-life and better quality of frozen foods, lower investments and running costs, higher energy savings, and enhanced environmental friendliness (Fikiin, 2003). 5.2 STATE OF THE ART AND CONVENTIONAL FREEZING MODES In the early twentieth century, many people were experimenting with mechanical and chemical methods to preserve food. As an industrial process, quick freezing began its history nearly 80 years ago when Clarence Birdseye found a way to flash-freeze foods and deliver them Emerging and Novel Freezing Processes 103 Shrunk cells and destructed tissue Ice crystals Cell membrane Water flux After Slow Freezing Less damaged tissue with maintained integrity Cell Natural Cells Crystallisation and Water Migration during Freezing After Quick Freezing Fig. 5.1 Crucial impact of freezing rate on the end product quality (Fikiin, 2003). to the public – one of the most important steps forward ever taken in the food industry. During his stay on the Arctic, Birdseye observed that the combination of ice, wind and low temperature almost instantly froze just-caught fish. More importantly, he also found that when such quick-frozen fish were cooked and eaten, they were scarcely different in taste and texture from how they would have been if fresh. After years of work, Birdseye invented a system that packed dressed fish, meat or vegetables into waxed-cardboard cartons, which were flash-frozen under pressure (US Patent No. 1,773,079, 1930). Then, he turned to marketing and a number of ventures have been initiated to manufacture, transport and sell frozen foods (e.g. construction of double-plate freezers and grocery display cases; lease of refrigerated boxcars for railway transport; and retail of frozen products in Springfield, Massachusetts, in 1930). These technology achievements constituted the world’s first cold chain for frozen foods (Fikiin, 2003). Thus, quick freezing has further been adopted as a widespread commercial method for long-term preservation of perishable foods, which improved both the health and convenience of virtually everyone in the industrialised countries. Freezing rate affects strongly the quality of frozen foods, in which the predominant water content should quickly be frozen in a finegrain crystal structure in order to prevent the cellular tissues and to inhibit rapidly the spoiling microbiologic and enzymatic processes (Fig. 5.1). Basic heat transfer considerations (Fikiin, 2003) clearly suggest that the desired shortening of freezing duration and a resulting high throughput of refrigerating equipment could be achieved by means of: (i) lower refrigerating medium temperature (which generally requires greater investment and running costs for the refrigeration machines to be employed); (ii) enhanced surface heat transfer coefficients (by increased refrigerating medium velocity and boundary layer turbulence, involvement of surface phase-change effects and less packaging); 104 Frozen Food Science and Technology (a) Evaporator plates Product to be frozen Expansion valve Compressor Condenser (b) (c) Freezer Plates Welding flange union Hydraulic cylinder Frame Plate angle Pressure plate Freezer plate Spacer Hose Lifting bolt Refrigerant header Oil purge valve (both headers) (d) Fig. 5.2 (e) Multi-plate freezing systems. and (iii) reduced size of the refrigerated objects (by freezing small products individually or appropriately cutting the large ones into minor pieces). As detailed in Chapters 4 and 6–8, air-blast and multi-plate freezers are most widespread, while air fluidising systems are used for IQF of small products (Figs. 5.2, 5.3 and 5.5). The application of cryogenic IQF (Fig. 5.4) is still very restricted because of the high price of the liquefied gases used. 5.2.1 Fluidised-bed freezing systems The air fluidisation (Fig. 5.5) was studied extensively and used commercially, with an increasing popularity, during the last 50 years (Fikiin et al., 1965, 1966, 1970; Fikiin, 1969, 1979, 1980). This freezing principle possesses many attractive features, including: r High freezing rate due to the small sizes and thermal resistance of the IQF products, great overall heat transfer surface of the fluidised foods and high surface heat transfer coefficients. Emerging and Novel Freezing Processes 105 (a) Tunnel Freezers (b) Spiral Freezers Fig. 5.3 Air-blast freezing systems. r Good quality of the frozen products that have an attractive appearance and do not stick together. r Continuity and possibilities for complete automation of the freezing process. In spite of these advantages the fluidisation freezing by air has some drawbacks, such as: r Necessity of two-stage refrigerating plants (using large quantities of CFC-, HCFC- or HFC-based refrigerants with significant ozone depletion or global warming potentials) to maintain an evaporation temperature of about −45◦ C, which needs high investment and power costs. (b) Liquid nitrogen spraying (a) Immersion in liquid nitrogen Fig. 5.4 Cryogenic freezing systems. 106 Frozen Food Science and Technology (a) Fluidized bed (b) ed ised b Fluid Cold air Frozen product out Blowers Air (c) Fig. 5.5 r r r r Fluidised-bed freezing systems. Lower surface heat transfer coefficients and freezing rates in comparison with the immersion methods. Need for a high speed and pressure airflow, which results in a great fan power consumption. Some moisture losses from the product surface and a rapid frosting of the air coolers, caused by the great temperature differential between the products and the evaporating refrigerant. Excessive sensitivity of the process parameters to the product shape, mass and sizes, which requires careful control, specific for every separate food commodity. 5.2.2 Freezing by immersion The immersion freezing in non-boiling liquid refrigerating media is a well-known method having several important advantages: high heat transfer rate, fine ice crystal system in foods, great throughput, low investments and operational costs (Tressler, 1968; Fleshland and Magnussen, 1990; Lucas and Raoult-Wack, 1998; Fikiin et al., 2001). Immersion applications have been limited because of the uncontrolled solute uptake by the refrigerated products and operational problems with the immersion liquids (high viscosity at low temperatures, difficulty in maintaining the medium at a definite constant concentration and free from organic contaminants). Recent achievements in the heat and mass transfer, physical chemistry, fluid dynamics and automatic process control make it possible to solve these problems and to develop advanced innovative immersion IQF systems (Fikiin and Fikiin, 1998, 1999, 2002, 2003a, 2003b; Fikiin, 2003). 