PRESSURE-ASSISTED FREEZING AND THAWING: PRINCIPLES AND POTENTIAL APPLICATIONS

The phase diagram of water as a function of temperature and pressure delimits distinct crystalline ice forms with different specific volumes, melting temperatures, and latent heats of fusion. The melting temperature of ice I decreases to -22°C when pressure increases to 207.5 MPa. It is possible to freeze a biological or food sample under pressure (obtaining ice I, III, V, VI, or VII), to enhance ice nucleation by fast pressure release, to keep a sample at subzero temperatures without ice crystal formation, to generate pressure through freezing, to reach the glassy state of water by fast cooling under pressure, or to thaw a frozen sample under pressure below 0°C. Fast pressure release from -10 or -20°C and 100 or 200 MPa (with a prior cooling step under pressure), called “pressure-shift freezing,” induces significant supercooling (as detected by fast data acquisition) and enhances uniform ice nucleation throughout the sample. When freezing is then completed at atmospheric pressure, different microscopy techniques reveal numerous small ice crystals with no specific orientation or marked size gradient. Crystals are smaller in pressure-shift frozen gels than in similarly frozen oil-in-water emulsions. In the latter, increasing solute concentrations in the aqueous phase tends to reduce ice crystal size. Modeling is proposed for pressure-shift freezing, although the supercooling and nucleation steps are not taken into account. Both freezing under various pressure levels and pressure-shift freezing are reported for gels (mainly heat-induced protein gels), emulsions, and plant and animal tissues. In spite of some discrepancies, gel or tissue structure and texture are generally better maintained after thawing, as compared to control samples frozen by air blast or immersion in a cooling medium at 0.1 MPa. Less liquid exudation is also observed. However, some protein denaturation is detected (unfolding of myofibrillar proteins, toughening of meat or seafood), especially when the initial cooling step is carried out at a high pressure level for a long time. Pressure application at subzero temperature is found to inactivate only some enzymes, but causes a significant degree of microbial inactivation for several species of micro-organisms. Freezing gels or vegetables under pressure with the formation of ice III, V, or VI appears to maintain tissue structure and texture, but the mechanisms for these effects are not fully understood. Pressure-assisted thawing markedly enhances the rate of thawing, mainly due to a greater ΔT between the subzero thawing temperature and that of the heating medium. Specific packaging and equipment requirements for pressure-assisted freezing and thawing are discussed. Suggestions are made for further studies on high pressure-subzero temperature treatments, such as the influence of sample size and composition; the effects on cell membranes; the reduced need for blanching before freezing; the viability of pressure-shift frozen cells, embryos, or organs; the mechanisms of protein denaturation; and texture-promoting effects, especially in ice creams.     

Effects of pressure-shift freezing and conventional freezing on model food gels

Summary Gels of agar, starch, ovalbumin, gelatin and an industrial β-lactoglobulin protein isolate, were frozen conventionally in a −30 °C freezer and by pressure-shift freezing at 200 MPa at −15 °C. Thawing was carried out conventionally at 20 °C and by the application of a pressure of 200 MPa. The microscopic structure and mechanical properties of the thawed gels were compared with those of the initial gels. Microscopic examination showed that pressure-shift freezing produces smaller and more uniform ice crystal damage than conventional freezing at −30 °C. The results also suggest that the freeze-thaw behaviour of food gels can be categorized into two general types: (1) gels which have a reduced gel strength as a result of mechanical damage to the gel microstructure caused by ice crystal formation, and (2) gels which have an enhanced gel strength, as a result of molecular structural changes that take place in the frozen state. Agar and gelatin were found to be typical of type (1) gels, whereas starch, β-lactoglobulin protein isolate and ovalbumin were found to be typical of type (2) gels. In the case of starch, retrogradation during thawing was found to be the most important factor.     

