High-Pressure Freezing

C H A P T E R

28 High-Pressure Freezing Pedro D. Sanz, Laura Otero Department of Processes, Institute of Food Science, Technology and Nutrition (ICTAN, CSIC), Madrid, Spain

28.1 INTRODUCTION Freezing is an excellent food preserving method that delays or prevents microbial, chemical, and physical alterations by reducing water activity (solidification to ice), microbial growth, and reaction velocities in enzymatic systems (temperature reduction imposes suboptimal conditions). To be suitable, a freezing method must preserve all the organoleptic characteristics of the fresh product (flavor, color, aroma, etc.) and also the nutritional value. However, the ice crystals formed in the process can severely damage the tissues, affect texture, and cause major drip loss during thawing. Moreover, these crystals are responsible for cellular disruptions that may accelerate food spoilage by making, for example, the contact between enzyme and substrate easier (Alizadeh et al., 2007a; Leygonie et al., 2012; Olivera and Salvadori, 2009). Ice forms in two stages: formation of ice nuclei, followed by growth of these nuclei. The distribution of ice crystal size throughout a sample depends on both the number of nuclei (or seeds) formed in the earlier phase and the rate of crystal growth, which influences the shape and size of the final crystals. In the first stage of freezing, the temperature of the product must fall well below the solideliquid equilibrium point to produce nucleation. This is an activated process driven by supercooling (the temperature decrease under the freezing point before nucleation). The greater the degree of supercooling, the larger is the number of nuclei created; thus, the rate of ice nucleation increases roughly tenfold for every degree of supercooling (Burke et al., 1975). Crystal growth takes place only once nucleation has occurred, through addition of water molecules to the nuclei already formed, and its rate mainly depends on the efficiency of latent heat removal (Kiani and Sun, 2011; Reid, 1983; Zaritzky, 2011). In slow freezing, where the sample temperature remains close to the solideliquid equilibrium curve

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for a long time, the rate of nucleation is low and only a few nuclei will be formed, leading to large ice crystals. On the other hand, in a system where the freezing rate is high, many nuclei will be produced, limiting the final crystal size. Large ice crystals formed during slow freezing generally cause extensive mechanical damage and reduce the maximum attainable food quality. Therefore, rapid freezing methods are, in general terms, preferred. However, when freezing large-sized foods, thermal gradients are established along the product, even when rapid freezing methods are used, and ice nucleation only occurs at the product surface, which is in close contact with the cooling medium. In the inner regions of the product, the supercooling required to produce ice nucleation is not achieved, and this results in the formation of large ice crystals that grow from the food surface to its center (Bevilacqua and Zaritzky, 1980; Woinet et al., 1998; Zaritzky, 2011). For decades, solutions focused on increasing the freezing rate, such as lowering the refrigerating medium temperature, enhancing surface heat transfer coefficients, or reducing the size of the products to be frozen, have been applied to avoid or limit this inconvenience (Fikiin, 2003; Reid, 2000). More recent approaches include a variety of strategies, such as the use of ultrasound, electric fields, or antifreeze proteins, to control ice crystallization (Kiani and Sun, 2011; Li and Sun, 2002; Petzold and Aguilera, 2009). Among them, high pressure is especially interesting mainly due to its potential for improving the kinetics of freezing and the characteristics of the ice crystals formed.

28.2 HIGH PRESSURE FOR FREEZING: PRINCIPLES AND EQUIPMENT Figure 28.1 shows the phase diagram of water. Ice I, the common ice at atmospheric pressure, is stable up to 210 MPa. Above this pressure, different ice

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80 Liquid water

40

Ice VII

Temperature (°C)

0

–40 Ice I

I IV

III

V

Ice VI

II –80

–120

Ice VIII

–160

Ice IX

0

600

1200

1800

2400

Pressure (MPa)

FIGURE 28.1

Phase diagram of water.

polymorphs, from ice II to ice XV, are thermodynamically stable depending on the pressure/temperature coordinates (Bridgman, 1912; Salzmann et al., 2006, 2009). The freezing point of water decreases with pressure up to 210 MPa. The opposite was observed above this level for ice types other than ice I (see Figure 28.1). According to the ClausiuseClapeyron equation, dT DV$Tk ¼ dP DH

(28.1)

where T is temperature and P is pressure, the negative slope of the liquid/ice I equilibrium line implies that the signs of the volume change DV and the latent heat DH in the above equation are different. In the case of liquidesolid phase transition, DH is negative, irrespective of the modification of the solid state. The variation of volume therefore must be positive for ice I. Liquid water is denser than ice I, which presents a hexagonal structure containing much empty space. This increase in volume upon freezing is mainly responsible for the damage incurred by biological systems when frozen. On the other hand, the volume increment in liquid-ice III, liquid-ice V, or liquid-ice VI phase changes is negative; also, the phase change enthalpy increment is negative and the slope of the melting curve is positive. We may therefore expect less damage to be caused by these ice polymorphs upon freezing. In principle, no solid (ordered) phase can exist in the liquid (disordered) domain, but the inverse is possible as a metastable state. This phenomenon, called

supercooling or undercooling, seems to be enhanced by pressure. The homogeneous nucleation temperature of water decreases even more than the melting temperature with pressure up to 210 MPa. The metastable range is thus significantly enlarged with the application of pressure. The minimum temperature at which it is possible to find supercooled water is therefore reduced from 40  C at atmospheric pressure to 92  C at about 210 MPa (Lu¨dermann, 1994). A deep look at the phase diagram of water shows that high pressure opens new opportunities for food freezing, thawing, and subzero-temperature nonfrozen storage (Deuchi and Hayashi, 1992; Hayashi, 1992; Kalichevsky et al., 1995; Philippon and Voldrich, 1993). In the 1990s and 2000s, many studies were carried out to establish the real potential of high pressure for food freezing (Cheftel et al., 2000; Kalichevsky et al., 1995; Le Bail et al., 2002; Otero and Sanz, 2011). However, no commercial applications have been yet developed, and all the experimental work described in this chapter has been carried out in lab and pilot-scale equipment. High-pressure freezing processes operate in pressureresistant vessels with thermally isolated thermostatic circuits to reach temperatures below 0  C. Packed foods are immersed in the pressure/cooling medium, pressurized, and frozen. The apparatus must be equipped with thermocouples to monitor the evolution of temperature during the process in both the product and the pressure medium, and also with, at least, a pressure gauge to measure pressure in the circuit. Different pressure/cooling media have been reported in the literature, either pure substances or mixtures: silicon oil (Luscher et al., 2005), propylene glycol (Teramoto and Fuchigami, 2000), glycol/water: 62/38, v/v (Kalichevsky-Dong et al., 2000), ethylene glycol/ water: 75/25, v/v (Otero and Sanz, 2000), ethanol/ water: 50/50, v/v (Chevalier et al., 2000c), ethanol/glycol: 20/ 80, v/v (Schlu¨ter et al., 1998), castor oil/ethanol: 15/85, v/v (Johnston, 2000) or propanediol/water: 55/45, v/v (Le´vy et al., 1999), among others. When choosing a pressure medium, it is necessary to take into account the freezing point of the fluid under pressure, its viscosity, and other thermophysical properties (heat capacity, thermal expansion coefficient, and specific volume) that influence pressure-induced temperature changes. Temperature control inside the pressure vessel is achieved by external cooling. The simplest mode uses a thermostatic bath, which is set at the desired temperature, and the pressure vessel is directly immersed in it (Fuchigami et al., 1997b; Schlu¨ter et al., 2004). However, this solution can only be used in lab equipment when the pressure vessel is small enough to fit in the bath. In other cases, the vessel must be fitted with a jacket in which a cooling medium is continuously circulated (Fuchigami et al., 1997a; Otero and Sanz, 2006).

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28.3 TYPES OF HIGH-PRESSURE FREEZING PROCESSES

28.3 TYPES OF HIGH-PRESSURE FREEZING PROCESSES According to the phase diagram of water, three different types of high-pressure freezing processes can be distinguished in terms of the way in which the phase transition occurs: 1. High-pressure-assisted freezing (HPAF) 2. High-pressure-shift freezing (HPSF) 3. High-pressure-induced freezing (HPIF) where pressure assisted means phase transition under constant pressure; pressure shift means phase transition due to a pressure release and pressure induced means phase transition initiated by a pressure increase and continued at constant pressure. This terminology was first suggested by Knorr et al. (1998), who presented different idealized freezing paths on the phase diagram of water. Before that, terms such as pressure-assisted freezing, pressure-supported freezing, or freezing under pressure were used in a general way to denote any kind of freezing process operated with help, support, or assistance from pressure.

28.3.1 High-Pressure-Assisted Freezing 28.3.1.1 Description of the Process HPAF is a freezing process in which phase transition occurs under constant pressure, above atmospheric pressure, while the temperature is lowered to below the corresponding freezing point. In this way, ice I or other ice polymorphs can be obtained (Figure 28.2). The process is identical to traditional freezing under atmospheric conditions, except that it takes place under pressure. According to Urrutia Benet et al. (2004), the term HPAF with no extra specifications must be used

FIGURE 28.2 Different high-pressure-assisted freezing processes producing different ice polymorphs: ice I (ABCDE) or ice III (A’B’C’D’E’). Bold lines represent the phase diagram of pure water. Figure taken from Otero and Sanz (2003).