5.3 INDIVIDUAL QUICK FREEZING OF FOODS BY HYDROFLUIDISATION AND PUMPABLE ICE SLURRIES The Hydrofluidisation Method (HFM) for fast freezing of foods was suggested and patented recently to overcome the drawbacks and to bring together the advantages of both air fluidisation and immersion food freezing techniques (Fikiin, 1985, 1992, 1994). The HFM uses a Emerging and Novel Freezing Processes 107 Hydrofluidisation Fig. 5.6 Possible arrangement of a HFM-based freezing system combining the advantages of both air fluidisation and immersion food freezing techniques (Fikiin and Fikiin, 1998, 1999): (1) charging funnel; (2) sprinkling tubular system; (3) refrigerating cylinder; (4) perforated screw; (5) double bottom; (6) perforated grate for draining; (8) sprinkling device for glazing; (7 and 9) netlike conveyor belt; (10 and 11) collector vats; (12) pump; (13 and 14) rough and fine filters; (15) cooler of refrigerating medium; (16) refrigeration plant. Reproduced by permission of the International Institute of Refrigeration: www.iifiir.org. circulating system that pumps the refrigerating liquid upwards, through orifices or nozzles, in a refrigerating vessel, thereby creating agitating jets. These form a fluidised bed of highly turbulent liquid and moving products, and thus evoke extremely high surface heat transfer coefficients. The principle of operation of a HFM freezing system is illustrated in Fig. 5.6. 108 Frozen Food Science and Technology (a) (b) 30 30 25 25 Ambient temperature = −15/−16°C Average jet velocity = 2 m s−1 15 10 5 Centre 0 −5 Scad (28 g) −10 Ambient temperature = −15/−16°C Average jet velocity = 2 m s−1 20 Temperature (°C) Temperature (°C) 20 15 10 Core 5 Peppers (30 g) 0 −5 −10 Surface −15 Surface −15 Green beans (11 g) Sprat (7 g) −20 −20 0 100 200 300 400 500 600 700 800 0 50 100 150 Time (s) 200 250 300 350 Time (s) Fig. 5.7 Experimental temperature histories during HFM freezing of fish (a) and vegetables (b) when using sodium chloride solution (without ice slurry) as a fluidising agent (Fikiin, 1992; Fikiin and Fikiin, 1998, 1999). Reproduced by permission of the International Institute of Refrigeration: www.iifiir.org. 5.3.1 Unfreezable liquid refrigerating media as fluidising agents Although various immersion techniques have been known for long time, until now hydrofluidisation principles have not been used for chilling and freezing of foods. Experiments on HFM freezing of small fish and some vegetables through an aqueous solution of sodium chloride showed a much higher freezing rate as compared to other IQF techniques (Fikiin, 1992, 1994). The maximal surface heat transfer coefficient achieved exceeded 900 W m−2 K−1 , while this was 378 W m−2 K−1 when immersing in running liquid, 432 W m−2 K−1 for sprinkling and 475 W m−2 K−1 for immersion with bubbling through (Fikiin and Pham, 1985). Even at a slight or moderate jet agitation and a comparatively high refrigerating medium temperature of about −16◦ C, the scad fish were frozen from 25◦ C down to −10◦ C in the centre in 6–7 minutes, sprat fish and green beans in 3–4 minutes and green peas within 1–2 minutes. As an illustration, Fig. 5.7 shows recorded temperature histories during hydrofluidisation freezing of scad and sprat fish, green beans and peppers. 5.3.2 Two-phase ice slurries as fluidising agents Pumpable ice slurries (known under different trade names, such as FLO-ICE, BINARY ICE, Slurry-ICE, Liquid ICE, Pumpable ICE or Fluid ICE) were proposed recently as environmentally benign secondary coolants circulated to the heat transfer equipment of refrigeration plants, instead of the traditional harmful CFC- or HCFC-based refrigerants (Ure, 1998; Pearson and Brown, 1998). Promising attempts to refrigerate foods by immersion in such slurries have already been carried out. For example, fish chilling in brine-based slurries has potential to replace the traditional use of ice flakes (Fikiin et al., 2001, 2002, 2005). A number of foods immersed in slurries with various ice contents are shown in Fig. 5.8. Emerging and Novel Freezing Processes (a) (b) (c) (d) (e) (f) 109 Fig. 5.8 Different foods immersed in slurries with various ice concentrations: (a) fruits; (b) vegetables; (c) chickens; (d), (e) and (f) fish (Fikiin et al., 2002). Reproduced by permission of the International Institute of Refrigeration: www.iifiir.org. Fikiin and Fikiin (1998, 1999) proposed, therefore, a novel method to enhance the advantages of the hydrofluidisation (described hereafter) by employing two-phase ice suspensions as fluidising media. The ice slurries reveal a great energy potential as HFM refrigerating media whose minute ice particles absorb latent heat when thawing on the product surface. Hence, the goal of the ice slurry involvement is to provide an enormously high surface heat transfer coefficient (of the order of 1000–2000 W m−2 K−1 or more), excessively short freezing time and uniform temperature distribution in the whole volume of the freezing apparatus. The combination of the HFM with the high heat transfer efficiency of the ice-slurry-based refrigerating media represents a new interdisciplinary research field whose development would advance essentially the refrigerated processing of foods. The HFM freezing with ice slurries can acquire a process rate approaching that of the cryogenic flash freezing modes. For instance, at a refrigerating ice-slurry temperature of −25◦ C and a heat transfer coefficient of 1000 W m−2 K−1 , strawberries, apricots and plums can be frozen from 25◦ C down to an average final temperature of −18◦ C within 8–9 minutes, raspberries, cherries and morellos within 1.5–3 minutes, and green peas, blueberries and cranberries within approximately 1 minute only. The general layout of an ice-slurry-based system for hydrofluidisation freezing is shown in Fig. 5.9. 