Pressure-shift freezing and its influence on texture,colour, microstructure and rehydration behaviour of potato cubes

Temperature changes during pressure-shift freezing (400 MPa) of potato cubes and its effects on the drip loss (weight and conductivity), texture (shear and compression tests), colour (L, a, b values), drying behaviour, rehydration properties (water uptake, texture after rehydration) and visible cell damage after thawing (micrographs) were investigated and compared with conventional freezing (0.1 MPa, -30 °C), subsequent frozen storage (-18 °C) or pressure treatment (400 MPa) at +15 :C. Pressure-shift freezing resulted in increased crystallization rates compared to conventional freezing at -30 °C. Crystallization and cooling to ?8 =C took 2.5 min during and after pressure release versus 17 min at atmospheric pressure. Drip loss was reduced from 12.0 to 10.8g/100g. Water uptake during 10 min of rehydration (93.9g/100g compared to 77.4g/100g and incomplete rehydration) and texture values were improved. Browning after thawing or after fluidized bed drying was reduced (increased a value, lower L value), suggesting partial enzyme inactivation during pressure treatment. Differences in colour and texture to the untreated controls were smaller after pressure-shift freezing than after conventional freezing. Cooling to ?30 °C after pressure-shift freezing did not significantly affect the results, whereas subsequent frozen storage at ?18 °C resulted in quality deterioration, as observed after frozen storage of conventionally frozen samples. The improved preservation of cell structure was demonstrated using scanning electron microscopy.     

High pressure–low temperature processing of foods: impact on cell membranes,texture, color and visual appearance of potato tissue

To design and optimize high pressure processes at low temperatures, a quantification of the effects of different processing steps on the food structure is required. Beside pressure-shift freezing, the processes of freezing to ice III and ice V, as well as storage at −27 °C and 250 MPa up to 24 h (metastable liquid state of water) of potato samples were examined. Analyses of the structural changes of the plant tissue included impedance measurements, texture analysis, color measurements and the evaluation of the optical appearance. Storage at subzero temperatures without phase transitions resulted in low membrane damage; however, cell lysis was triggered. Freezing to ice III resulted in the lowest damaging effect on the tissue compared to the other phase transition processes investigated. Samples frozen to ice V and pressure-shift frozen were more deteriorated compared to those frozen to ice III. However, considerable improvements compared to conventional freezing were found. The direction of solid–solid phase transitions (phase transition of ice I to ice III or phase transition of ice III to ice I) influenced the result of high pressure–low temperature processing significantly.Industrial relevanceIt was previously shown that pressure supported phase transitions of ice I like pressure shift freezing are able to preserve the fragile stucture of biological samples like food better than conventional freezing. The present study extends the knowledge of pressure supported phase transitions to a higher pressure domain with the participation of other ice modifications. The authors demonstrate the influences of high pressure phase transitions of water on plant tissue material depending on the processing conditions. The study opens the way to new industrial processing concepts based on high pressure low temperature applications.     

Microstructure of high-pressure vs. atmospheric frozen starch gels

The aim of this study was to find out whether the ice crystal size of a starch gel, a model food system, could be reduced by high-pressure freezing compared with freezing at atmospheric pressure. The size and number of pores in thawed gels was determined by light microscopy and image analysis, and was taken as an indirect measure of ice crystals formed during the different freezing processes studied.The pore size and the total area occupied by the pores were clearly reduced by high-pressure freezing at 150–240 MPa compared with freezing at atmospheric pressure at the same cooling rate. The pore size in the high-pressure (nor in the atmospheric) frozen gels did not increase during a storage time of 3 months at − 24 °C (still air) at atmospheric pressure.Industrial relevanceHigh-pressure processing at subzero temperatures is not yet industrially applied. More evidence on the benefits of high-pressure freezing or thawing on the quality of real food materials as well as development of processing equipment is needed for commercialization of the processes. This study demonstrates that the pore size of frozen and thawed starch gels can be reduced by high-pressure freezing compared with freezing at atmospheric pressure. The reduced pore size was assumed to be a result of smaller ice crystals formed in the high-pressure freezing process. Based on this study, no conclusions can be drawn on the possibility of high-pressure freezing to improve the quality of real foods of a more complex composition and structure.     

Conventional freezing plus high pressure-low temperature treatment: Physical properties, microbial quality and storage stability of beef meat