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exclusively to refer to ice I. When a higher ice modification is frozen, this should be explicitly indicated. It is generally accepted that pressure strongly affects the degree of supercooling needed to initiate nucleation (Luscher et al., 2005; Schlu¨ter et al., 2004). However, it is important to note that the supercooling that takes place experimentally depends, not only on the pressure level, but also on many factors such as the product, its size, the container in which it is located, and the cooling rate. Even when all these factors are controlled, results can be nonrepetitive due to the stochastic nature of the nucleation phenomenon (Chen and Lee, 1998; Heneghan et al., 2002). For these reasons, Schlu¨ter et al. (2004) proposed defining a temperature range in which nucleation might occur in order to predict the required conditions for freezing foods under pressure. As an example, in potato tissue, Knorr et al. (1998) needed only negligible supercooling to obtain ice I when freezing at 70 MPa/17  C. However, greater supercooling was found at 100 MPa/19  C (about 5  C) and 140 MPa/23  C (about 7  C). In later experiments, this same group observed negligible supercooling in freezing experiments at 140 MPa and 209 MPa, maintaining the freezing medium at 25  C lower than the corresponding freezing point (Schlu¨ter et al., 2004). For other kinds of ice, greater supercooling seems to be required to initiate nucleation. Knorr’s group concluded that the formation of ice III in potato tissue was only possible after minimum supercooling of 15  C (Luscher et al., 2005; Schlu¨ter and Knorr, 2002). In general terms, the higher the pressure, the greater the degree of supercooling that is needed (Schlu¨ter et al., 2004). Fuchigami et al. (2002) needed negligible supercooling to freeze tofu at 0.1 and 100 MPa (pressure medium at 20  C), but about 9  C of supercooling was needed to obtain ice under 686 MPa. Molina-Garcı´a et al. (2004) also needed high degrees of supercooling to obtain ice VI in water and meat samples at 700 MPa (13  3  C and 10  3  C, respectively). This degree of supercooling was approximately twice that needed for ice I freezing at atmospheric pressure in identical experimental conditions. Therefore, supercooling is essential when designing freezing experiments under pressure. Also, experimental temperatureepressure values need to be precisely monitored to ensure that freezing has actually occurred. Pressure increases or decreases (related to volume changes) in addition to sudden temperature changes have been effectively used as indicators of phase transition events (Knorr et al., 1998; Molina-Garcı´a et al., 2004). Without these measurements, there is no way to be certain that a phase change has really occurred. In this connection, earlier experiments by Fuchigami’s group in carrots (Fuchigami et al., 1996, 1997a,b), tofu (Fuchigami and Teramoto, 1996, 1997; Fuchigami et al.,

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1998b), and agar gel (Fuchigami and Teramoto, 1998) present some uncertainty. In HPAF, cooling of the sample proceeds from surface to center, and the process is governed by thermal gradients as in classical freezing processes at atmospheric pressure. Thus, ice nucleation only occurs in the outer zone of the product that is in close contact with the cooling medium. Therefore, the resulting ice crystals are large, needle-shaped, radially oriented, and present a marked size gradient from sample surface to center (Fuchigami and Teramoto, 1997; Le´vy et al., 1999). Once the freezing plateau is complete and the sample reaches the final temperature, the pressure is released. If ice I has formed (ABCDE in Figure 28.2), the sample temperature will drop after expansion. If another ice polymorph was produced (A’B’C’D’E’ in Figure 28.2), there will be a solidesolid phase transition to ice I upon pressure release to atmospheric conditions (Cheftel et al., 2000; Fuchigami et al., 1997a, 2002; Fuchigami and Teramoto, 2003; Luscher et al., 2005). The latent heat of water decreases with pressure up to 210 MPa (Hobbs, 1974). In this connection, Knorr et al. (1998) reported reductions in the freezing time with increasing pressure (0.1e210 MPa) as compared to atmospheric conditions. When comparing freezing times, it is essential that the thermal gradient between the freezing point of the sample and the cooling medium (DTrefrig) remains constant, since the freezing point also decreases with pressure up to 210 MPa (see Figure 28.1). If the cooling medium temperature is kept constant, DTrefrig will be smaller when pressure is increased (freezing point decreases), and the freezing times will increase (Barry et al., 1998; Ferna´ndez et al., 2006a; Le´vy et al., 1999). 28.3.1.2 Quality of Pressure-Assisted Frozen Foods Observations of the structure of different highpressure-assisted frozen products (ice I or other ice polymorphs) show that the crystal size always grows from the sample surface to the center as a result of the thermal gradients established during the phase transition (Ferna´ndez et al., 2007a; Fuchigami and Teramoto, 1997; Le´vy et al., 1999). Less damage to sample structure is expected from ice polymorphs other than ice I, mainly because of their corresponding decrease in volume during the phase transition. However, these types of ice will revert to ice I during pressure release at below 0  C, thus canceling out the expected advantages. In this connection, Fuchigami and Teramoto (2003) found no significant improvements in texture and structure of high-pressure-assisted frozen gellan gum gels to ice V or ice VI (600e686 MPa and 20  C) with respect to those frozen at atmospheric pressure or 100 MPa to ice

I. Similar results were found when freezing agar gels. Visual appearance, exudate, and texture of highpressure-assisted frozen gels to different ice polymorphs did not differ after thawing (Fuchigami and Teramoto, 1998; Fuchigami et al., 2006). Various authors have designed freezing/thawing experiments under constant pressure to avoid passing through the ice I phase in the expansion (Fuchigami and Teramoto, 1998; Fuchigami et al., 1998b; Luscher et al., 2005; Molina-Garcı´a et al., 2004). High-pressureassisted frozen samples can be stored at constant pressure and then thawed under the same constant pressure (by raising the temperature) when desired. Experiments of this kind make it possible to study the real effect of a certain ice polymorph while excluding the added effect of the solidesolid phase transition to ice I. For example, Molina-Garcı´a et al. (2004) froze meat samples to ice VI at 700 MPa/0  C and fixed them under pressure (with Carnoy fixing agent). After 14 h in the frozen state, the temperature was raised; then the sample was thawed at 700 MPa and the pressure was released to atmospheric conditions. Ice VI freezesubstitution microscopy showed no traces of ice on muscle fibers compared to the extensive damage caused by ice I freezing (Figure 28.3). Luscher et al. (2005) have proved that freezing to ice III (at 320 MPa) produced less damage to potato tissue than freezing to ice I (at 0.1 or 200 MPa) or to ice V (at 400 MPa). These authors maintained the same DTrefrig in all cases, taking into account pressure-induced depression of the freezing point. Post-freezing damage in cellular membranes seems to be influenced by two factors: the duration of the phase transition and the volume change of water during it. Luscher et al. (2005) have shown that ice I is the most harmful ice for cellular membranes of potato tissue. There was more damage at 200 MPa (volume increase of 0.13 cm3/g) than at atmospheric pressure (volume increase of 0.09 cm3/g). Ice V at 400 MPa (volume increase of 0.07 cm3/g) caused less damage, and ice III at 320 MPa (volume increase of 0.03 cm3/g) was found to be the least destructive. Thus, a phase change with a volume decrease is less destructive for cellular membranes than a phase change with a volume increase, but a larger negative change in volume is more destructive than a smaller negative change. The same authors found that after freezing to ice III/thawing under 320 MPa, the potato texture was quite close to that of the untreated control, and there was slightly less browning compared to conventional freezing and thawing. 28.3.1.3 Use of Additives There are only a few studies on the effect of additives on the kinetics of HPAF and the resulting ice crystals. Several solutes or macromolecules are known to affect ice nucleation and crystal growth at atmospheric

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FIGURE 28.3 Freeze-substitution micrographs of pork meat (loin). (a) Fresh. (b) Frozen to ice VI (at 700 MPa and 0  C). (c) Frozen to ice I (at 0.1 MPa and 18  C). (d) Treated at 700 MPa and 25  C. The bar indicates 100 mm. Figure taken from Molina-Garcı´a et al. (2004).

pressure, especially sugars and hydrocolloids. Sugars inhibit crystal growth; they also modify freezing and glass transition temperatures and the amount of frozen water at a given temperature. Fuchigami et al. (2002) studied the effect of 0%, 2.5%, or 5% trehalose in pressureassisted frozen tofu at 100 and 686 MPa/20  C. The addition of trehalose was effective in decreasing the size of the ice crystals, in preventing tofu from becoming too firm, and in improving the mouthfeel (texture) of pressure-frozen tofu. These authors also found that the addition of sucrose (0%, 5%, 10%, and 20%) was also effective in improving the quality of pressure-assisted frozen agar (Fuchigami and Teramoto, 1998; Fuchigami et al., 2006) and gellan gum gels (Fuchigami and Teramoto, 2003). Hydrocolloids such as gelatin, carrageenan, locust bean gum, or alginate are commonly used to control ice recrystallization during frozen storage in ice creams and other desserts. These stabilizers have little or no effect either on heterogeneous nucleation or on freezing temperatures, and they do not change the amount of freezable water or the glass transition temperature.

Ferna´ndez et al. (2007a) studied the effect of some hydrocolloids on the ice crystal size of sucrose solutions frozen under pressure. No beneficial effects were observed after addition of either 0.3% (w/w) guar gum or a 0.3% (w/w) mixture of locust bean and xanthan gum (1:1) to a 16% (w/w) sucrose solution. The degree of supercooling needed to initiate nucleation during freezing at 100 MPa/22  C significantly increased in samples containing hydrocolloids, and, consequently, phase transition times were reduced. But this did not produce the expected effect on the ice crystals. The addition of hydrocolloids induced the enlargement of the ice crystal size distribution toward higher size classes, and guar gum addition substantially increased the mean equivalent diameter of the ice crystals.

28.3.2 High-Pressure-Shift Freezing 28.3.2.1 Description of the Process HPSF denotes a freezing process in which phase transition is initiated after a pressure release (Knorr et al.,

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FIGURE 28.4 High-pressure-shift freezing processes. ABCDE: rapid expansion; ABC12E: slow expansion. Bold lines represent the phase diagram of pure water. PN: nucleation pressure. DTsup: extent of supercooling reached before nucleation. Figure adapted from Otero and Sanz (2003).