5.3.3 Advantages of the hydrofluidisation freezing As described above, the novelty of the hydrofluidisation method lies in the involvement of unfreezable liquids or pumpable ice slurries as fluidising agents. It is well known that the immersion freezing history began with use of brines to freeze fish, vegetables and meat. Binary or ternary aqueous solutions containing soluble carbohydrates (e.g. sucrose, invert sugar, glucose (dextrose), fructose and other mono- and disaccharides) with additions of ethanol, salts, glycerol, etc., have been studied as possible immersion media. There are practically unlimited possibilities to combine constituents and to formulate appropriate multi-component HFM refrigerating media based on one-phase liquids or two-phase ice slurries, which have 110 Frozen Food Science and Technology Product in Hydrofluidisation freezer Filter system Ice slurry tank Compressor Condenser Refrigeration unit Ice slurry generator Pump Product out Expansion valve Fig. 5.9 Schematic diagram of an ice-slurry-based hydrofluidisation system HyFloFreeze® (Fikiin and Fikiin, 1998, 1999). Reproduced by permission of the International Institute of Refrigeration: www.iifiir.org. to be both product- and environment-friendly and to possess a viscosity low enough in terms of pumpability and good hydrofluidisation. The main advantages of the hydrofluidisation over the conventional freezing modes can be summarised as follows: r The HFM affords a very high heat transfer rate with a small product–medium temperature difference. The evaporation temperature can be maintained much higher (at −25/−30◦ C) by a single-stage refrigerating machine with much higher COP and nearly two times lower investments and power costs as compared to the conventional air fluidisation. Cold dissipation through the freezer walls is also lower. The water flow rate or fan power consumption for cooling the condenser decrease as well, due to the reduced mechanical work of the single-stage unit. r The critical zone of water crystallisation (from −1◦ C to −8◦ C) is quickly passed through, that ensures a fine ice crystal structure in foods preventing the cellular tissues from perceptible damage. r The product surface freezes immediately in a solid crust that hampers the osmotic transfer and gives an excellent appearance. The mass losses tend to zero, while in air freezing tunnels the moisture losses are usually 2–3%. r New delicious products can easily be formulated by using some selected product-friendly HFM media (for example, fruits frozen in syrup-type sugar solutions turn into dessert products with beneficial effect on colour, flavour and texture). Such media can also include Emerging and Novel Freezing Processes 111 Fig. 5.10 HyFloFreeze® prototype: hydrofluidised bed of highly turbulent ice slurry (Fikiin, 2003). Reproduced by permission of the International Institute of Refrigeration: www.iifiir.org. r appropriate antioxidants, flavourings and micronutrients to extend the shelf-life of the products and to improve their nutritional value and sensory properties. The HFM freezers use environmentally-friendly secondary coolants (for instance, syruptype aqueous solutions and ice slurries) and the refrigerant is closed in a small isolated system, in contrast to the common air fluidisation freezers where large quantities of harmful HCFCs or expensive HFCs circulate to remote evaporators with a much greater risk for emission to the environment. r Fluidised state is acquired with low velocity and pressure of the fluid jets due to the Archimedes forces and buoyancy of the products, which leads to both energy savings and minimum mechanical action on the foods. r The operation is continuous, easy to maintain, convenient for automation and the labour costs are substantially low. Further processing or packaging of the HFM-frozen products is considerably easier since they emerge from the freezer in a ‘free-flowing’ state. r Ice-slurry-based HFM agents may easily be integrated into systems for thermal energy storage, accumulating ice slurry during the night at cheap electricity charges. The top view photos in Fig. 5.10 show how a hydrofluidised bed of highly turbulent ice slurry is formed inside the HyFloFreeze prototype’s freezing compartment. 5.3.4 International research co-operation Two main innovative aspects of the suggested HFM freezing technique can clearly be distinguished: (i) employment of unfreezable liquids as fluidising agents and (ii) use of pumpable ice slurries as fluidising media. This freezing principle provides an extremely high heat transfer rate, short freezing times, great throughput and better product quality at refrigerating temperatures maintained by a single-stage refrigeration machine. Thus, nearly two times lower investments and power costs are necessary as compared with the popular individual quick freezing methods. Moreover, such hydrofluidisation freezing systems are less hazardous from the environmental viewpoint, since the refrigerant is closed in a small isolated circuit only. The emerging HFM technology has drawn the attention of a number of academics and industrialists. The identification of optimal design specifications for HFM freezing systems requires an interdisciplinary approach of researchers with complementary skills. The HyFloFreeze project was, therefore, funded by the European Commission and performed by an international research consortium of six participating organisations (four universities and two SMEs) from Belgium, Bulgaria, Russia and the UK (Fikiin, 2003). 112 5.4 Frozen Food Science and Technology HIGH-PRESSURE FREEZING Non-thermal food processing techniques (e.g. pulse-electric field pasteurisation, highintensity pulsed lights, high-intensity pulsed-magnetic field, ozone treatment) are presently regarded with special interest by the food industry. Among them, high-pressure processing is gaining in popularity with food processors because of its food preservation capability and potential to achieve interesting functional effects. Under high pressure pathogenic microorganisms can be inactivated with minimal heat treatment, which results in almost complete retention of nutritional and sensory characteristics of fresh foods, without sacrificing their shelf-life. Other advantages over traditional thermal processing include reduced process times; minimal heat damage problems; retention of freshness, flavour, texture, and colour; lack of vitamin C loss and tangible changes in food during pressure-shift freezing (due to reduced crystal size and multiple ice-phase forms); and minimal undesirable functionality alterations. However, the spore inactivation is a major challenge as methods for full inactivation of spores under pressure are yet to be developed. Hence, another group of research activities worldwide focus on different techniques for treatment of foods by high hydrostatic pressure, including high-pressure-aided freezing and thawing. A number of products (such as jams and fruit juices) have been processed under high pressure