Meat high-hydrostatic pressure treatment causes severe decolouration, preventing its commercialisation due to consumer rejection. Novel procedures involving product freezing plus low-temperature pressure processing are here investigated. Room temperature (20 °C) pressurisation (650 MPa/10 min) and air blast freezing (−30 °C) are compared to air blast freezing plus high pressure at subzero temperature (−35 °C) in terms of drip loss, expressible moisture, shear force, colour, microbial quality and storage stability of fresh and salt-added beef samples (Longissimus dorsi muscle). The latter treatment induced solid water transitions among ice phases. Fresh beef high pressure treatment (650 MPa/20 °C/10 min) increased significantly expressible moisture while it decreased in pressurised (650 MPa/−35 °C/10 min) frozen beef. Salt addition reduced high pressure-induced water loss. Treatments studied did not change fresh or salt-added samples shear force. Frozen beef pressurised at low temperature showed L, a and b values after thawing close to fresh samples. However, these samples in frozen state, presented chromatic parameters similar to unfrozen beef pressurised at room temperature. Apparently, freezing protects meat against pressure colour deterioration, fresh colour being recovered after thawing. High pressure processing (20 °C or −35 °C) was very effective reducing aerobic total (2-log₁₀ cycles) and lactic acid bacteria counts (2.4-log₁₀ cycles), in fresh and salt-added samples. Frozen + pressurised beef stored at −18 °C during 45 days recovered its original colour after thawing, similarly to just-treated samples while their counts remain below detection limits during storage.     

Preservation of Microstructure in Peach and Mango during High-pressure-shift Freezing

A histological technique was used to evaluate modifications on the microstructure of peach and mango due to classical methods of freezing and those produced by high-pressure-shift freezing (HPSF). With the high-pressure-shift method, samples are cooled under pressure (200 MPa) to -20°C without ice formation, then pressure is released to atmospheric pressure (0.1 MPa). The high level of supercooling (approximately 20°C) leads to uniform and rapid ice nucleation throughout the volume of the specimen. This method maintained the original tissue structure to a great extent. Since problems associated with thermal gradients are minimized, high-pressure-shift freezing prevented quality losses due to freeze-cracking or large ice crystal presence.     

Dendritic growth of ice crystals during the freezing of beef

The morphology adopted by ice in frozen tissues is accepted as one of the factors responsible for freezing damage. For this reason ice nucleation and growing mechanisms have been extensively studied. However, under the conditions used in the industry, where important temperature gradients exist, the classical analysis of nucleation and growth, depending on the supercooling, is complicated by the dendritic growth of crystals as well as by the possibility of the location of the ice crystal inside or outside the cells. In the present paper experiments which verify the existence of dendritic ice growth during the freezing of beef are described. The dendritic growth rate of ice in beef is measured as a function of the supercooling and an analysis of the expected mechanism, according to freezing conditions, is also provided.     

Ice VI freezing of meat: supercooling and ultrastructural studies

While "classical" freezing (to ice I) is disruptive to the microstructure of meat, freezing to ice VI has been found to preserve it. Ice VI freeze-substitution microscopy showed no traces of structural alteration on muscle fibres compared with the extensive damage caused by ice I freezing. The different signs of the freezing volume changes associated with these two ice phases is the most likely explanation for the above effects. Ice VI exists only at high pressure (632.4-2216 MPa) but can be formed and kept at room temperature. It was found that its nucleation requires a higher degree of supercooling than ice I freezing does, both for pure water and meat. Monitoring of the freezing process (by temperature and/or pressure measurements) is, thus, essential. The possible applications of ice VI freezing for food and other biological materials and the nucleation behaviour of this ice phase are discussed.     

Effects of freezing and thawing processes on the quality of Atlantic salmon (Salmo salar) fillets

ABSTRACT:  High-pressure processing is finding a growing interest in the food industry. Among the advantages of this emerging process is the ability to favorably freeze and thaw food. This study aims at comparing the effect of different freezing and thawing processes on the quality of Atlantic salmon fillets. Atlantic salmon ( Salmo salar ) samples were frozen by Pressure-Shift Freezing (PSF, 200 MPa, −18 °C) and Air-Blast Freezing (ABF, −30 °C, 4 m/s). Samples were stored 1 mo at −20 °C and then subjected to different thawing treatments: Air-Blast Thawing (ABT, 4 °C, 4 m/s), Immersion Thawing (IMT, 20 °C), and Pressure-Assisted Thawing (PAT, 200 MPa, 20 °C). Changes in texture, color, and drip loss were investigated. The toughness of the PSF samples was higher than that of the ABF sample. The modification of color was more important during high-pressure process than during the conventional process. The PSF process reduced thawing drip compared with ABF. The presence of small ice crystals in the pressure-shift frozen sample is probably the major reason leading to the reduced drip volumes. The freezing process was generally much more influent on quality parameters than the thawing process. These results show the interaction between freezing and thawing processes on selected quality parameters.     

