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5 Emerging and Novel Freezing Processes

Kostadin Fikiin

5.1 NEED FOR PROCESS INNOVATIONS IN THE FOODFREEZING INDUSTRY TO IMPROVE THE HUMANWELL-BEING AND QUALITY OF LIFE

As is known, heat and cold are of the same physical nature. In spite of this, they have playeddifferent roles in the development of human civilisation. Prometheus, the mythological herowho bestowed the divine fire of Olympus to mankind, is glorified in immortal poeticaland musical works. However, the pioneers who created artificial refrigeration and gave it tohumanity have not yet been praised in a work of art as a token of gratitude. For millennia, coldhas primarily been associated with winter, diseases and people’s misery, rather than with itsproven 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 resourcesof humankind. The worldwide food output amounts nearly 5 billion tonnes per year, some2 billion of which need refrigerated processing, but only 400 million are effectively re-frigerated. Chilling is an indispensable element of almost all post-harvest or post-mortemtechniques for handling food commodities of plant or animal origin, while freezing has beenestablished and recognised as the paramount commercial method for long-term preservationof the natural quality attributes of perishable foods, thereby forming a substantial part of theglobal economy and the well-being of citizens. In terms of money, every year the global in-vestment in refrigerating equipment is over US$ 170 billion, while all refrigerated foodstuffscost US$ 1200 billion (which exceeds 3.5 times the US military budget). Some 700–1000million household refrigerators and 300,000,000 m3 of cold-storage facilities are availableover the world. The annual global production of various frozen foods is about 50 milliontonnes (plus 20 million tonnes of ice creams and 30 million tonnes of fish), with a remarkablegrowth of 10% every year. Refrigeration thus accounts for about 15% of the worldwide elec-tricity 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/WHOCodex Alimentarius Commissions and played an important role in the historic world leaders’summits in Montreal, Kyoto and Johannesburg (devoted to ozone-depletion, global warmingand sustainable development). Refrigeration and the cold chain are among the top priorities ofthe US Presidential Council for Food Safety. While the economies of industrialised nationswaste 25–30% of their perishable food production because of imperfect or lacking coldchain, the disastrous dimension of such agricultural losses in developing countries is a majorcontributor to malnutrition.

Frozen Food Science and Technology. Edited by Judith A. Evans

© 2008 Blackwell Publishing Ltd, ISBN: 978-1-4051-5478-9

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102 Frozen Food Science and Technology

The twenty-first century poses new challenges for the global frozen food sector, whichcan be summarised as follows:� Although the promising capabilities of several emerging freezing technologies (detailed in

this chapter) attract the attention of researchers and industrialists, many such innovationsremain unimplemented in the common industrial practice.� Both conventional and novel freezing techniques have a substantial potential for fur-ther optimisation by involving advanced modelling and experimental tools and enrichingtheoretical understanding of underlying phenomena (e.g. heat transfer, fluid flow andbiochemical processes).� The manufacture of frozen commodities and related scientific research are still insuffi-ciently attractive for high-skilled experts and young specialists as compared with hi-techbranches (e.g. information technologies, electronics, communications, biotechnologies,etc.).� Developing countries around the world need more refrigeration capacities and inexpensivefood 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 nutritionallyvaluable preservation techniques should not create a false sense of total security and shouldnot 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 num-ber of physical and biochemical reactions still occur and are accentuated if proper processingand handling conditions are not maintained throughout the entire chain for production, stor-age, transport, distribution, retail and household handling of various frozen commodities ofplant or animal origin by using an integral farm-to-table approach.

Contemporary food industry promotes research and innovations, which primarily dealwith the sustainable processing, preservation and supply of safe, high-quality, healthier foodsand beverages for the consumers. In that context, food-freezing technologies are a crucialand underexploited element of the sustainable food production and preservation and relatedfood chain management. This importance perceptibly increases as a result of the ongoingextension of traditional food chains into new markets and emerging economies around theworld. Essential measures should, therefore, be undertaken to raise professional competenceand encourage a stronger public commitment to food freezing investigations. It is vital to makepublic authorities and food policy makers more aware of the topical professional endeavoursof refrigeration scientists and industrialists around the world. Relevant funding agencies(such as the European Commission) supported, therefore, a number of successful freezing-related research projects whose deliverables and industrial implementation result in reducedpost-harvest losses, extended shelf-life and better quality of frozen foods, lower investmentsand running costs, higher energy savings, and enhanced environmental friendliness (Fikiin,2003).