in Japan. There have been 10–15 types of pressurised foods on the Japanese market but several have recently disappeared, while the remaining ones are too specific to excite a substantial commercial interest. Examples of pressurised products in Europe and US are: (i) orange juice (Pernod Ricard Company, France); (ii) acidified avocado purée (Avomex Company, USA and Mexico); and (iii) sliced ham (Espuna Company, Spain). Volumes produced are still very small and some current European food regulations slowed down the launching of new pressurised products because of legislative problems. The so-called cryofixation is a physical method for immobilisation of biological materials by ultra-quick freezing. Unlike the chemical fixation, it preserves thoroughly the ultrastructural morphology, much closer to the natural state of the cell tissue. This results in fast preservation of morphological details without artificial damage, less cross-linking of proteins by aldehyde fixation and reduced masking of the antigenic sites. The water phase diagram (Fig. 5.11) shows that at atmospheric pressure crystalline ice will build up at around 0◦ C and this water crystallisation leads to some rupture of the biological structures (Fig. 5.1). The cryofixation aims, therefore, to avoid such crystallisation-caused damages. At very high freezing rates particles and large molecules in water serve as cores for a heterogeneous nucleation, i.e. water becomes solid in a vitreous state and does not show a crystalline structure. The necessary freezing rates can only be achieved for very thin layers of 5–25 µm during freezing at atmospheric pressure. This restriction could be overcome through a depression of the initial freezing (cryoscopic) point of water by adding chemical cryoprotectants or by increasing the ambient pressure. At a pressure of 200 MPa the freezing point drops to about −22◦ C (see Fig. 5.11), which enables a depth of vitrification of about 200 µm, so that objects with a thickness of up to 0.4–0.6 mm could be well frozen. Consequently, the main promising features of high-pressure freezing are as follows: r r r Freezing point depression and reduced latent heat of phase change; Short freezing times and resulting benefits (e.g. microcrystalline or vitreous ice); Inactivation of micro-organisms and enzymes, and structure modifications with no essential changes of nutritional and sensory quality. Emerging and Novel Freezing Processes 113 10 A B I H C K Temperature (°C) 0 Liquid −10 D E Ice I −20 Ice V Ice III F G Ice II −30 0 100 200 300 400 500 Pressure (MPa) Fig. 5.11 Illustrates various paths of changing the food physical state by external manipulations of temperature or pressure, while Figs. 5.12 and 5.13 show temperature- and pressure-dependent thermal properties of potatoes during pressure-assisted freezing (Schlüter et al., 2000). Water phase diagram and high pressure effects on the phase transitions: A–B–C–D–C–B–A Subzero storage without freezing A–B–H–I Pressure-assisted1 freezing I–H–B–A Pressure-assisted1 thawing A–B–C–D–E Pressure-shift2 freezing E–D–C–B–A Pressure-induced3 thawing A–B–C–D–G–F Freezing to ice III F–G–D–C–B–A Thawing of ice III A – B – C – K – ice VI Freezing above 0◦ C 1 assisted: phase transition at constant pressure 2 shift: phase transition due to pressure change 3 induced: phase transition initiated with pressure change and continued at constant pressure Reproduced by permission of the International Institute of Refrigeration: www.iifiir.org The future will reveal soon whether the current achievements in this field are more likely to stay in the laboratories or they could be implemented as a common industrial practice. 5.5 MAGNETIC RESONANCE FREEZING As already discussed, the conventional refrigeration equipment provides freezing rates which, as a rule, are insufficient to eliminate completely undesirable water migration and mass transfer within a food product undergoing freezing. Realising this circumstance, researchers decided that if water could somehow be retained within the cells while freezing, then the cells would not become dehydrated and foodstuff could keep its original attributes and freshness. A system for magnetic resonance freezing (MRF) preventing such cellular dehydration could be regarded as composed of a common freezer and a special magnetic resonance device. The MRF process (Fig. 5.14) is then applied with the following two steps (Mohanty, 2001). 114 Frozen Food Science and Technology 1E+05 9E+04 0.1 MPa Cp 8E+04 7E+04 cp (J kg−1 K−1) 130 MPa 200 MPa 6E+04 5E+04 4E+04 3E+04 2E+04 1E+04 −30 −25 −20 −15 −10 0 −5 5 10 Temperature (°C) Fig. 5.12 Apparent specific heat capacity of potato tissue at different pressures. Reproduced by permission of the International Institute of Refrigeration: www.iifiir.org. 1.8 λ 130 MPa 200 MPa 0.1 MPa λ (W m−1 K−1) 1.6 1.4 1.2 1.0 0.8 0.6 −35 −30 −25 −20 −15 −10 −5 0 5 10 Temperature (°C) Fig. 5.13 Thermal conductivity of potato tissue at different pressures. Reproduced by permission of the International Institute of Refrigeration: www.iifiir.org. Emerging and Novel Freezing Processes 115 Temperature (°C) 10 0 Conventional Blast Freezer −10 −20 −30 −40 −50 MRF System −60 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 Time (min) Fig. 5.14 Product freezing curves for conventional and MRF equipment (Mohanty, 2001). Step 1: Food undergoes continuous magnetic wave vibrations, which provide for: • Impeding the crystallisation; • Supercooling below the initial freezing point. Step 2: After a suitable product-specific period of time the magnetic fields are abruptly removed with many resulting quality benefits for the frozen end product, e.g.: • Uniform flash freezing of the entire food volume; • Quick passing through the critical temperature zone of intense water crystallisation (between −1◦ C and −6◦ C); • Fine ice structure in foods; • No water migration and undesirable mass transfer phenomena; • No cellular dehydration; • Avoiding cracks and related damages; • Protected integrity of food tissues. At present MRF data are still kept as a confidential know-how of a number of companies, while MRF equipment still needs to prove its claimed advantages and capabilities through extensive tests within a sufficiently representative industrial environment. 