食品的低温高压处理技术及其研究进展

文中介绍了低温高压处理技术的基本原理、应用范围和研究进展。在 0～ 6 32 4MPa范围内 ,高压下水的冻结点均较常压下的低 ,并在低于 0℃的温度下形成一个水的不冻结区域。高压还使水的体积收缩、温度升高。低温高压处理技术可应用于食品杀菌和抑酶、高压冻结和高压解冻、低温高压不冻结贮藏。低温高压具有比常温高压更好的杀菌效果 ;高压冻结和高压解冻可缩短食品冻结和解冻的时间、改善冻藏食品的品质 ;低温高压下的不冻结贮藏能更好地保持食品原有的风味和质地     

Bacterial Extracellular Ice Nucleator Effects on Freezing of Foods

Extracellular ice nucleators (ECINs) were incorporated into foods and subjected to subzero freezing. Time-temperature profiles, ice-formation patterns and textures were examined by thermocouple, microscopy and texture analyzer. Onset temperatures (initial freezing), enthalpies and freezing rates were measured by DSC. Addition of ECINs to liquid foods elevated ice nucleation temperatures and promoted freezing. Solid or semisolid products frozen with ECINs resulted in a fiber-like texture. These effects were more apparent at –10°C or higher. Differential scanning calorimetry revealed onset temperatures were increased 11°C by addition of ECINs, but length of time to complete the phase transition was extended at constant cooling rates. Results indicated that ECINs can be used instead of whole bacterial cells for efficient freezing and textural modification.     

Size and location of ice crystals in pork frozen by high-pressure-assisted freezing as compared to classical methods

In high-pressure-assisted freezing, samples are cooled under pressure (200 MPa) to - 20 °C without ice formation then pressure is released (0.1 MPa) and the high super-cooling reached (approx. 20 °C), promotes uniform and rapid ice nucleation. The size and location of ice crystals in large meat pieces (Longissimus dorsi pork muscle) as a result of high-pressure-assisted freezing were compared to those obtained by air-blast and liquid N(2). Samples from the surface and centre of the frozen muscle were histologically analysed using an indirect technique (isothermal-freeze fixation). Air-blast and cryogenic fluid freezing, having thermal gradients, showed non-uniform ice crystal distributions. High-pressure-assisted frozen samples, both at the surface and at the central zones, showed similar, small-sized ice crystals. This technique is particularly useful for freezing large pieces of food when uniform ice crystal sizes are required.     

Controlled ice nucleation under high voltage DC electrostatic field conditions

The quality of frozen food and the preservation of the microstructure of the tissue depend on the nucleation rate. As the nucleation rate is related to supercooling, the control of supercooling is highly desirable. In this study, a novel approach is proposed to control ice nucleation using high electrostatic field. Distilled water has been considered as a model food. An original experimental set-up, consisting of pair of plate electrodes and cooling–heating system (Peltier element), has been designed. During sample cooling, high DC voltage was applied to a flat electrode located above the sample. The electrostatic field used in the experiments ranged from 0 V/m to 6.0 × 10⁶ V/m. It was found that with the increase of applied voltage, nucleation temperature was shifted towards higher values. Other tests were conducted to clarify the capability of electrostatic field to induce ice nucleation at a desired supercooling degree. Our experimental results revealed that utilization of sufficiently high electrostatic field is a reliable method to control ice nucleus formation.     

Texture and structure of high-pressure-frozen gellan gum gel

To determine the effects of sucrose and high-pressure-freezing, unsubstituted form-gellan gum gels with 0, 5, 10 or 20% sucrose were frozen at 0.1–686 MPa and −20 °C. Gels were frozen during pressurization at 0.1, 100, 600–686 MPa. However, at 200–500 MPa, gels did not freeze but froze during pressure release (pressure-shift-freezing). On pressure release, a sharp rise in sample temperature was observed for the samples between 200 and 500 MPa. This was a consequence of the exothermic freezing event. Thus, appearance and structure of gels frozen at 200–500 MPa were better than other treated samples due to quick freezing. However, when gels were frozen at 0.1–686 MPa, rupture stress decreased remarkably and strain increased. Texture of pressure-shift-frozen gel was somewhat better than that of gels frozen in freezers (−20, −30 or −80 °C) at atmospheric pressure. Consequently pressure-shift-freezing was more effective. It was found that the addition of sucrose to gels was effective in improving the quality of frozen gellan gum gels.     