5.2 STATE OF THE ART AND CONVENTIONALFREEZING MODES

In the early twentieth century, many people were experimenting with mechanical and chemicalmethods to preserve food. As an industrial process, quick freezing began its history nearly80 years ago when Clarence Birdseye found a way to flash-freeze foods and deliver them

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Emerging and Novel Freezing Processes 103

Cell

Cell membrane

Natural Cells Crystallisation and Water Migrationduring Freezing

After Quick Freezing

Less damaged tissue with maintained integrity

After Slow Freezing

Shrunk cells and destructed tissue

Ice crystalsWater flux

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 lowtemperature almost instantly froze just-caught fish. More importantly, he also found thatwhen such quick-frozen fish were cooked and eaten, they were scarcely different in taste andtexture from how they would have been if fresh. After years of work, Birdseye invented asystem that packed dressed fish, meat or vegetables into waxed-cardboard cartons, which wereflash-frozen under pressure (US Patent No. 1,773,079, 1930). Then, he turned to marketingand 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 refrigeratedboxcars for railway transport; and retail of frozen products in Springfield, Massachusetts,in 1930). These technology achievements constituted the world’s first cold chain for frozenfoods (Fikiin, 2003).

Thus, quick freezing has further been adopted as a widespread commercial method forlong-term preservation of perishable foods, which improved both the health and convenienceof virtually everyone in the industrialised countries. Freezing rate affects strongly the qualityof frozen foods, in which the predominant water content should quickly be frozen in a fine-grain crystal structure in order to prevent the cellular tissues and to inhibit rapidly the spoilingmicrobiologic and enzymatic processes (Fig. 5.1).

Basic heat transfer considerations (Fikiin, 2003) clearly suggest that the desired shorteningof freezing duration and a resulting high throughput of refrigerating equipment could beachieved by means of: (i) lower refrigerating medium temperature (which generally requiresgreater investment and running costs for the refrigeration machines to be employed); (ii)enhanced surface heat transfer coefficients (by increased refrigerating medium velocity andboundary layer turbulence, involvement of surface phase-change effects and less packaging);

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104 Frozen Food Science and Technology

Evaporator plates

Product to be frozen

Expansion valve

Condenser

Compressor

HydrauliccylinderWelding flange

unionPressureplate

Freezerplate

Spacer

Liftingbolt

Freezer Plates

Frame

Plateangle

Hose

Refrigerantheader Oil purge valve

(both headers)

(a)

(b)

(d)

(c)

(e)

Fig. 5.2 Multi-plate freezing systems.

and (iii) reduced size of the refrigerated objects (by freezing small products individually orappropriately 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). Theapplication of cryogenic IQF (Fig. 5.4) is still very restricted because of the high price of theliquefied gases used.

5.2.1 Fluidised-bed freezing systems

The air fluidisation (Fig. 5.5) was studied extensively and used commercially, with an in-creasing popularity, during the last 50 years (Fikiin et al., 1965, 1966, 1970; Fikiin, 1969,1979, 1980). This freezing principle possesses many attractive features, including:� 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 transfercoefficients.

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Emerging and Novel Freezing Processes 105

TunnelFreezers

Spiral Freezers

(a)

(b)

Fig. 5.3 Air-blast freezing systems.� Good quality of the frozen products that have an attractive appearance and do not sticktogether.� 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:� Necessity of two-stage refrigerating plants (using large quantities of CFC-, HCFC- orHFC-based refrigerants with significant ozone depletion or global warming potentials) tomaintain an evaporation temperature of about −45◦C, which needs high investment andpower costs.

Liquid nitrogen spraying

Immersion in liquid nitrogen

(a)

(b)

Fig. 5.4 Cryogenic freezing systems.