1998). Figure 28.4 shows two different HPSF processes (paths ABCDE and ABC12E, respectively). The only difference between them is the rate of pressure release: rapid in process ABCDE and slow in process ABC12E. The sample to be frozen is introduced into the highpressure vessel and compressed up to the desired pressure level (path AB in Figure 28.4). Then, it is cooled under pressure below 0  C (path BC in Figure 28.4) but kept in unfrozen state according to the phase diagram of water superimposed in Figure 28.4. Once the desired temperature is reached in the whole product, pressure is released either slowly over several minutes (Fuchigami et al., 1997b; Le´vy et al., 1999) or quickly in a matter of seconds (Kanda et al., 1992; Le´vy et al., 1999; Otero et al., 1998), inducing uniform supercooling throughout the sample due to the isostatic nature of pressure (paths C12E or CD in Figure 28.4, respectively). Supercooling and nucleation can occur under atmospheric conditions after the pressure release (point D in Figure 28.4), but also during the expansion, under pressure (point 1 in Figure 28.4), due to the stochastic nature of the nucleation process (Le´vy et al., 1999; Otero and Sanz, 2006; Thiebaud et al., 2002). In general terms, the faster the pressure release, the lower the nucleation pressure (PN) is and the greater the extent of supercooling (DTsup) that is caused (Thiebaud et al., 2002). Moreover, PN generally increases with higher initial pressures and lower temperatures (Otero and Sanz, 2006). These statements must be only considered as general trends since, as previously mentioned, there are many factors involved in the tendency of a system to supercool, and nucleation is a nonrepetitive phenomenon (Heneghan et al., 2002). Supercooling, under pressure or in atmospheric conditions, produces quasi-instantaneous and uniform nucleation throughout the sample (whatever its form and size), which raises sample temperature to the

corresponding freezing point due to the release of latent heat. A percentage of water is instantaneously frozen. The higher the pressure and the lower the temperature before expansion, the larger is the extent of supercooling reached, the more ice is formed, and hence the shorter the plateau time is for a given cooling temperature (Chevalier et al., 2000a; Otero and Sanz, 2000, 2006; Zhu et al., 2005b). Freezing is then completed at atmospheric pressure. It is generally accepted that no further nucleation occurs at this stage. Many efforts have been made in the literature to evaluate the amount of ice instantaneously frozen after expansion. Direct measurements are rather difficult to carry out inside pressure vessels, and, therefore, most authors have employed indirect methods like calorimetric techniques (Chevalier et al., 2001; Zhu et al., 2005a, 2006b). By using high-pressure calorimeters, it is possible to measure the latent heat released after expansion and relate it to the amount of ice formed (Zhu et al., 2005a, 2006b). Otero et al. (2009) proposed an easier and cheaper procedure that involves measuring the thermal evolution of the sample after expansion and simple mass balances. All the studies conclude that the amount of ice formed after expansion, from given pressure/temperature conditions, depends on the water content of the sample, but the percentage ice, which is a ratio based on the total mass of water present in the sample, is the same for any product (Otero et al., 2009; Zhu et al., 2005a, 2006b). According to all the measurements made, the maximum percentage of ice that can be quasi-instantaneously obtained by HPSF is close to 30%, after expansion from 210 MPa and 22  C (Otero et al., 2009; Zhu et al., 2005a, 2006b). These conditions correspond to the triple point of liquid/ice I/ice III in the phase diagram of water, and they are usually assumed to be the optimal conditions for HPSF, which maximize ice crystallization after expansion. Nevertheless, the existence of liquid metastable phases in the phase diagram of water and waterbased food products, above the prolongation of the ice I and ice V melting curves, has been experimentally proved (Evans, 1967; Schlu¨ter and Knorr, 2002; Schlu¨ter et al., 2004), and this opens a new range of possibilities in HPSF. In this connection, Schlu¨ter et al. (2004) has suggested the possibility of carrying out expansions from metastable zones to increase the amount of frozen water after the pressure release, taking advantage of the considerable supercooling needed to initiate nucleation under pressure. They proposed the theoretical triple point liquid/ice I/ice V as an optimized onset point for HPSF, which allows a considerable temperature lowering. In potato, for example, it means expansions from 240 MPa and 28  C, which would increase supercooling by about 30% (Schlu¨ter and Knorr, 2002; Schlu¨ter et al., 2004) and reduce the phase transition times (Urrutia Benet et al., 2007).

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28.3 TYPES OF HIGH-PRESSURE FREEZING PROCESSES

Total freezing times in HPSF experiments are longer than in conventional freezing at atmospheric pressure, since the cooling step under pressure, before nucleation (path BC in Figure 28.4), must be taken into account even though this is not actual freezing (Le´vy et al., 1999; Otero and Sanz, 2006). On the other hand, the phase transition time is much shorter due to the percentage of water instantaneously frozen after expansion. Various researchers have reported major reductions of the freezing plateau with respect to conventional freezing in HPSF experiments: 28% in b-lactoglobulin gels from 207 MPa/20  C (Barry et al., 1998); 20% in oil-inwater emulsions from 207 MPa/18  C (Le´vy et al., 1999); and 14.3% and 15.8% in meat samples from 100 MPa/11  C and 200 MPa/12  C, respectively (Massaux et al., 1999). These reductions must be attributed not only to the instantaneous freezing of water after expansion, but also to the temperature drop in the pressure medium after pressure release. Pressure media that undergo large temperature changes with pressure may therefore be useful in HPSF processes, since the corresponding temperature drop after expansion aids the removal of latent heat from the sample (KalichevskyDong et al., 2000; Otero and Sanz, 2006). After expansion, freezing is completed at atmospheric pressure, usually inside the pressure vessel where it is difficult to remove the latent heat of crystallization efficiently. Therefore, important thermal gradients can be established during the crystal growth step that can induce slow freezing rates, especially at the center of large samples. Phase transition times can be considerably reduced by removing the sample from the pressure vessel, just after expansion, and completing the freezing process using a more efficient heat removal method (Chevalier et al., 2000c; Fuchigami et al., 1997b). Moreover, this methodology also allows a full exploitation of the pressure processing facilities, because the product only remains in the vessel when the pressure is being applied. 28.3.2.2 Quality of High-Pressure-Shift Frozen Foods The main advantage of HPSF lies in the large extent of supercooling reached after expansion, which promotes the instantaneous crystallization of a relatively large percentage of water throughout the product. Observations of ice crystal imprints, either by cryoscanning electron microscopy or by light microscopy, show small ice crystals of granular shape, without specific orientation, dispersed throughout the sample, demonstrating that ice nucleation occurs throughout the product and not only on the surface during expansion (Kanda et al., 1992; Le´vy et al., 1999; Martino et al., 1998). As a result, there is none of the damage to tissues and cell structures that is typically caused by

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large crystals in conventional freezing under atmospheric conditions, there is less drip loss, and the resulting textures are generally better (Martino et al., 1998; Otero et al., 1998; Sequeira-Munoz et al., 2005). As noted in the previous section, the depressurization rate can influence the extent of supercooling, and, consequently, it should affect the ice nucleation rate and the ice crystals formed. Microstructural studies in oil-in-water emulsions frozen by either slow or fast pressure release confirm this hypothesis (Le´vy et al., 1999). Fast expansions produced smaller ice clusters than those formed after slow expansions; therefore, quick expansions seem to be better from the standpoint of quality. After expansion, however, freezing is completed at atmospheric pressure, and, as mentioned in the previous section, thermal gradients are established during the crystal growth step. In this phase, the removal of latent heat is slower, and there is a risk of recrystallization in the central parts of the sample. This phase has, therefore, been identified as the crucial step of texture loss and tissue damage during HPSF (Buggenhout et al., 2006a). Using image analysis, Thiebaud et al. (2002) confirmed that each ice crystal cluster present in a pressure shift frozen oil-in-water emulsion derived from the aggregation of smaller ice crystals, probably produced during ice crystal growth at atmospheric pressure. Micrographs of different products show that ice crystal diameters slightly grow from the surface to the center of the sample (Fuchigami and Teramoto, 1996, 1997; Le´vy et al., 1999; Martino et al., 1998; Thiebaud et al., 2002). Therefore, latent heat must be removed quickly if the initial advantages of the nucleation step are to be maintained. This can be achieved, as noted above, by removing the sample from the pressure vessel, just after expansion, and completing the freezing process using a more efficient heat removal method. HPSF has been applied both to model systems and to real foods. Comparisons between the effects of HPSF and conventional freezing in different food models (oil-in-water emulsions and b-lactoglobulin, agar, starch, gelatin, ovalbumin, or gellan gum gels) confirm that HPSF produces smaller and more uniform ice crystals, with no specific orientation (Barry et al., 1998; Chevalier et al., 2000a; Ferna´ndez et al., 2006a; Fuchigami and Teramoto, 1998, 2003; Fuchigami et al., 2006; Kalichevsky-Dong et al., 2000; Le´vy et al., 1999; Le´vy et al., 2000; Lille and Autio, 2005, 2007; Thiebaud et al., 2002; Zhu et al., 2005b). Ice crystal size decreases when the pressure and the degree of supercooling increases (Chevalier et al., 2000a; Lille and Autio, 2005; Zhu et al., 2005b). In frozen gels, this results in more homogeneous and robust structures with small pores that induce less damage to their general appearance and less syneresis (Barry et al., 1998; Fuchigami and Teramoto, 1998, 2003; Fuchigami et al., 2006;