5.6 AIR-CYCLE-BASED FREEZING SYSTEMS The majority of existing food refrigeration equipment has been designed to use halogenated hydrocarbons (CFCs and HCFCs) whose emissions to the environment are damaging the ozone layer and contributing significantly to global warming. Manufacture and import of CFCs is banned in most of the world and many HCFC refrigerants are only short-term replacements, often being more expensive and less efficient. The vapour-compression systems have refrigerant leakage rates to the environment of 15% of the total charge per annum, thus leading to stratospheric ozone depletion and climate change. Some other non-CFC alternatives called natural refrigerants (e.g. ammonia, propane, butane, isobutane, carbon dioxide and water) are also being employed or examined. Ammonia is the most common of the alternative refrigerants but is toxic and not always suitable for all refrigeration applications. Natural gas–based refrigerants involve a safety hazard because of their high flammability. 116 Frozen Food Science and Technology Q T P2 2 2 3 P1 Work 1 3 4 1 4 S Fig. 5.15 Q The Air Cycle on a T–S (Temperature–Entropy) diagram and shown diagrammatically. Transcritical CO2 cycles needs high pressures and complicated equipment. As a rule, important modifications to the existing refrigerating plants are required to introduce green technologies in the food refrigeration sector. The principle of the air cycle is that when air is compressed its temperature and pressure increase (1–2) as shown in Fig. 5.15. Heat is removed from the compressed air at constant pressure and its temperature is reduced (2–3). Air is then expanded and its temperature decreases (3–4). The air further absorbs heat (gaining temperature) from the process at constant pressure (4–1) where it starts the cycle again (Evans et al., 2005). Air-cycle refrigeration is an environmentally friendly alternative to the conventional vapour-compression systems. Originally used in the nineteenth century, the technology employed slow-speed reciprocating compressors and low-efficiency expanders. The poor energy efficiency and comparatively high cost of such machinery was a major factor for the replacement of these systems with vapour-compression equipment. However, the remarkable progress in turbine technology since that time (Horlock, 2003), along with the development of air bearings and ceramic components, provided dramatic improvements in efficiency. The air-cycle use for aircraft and railway carriage air-conditioning (which affords equipment simplicity, compactness and robustness) and the development of high-speed rotary compressors and expanders have greatly improved the cycle performance and reliability (Fig. 5.16). Hot air heat exchanger Motor Fig. 5.16 Bootstrap Expansion turbine Cold air heat exchanger Powered compressor An air-cycle unit for automotive applications (Normalair-Garrett Ltd.) Emerging and Novel Freezing Processes 117 Combining this with newly available compact heat exchangers with highly enhanced heat transfer characteristics makes competition with existing vapour-compression systems rather attractive. Air-cycle systems could be viable for many industrial applications (Gigiel et al., 1996; Van Gerwen and Verschoor; 1996; Verschoor, 2001). An air-cycle system can simultaneously produce both heat and cold, the advantage being that heating is provided at higher temperatures as compared to the conventional vapour-cycle heat-pump system. Frequency control of the motor speed permits a smooth variation of the operating capacity depending on the heat loads. On the other hand, mass production techniques make the air systems much cheaper than they were in the past. Air-cycle refrigeration is one of the most attractive ways for solving the environmental problems since the working fluid (air) is completely natural, free and totally environmentally benign. This environmental friendliness, reliability, minimal maintenance and efficiency can successfully be transferred to the food refrigeration industry (Gigiel et al., 1992; Fikiin et al., 1998; Russell et al., 2001, 2001). The energy efficiency can additionally be enhanced by combining the air cycle with systems for thermal energy storage. Feasibility studies have shown that such type of equipment may compete (in terms of energy and investment costs) with vapour-compression systems, coupled with conventional water heating devices (Gigiel et al., 1996; Van Gerwen and Verschoor, 1996). Both open and closed air-circulation systems can be employed to freeze foods. In the open cycle atmospheric air normally enters the system and returns to the atmosphere after being used for heating and cooling purposes (Shaw et al., 1995). The air as a refrigerant is in a direct contact with the refrigerated products, avoiding any intermediate heat exchangers, pipelines, secondary coolants and related energy dissipation. However, the running of such systems is strongly dependent on the atmospheric conditions, which results in more difficult control. Atmospheric air contains water vapours, which may freeze at low temperatures. Obviously, the open systems also require turbo-machines with oil-free air bearings. In the closed cycle dry air is circulated around the closed system, similarly to a conventional vapourcompression machine. The advantage of a closed air cycle is that the load on the system can be matched exactly without any change in the efficiency. This is because the efficiency of the rotating machinery used in air-cycle plants depends only on the velocities of the air and not its density. The change of pressure in the system overall results in corresponding changes in the air density and mass flow. Thus the refrigeration and the heating effects from the air-cycle plant can vary in proportion to the mass flow, without change in the efficiency, in contrast to common vapour-compression systems. Recent developments and improvements in air-cycle equipment enable a wider practical exploitation of this emerging technology for food freezing applications. The benefits are greater reliability, reduced maintenance, simple and compact design, reduced overall labour costs, improved control of product quality, along with using an environmentally benign and free refrigerant unlike the conventional heating and cooling systems. Fast freezing through low-temperature air dramatically reduces freezing times and has many advantages (e.g. flexibility, savings in freezer footprint and higher product quality). With a conventional vapour-compression plant, freezing is limited to refrigerating air temperatures above −40◦ C. Freezing at lower refrigerating temperatures is limited to cryogens, such as nitrogen or carbon dioxide that are expensive throw away liquefied gases. The primary reason for using an air cycle in food freezing is that it can greatly increase the range of operating conditions available (Evans et al., 2005). Lower refrigerating temperatures result in faster freezing, improved food quality and either reduced freezer size or larger throughput through an existing freezer (if the necessary refrigeration capacity is duly provided). 