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106 Frozen Food Science and Technology

Fluidized bed

Blowers

Cold air

Air

(a) (b)

(c)

Frozenproduct

out

Fluidised bed

Fig. 5.5 Fluidised-bed freezing systems.� Lower surface heat transfer coefficients and freezing rates in comparison with the immer-sion 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 evaporatingrefrigerant.� Excessive sensitivity of the process parameters to the product shape, mass and sizes, whichrequires 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 methodhaving several important advantages: high heat transfer rate, fine ice crystal system in foods,great throughput, low investments and operational costs (Tressler, 1968; Fleshland andMagnussen, 1990; Lucas and Raoult-Wack, 1998; Fikiin et al., 2001). Immersion applica-tions have been limited because of the uncontrolled solute uptake by the refrigerated productsand operational problems with the immersion liquids (high viscosity at low temperatures,difficulty in maintaining the medium at a definite constant concentration and free from or-ganic 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 todevelop advanced innovative immersion IQF systems (Fikiin and Fikiin, 1998, 1999, 2002,2003a, 2003b; Fikiin, 2003).

5.3 INDIVIDUAL QUICK FREEZING OF FOODS BYHYDROFLUIDISATION AND PUMPABLE ICE SLURRIES

The Hydrofluidisation Method (HFM) for fast freezing of foods was suggested and patentedrecently to overcome the drawbacks and to bring together the advantages of both air fluidis-ation and immersion food freezing techniques (Fikiin, 1985, 1992, 1994). The HFM uses a

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Emerging and Novel Freezing Processes 107

Hydrofluidisation

Fig. 5.6 Possible arrangement of a HFM-based freezing system combining the advantages of both airfluidisation 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) perforatedgrate for draining; (8) sprinkling device for glazing; (7 and 9) netlike conveyor belt; (10 and 11) collectorvats; (12) pump; (13 and 14) rough and fine filters; (15) cooler of refrigerating medium; (16) refrigerationplant. 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 highlyturbulent liquid and moving products, and thus evoke extremely high surface heat transfercoefficients. The principle of operation of a HFM freezing system is illustrated in Fig. 5.6.

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108 Frozen Food Science and Technology

−20

−15

−10

−5

0

5

10

15

20

25

30

0 50 100 150 200 250 300 350

Time (s)

Ambient temperature = −15/−16°CAverage jet velocity = 2 m s−1

Core

Surface

Green beans (11 g)

Peppers (30 g)

−20

−15

−10

−5

0

5

10

15

20

25

30

0 100 200 300 400 500 600 700 800

Time (s)

Tem

per

atu

re (

°C)

Tem

per

atu

re (

°C)

Scad (28 g)

Sprat (7 g)

Centre

Surface

Ambient temperature = −15/−16°CAverage jet velocity = 2 m s−1

(a) (b)

Fig. 5.7 Experimental temperature histories during HFM freezing of fish (a) and vegetables (b) whenusing 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 asfluidising agents

Although various immersion techniques have been known for long time, until now hydroflu-idisation principles have not been used for chilling and freezing of foods. Experiments onHFM freezing of small fish and some vegetables through an aqueous solution of sodium chlo-ride 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−1for sprin-kling 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 tem-perature of about −16◦C, the scad fish were frozen from 25◦C down to −10◦C in the centrein 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 hydrofluidisationfreezing 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 environ-mentally benign secondary coolants circulated to the heat transfer equipment of refrigerationplants, instead of the traditional harmful CFC- or HCFC-based refrigerants (Ure, 1998;Pearson and Brown, 1998). Promising attempts to refrigerate foods by immersion in suchslurries have already been carried out. For example, fish chilling in brine-based slurries haspotential to replace the traditional use of ice flakes (Fikiin et al., 2001, 2002, 2005). A numberof foods immersed in slurries with various ice contents are shown in Fig. 5.8.

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Emerging and Novel Freezing Processes 109

(a)

(d) (e) (f)

(b) (c)

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 ofRefrigeration: www.iifiir.org.