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Kalichevsky-Dong et al., 2000). Regarding texture, gels can be categorized into two general types: (1) gels that have reduced gel strength as a result of the mechanical damage caused by ice crystal formation, for example, agar, gelatin, and gellan gum gels (Fuchigami and Teramoto, 1998, 2003; Fuchigami et al., 2006; KalichevskyDong et al., 2000); and (2) gels that have an enhanced gel strength as a result of the molecular structural changes that take place in the frozen stage, for example, gels of b-lactoglobulin protein isolate and ovalbumin (Barry et al., 1998; Kalichevsky-Dong et al., 2000). In either case, the texture is better than conventionally frozen gels (Fuchigami and Teramoto, 1998; Fuchigami et al., 2006). HPSF has also been applied to different food gels, such as tofu (Fuchigami et al., 2002; Fuchigami and Teramoto, 1996, 1997; Fuchigami et al., 1998b; Kanda et al., 1992), konnyakudthat is, konjac glucomannan gel (Teramoto and Fuchigami, 2000)dand cheddar and mozzarella cheeses (Johnston, 2000) with different results. Highpressure-shift frozen tofu maintains its structure after thawing better than the conventionally frozen product, and drip losses are also considerably reduced (Fuchigami et al., 2002; Fuchigami and Teramoto, 1996, 1997; Kanda et al., 1992). Moreover, the appearance, mouthfeel, texture, and sensory properties are also better preserved (Fuchigami et al., 2002; Fuchigami and Teramoto, 1996, 1997; Kanda et al., 1992). On the other hand, HPSF is not effective in improving the texture and structure of frozen konnyaku. As occurs in conventional freezing, high-pressure-shift frozen konnyaku loses the minute structure of the gel network and its elastic properties, and the product becomes hard (Teramoto and Fuchigami, 2000). Rheology of cheddar and mozzarella cheeses is also negatively affected after HPSF (Johnston, 2000). Examination of high-pressure-shift frozen plant tissues like potatoes (Koch et al., 1996; Luscher et al., 2005; Urrutia Benet et al., 2006, 2007), carrots (Fuchigami et al., 1997a,b), broccoli (Ferna´ndez et al., 2006b), whole eggplants (Otero et al., 1998), or whole peaches and mangoes (Otero et al., 2000a) indicated that the microstructure of the tissues was better after pressure-shift freezing than after conventional freezing in atmospheric conditions. The shorter phase transition time in HPSF processes, combined with the formation of smaller ice crystals, caused less damage to the membranes (related to the turgor pressure) and the framework of the cell walls than that during conventional freezing, although ice I was formed (Luscher et al., 2005). Moreover, in large-sized samples, no substantial differences in the microstructure at the center and the surface of the frozen product are observed, probably due to the instantaneous and uniform ice crystallization that occurs in the samples after expansion (Otero et al., 1998, 2000a). All these benefits lead to minimized textural damage and drip

losses in products like potatoes (Carbonell et al., 2006; Koch et al., 1996; Luscher et al., 2005; Urrutia Benet et al., 2007) or eggplants (Otero et al., 1998), in which texture mainly depends on the integrity of the cell walls. However, in those products in which turgor pressure determines the texture, HPSF results are not so good, even though it produces less histological damage than conventional freezing. As an example, Fuchigami et al. (1998a) observed that HPSF from 200 to 400 MPa/20  C produced less damage in Chinese cabbage than pressure-assisted freezing. However, there was a pronounced increase of softness in all pressurefrozen samples, which became flexible and the crispness of the raw product was lost. Buggenhout et al. (2006b) found similar results in strawberries. Cryo-SEM micrographs, made in high-pressure-shift frozen broccoli samples, show the rupture of vacuoles, which should be strongly related to the loss of turgor pressure observed (Ferna´ndez et al., 2006b). Moreover, in products of high porosity or high air content, such as strawberries, air bubbles can also cause serious tissue damage during pressurization (Buggenhout et al., 2008). As for color, fruits and vegetables are not directly affected by HPSF. However, during thawing, important color changes can occur due to enzymatic activity if the samples were not previously blanched (Koch et al., 1996; Luscher et al., 2005; Urrutia Benet et al., 2006, 2007). Obviously, these changes are considerably less pronounced than those produced in nonblanched, conventionally frozen products. Thus, Luscher et al. (2005) detected considerably less browning in high-pressureshift frozen (250 MPa and 27  C) potato samples than in samples frozen to ice I and ice III by HPAF or in conventionally frozen samples. Conventional freezing in animal tissues mainly produces deterioration in texture, color, and flavor, which can be attributed to protein denaturation. One of the factors in this protein denaturation is the size of the ice crystals formed. Large ice crystals produce cellular disruption that may induce interaction among enzymes, lipids, and proteins, and lead to protein denaturation and lipid degradation. Moreover, drip losses are important, inducing losses of moisture that affect the quality of the frozen product. Pressure-shift freezing in animal tissues like pork muscle (Hansen et al., 2003; Martino et al., 1998; Otero et al., 1997a; Zhu et al., 2004b), Norway lobster (Chevalier et al., 2000b), turbot fillets (Chevalier et al., 2000c), Atlantic salmon (Alizadeh et al., 2007a,b, 2009; Zhu et al., 2003), or sea bass (Tironi et al., 2007) produced small, rounded ice crystals evenly distributed throughout the product. The higher the pressure employed in the HPSF process, the smaller and rounder are the ice crystals formed (Alizadeh et al., 2007a; Zhu et al., 2003, 2004b). These small ice crystals produce less drip losses after thawing than those observed in

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conventionally frozen products (Alizadeh et al., 2007b; Chevalier et al., 2000c; Hansen et al., 2003; Massaux et al., 1999; Zhu et al., 2004a). However, several studies show that drip losses after centrifugation are higher in pressure-shift-frozen than in raw and conventionally frozen fish and meat products (Ferna´ndez-Martı´n et al., 2000; Massaux et al., 1999; Tironi et al., 2007). Moreover, an increase in toughness has been described in different high-pressure-shift frozen products such as beef (Ferna´ndez-Martı´n et al., 2000), pork (Zhu et al., 2004a), Norway lobsters (Chevalier et al., 2000b), turbot (Chevalier et al., 2000c), and Atlantic salmon fillets (Alizadeh et al., 2007b) when compared to conventionally frozen samples. All these changes, which increase with increasing pressure (Zhu et al., 2004b), can be related with the important protein denaturation observed in different products. Thus, all the studies agree that pressure levels usually employed in HPSF induce denaturation in the myofibrillar and sarcoplasmic proteins of animal tissues, while collagen remains practically unaltered (Chevalier et al., 2000bed; Ferna´ndez-Martı´n et al., 2000; Hansen et al., 2003; Tironi et al., 2007; Zhu et al., 2004a). Transmission electron microscopy images from highpressure-shift frozen beef and pork muscles reveal major ultrastructural damage (Ferna´ndez-Martı´n et al., 2000). Thus, substantial destruction of sarcomeres (broken A-bands with extinction of M-line and H-zone, and loss of defined filamentous structuredmyosin dissociation; broken I-bands and thickening of Z-line by collapse of the I-band and deposits of dense material) and marked fragmentation of myofibrils are clearly observed. It has been widely proved that this microstructural damage is induced by pressure, and, therefore, all the advantages obtained from the small and round ice crystals formed after HPSF are lost. Regarding color, pressure-shift freezing produces whitening of thawed meat and fish products (Alizadeh et al., 2007b; Chevalier et al., 2000d; Ferna´ndez-Martı´n et al., 2000; Hansen et al., 2003; Massaux et al., 1999; Tironi et al., 2007; Zhu et al., 2004a), lending them the appearance of cooked muscle. Thus, a marked increase in lightness is observed, while redness is usually reported to decrease. Goutefongea et al. (1995) attributed this shift toward lighter tones to pressure-induced coagulation of sarcoplasmic and/or myofibrillar proteins. However, it is important to note that these color changes are only really apparent after thawing. Thus, Chevalier et al. (2000c) reported that the visual appearance of turbot fillets frozen either by air blast or by pressure release were similar in the frozen and the cooked state. It is clear from the above that, when freezing animal tissues by pressure release, pressure can induce protein denaturation, even at 200 MPa, and, therefore, protein modifications may cancel out the benefit of the

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formation of smaller ice crystals during pressure-shift freezing. 28.3.2.3 Use of Additives Some studies have looked at the effect of additives in high-pressure-shift frozen food models, food gels, and vegetables. In general terms, the addition of sugars, such as sucrose, fructose, or thehalose, induces a decrease in the size of the ice crystals formed and produces more spherical shapes (Fuchigami et al., 2002; Fuchigami and Teramoto, 1998, 2003; Fuchigami et al., 2006; Thiebaud et al., 2002). Moreover, it appears to also be effective in preventing syneresis and improving the texture (Fuchigami and Teramoto, 2003; Fuchigami et al., 2006). Thus, amounts of 2.5e5% trehalose in high-pressure-shift frozen tofu have been found to improve the visual appearance and mouthfeel of the product, but some sweetness can be detected (Fuchigami et al., 2002). On the other hand, different hydrocolloids, such as sodium alginate, guar gum, locust bean gum, and xanthan gum have been reported to present different behavior when added to food models. Thus, locust bean gum plus xanthan gum are effective in reducing the ice crystal size in sucrose solutions (Ferna´ndez et al., 2007a), while sodium alginate or guar gum fails in sucrose solutions and oil-in-water emulsions (Ferna´ndez et al., 2007a; Thiebaud et al., 2002). As previously commented, some vegetable foods can undergo serious tissue damage during pressurization and/or freezing. Therefore, different pretreatments (thermal or high-pressure pretreatments combined or not combined with calcium soaking, vacuum infusion with pectin methylesterase (PME) and calcium, among others) have been investigated for their effect on the structural integrity of plant cell walls (Buggenhout et al., 2005, 2006a,b, 2008; Castro et al., 2007). As an example, vacuum infusion with PME/Ca has been found to be effective in minimizing structural damage, preserving firmness, and reducing drip losses in highpressure-shift frozen strawberries (Buggenhout et al., 2006b, 2008). The effectiveness of these treatments, not so clear when used with conventional freezing methods, seems to be caused by the increased cross-links in the middle lamella between adjacent cells and the uniform nucleation produced in HPSF (Buggenhout et al., 2005, 2006b; Castro et al., 2007). 28.3.2.4 Effects of Frozen Storage It is well known that frozen foods undergo quality losses during storage. These losses are mainly due to cellular damage produced during the freezing process and on the conditions in which the frozen food is stored. Temperature fluctuations increase the size of the ice crystals and reduce their total number, but, even under