118 Frozen Food Science and Technology Fig. 5.17 Schematic diagram of an open air-cycle system for quick freezing of foods: C1 = primary compressor, C2 = bootstrap, T = turbine, M = motor (Gigiel et al., 2004). Reproduced by permission of the International Institute of Refrigeration: www.iifiir.org. An air-cycle system for food freezing is presented schematically in Figure 5.17 (Gigiel et al., 2004), while Figure 5.18 shows how the compressor, expander and driving motor can be assembled in a single compact unit with a powered bootstrap and common power supply (Kulakov et al., 1999; Gigiel et al., 2004; Kikuchi et al., 2005). A number of theoretical studies have indicated the potential for air cycle in food processing operations (Gigiel et al., 1992; Russell et al., 2000, 2001). With conventional vapour Fig. 5.18 Internal view of a single-shaft oil-less turbo-machine (Courtesy of Velis Refrigeration, Russia). Emerging and Novel Freezing Processes 119 compression plant the refrigerating air temperature ranges between ambient and −40◦ C. With air cycle, there is no limit on the cold side temperature above the liquefaction point and therefore the processes can be designed to suit the food and not the available equipment (Evans et al., 2005). Theoretically, air temperatures up to 300◦ C on the hot side can also be obtained at the same time, suitable for direct cooking or the production of steam. The established public opinion is that the air-cycle is inefficient as it has a low COP. It is fairly mentioned in almost every manual, that air-cycle possesses low thermodynamic efficiency when using it solely for cooling within a temperature range common for the commercial food refrigeration. Nevertheless, thermodynamic evidence shows (Kulakov et al., 1999) that air-cycle refrigeration may reach or exceed the performance of vapour-compression systems in two practically valuable cases: (i) at low refrigerating temperatures (approaching or falling within the cryogenic range), and (ii) when using the air cycle for both cooling and heating, i.e. as a heat pump. In the 1940s Peter Kapitza (Nobel Laureate in physics) introduced for the first-time turbo-technologies for mass production of liquid oxygen at low pressures. His enemies subjected him to malicious attacks at a governmental level because this process has been energy inefficient as compared to common high-pressure cryogenic cycles. However, because of the mass production technologies and the lack of breakdowns, the ‘energy inefficient’ Kapitza process proved to have much higher economic efficiency than alternative cycles (efficient in terms of running costs only). Remarkable progress has recently been achieved as advanced turbo-technologies lie in the basis of the modern power generation and aeronautics – major players, such as Rolls Royce and Boeing, are continuously generating new knowledge in the field (Horlock, 2003). Consequently, air-cycle food freezing has no intrinsic detrimental ozone depletion and global warming effects. Air cycle is capable of operating at low refrigerating temperatures (close to the cryogenic range), where it outperforms conventional vapour-compression systems (efficient above −40◦ C) and CO2 -based systems (used above −54◦ C). Furthermore, the high-velocity airflow exiting the turbo-expander can directly be impinged on the food, thereby ensuring high heat transfer coefficients (over 140 W m−2 K−1 ). Thus, along with enhanced air-blast freezing, an open cycle can perfectly serve to freeze foods by air-cycle-based fluidisation or air impingement, both associated with high process rates and substantially improved quality of the end product. 5.7 OTHER UNCONVENTIONAL FREEZING METHODS AND CONCLUDING REMARKS Alongside the emerging and novel freezing techniques mentioned in this chapter (e.g. hydrofluidisation; immersion freezing with smart agitation modes; application of ice slurries or air impingement; air-cycle-based freezing; flash-freezing cryogenic methods; high-pressure shift freezing and magnetic resonance freezing), a number of promising freezing process innovations have also been launched (such as ultrasonic freezing, dehydrofreezing, use of antifreezes and ice nucleation proteins; freeze drying, partial freezing, vacuum and heat pipe applications; solar, thermionic; magnetocaloric, electrocaloric and thermoacoustic refrigeration). Some of these freezing principles are presently small scale only and still unlikely to be implemented for commercial refrigeration in a short-term perspective. Novel technologies with long-term implementation perspective include magnetic and acoustic Stirling refrigeration. Magnetic refrigeration is based on the magnetocaloric effect 120 Frozen Food Science and Technology Hot heat exchanger Magnets Magnetocaloric wheel Hot heat exchanger Cold heat exchanger (a) Drive Rotation (b) Fig. 5.19 (a) Possible arrangement of a rotary magnetic refrigerator and (b) the world’s first commercial thermoacoustic refrigerator for ice cream. (MCE). Magnetocaloric materials change temperature in response to an applied magnetic field and can therefore be used to cool. Although this is not dissimilar to a traditional vapourcompression cycle (where heat is removed at the condenser and gained at the evaporator), unlike conventional systems, no potentially harmful working fluid is used. The MCE peaks at around the Curie temperature and (due to recent discoveries of materials with high Curie temperatures around ambient temperature and materials with giant MCE) refrigeration devices working at ambient temperatures are being developed (Fig. 5.19a). As a rule, magnetic refrigeration possesses high energy efficiency and, in spite of the comparatively low refrigeration capacities achieved so far, the technology is rapidly developing and might soon become a viable alternative for cooling in small household and retail refrigerators. IIR has recently created a dedicated Working Party on Magnetic Cooling, which holds regular IIR conferences on magnetic refrigeration at room temperature as an important forum to