Fikiin and Fikiin (1998, 1999) proposed, therefore, a novel method to enhance the advan-tages of the hydrofluidisation (described hereafter) by employing two-phase ice suspensionsas fluidising media. The ice slurries reveal a great energy potential as HFM refrigeratingmedia 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 heattransfer coefficient (of the order of 1000–2000 W m−2 K−1 or more), excessively short freez-ing 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 re-frigerating media represents a new interdisciplinary research field whose development wouldadvance essentially the refrigerated processing of foods. The HFM freezing with ice slur-ries can acquire a process rate approaching that of the cryogenic flash freezing modes. Forinstance, at a refrigerating ice-slurry temperature of −25◦C and a heat transfer coefficientof 1000 W m−2 K−1, strawberries, apricots and plums can be frozen from 25◦C down to anaverage final temperature of −18◦C within 8–9 minutes, raspberries, cherries and morelloswithin 1.5–3 minutes, and green peas, blueberries and cranberries within approximately 1minute only. The general layout of an ice-slurry-based system for hydrofluidisation freezingis 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 ofunfreezable liquids or pumpable ice slurries as fluidising agents. It is well known that theimmersion freezing history began with use of brines to freeze fish, vegetables and meat. Binaryor 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 practicallyunlimited possibilities to combine constituents and to formulate appropriate multi-componentHFM refrigerating media based on one-phase liquids or two-phase ice slurries, which have

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110 Frozen Food Science and Technology

Hydrofluidisation freezer

Product out

Product in

Filt

er s

yste

m

Pu

mp

Ice slurrytank

Ice

slu

rry

gen

erat

or

Compressor

Expansion valve

Refrigeration unit

Con

dens

er

Fig. 5.9 Schematic diagram of an ice-slurry-based hydrofluidisation system HyFloFreeze® (Fikiin andFikiin, 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 termsof pumpability and good hydrofluidisation.

The main advantages of the hydrofluidisation over the conventional freezing modes canbe summarised as follows:� 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 timeslower investments and power costs as compared to the conventional air fluidisation. Colddissipation through the freezer walls is also lower. The water flow rate or fan powerconsumption for cooling the condenser decrease as well, due to the reduced mechanicalwork of the single-stage unit.� 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 per-ceptible damage.� The product surface freezes immediately in a solid crust that hampers the osmotic transferand gives an excellent appearance. The mass losses tend to zero, while in air freezingtunnels the moisture losses are usually 2–3%.� New delicious products can easily be formulated by using some selected product-friendlyHFM media (for example, fruits frozen in syrup-type sugar solutions turn into dessertproducts with beneficial effect on colour, flavour and texture). Such media can also include

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Emerging and Novel Freezing Processes 111

Fig. 5.10 HyFloFreeze® prototype: hydrofluidised bed of highly turbulent ice slurry (Fikiin, 2003). Re-produced by permission of the International Institute of Refrigeration: www.iifiir.org.

appropriate antioxidants, flavourings and micronutrients to extend the shelf-life of theproducts and to improve their nutritional value and sensory properties.� The HFM freezers use environmentally-friendly secondary coolants (for instance, syrup-type aqueous solutions and ice slurries) and the refrigerant is closed in a small isolatedsystem, in contrast to the common air fluidisation freezers where large quantities of harmfulHCFCs or expensive HFCs circulate to remote evaporators with a much greater risk foremission to the environment.� Fluidised state is acquired with low velocity and pressure of the fluid jets due to theArchimedes forces and buoyancy of the products, which leads to both energy savings andminimum mechanical action on the foods.� The operation is continuous, easy to maintain, convenient for automation and the labourcosts are substantially low. Further processing or packaging of the HFM-frozen productsis considerably easier since they emerge from the freezer in a ‘free-flowing’ state.� Ice-slurry-based HFM agents may easily be integrated into systems for thermal energystorage, 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 slurryis 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 distin-guished: (i) employment of unfreezable liquids as fluidising agents and (ii) use of pumpableice slurries as fluidising media. This freezing principle provides an extremely high heattransfer rate, short freezing times, great throughput and better product quality at refrigerat-ing temperatures maintained by a single-stage refrigeration machine. Thus, nearly two timeslower investments and power costs are necessary as compared with the popular individualquick freezing methods. Moreover, such hydrofluidisation freezing systems are less haz-ardous from the environmental viewpoint, since the refrigerant is closed in a small isolatedcircuit only.