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isothermal conditions, recrystallization phenomena can take place during storage. Unfortunately, the literature reports very little information about the behavior of ice crystals in high-pressure-shift frozen products during storage. The existing studies suggest similar recrystallization kinetics to conventional freezing (Ferna´ndez et al., 2008). No significant changes in the ice crystal size of fish fillets frozen by pressure release have been found after storage for relative long periods (Alizadeh et al., 2007a; Chevalier et al., 2000c). Moreover, after appropriate frozen storage, different quality parameters, such as drip losses after thawing, texture, or color, have been found to remain unaffected in meat and fish products (Chevalier et al., 2000c,d; Hansen et al., 2003; Sequeira-Munoz et al., 2005). On the other hand, the effects on the lipid fraction seem to depend on the product and the pressure level employed in the freezing process. Lipid oxidation during frozen storage was found to be inhibited in carp fillets (Sequeira-Munoz et al., 2005) while it was enhanced in turbot fillets (Chevalier et al., 2000d), both frozen by pressure release from 140 MPa/14  C. Moreover, in contrast to conventional freezing, lipid hydrolysis was either retarded or inhibited, probably due to the denaturation of phospholipases under pressure (Sequeira-Munoz et al., 2005). Results with vegetable foods differ according to the product and the pretreatments applied. Frozen storage has been found to increase drip losses and induce a considerable softening in high-pressure-shift frozen potato cubes (Koch et al., 1996) and carrots (Buggenhout et al., 2005), but no detrimental effects were described in thermal/pressure-pretreated peppers (Castro et al., 2007) and PME/Ca-infused strawberries (Buggenhout et al., 2006b). All the results described above seem to show that the quality of high-pressure-shift frozen products can be well preserved during storage, but further studies are needed for more conclusive results.

28.3.3 High-Pressure-Induced Freezing HPIF refers to a freezing process in which the phase transition is initiated by a pressure increase and then completed under pressure. Figure 28.5 shows a schematic view of this process. In a first step, a sample is compressed to an intermediate pressure level and cooled under pressure. At this point, the sample remains in an unfrozen state. Then, pressure is increased up to the definitive level and a phase transition is induced. This process was first described by Urrutia Benet et al. (2004), but after their experiments with potatoes, no further references have been found in the literature. HPIF is interesting from a theoretical point of view, but, in practice, it suffers the same problems as

FIGURE 28.5 High-pressure-induced freezing process. Bold lines represent the phase diagram of pure water.

pressure-assisted freezing to ice polymorphs higher than ice I. As Figure 28.5 shows, ice I cannot be obtained since the melting curve liquid/ice I decreases with pressure. Therefore, after HPIF, only higher ice modifications are produced. Then, the frozen products should be stored and thawed under pressure to benefit from higher ice densities. However, all the advantages achieved will be lost in the expansion that occurs when solidesolid phase transitions to ice I take place.

28.4 MICROBIAL AND ENZYMATIC INACTIVATION AFTER HIGH-PRESSURE FREEZING It is widely accepted that conventional freezing and subsequent frozen storage of foods do not induce significant inactivation of most detrimental microorganisms. On the other hand, high-pressure processing has been proved to be an effective mode of microbial inactivation if appropriate process conditions are chosen (Farkas and Hoover, 2000; Patterson, 2005; Smelt, 1998). Some references in the literature indicate that pressurization at subzero temperatures, without phase transition, may be more effective for microbial inactivation than pressure processing at room temperature (Hashizume et al., 1995; Moussa et al., 2006; Noma et al., 2002; Perrier-Cornet et al., 2005; Picart et al., 2004, 2005; Reyns et al., 2000; Ritz et al., 2008; Takahashi, 1992).There are also some articles in the literature about the microbial inactivation achieved by pressure treatments at subzero temperatures in products previously frozen at atmospheric conditions (Ferna´ndez et al., 2007b; Luscher et al., 2004; Picart et al., 2005; Shen et al., 2005, 2009; Vaudagna et al., 2012). However,

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studies on the effects of high-pressure freezing on microbial inactivation of food are still scarce. The results obtained show that both high-pressure-assisted (Hayakawa et al., 1998; Park et al., 2008) and HPSF (Ballestra et al., 2010; Choi et al., 2008b; Park et al., 2008; Picart et al., 2004, 2005; Pre´stamo et al., 2007) are able to reduce microbial counts. The effectiveness of the treatment depends on the characteristics of the product (composition, pH, water activity, among others), on the process conditions (pressure level applied, holding time, temperature), and on the type of microorganism. In general terms, the higher the pressure level and the longer the holding time applied, the higher the microbial inactivation achieved (Choi et al., 2008b; Park et al., 2008; Pre´stamo et al., 2007). Moreover, in HPSF, the rate at which expansion is made is also a relevant parameter (Picart et al., 2004, 2005). Thus, slow expansions, made in 18 min, have been found to be more efficient in reducing Listeria innocua, Micrococcus luteus, and Pseudomonas fluorescens counts than quick expansions in highpressure-shift frozen minced salmon at 207 MPa/ 21  C. This could be due not only to the increased time under pressure but also to the formation of larger ice crystals, since, as previously commented, slow pressure release induces less undercooling (Picart et al., 2004, 2005). The response to a same high-pressure freezing process can be greatly different with different microorganisms and different medium characteristics (Ballestra et al., 2010; Choi et al., 2008b; Hayakawa et al., 1998). As an example, reductions of 0.8, 3.5, and 3.8 log cycles have been reported for Staphylococcus aureus KCCM 11335, Escherichia coli ATCC 25922, and Listeria monocytogenes HTCC 19115, inoculated in milk, after HPSF at 200 MPa/18  C (holding time under pressure: 30 min) while these values increased up to 1.3, 6.1, and 4.0 log cycles, respectively, in different inoculated buffer solutions (Choi et al., 2008b). There are only a few studies in the literature on enzymatic inactivation after high-pressure freezing. Indrawati et al. (1998) studied inactivation in enzyme solutions after HPSF (200e400 MPa, 22/10  C) and assisted freezing (350 MPa/22  C and 400 MPa/22  C). They found that polyphenoloxidase (PPO) was not inactivated, and pectin methylesterase was only inactivated slightly. Alpha-amylases from Bacillus subtilis and peroxidase (POD) were slightly and reversibly inactivated, and lipoxygenase was irreversibly inactivated (60% inactivation at 200 MPa). In real foods, there are also some studies focused on vegetables since, as explained below, HPSF presents some relevant advantages in these products. HPSF has been found to be unable to completely inactivate PPO and POD in different substrates such as broccoli (Pre´stamo et al., 2004) or potato (Pre´stamo et al., 2005; Urrutia Benet et al., 2007). The reduction observed in the PPO activity after HPSF depends both on the

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product and on the pressure/temperature conditions employed. In general terms, the higher the pressure of the treatment, the lower is the remaining PPO activity (Pre´stamo et al., 2005; Urrutia Benet et al., 2007). Incomplete PPO inactivation after high-pressure freezing produces considerable browning in vegetables after thawing (Koch et al., 1996; Urrutia Benet et al., 2006, 2007); therefore, a blanching treatment previous to high-pressure freezing of fruits and vegetables is highly recommended.

28.5 MODELING HIGH-PRESSURE FREEZING PROCESSES Modeling food freezing processes, even at atmospheric pressure, is a difficult task (Delgado and Sun, 2001; Pham, 2006). Food products usually have complex shape, size and composition. Initial freezing temperatures are variable and may be lower than 0  C, depending on the sample composition. Water is not totally available for freezing and it solidifies within a range of freezing temperatures rather than at a freezing point, due to the freeze concentration of solutes (Franke, 2000). Moreover, in selecting thermal properties, identification of values for foods is difficult because these change as the freezing process progresses, as does the heat transfer coefficient, which has a significant influence on the freezing time (Hung, 1990). Changes in thermophysical properties are especially sharp near the initial freezing point, leading to a highly non-linear differential equation that is difficult to solve (Pham, 2006). All these difficulties considerably increase when the phase change takes place under pressure or by fast pressure release (Otero and Sanz, 2003). High-pressure freezing processes are driven by both pressure and temperature. The pressure-dependent physical properties of the substances involved and the convective motion of the pressure medium significantly influence the heat transfer (Hartmann et al., 2003). Moreover, there are temperature variations produced by pressure changes, especially in HPSF processes, where there is considerable supercooling after expansion. In this process, the sudden ice nucleation produced after pressure release complicates the modeling task even further.