report current findings and a valuable source of up-to-date information (alongside a recent special issue of the International Journal of Refrigeration, edited by Auracher and Egolf, 2006). Backhaus and Swift (1999) developed and reported a regenerator-based thermoacoustic Stirling heat engine, which was more efficient than previous acoustic engines. In thermoacoustic refrigerators sound is used to generate pressure and alternatively compress and expand a gas (usually helium). When the gas compresses it heats up and when it expands it cools. The gas moves backwards and forwards along a tube stopping to reverse direction when the gas reaches either maximum compression or expansion. The Stirling engine principle has been known for almost 200 years but unlike mechanical Stirling machines the acoustic system uses no moving parts. By incorporating plates into the tube the system efficiency can further be enhanced. A small 200-litre prototype of commercial thermoacoustic freezer was developed for the first time by Poese et al. (2004) for Ben and Jerry’s Homemade (Fig. 5.19b). Further improvements in the design of acoustic refrigerators have been reported extensively by Backhaus and Swift (2000, 2004), So et al. (2006), Matveev et al. (2007) and Ueda et al. (2003). A number of further publications which feature interesting food freezing innovations in more detail, are readily available for the stakeholders of refrigeration science and industry (see for instance, James et al., 2000; Magnussen et al., 2000; Sun, 2001; 2005). Emerging and Novel Freezing Processes 121 ACKNOWLEDGEMENTS Figures 5.1–5.19 include illustrations of R.P. Singh, O. Schlüter, Air Products and Chemicals, Advanced Freezers, Frigoscandia, Normalair-Garrett, Food Refrigeration and Process Engineering Research Centre, Velis Refrigeration, Pennsylvania State University and Ames Laboratory, whose contribution is gratefully acknowledged. ACRONYMS CFC COP FAO HCFC HFC HFM IIR IQF MCE MRF SME WHO Chlorofluorocarbon Coefficient of performance Food and Agriculture Organisation of the United Nations Hydrochlorofluorocarbon Hydrofluorocarbon Hydrofluidisation method International Institute of Refrigeration Individual quick freezing Magnetocaloric effect Magnetic resonance freezing Small or medium enterprise World Health Organisation REFERENCES Auracher, H. and Egolf, P.W., eds. (2006). Magnetic refrigeration at room temperature. International Journal of Refrigeration 29(8), 1235–1394. Backhaus, S. and Swift, G.W. (1999). A thermoacoustic Stirling heat engine. Nature 399, 335–338. Backhaus, S. and Swift, G.W. (2000). A thermoacoustic-Stirling heat engine: detailed study. Journal of the Acoustical Society of America 107(6), 3148–3166. Backhaus, S. and Swift, G.W. (2004). A resonant, self-pumped, circulating thermoacoustic heat exchanger. Journal of the Acoustical Society of America 116(5), 2923–2938. Evans, J.A., Gigiel, A.J. and Brown, T. (2005). Fast freezing using air cycle technologies. In: Proceedings of EU Workshop ‘EUROFREEZE 2005: Individual Quick Freezing of Foods’, Sofia (Bulgaria), 13–15 January 2005, European Commission. Fikiin, A.G. (1969). Congélation de fruits et de légumes par fluidisation. Proceedings Budapest Conference, IIF/IIR: pp. 197–203. Fikiin, A.G. (1979). Bases théoriques du procédé de fluidisation lors de l’intensification de la congélation des fruits et des légumes. Proceedings 15th International Congress Refrigeration, Venice, 4, pp. 221–230. Fikiin, A.G. (1980). Physical conditions of fluidized-bed freezing of fruits and vegetables. Kholodilnaya Tekhnika/Refrigeration Engineering, 7, 59–61 (in Russian) Fikiin, A.G. (1985). Method and system for immersion cooling and freezing of foodstuffs by hydrofluidization. Invention Certificate No. 40164, Bulgarian Patent Agency INRA. Fikiin, A.G. (1992). New method and fluidized water system for intensive chilling and freezing of fish. Food Control 3(3), 153–160. Fikiin, A.G. (1994). Quick freezing of vegetables by hydrofluidization. In: New Applications of Refrigeration to Fruit and Vegetables Processing – Proceedings of IIR Conference, Istanbul (Turkey), Refrigeration Science and Technology, International Institute of Refrigeration, 1994-3, pp. 85–91. Fikiin, A.G. and Pham, V.H. (1985). System for examination of heat transfer regimes during hydrorefrigeration of foodstuffs. Invention Certificate No. 39749, Bulgarian Patent Agency INRA. 122 Frozen Food Science and Technology Fikiin, A.G., Ditchev, S.P. and Fikiina, I.K. (1966). Principal parameters characterising the fluidisation of fruit and vegetable layers. Kholodilnaya Tekhnika/Refrigeration Engineering 11, 33–37 (in Russian) Fikiin, A.G., Ditchev, S.P. and Karagerov, D.I. (1965). Fluidized bed freezing system for fruits and vegetables with various dimensions. Invention Certificate No. 10967, Bulgarian Patent Agency INRA. Fikiin, A.G., Ditchev, S.P. and Karagerov, D.I. (1970). Fluidized bed freezing apparatus AZF. Kholodilnaya Tekhnika/Refrigeration Engineering 7, 55–58 (in Russian). Fikiin, K.A. (2003). Novelties of Food Freezing Research in Europe and Beyond. Flair-Flow Europe Synthetic Brochure for SMEs No.10 (ISBN: 2-7380-1145-4), INRA: Institut National de la Recherche Agronomique, Paris (France), p. 55. Fikiin, K.A. and Fikiin, A.G. (1998). Individual quick freezing of foods by hydrofluidisation and pumpable ice slurries. In: K. Fikiin, ed. Advances in the Refrigeration Systems, Food Technologies and Cold Chain. IIR Proceedings Series ‘Refrigeration Science and Technology’, 1998-6: pp. 319–326 (published also in the AIRAH Journal, 2001, 55(11), pp. 15–18). Fikiin, K.A. and Fikiin, A.G. (1999). Novel cost-effective ice-slurry-based technology for individual quick freezing of foods by hydrofluidisation. CD-Rom Proceedings of the 20th International Congress of Refrigeration, Sydney (Australia), ICR Paper No. 271. Fikiin, K.A. and Fikiin, A.G. (2002). Congelación individual rápida de alimentos por hidrofluidificación y compuestos de hielo. Frı́o, Calor y Aire Acondicionado (Madrid) 30(334), 22–27. Fikiin, K.A. and Fikiin, A.G. (2003a). Quick freezing of foods by hydrofluidisation and pumpable ice suspensions. Kholodilnaya Tekhnika/Refrigeration Engineering (1), 22–25 (in Russian). Fikiin, K.A. and Fikiin, A.G. (2003b). L’Ice Slurry (ghiaccio binario) per una surgelazione veloce degli alimenti – Surgelamento singolo degli alimenti mediante idrofluidizzazione. Industria & Formazione per il Tecnico della Refrigerazione e Climatizzazione (Milano) (3), 36–39. Fikiin, K.A., Fikiin, A.G., Russell, S.L. and Fitt, P.W. (1998). Shelf-life extension of Bulgarian yoghurt by using a novel environment-friendly air-cycle integrated system for thermal and refrigerated processing. In: Shelf-Life Prediction – Proceedings of the EU COPERNICUS Workshop, Wageningen (the Netherlands), pp. 159–168. Fikiin, K.A., Kaloyanov, N.G., Filatova, T.A. and Sokolov, V.N. (2002). Fine-crystalline ice slurries as a basis of advanced industrial technologies: state of the art and future prospects. Refrigeration Business (Moscow) (7), 4–11 (in Russian). Fikiin, K.A., Wang, M.