The emerging HFM technology has drawn the attention of a number of academics andindustrialists. The identification of optimal design specifications for HFM freezing sys-tems requires an interdisciplinary approach of researchers with complementary skills. TheHyFloFreeze project was, therefore, funded by the European Commission and performed byan international research consortium of six participating organisations (four universities andtwo SMEs) from Belgium, Bulgaria, Russia and the UK (Fikiin, 2003).

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112 Frozen Food Science and Technology

5.4 HIGH-PRESSURE FREEZING

Non-thermal food processing techniques (e.g. pulse-electric field pasteurisation, high-intensity pulsed lights, high-intensity pulsed-magnetic field, ozone treatment) are presentlyregarded with special interest by the food industry. Among them, high-pressure processingis gaining in popularity with food processors because of its food preservation capability andpotential to achieve interesting functional effects. Under high pressure pathogenic micro-organisms can be inactivated with minimal heat treatment, which results in almost completeretention of nutritional and sensory characteristics of fresh foods, without sacrificing theirshelf-life. Other advantages over traditional thermal processing include reduced processtimes; 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 toreduced crystal size and multiple ice-phase forms); and minimal undesirable functionalityalterations. However, the spore inactivation is a major challenge as methods for full inacti-vation of spores under pressure are yet to be developed. Hence, another group of researchactivities worldwide focus on different techniques for treatment of foods by high hydrostaticpressure, including high-pressure-aided freezing and thawing.

A number of products (such as jams and fruit juices) have been processed under highpressure in Japan. There have been 10–15 types of pressurised foods on the Japanese marketbut several have recently disappeared, while the remaining ones are too specific to excitea substantial commercial interest. Examples of pressurised products in Europe and US are:(i) orange juice (Pernod Ricard Company, France); (ii) acidified avocado puree (AvomexCompany, USA and Mexico); and (iii) sliced ham (Espuna Company, Spain). Volumes pro-duced are still very small and some current European food regulations slowed down thelaunching of new pressurised products because of legislative problems.

The so-called cryofixation is a physical method for immobilisation of biological materi-als by ultra-quick freezing. Unlike the chemical fixation, it preserves thoroughly the ultra-structural morphology, much closer to the natural state of the cell tissue. This results in fastpreservation of morphological details without artificial damage, less cross-linking of proteinsby 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◦Cand 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 highfreezing rates particles and large molecules in water serve as cores for a heterogeneous nu-cleation, 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 duringfreezing at atmospheric pressure. This restriction could be overcome through a depressionof the initial freezing (cryoscopic) point of water by adding chemical cryoprotectants or byincreasing 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 objectswith 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:� 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 essen-tial changes of nutritional and sensory quality.

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Emerging and Novel Freezing Processes 113

Liquid

Ice IIce III

Ice II

Ice V

Pressure (MPa)

Tem

pera

ture

(°C

)

0

10

0

−10

−20

−30100 200 300 400 500

A B C K

H

D

G

I

E

F

Fig. 5.11 Illustrates various paths of changing the food physical state by external manipulations oftemperature or pressure, while Figs. 5.12 and 5.13 show temperature- and pressure-dependent thermalproperties of potatoes during pressure-assisted freezing (Schluter et al., 2000).

Water phase diagram and high pressure effects on the phase transitions:A – B – C – D – C – B – A Subzero storage without freezingA – B – H – I Pressure-assisted1 freezingI – H – B – A Pressure-assisted1 thawingA – B – C – D – E Pressure-shift2 freezingE – D – C – B – A Pressure-induced3 thawingA – B – C – D – G – F Freezing to ice IIIF – G – D – C – B – A Thawing of ice IIIA – B – C – K – ice VI Freezing above 0◦C

1 assisted: phase transition at constant pressure2 shift: phase transition due to pressure change3 induced: phase transition initiated with pressure change and continued at constant pressureReproduced 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 tostay 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 masstransfer within a food product undergoing freezing. Realising this circumstance, researchersdecided that if water could somehow be retained within the cells while freezing, then thecells would not become dehydrated and foodstuff could keep its original attributes and fresh-ness. A system for magnetic resonance freezing (MRF) preventing such cellular dehydrationcould be regarded as composed of a common freezer and a special magnetic resonance de-vice. The MRF process (Fig. 5.14) is then applied with the following two steps (Mohanty,2001).