28.5.1 Initial Freezing Point under Pressure The initial freezing point of food under pressure is one of the most important parameters when modeling high-pressure freezing processes, since it defines a discontinuity in all the thermal properties. However, there is little and disperse data about this in the literature, primarily because there is not much suitable equipment available to do the measurements, and these are

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difficult to do. Pressure-temperature phase diagrams have been reported for sodium chloride and sucrose aqueous solutions (Guignon et al., 2005), tuna fish and surimi (Takai et al., 1991), potato (Schlu¨ter and Knorr, 2002), tomato paste (Denys, 2000), different gelatin gels (Chevalier et al., 2000a; Guignon et al., 2008), and broccoli (Guignon et al., 2008). Additionally, some isolated data points can be found in assisted freezing or thawing processes for particular food and food models: b-lactoglobulin gel (Barry et al., 1998), oil in water emulsion (Le´vy et al., 1999), ground beef (Zhao et al., 1998), whiting (Chevalier et al., 1999), and salmon flesh (Schubring et al., 2003). Different methods have been applied to detect the initial freezing point of food under pressure, with time-temperature data recording under constant pressure being the simplest (Schlu¨ter and Knorr, 2002; Takai et al., 1991). In the last decade, high-pressure calorimetry has been employed, working at constant temperature and varying pressure, to explore phase transition phenomena in aqueous solutions (Chourot et al., 2000) and real food (Zhu et al., 2004c, 2006a). Moreover, Guignon et al. (2008) presented a procedure to determine the pressure-temperature phase diagrams of aqueous solutions, avoiding supercooling and cryoconcentration problems by using a solution-ice mixture as a starting point. Taking into account that experimental measurements are laborious and there exists almost an infinite number of food products with different composition, several mathematical models have been developed to predict phase diagrams of food: polynomial and Simon-like equations, the linear additive model, artificial neuronal networks, and models based on the modified Raoult’s law and on the Robinson and Stokes equation (Guignon et al., 2008). Among them, the linear additive model, which considers the melting curves of real food as parallel to those of pure water, is the most employed (Denys, 2000; Otero et al., 2006). Guignon et al. (2008) and Otero et al. (2010a) have made comprehensive compilations of all the models mentioned, and these can be consulted by the interested reader; therefore, no more information is given here.

28.5.2 Thermophysical Properties under Pressure Another difficulty found when modeling heat transfer in high-pressure freezing processes is the lack of appropriate thermophysical properties of the materials involved over a wide range of pressures and temperatures, that is, above and below the corresponding freezing points (Juliano et al., 2011; Otero and Sanz, 2003). The determination of these properties under pressure using conventional techniques is

hampered by practical problems. Therefore, it is necessary to adapt existing techniques and devices and to develop new instruments capable of working under high-pressure conditions (Barbosa-Ca´novas and Rodrı´guez, 2005; Denys and Hendrickx, 1999; Le Bail et al., 2001). In spite of the enormous efforts made in the last years, thermophysical data of pressure media, packages, and foodstuffs under pressure are, even nowadays, scarce, and, therefore, more research in this area is needed. Pressure media employed at subzero temperatures include, as commented above, pure substances like ethanol, silicon oil, and castor oil, or mixtures like propanediol/water or ethanol/water, among others. In the literature there is some thermophysical data at high pressure for some, but not all, of these fluids (Fo¨rst et al., 2000; Guignon et al., 2010; Kubota et al., 1987; Makita, 1984; Sun et al., 1988, 1991). The fluid most exhaustively studied is, without any doubt, liquid water. Water is of especial interest not only as a usual component of many pressure media, but also because all raw foods contain water. In 2009, the International Association for the Properties of Water and Steam (IAPWS) published a revised release on the IAPWS Formulation 1995 for the Thermodynamic Properties of Ordinary Water for General and Scientific Use (IAPWS). The National Institute of Standards (NIST) has incorporated this formulation in the computer program NIST Standard Reference Database 10: NIST/ ASME Steam Properties, a pay software implementation. A limited set of properties based on this formulation is freely available from NIST on the Web at the NIST WebBook (Lemmon et al., 2001). Wagner (2000) also offers the software FLUIDCAL, which enables the user to calculate more than 25 different thermodynamic properties of more than 70 substances, including water. The calculation of water properties is based on IAPWS Formulation 1995, and FLUIDCAL is available free of charge for universities provided that the software is used only for pure teaching and research purposes. However, the IAPWS formulation is only valid in the stable fluid region of water, from the melting-pressure curve to 1000  C at pressures up to 1000 MPa, and, therefore, it does not include data for supercooled water. Otero et al. (2002) have published a review of some interrelated thermophysical properties of liquid water and ice I (thermal expansion coefficient a, isothermal compressibility coefficient b, specific volume V, and specific isobaric heat capacity cp) in a pressure/temperature range of interest for high-pressure food processing, including supercooled water. The corresponding calculation routines are publicly available on the Internet (Otero et al., 2002, 2010b). However, accurate thermophysical properties of real foods under pressure cannot be derived from

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only the equations governing pure water. Food comprises other phases, in which different types of bonding determine other thermophysical properties. Also, substances dissolved or suspended in the actual water phase alter its properties. The validity of these properties is even affected by the physical structuring and microcompartmentalization characterizing many food products and derived from cellular structure, polymers networks, or emulsion phenomena. At present, some experimental determinations of density (Aparicio et al., 2011b; Denys et al., 2000b; Guignon et al., 2009, 2012; Min et al., 2010), thermal expansion coefficient (Aparicio et al., 2011a; Denys et al., 2000a,b; Guignon et al., 2009), isothermal compressibility coefficient (Aparicio et al., 2011b; Guignon et al., 2009, 2012; Min et al., 2010), specific heat (Aparicio et al., 2011a; Zhu et al., 2007), and thermal conductivity (Denys and Hendrickx, 1999; Ramaswamy et al., 2007; Shariaty-Niassar et al., 2000; Werner et al., 2007; Zhu et al., 2007) have been described in different food models and real foods under pressure. The methods employed are described in depth in their respective papers. Taking into account the difficulties that exist for the experimental determination of the thermophysical properties of food under pressure, different approaches have also been used in the literature for their prediction. Miles and Morley (1978) used thermodynamic equations to estimate the effect of hydrostatic pressure on some of the thermal and physical properties of frozen foods. Other authors have shifted the thermophysical data at atmospheric pressure according to the freezing point depression associated with the pressure applied (Denys et al., 1997; Otero et al., 2006; Schlu¨ter et al., 1999). Denys et al. (2000c) adjusted the apparent specific heat curve, reducing its height or width to simulate the well-known reduction of latent heat of pure water at high pressure (Bridgman, 1912; Hobbs, 1974). Hartmann and Delgado (2003) and Abdul Ghani and Farid (2007) considered that the values of a given physical property for water and the food of interest maintained a constant ratio at atmospheric and elevated pressures. Other approximations have been made from real experiments and numerical simulations, using inverse methods (Denys et al., 2000c; Kowalczyk et al., 2005; Schlu¨ter et al., 2004). More recently, Otero et al. (2010a) presented a group of generic models able to predict some thermophysical properties of food under pressure (density, specific heat, thermal conductivity, apparent specific heat, and enthalpy) from its composition. If the composition of the product is not known, an alternative is proposed if some thermal data at atmospheric conditions are available. All the routines have been compiled and can be freely downloaded from the Internet (Otero et al., 2010a,b).

28.5.3 Temperature Variation after an Adiabatic Pressure Change An important factor that must be taken into account when modeling high-pressure freezing processes is the temperature variations produced by pressure changes. Pressurization/depressurization always induces a temperature increase/decrease due to the work of compression/expansion in both the food and the pressurizing fluid. When modeling, this temperature variation produced by an adiabatic pressure change can be calculated by means of the following well-known thermodynamic equation: dT Tk $V$a ¼ dP cp

(28.2)

where T is the temperature (K), P is the pressure (Pa), V is the specific volume (m3/kg), a is the thermal expansion coefficient (K1), and cp is the specific heat capacity (J/kg K). Under adiabatic conditions, dT/dP depends only on the initial temperature of the product and its composition defined by the corresponding specific volume, thermal expansion coefficient, and specific heat capacity. But, since these parameters depend on T and P, the calculation of dT/dP is complex. Many studies related to the compression heating of different pressure-transmitting media (water, ethylene glycol, ethanol, water/glycol, and water/propyleneglycol mixtures), foods (orange juice, whole and skim milk, egg yolk and white, olive and soybean oils, mayonnaise, avocado, beef, chicken and salmon, among others), and even plastic polymers (high-density polyethylene, polypropylene, polytetrafluoroethylene) can be found in the literature (Ardia et al., 2004; Balasubramanian and Balasubramaniam, 2003; Buzrul et al., 2008; Knoerzer et al., 2010a,b; Landfeld et al., 2011; Patazca et al., 2007; Rasanayagam et al., 2003; Ting et al., 2002; Wang et al., 2009). Regarding expansions, Otero et al. (2000b) found that theoretically calculated dT/dP values for liquid water after quasi-adiabatic expansions were in good agreement with experimental data. When modeling high-pressure freezing processes, it is essential to bear in mind that the different thermophysical properties of the products inside the pressure vessel will induce different temperature increments after a pressure change. For example, several studies show that fats and oils have high compression heating factors, with temperature increases, up to 8.7  C/ 100 MPa as compared to 2e3  C/100 MPa for water (Patazca et al., 2007; Rasanayagam et al., 2003).Therefore, after a pressure change, heat exchanges between the walls of the pressure vessel, the pressure medium, the packaging material, and the product will take place, greatly influencing the temperature evolution in the sample. All these considerations are particularly

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important in HPSF, especially after expansion, when a phase transition occurs in the product. As an example, the temperature of an ethanol/water mixture decreases from 14  C to 24.4  C after an expansion from 140 MPa to atmospheric pressure, while the temperature of the sample increases to its initial freezing point due to ice nucleation (Chevalier et al., 2000d). These large and quick temperature changes must be taken into account when modeling, since they will strongly influence the subsequent temperature evolution in the sample.