Characterization of Ice Crystals in Pork Muscle Formed by Pressure-shift Freezing as Compared with Classical Freezing Methods

ABSTRACT: Cylindrical specimens of fresh pork muscle packed in plastic bags were frozen by air blaster freezing (ABF), liquid immersion freezing (LIF), and pressure-shift freezing (PSF) (100 to 200 MPa). Sample internal temperature and phase transformations were monitored at center, midway, and surface locations. ABF and LIF resulted in large irregular ice crystals, causing serious muscle structure deformation. PSF ice crystals were generally small and regular, but differed along the radial direction. Near the surface, there were many fine and regular intracellular ice crystals with well-preserved muscle tissue. From midway to the center, ice crystals were larger in size and located extracellularly. Ice crystal formation was affected by super-cooling during/after depressurization and subsequent freezing.     

Effects of pressure-shift freezing and pressure-assisted thawing on sea bass (Dicentrarchus labrax) quality

ABSTRACT:   Effects of pressure-shift freezing and/or pressure-assisted thawing on the quality of sea bass muscle were evaluated and compared with conventional (air-blast) frozen and thawed samples. Microstructural analysis showed a marked decrease of muscle cell damage for pressure-assisted frozen samples. According to differential scanning calorimetry (DSC), protein extractability, and SDS-PAGE results, high-pressure treatment (200 MPa) produced a partial denaturation with aggregation and insolubilization of the myosin, as well as alterations of the sarcoplasmic proteins. Only small differences between high-pressure processes (freezing or/and thawing) were registered. High-pressure-treated systems led to a decrease of water holding capacity but differences between high-pressure and conventional methods disappeared after cooking. Muscle color showed important alterations due to high-pressure treatments (increasing L * and b *).     

Recent developments in novel freezing and thawing technologies applied to foods

This article reviews the recent developments in novel freezing and thawing technologies applied to foods. These novel technologies improve the quality of frozen and thawed foods and are energy efficient. The novel technologies applied to freezing include pulsed electric field pre-treatment, ultra-low temperature, ultra-rapid freezing, ultra-high pressure and ultrasound. The novel technologies applied to thawing include ultra-high pressure, ultrasound, high voltage electrostatic field (HVEF), and radio frequency. Ultra-low temperature and ultra-rapid freezing promote the formation and uniform distribution of small ice crystals throughout frozen foods. Ultra-high pressure and ultrasound assisted freezing are non-thermal methods and shorten the freezing time and improve product quality. Ultra-high pressure and HVEF thawing generate high heat transfer rates and accelerate the thawing process. Ultrasound and radio frequency thawing can facilitate thawing process by volumetrically generating heat within frozen foods. It is anticipated that these novel technologies will be increasingly used in food industries in the future.     

MECHANICAL PROPERTIES OF WEAK LOCUST BEAN GUM (LBG) GELS UNDER CONTROLLED RAPID FREEZE-THAWING

Locust bean gum (LBG) solutions and gels were studied in compression, and stress-relaxation was monitored by an Instron universal testing machine (UTM). The strength of the formed gels under freeze-thaw treatment was dependent upon the freezing and thawing rates and the temperature of freezing. LBG solution which froze at a rate of 50C/min was stiffer than those samples which froze at a rate of 1C/min. LBG solution thawed at a rate of 1C/min was stiffer than those samples which were thawed at 10C/min. Very high freezing rates may reduce the size of the formed ice crystals (less disturbance to network formation), while slow thawing may induce the favored gelation. LBG solutions frozen to -60C were stiffer than those frozen to -20C. A short holding time (up to 90 min) at the freezing temperature did not influence the strength of the cryogels. These parameters are important in building products which require a priori knowledge of their texture.     

Impact of Freezing Process on Salt Diffusivity of Seafood: Application to Salmon (Salmo salar) Using Conventional and Pressure Shift Freezing

The depression of the melting temperature of ice under pressure permits to obtain a rapid freezing of foods. The expected benefit lies in reduced water diffusion from the intra- toward the extracellular media, resulting in a reduced drip loss during thawing. Beside, the modification of the cellular structure induced by ice formation may affect the mass diffusivity of the flesh. In the present study, salmon was used as a model food. Slabs of salmon (1-cm thick) were frozen using blast air and pressure shift freezing at 200 MPa. The impact of the freezing process on the mass diffusivity of salt was evaluated using an aqueous solution (NaCl, 3% w/w). Results indicate that the effective mass diffusivity was slightly increased in comparison to non-frozen flesh when a rapid freezing process was used. This may be attributed to a change in the permeability of cell membranes caused by freezing and high pressure.