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114 Frozen Food Science and Technology

1E+04

2E+04

3E+04

4E+04

5E+04

6E+04

7E+04

8E+04

9E+04

1E+05

−30 −25 −20 −15 −10 −5 0 5 10

Cp0.1 MPa

130 MPa200 MPa

Temperature (°C)

c p (

J kg

−1 K

−1)

Fig. 5.12 Apparent specific heat capacity of potato tissue at different pressures. Reproduced by permis-sion of the International Institute of Refrigeration: www.iifiir.org.

0.6

0.8

1.0

1.2

1.4

1.6

1.8

−35 −30 −25 −20 −15 −10 −5 0 5 10

λ0.1 MPa130 MPa200 MPa

Temperature (°C)

λ (W

m−1

K−1

)

Fig. 5.13 Thermal conductivity of potato tissue at different pressures. Reproduced by permission of theInternational Institute of Refrigeration: www.iifiir.org.

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Emerging and Novel Freezing Processes 115

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75

Time (min)

Conventional Blast Freezer

MRF System

10

0

−10

−20

−30

−40

−50

−60

Tem

pera

ture

(°C

)

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 abruptlyremoved 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 crystallisa-tion (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 throughextensive 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 halogenatedhydrocarbons (CFCs and HCFCs) whose emissions to the environment are damaging theozone layer and contributing significantly to global warming. Manufacture and import ofCFCs is banned in most of the world and many HCFC refrigerants are only short-term re-placements, often being more expensive and less efficient. The vapour-compression systemshave refrigerant leakage rates to the environment of 15% of the total charge per annum, thusleading to stratospheric ozone depletion and climate change. Some other non-CFC alterna-tives called natural refrigerants (e.g. ammonia, propane, butane, isobutane, carbon dioxideand water) are also being employed or examined. Ammonia is the most common of the al-ternative 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.

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116 Frozen Food Science and Technology

Q

S Q

T

Work

P1

P2

1

2

3

41

23

4

Fig. 5.15 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 greentechnologies in the food refrigeration sector.

The principle of the air cycle is that when air is compressed its temperature and pressureincrease (1–2) as shown in Fig. 5.15. Heat is removed from the compressed air at constantpressure and its temperature is reduced (2–3). Air is then expanded and its temperaturedecreases (3–4). The air further absorbs heat (gaining temperature) from the process atconstant pressure (4–1) where it starts the cycle again (Evans et al., 2005).

Air-cycle refrigeration is an environmentally friendly alternative to the conventionalvapour-compression systems. Originally used in the nineteenth century, the technology em-ployed slow-speed reciprocating compressors and low-efficiency expanders. The poor en-ergy efficiency and comparatively high cost of such machinery was a major factor for thereplacement of these systems with vapour-compression equipment. However, the remarkableprogress in turbine technology since that time (Horlock, 2003), along with the developmentof air bearings and ceramic components, provided dramatic improvements in efficiency. Theair-cycle use for aircraft and railway carriage air-conditioning (which affords equipmentsimplicity, compactness and robustness) and the development of high-speed rotary compres-sors and expanders have greatly improved the cycle performance and reliability (Fig. 5.16).

BootstrapHot air heatexchanger

Motor Poweredcompressor

Expansionturbine Cold air heat

exchanger

Fig. 5.16 An air-cycle unit for automotive applications (Normalair-Garrett Ltd.)

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Combining this with newly available compact heat exchangers with highly enhanced heattransfer characteristics makes competition with existing vapour-compression systems ratherattractive.