28.5.4 Convective Phenomena Different authors have shown the importance of convective phenomena in high-pressure processes. During compression, forced convection is induced when pressure medium is pumped into the vessel. Moreover, as mentioned in the previous section, thermal gradients are also induced due to the different temperature increments experimented by the product, its package, and the pressure medium. The same occurs after expansion. All these phenomena lead to a spatial and temporal distribution of the temperature and the fluid velocity during the process, as observed by Pehl et al. (2000). Delgado’s group studied its consequences in the inactivation of enzymes and microorganisms. They proved that the uniformity of a high-pressure effect can be disturbed by convective and conductive heat and mass transport conditions that are affected by parameters such as the compression rate, the size of the pressure chamber, or the fluid viscosity (Hartmann, 2002; Hartmann and Delgado, 2002, 2003; Hartmann et al., 2003, 2004; Kilimann et al., 2006; Rauh et al., 2009). Other authors confirmed that convection currents in the pressure medium play an important role in the thermal evolution of the processed samples, especially when the filling ratio in the pressure vessel is low (Chen et al., 2007; Otero et al., 2007b). First models on high-pressure freezing only considered heat conduction, since combining fluid motion and phase transition phenomena when modeling is a very difficult task (Denys et al., 1997, 2000c; Sanz and Otero, 2000; Schlu¨ter et al., 1998). Delgado’s group was the first to model high-pressure freezing processes taking into account convective effects in a liquid sample (Kowalczyk et al., 2003, 2004). In the following sections, different HPAF and HPSF models, developed in the last two decades, are discussed in depth.

28.5.5 Modeling HPAF Processes HPAF takes place under pressure, but in essence the freezing process is the same as in atmospheric conditions. It is governed by thermal gradients between the cooling medium and the sample, as occurs in

classical freezing. So, any model developed for a classical freezing process at atmospheric pressure should be valid to reproduce the process under pressure if some relevant points are kept in mind. The model might take into account the temperature/pressuredependent thermophysical properties of the product and the pressure medium. Moreover, the latent heat and the freezing point of the sample also vary with pressure and must be considered. In addition, it is essential to include in the model the temperature change that the system (sample and pressurizing fluid) undergoes after an adiabatic compression or expansion. As mentioned above, first modeling studies in the literature only included heat conduction phenomena. As an example, Schlu¨ter et al. (1998) assumed radial symmetry and one-dimensional heat conduction when modeling thermal exchanges in potato cylinders subjected to HPAF. As a computational domain, they only considered the solid sample, and they assumed convective heat transfer at the surface as a boundary condition. Values of apparent specific heat capacity and thermal conductivity were considered as pressure and temperature dependent. Denys et al. (2000c) employed an explicit two-dimensional finite-difference scheme to simulate HPAF of TyloseÒ cylinders. Only heat conduction was considered in the model, and the effect of convection currents was minimized by using a sample holder with the maximum radial dimension allowed by the internal volume of the high-pressure vessel. Later, Kowalczyk et al. (2003, 2004) analyzed phase transitions in liquid systems under pressure. These authors investigated fluid dynamics and thermodynamics in water during the HPAF process, and included fluid convection when modeling. They employed the enthalpy-porosity model reformulated in terms of conservation equations of mass, momentum, and energy for a compressible medium complemented with equations of state for liquid water and ice. Moreover, pressure- and temperature-dependent thermophysical properties and viscosity of water and ice were considered. They found a very good agreement between the experiments and the numerical predictions. Kowalczyk et al. (2003, 2004) observed that convective fluid motion has a major influence on the formation of the ice front. Moreover, due to the vanishing density anomality of water under pressure, natural convection exhibits a different behavior than at atmospheric conditions. The importance of convective fluid motion during phase changes under pressure was ¨ zmutlu et al. (2006) who subsequently confirmed by O employed high pressure digital particle image velocimetry and thermography (HP-DPIV and HP-DPIT) to visualize temperature-convective fields during highpressure-assisted thawing of water.

V. INNOVATIONS IN FOOD REFRIGERATION

529

28.5 MODELING HIGH-PRESSURE FREEZING PROCESSES

28.5.6 Modeling HPSF Processes Different authors have made attempts to model HPSF processes. The main difficulty that they have encountered is how to model the phenomenon of uniform nucleation induced by pressure release and calculate the amount of ice instantaneously formed just after expansion. As Table 28.1 shows, initial models considered that the sample extended over its melting curve in the adiabatic expansion (no supercooling is considered), although in practice it has been proved that metastable conditions are always reached after the pressure release (Sanz et al., 1997). Otero et al. (1997b) built a model to calculate the amount of ice (mi) produced after expansion based on an analysis of the variation of the thermodynamic properties of ice I/water along its melting curve (Chizhov and Nagornov, 1991). These authors obtained 36% instantaneous ice after an expansion from 210 MPa down to atmospheric pressure. Later, Chevalier et al. (2001), following this same outline, calculated the ice fraction formed in water samples after different pressure releases and compared the results with calorimetric determinations. Their experimental results were consistently higher (between 13% and 23%) than the numerically calculated values, probably because the ice ratio could be increased by handling the samples prior to calorimetric measurements. Subsequent models considered supercooling (DTsup) reached after expansion. Table 28.1 shows various approaches used by different authors to calculate the

TABLE 28.1

amount of ice (mi) produced after pressure release (Barry et al., 1998; Le Bail et al., 1997; Otero and Sanz, 2000). All of them are more or less sophisticated heat balances that assume that the latent heat (L) released by nucleation is equal to the sensible heat absorbed by the sample in transition from metastable conditions to its freezing point at the nucleation pressure. These models clearly show that the higher DTsup, the more ice is formed. Table 28.2 shows the amount of ice produced after expansions from different coordinates over the melting curve of water. The theoretical supercooling reached after expansion was calculated according to Eqn (28.2) and incorporated in Eqn (28.6) in Table 28.1, on the assumption that nucleation occurred at atmospheric pressure. Results differ slightly from those presented by Otero and Sanz (2000), since an improvement made by Otero et al. (2000b) has been introduced in the calculus of the theoretical supercooling. HPSF experiments performed at pressure-temperature conditions on the melting curve liquid-ice I produce the maximum ice ratio, which is theoretically possible to obtain for a given pressure. Nevertheless, for a given temperature, maximum supercooling is reached after expansion from the maximum pressure possible in liquid state according to the phase diagram of water. Under these conditions, the amount of ice formed will be maximized for a given temperature. This maximum pressure was calculated for different temperatures in Table 28.2 (Wagner et al., 1994), and the amount of ice from these new pressure/

Various Approaches Used to Calculate the Amount of Ice Instantaneously Produced after Adiabatic Expansion in HighPressure-Shift Freezing Processes by Different Authors1

Supercooling Phenomena Not taken into account

Equation


dV dP

 S

References
 
 vVi 2$Tk vVi $ ¼ mi $ þ $ðVw  Vi Þ vP T L vP P  
 cpi $Tk vVw $ðVw  Vi Þ2 þ ð1  mi Þ$  2 vP T L
  2$Tk vVw cpw $Tk 2 Þ $ þ $ðVw  Vi Þ $ðV  V w i L vP P L2

Chizhov and Nagornov (1991), Otero et al. (1997b)

(28.3) mw $cpw $DTsup ¼ L$mi

(28.4)

Le Bail et al. (1997)

    mi $cpi $DTsup þ ð1 mi Þ$cpw $DTsup ¼ L$mi

(28.5)

Barry et al. (1998)

i h i mi $c pi $DTsup þ ð1 mi Þ$c pw $DTsup ¼ L$mi

(28.6)

Otero and Sanz (2000)

Taken into account

Taken into account

Taken into account

h

1

mi denotes mass of ice and mw denotes mass of liquid water.

V. INNOVATIONS IN FOOD REFRIGERATION

530 TABLE 28.2

28. HIGH-PRESSURE FREEZING

Amount of Ice Instantaneously Formed after Adiabatic Expansions from Different Pressure/ Temperature Coordinates

From Melting Curve

From Extreme Pressure Conditions (According to the Phase Diagram of Water)

Initial Temperature ( C)

Pressure (MPa)

Ice Formed (%)

Pressure (MPa)

Ice Formed (%)

22.0

210

24.8

210

24.8

20.7

200

23.5

232.3

23.7

19.4

190

22.1

262.7

23.3

18.1

180

20.2

303.2

23.1

16.9

170

19.4

351.9

23.08

15.6

160

18.1

368.3

22.66

14.4

150

16.8

384.9

21.87

13.2

140

15.5

401.6

21.13

12.1

130

14.2

418.4

20.79

11.0

120

13.0

435.2

20.12

9.9

110

11.7

452.1

19.47

8.8

100

10.5

469.0

19.20

temperature coordinates was calculated again. It is interesting to note that, at least in theory, large amounts of frozen water could be obtained after expansions from relatively high temperatures if the pressure is increased. Experiments made by Otero and Sanz (2006) confirm these theoretical results. Sanz and Otero (2000) applied a mathematical model in three steps (precooling, phase change, and tempering) to give a comprehensive idea of HPSF processes and compare them to traditional freezing in atmospheric conditions. They employed a large cylindrical sample of agar gel containing 99% water. The precooling and tempering stages were solved using the transient state heat transfer equations for finite cylinders presented by Chung and Merritt (1991). Moreover, pressure-dependent thermophysical properties of water were considered (Otero et al., 2002). To determine the time needed to complete the phase change stage, a modified Plank’s equation (De Michelis and Calvelo, 1983) was used, taking into account the amount of ice instantaneously produced after the adiabatic expansion (Table 28.1). The complete model by Sanz and Otero (2000) satisfactorily reproduced experimental freezing processes, both at atmospheric pressure and by HPSF, taking into account that its main purpose was to serve as a comprehensive tool for comparing both processes.