-J., Kauffeld, M. and Hansen, T.M. (2005). Direct contact chilling and freezing of foods in ice slurries – Chapter 9. In: M. Kauffeld, M. Kawaji and P.W. Egolf, eds. Handbook on Ice Slurries – Fundamentals and Engineering. International Institute of Refrigeration, pp. 251–271. Fikiin, K.A., Tsvetkov, O.B., Laptev, Yu.A., Fikiin, A.G. and Kolodyaznaya, V.S. (2001). Thermophysical and engineering issues of the immersion freezing of fruits in ice slurries based on sugar-ethanol aqueous solutions. Proceedings of the Third IIR Workshop on Ice Slurries, Lucerne (Switzerland). International Institute of Refrigeration, pp. 147–154 (published also in EcoLibrium – Journal of AIRAH: Australian Institute of Refrigeration, Air Conditioning and Heating, 2003, 2(7), 10–15). Fleshland, O. and Magnussen, O. M. (1990). Chilling of farmed fish. Proceedings Aberdeen Conference, IIF/IIR: pp. 185–192. Gigiel, A.J., Chauveron, S. and Fitt, P. (1992). Air as a replacement for CFC refrigerants. IIR Congress ‘Cold ‘92’, Buenos Aires, Argentina, 7–9 Sept. Gigiel, A., Gibbs, R., Butler, D. and Holder, D. (1996). Air-cycle technologies for building services. IIR Proceedings Series ‘Refrigeration Science and Technology’, International Institute of Refrigeration – Paris, 1996-3: 687–696. Gigiel, A., Polonara, F. and Di Nicola, G. (2004). Rapid freezing of food in an open air cycle freezer. In: Natural Working Fluids 2004 – 6th IIR Gustav Lorentzen Conference. IIR Proceedings Series ‘Refrigeration Science and Technology’, International Institute of Refrigeration, 2004–2. Horlock, J. (2003). Advanced Gas Turbine Cycles. Elsevier Science, p. 405. IIR (1986). Recommendations for the Processing and Handling of Frozen Foods. Paris: International Institute of Refrigeration, p. 419. IIR (1999). Control of the Cold Chain for Quick-Frozen Foods. Handbook. Paris: International Institute of Refrigeration, p. 94. James, C., Ketteringham, L. and James, S.J. (2000). Enhanced heat transfer in food chilling, freezing and thawing using heat pipes. In: K. Fikiin, ed. Advances in the Refrigeration Systems, Food Technologies and Cold Chain. IIR Proceedings Series ‘Refrigeration Science and Technology’, 1998/6, pp. 327–333. Kennedy, C.J., ed. (2000). Managing Frozen Foods. Cambridge: Woodhead Publishing, p. 286. Emerging and Novel Freezing Processes 123 Kikuchi, S., Okuda, S., Igawa, H., Morii, S., Mitsuhashi, M. and Higashimori, H. (2005). Development of an air-cycle system for refrigeration. Mitsubishi Heavy Industries Technical Review 42(4), 1–4. Kulakov, V.M., Vereschagin, M.P., Kulakov, V.V. and Vereschagin, M.M. (1999). Air turbo-refrigerating machines. Refrigeration Business 6: 5–10 (in Russian). Lucas, T. and Raoult-Wack, A.L. (1998). Immersion chilling and freezing in aqueous refrigerating media: review and future trends. International Journal of Refrigeration 21(6), 419–429. Magnussen, O.M., Nordtvedt, T.S. and Torstveit, A.K. (2000). Use of partial freezing in the cold chain. In: K. Fikiin, ed. Advances in the Refrigeration Systems, Food Technologies and Cold Chain. IIR Proceedings Series ‘Refrigeration Science and Technology’, 1998/6, pp. 363–370. Matveev, I.K., Backhaus, S. and Swift, G.W. (2007). Analytical solution for temperature profiles at the ends of thermal buffer tubes. International Journal of Heat and Mass Transfer 50, 897–901. Mohanty, P. (2001). Magnetic resonance freezing system. AIRAH Journal 55(6), 28–29. Pearson, S.F. and Brown, J. (1998). Use of pumpable ice to minimise salt uptake during immersion freezing. Proceedings Oslo Conference, IIF/IIR: pp. 712–722. Poese, M.E., Robert, W.M., Garrett, S.L., van Gerwen, R. and Gosselin, P. (2004). Thermoacoustic refrigeration for ice cream sales. 6th IIR Gustav Lorentzen Conference on Natural Working Substances, Glasgow, Aug 29–Sept 1, 2004. Russell, S.L., Gigiel, A.J. and James, S.J. (2001). Progress in the use of air cycle technology in food refrigeration and especially retail display. AIRAH Journal 55(11), 20–25. Schlüter, O., George, S., Heinz, V. and Knorr, D. (2000). Phase transitions in model foods, induced by pressureassisted freezing and pressure-assisted thawing. In: K. Fikiin, ed. Advances in the Refrigeration Systems, Food Technologies and Cold Chain. IIR Proceedings Series ‘Refrigeration Science and Technology’, 19986, pp. 240–248. Shaw, R., Kiczek, E. and Rossman, J. (1995). COLDBLAST air-cycle freezing system. Proceedings of the 19th International Congress of Refrigeration, The Hague, 3a: 684–691. So, J.H., Swift, G.W. and Backhaus, S. (2006). An internal streaming instability in regenerators. Journal of the Acoustical Society of America 120(4), 1898–1909. Sun, Da-Wen, ed. (2001). Advances in Food Refrigeration. Surrey: Leatherhead Publishing, p. 482. Sun, Da-Wen, ed. (2005). Handbook of Frozen Food Processing and Packaging. Boca Raton, FL: CRC Press, Taylor & Francis Group, p. 737. Tressler, D.K. (1968). Food freezing systems. In: D.K. Tressler, W.B. Van Arsdel and M.J. Copley, eds. The Freezing Preservation of Foods, Vol. 1. Westport, CT: AVI Publishing, pp. 120–152. Verschoor, M., ed. (2001). Guidelines for the Application and Design of Air-cycle Systems for Heating, Ventilating and Air-Conditioning in Buildings. TNO Environment, Energy and Process Innovation, p. 120. Van Gerwen, R. and Verschoor, M. (1996). Feasibility of air cycle systems for building air conditioning systems. IIR Proceedings Series ‘Refrigeration Science and Technology’, International Institute of Refrigeration – Paris, 1996-3: pp. 677–685. Ueda, U., Biwa, T., Yazaki, T. and Mizutani, U. (2003). Construction of a thermoacoustic Stirling cooler. Physica B 329, 1600–1601. Ure, Z. (1998). Slurry-ice based cooling systems. In: K. Fikiin, ed. Advances in the Refrigeration Systems, Food Technologies and Cold Chain. IIR Proceedings Series ‘Refrigeration Science and Technology’, 1998-6: pp. 172–179.