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 simultaneouslyproduce both heat and cold, the advantage being that heating is provided at higher temperaturesas compared to the conventional vapour-cycle heat-pump system. Frequency control of themotor speed permits a smooth variation of the operating capacity depending on the heatloads. On the other hand, mass production techniques make the air systems much cheaperthan they were in the past. Air-cycle refrigeration is one of the most attractive ways forsolving the environmental problems since the working fluid (air) is completely natural, freeand totally environmentally benign. This environmental friendliness, reliability, minimalmaintenance 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 canadditionally 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 energyand investment costs) with vapour-compression systems, coupled with conventional waterheating devices (Gigiel et al., 1996; Van Gerwen and Verschoor, 1996).

Both open and closed air-circulation systems can be employed to freeze foods. In theopen cycle atmospheric air normally enters the system and returns to the atmosphere afterbeing used for heating and cooling purposes (Shaw et al., 1995). The air as a refrigerant isin a direct contact with the refrigerated products, avoiding any intermediate heat exchangers,pipelines, secondary coolants and related energy dissipation. However, the running of suchsystems is strongly dependent on the atmospheric conditions, which results in more difficultcontrol. 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 theclosed cycle dry air is circulated around the closed system, similarly to a conventional vapour-compression machine. The advantage of a closed air cycle is that the load on the system canbe matched exactly without any change in the efficiency. This is because the efficiency of therotating machinery used in air-cycle plants depends only on the velocities of the air and notits density. The change of pressure in the system overall results in corresponding changes inthe air density and mass flow. Thus the refrigeration and the heating effects from the air-cycleplant can vary in proportion to the mass flow, without change in the efficiency, in contrast tocommon vapour-compression systems.

Recent developments and improvements in air-cycle equipment enable a wider practi-cal exploitation of this emerging technology for food freezing applications. The benefitsare greater reliability, reduced maintenance, simple and compact design, reduced overalllabour costs, improved control of product quality, along with using an environmentally be-nign and free refrigerant unlike the conventional heating and cooling systems. Fast freezingthrough low-temperature air dramatically reduces freezing times and has many advantages(e.g. flexibility, savings in freezer footprint and higher product quality). With a conventionalvapour-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 carbondioxide that are expensive throw away liquefied gases. The primary reason for using an aircycle 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 foodquality and either reduced freezer size or larger throughput through an existing freezer (if thenecessary refrigeration capacity is duly provided).

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118 Frozen Food Science and Technology

Fig. 5.17 Schematic diagram of an open air-cycle system for quick freezing of foods: C1 = primarycompressor, C2 = bootstrap, T = turbine, M = motor (Gigiel et al., 2004). Reproduced by permission ofthe International Institute of Refrigeration: www.iifiir.org.

An air-cycle system for food freezing is presented schematically in Figure 5.17 (Gigielet al., 2004), while Figure 5.18 shows how the compressor, expander and driving motor canbe 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 process-ing 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).

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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 pointand 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 beobtained 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 thermodynamicefficiency when using it solely for cooling within a temperature range common for thecommercial food refrigeration. Nevertheless, thermodynamic evidence shows (Kulakov et al.,1999) that air-cycle refrigeration may reach or exceed the performance of vapour-compressionsystems in two practically valuable cases: (i) at low refrigerating temperatures (approachingor falling within the cryogenic range), and (ii) when using the air cycle for both coolingand 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 lowpressures. His enemies subjected him to malicious attacks at a governmental level becausethis process has been energy inefficient as compared to common high-pressure cryogeniccycles. However, because of the mass production technologies and the lack of breakdowns,the ‘energy inefficient’ Kapitza process proved to have much higher economic efficiencythan alternative cycles (efficient in terms of running costs only). Remarkable progress hasrecently been achieved as advanced turbo-technologies lie in the basis of the modern powergeneration and aeronautics – major players, such as Rolls Royce and Boeing, are continuouslygenerating new knowledge in the field (Horlock, 2003).