Nevertheless, complex situations (for example, complex geometries, or slow expansions where supercooling and nucleation occur under pressure before reaching atmospheric conditions, or freezing small samples where convective phenomena in the pressure medium gain importance, etc.) and more accurate predictions requires the use of numerical methods. Denys et al. (1997) used an explicit two-dimensional finite-difference scheme to simulate HPSF of a TyloseÒ cylinder according an apparent specific heat formulation. The computational domain included the TyloseÒ sample and the pressure medium, but only heat conduction was considered. Temperature increases or decreases of the high-pressure medium during adiabatic compression or expansion were calculated according to Eqn (28.2), and apparent specific heat and thermal conductivity under pressure were estimated, shifting the known data at atmospheric conditions on the temperature scale depending on the prevalent pressure. Moreover, to deal with the problem of instantaneous ice formation, the model assumed that the sensible heat required for the temperature increase in the expansion is subtracted from the total enthalpy of the sample. The authors found good agreement with experimental data, but they concluded that an improvement needed is the contribution of convection heat transfer by the pressure medium. As noted before, apparent specific heat presents a strong discontinuity at the freezing point, and this involves an important difficulty when modeling freezing processes according an apparent specific heat formulation. In this sense, Pham (2006) suggested that a simpler and more efficient way to model HPSF processes is to use an enthalpy formulation. Norton et al. (2009) developed a one-dimensional finite-difference numerical model based on the enthalpy formulation to simulate HPSF of TyloseÒ, agar gel, and pork. This uses Schwartzberg’s equation (Schwartzberg, 1976) to determine both the initial freezing point and the temperature evolution below freezing. The model predicts temperature profiles, freezing times, and instantaneous ice formation after pressure release by considering the shifting effect of pressure on the enthalpy curve, initial freezing temperature, and thermophysical properties. Moreover, the modeling approach also includes the temperature changes due to adiabatic compressions and expansions. Norton et al. (2009) compared model predictions to experimental data in the literature and found quite satisfactory results. Fluid convection currents, not taken into account by the model, could be responsible for the small divergences found. In this sense, Smith et al. (2012) presented a generalized enthalpy model in which convection in the pressure medium is accounted for by solving the NaviereStokes equations.

V. INNOVATIONS IN FOOD REFRIGERATION

28.6 FUTURE PERSPECTIVES

28.6 FUTURE PERSPECTIVES In spite of the exciting possibilities that high-pressure freezing offers, some obstacles make it difficult to introduce this technology into the food industry. Currently, industrial pressure vessels, able to work at subzero temperatures, do not exist in the market. Cooling conventional pressure equipment would be a difficult task due to their large size. It is important to note that today, the use of high pressure is not an impediment, but the long processing times are the limiting factors and they must be reduced to make the processes competitive. Therefore, introducing high-pressure freezing into the food industry would involve a previous development of suitable industrial high-pressure equipment that allows quick cooling and continuous or semicontinuous processes to make the technology attractive. In this sense, high-pressure tubular reactors inspired by those employed in the chemical industry to produce lowdensity polyethylene could be a solution (Otero et al., 2007a). Of all the high-pressure freezing techniques discussed herein, it is clear that only HPSF could present some applications for future use by the food industry. However, the implementation of this process will depend on the capabilities of HPSF to solve real problems. Current markets offer a wide variety of frozen foods of good quality. It is probably fair to say that there is probably little interest in freezing these products by pressure release as demonstrated by the existing difficulties of implementing HPSF in the industry. HPSF would only be justified in products of high added value that are not usually frozen due to their delicate microstructure, for example. Even in these cases, HPSF would be only recommended if correct management of the cold chain can be guaranteed from the producer up to the final consumer. If this does not happen, recrystallization phenomena will take place and all the efforts to achieve small ice crystals in the product will be lost. Nevertheless, HPSF can be envisaged not only as a new technology to directly obtain frozen foods but also as a unit operation to be integrated in more complex processes. Thus, some industrial applications in which ice crystallization is a critical step (ice cream elaboration, cryoconcentration of fluid food, or freeze drying) could be important beneficiaries of HPSF (Choi et al., 2008a; Otero et al., 2012; Sanz et al., 2011; Volkert et al., 2012). In this regard, important food companies have developed patented processes to elaborate ice creams and frozen desserts by combining high pressure and subzero temperatures (Keenan et al., 1998; Nestec et al., 2007). Volkert et al. (2012) explored different ways of taking advantage of high-pressure low-temperature technology

531

in aerated frozen products. In these kinds of products, both the size of the ice crystals and the incorporated air cells essentially affect the food quality. The authors found that good air cell recovery and improved sensorial properties are achieved after HPSF (360 MPa/25  C) of a sugar-rich, dairy-based food foam. They concluded that HPSF processing offers interesting options for aerated products, but more research work is needed to gain deeper understanding of the phenomena involved. Recent studies show that HPSF can also be conceived as a substitute for the crystallization step in conventional freeze concentration systems (Otero et al., 2012; Sanz et al., 2011). This step is usually performed in scraped surface heat exchangers working at very low temperatures. HPSF allows working at temperatures considerably higher, thanks to the adiabatic cooling produced in the sample after expansion, just before nucleation. Moreover, in HPSF, crystallization occurs instantaneously throughout the product and not only at the surface in direct contact with the walls of the heat exchanger. Therefore, the product can be obtained, at the desired concentration level, just after expansion. Experiments with orange juice show that ice crystals formed in HPSF are rounder and larger than those usually obtained in scraped surface heat exchangers. This gives important advantages in the subsequent separation of the ice crystals from the concentrated product. HPSF has also been envisaged as a useful technology for freeze drying. In this sense, Choi et al. (2008a) studied the effect of HPSF on the stability of freeze-dried nanocapsules and concluded that, at 10  C, HPSF was more effective than conventional freezing. All these preliminary results indicate that HPSF could have interesting applications in different processes of the food industry in which ice crystallization is involved, but much more research is needed. Finally, it is interesting to note that when a new processing technology is introduced in the industry, consumer acceptance is one of the key issues for its future success. Lampila and La¨hteenma¨ki (2007) surveyed consumers’ attitudes toward high-pressure freezing of food in The Netherlands, Belgium, Spain, and Finland. They found that even though the term high-pressure freezing was weakly recognized, attitudes toward the processing method were neutral and no negative images were detected in the consumers’ minds. When given some information about the process, consumers considered the application of HPSF as appropriate, especially if it had advantageous consequences for the product. This gives a good starting point for the future implantation of HPSF in the food industry.

V. INNOVATIONS IN FOOD REFRIGERATION

532

28. HIGH-PRESSURE FREEZING

28.7 CONCLUSIONS The advantages of high-pressure freezing processes for product quality compared to conventional methods stem from two main parameters: reduced duration of the phase transition and less mechanical stress during formation of ice crystals. When freezing at constant pressure (atmospheric conditions and HPAF), nucleation occurs only near the sample surface, in close contact with the cooling medium. When the latent heat of crystallization is removed, ice crystals grow from the surface to the center of the product. The final ice crystals are needle-shaped and radially oriented. HPAF to ice I offers some advantages because the latent heat of the water decreases with pressure. However, the freezing point also decreases, and so the temperature of the cooling medium must be lowered to keep the DTrefrig constant. HPAF to dense forms of ice produces a quality improvement; however, to preserve that improvement, frozen storage and thawing must take place under pressure to avoid a solidesolid phase transition to ice I during expansion. Transferring this technology to an industrial scale is therefore a challenge, since continuous storage under high pressure at low temperature seems to be a cost-intensive technology. HPSF entails homogeneous supercooling throughout the sample after the expansion that induces uniform nuclei formation in the product. When the latent heat is removed, nuclei grow to form small ice crystals, which are granular in shape, and without specific orientation. Phase transition times are always shorter than in atmospheric conditions, and important quality improvements have been reported for many products. When optimizing the process, pressure and temperature should be adjusted to maximize the amount of ice formed just after expansion and, therefore, minimize the length of the freezing plateau. Lowering temperature before pressure release is an option, but increasing pressure could represent an effective and rapid alternative to considerably increase the percentage of frozen water. In this case, the microbial inactivation that could be induced in the frozen product can be considered as an added value to the freezing process. High-pressure freezing is an emerging technology still under research. Introducing this technology in the food industry implies the development of suitable high-pressure equipment that allows continuous or semicontinuous processes. A lot of research is still needed to identify all the opportunities that offer both pressure-assisted and pressure-shift freezing. For example, special attention must be paid to ice VI, which can be maintained under pressure at ambient temperature. Recent studies show that HPSF can be used not only to directly obtain frozen foods but also as a unit

operation to be integrated in other processes of the food industry in which ice crystallization is a critical step. Future efforts should be focused on clearly understanding the effects of high-pressure/subzero temperature on food components, studying the kinetics of microbial and/or enzymatic inactivation, developing equipment adapted to the industry, evaluating the stability of ice crystals during storage, and assessing the economic viability of the processes, among others.

NOMENCLATURE cp Specific isobaric heat capacity (J/kg K) DH Enthalpy change (J/kg) L Latent heat (J/kg) m Mass fraction P Pressure (Pa) T Temperature ( C) Tk Temperature (K) DTrefrig Thermal gradient between the freezing point of the sample and the cooling medium ( C) DTsup Supercooling extent ( C) V Specific volume (m3/kg)

Greek symbols a Thermal expansion coefficient (K1) b Isothermal compressibility coefficient (Pa1)

Subindexes N Nucleation i Ice w Water

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