Consequently, air-cycle food freezing has no intrinsic detrimental ozone depletion andglobal warming effects. Air cycle is capable of operating at low refrigerating temperatures(close to the cryogenic range), where it outperforms conventional vapour-compression sys-tems (efficient above −40◦C) and CO2-based systems (used above −54◦C). Furthermore, thehigh-velocity airflow exiting the turbo-expander can directly be impinged on the food, therebyensuring high heat transfer coefficients (over 140 W m−2 K−1). Thus, along with enhancedair-blast freezing, an open cycle can perfectly serve to freeze foods by air-cycle-based fluidis-ation or air impingement, both associated with high process rates and substantially improvedquality of the end product.

5.7 OTHER UNCONVENTIONAL FREEZING METHODS ANDCONCLUDING REMARKS

Alongside the emerging and novel freezing techniques mentioned in this chapter (e.g. hy-drofluidisation; immersion freezing with smart agitation modes; application of ice slurries orair impingement; air-cycle-based freezing; flash-freezing cryogenic methods; high-pressureshift freezing and magnetic resonance freezing), a number of promising freezing processinnovations have also been launched (such as ultrasonic freezing, dehydrofreezing, use ofantifreezes and ice nucleation proteins; freeze drying, partial freezing, vacuum and heat pipeapplications; solar, thermionic; magnetocaloric, electrocaloric and thermoacoustic refriger-ation). Some of these freezing principles are presently small scale only and still unlikely tobe implemented for commercial refrigeration in a short-term perspective.

Novel technologies with long-term implementation perspective include magnetic andacoustic Stirling refrigeration. Magnetic refrigeration is based on the magnetocaloric effect

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120 Frozen Food Science and Technology

Hot heat exchanger

Magnets

Magnetocaloricwheel

Hot heatexchanger

Cold heatexchanger Drive Rotation

(a) (b)

Fig. 5.19 (a) Possible arrangement of a rotary magnetic refrigerator and (b) the world’s first commercialthermoacoustic refrigerator for ice cream.

(MCE). Magnetocaloric materials change temperature in response to an applied magneticfield and can therefore be used to cool. Although this is not dissimilar to a traditional vapour-compression 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 peaksat around the Curie temperature and (due to recent discoveries of materials with high Curietemperatures around ambient temperature and materials with giant MCE) refrigeration de-vices working at ambient temperatures are being developed (Fig. 5.19a). As a rule, magneticrefrigeration possesses high energy efficiency and, in spite of the comparatively low re-frigeration capacities achieved so far, the technology is rapidly developing and might soonbecome a viable alternative for cooling in small household and retail refrigerators. IIR hasrecently created a dedicated Working Party on Magnetic Cooling, which holds regular IIRconferences on magnetic refrigeration at room temperature as an important forum to re-port current findings and a valuable source of up-to-date information (alongside a recentspecial issue of the International Journal of Refrigeration, edited by Auracher and Egolf,2006).

Backhaus and Swift (1999) developed and reported a regenerator-based thermoacousticStirling heat engine, which was more efficient than previous acoustic engines. In thermoa-coustic refrigerators sound is used to generate pressure and alternatively compress and expanda 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 whenthe gas reaches either maximum compression or expansion. The Stirling engine principlehas been known for almost 200 years but unlike mechanical Stirling machines the acousticsystem uses no moving parts. By incorporating plates into the tube the system efficiency canfurther be enhanced. A small 200-litre prototype of commercial thermoacoustic freezer wasdeveloped 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 extensivelyby 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 inmore 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).

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ACKNOWLEDGEMENTS

Figures 5.1–5.19 include illustrations of R.P. Singh, O. Schluter, Air Products and Chemi-cals, Advanced Freezers, Frigoscandia, Normalair-Garrett, Food Refrigeration and ProcessEngineering Research Centre, Velis Refrigeration, Pennsylvania State University and AmesLaboratory, whose contribution is gratefully acknowledged.

ACRONYMS

CFC ChlorofluorocarbonCOP Coefficient of performanceFAO Food and Agriculture Organisation of the United NationsHCFC HydrochlorofluorocarbonHFC HydrofluorocarbonHFM Hydrofluidisation methodIIR International Institute of RefrigerationIQF Individual quick freezingMCE Magnetocaloric effectMRF Magnetic resonance freezingSME Small or medium enterpriseWHO World Health Organisation

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