D. Chevalier

Effects of high pressure treatment (100–200 MPa) at low temperature on turbot (Scophthalmus maximus) muscle

Turbot fillets were subjected to high pressure treatments at 100, 140, 180 and 200 MPa for 15 and 30 min at 4°C. The influence of such treatments on the lipid and protein stability and on color was studied. It appeared that color, protein stability and lipid oxidation phenomena were influenced both by the pressure level and pressure holding time. The oxidative stability of lipids was particularly affected from 180 MPa as measured by the thiobarbituric acid number. Differential scanning calorimetry showed a full denaturation of myosin at 200 MPa and the appearance of a new structure from the treatment at 100 MPa for 30 min.

Freezing and ice crystals formed in a cylindrical food model: part I. Freezing at atmospheric pressure

Cylindrical gelatin gels were frozen at atmospheric pressure with different operating conditions (air-blast freezing at different air temperatures and brine freezing). A method to calculate a local freezing rate was proposed to take into account the variation of freezing rate as a function of the radius. A linear evolution of the local freezing rate according to the radius was observed whatever the freezing process was. Frozen gels were freeze-dried and sliced perpendicularly to the heat flux. The ice crystal marks were measured according to the radial position with image analysis software. Each radial distribution of ice crystal size was characterised by the mean representative diameter. A linear regression permitted to link the ice crystal mean representative diameter to the radial position. On addition, the variation in the mean diameter with the local freezing rate was fitted by a power law.

Effect of freezing conditions and storage on ice crystal and drip volume in turbot (Scophthalmus maximus): Evaluation of pressure shift freezing vs. air-blast freezing

Abstract Turbot fillets were frozen either by pressure shift freezing (PSF, 140 MPa, −14°C) or by air-blast freezing (ABF), and then stored at −20°C for 75 days. Smaller and more regular intracellular ice crystals were observed in fillets frozen by PSF compared with air-blast frozen ones. Ice crystals area in PSF samples was approximately 10 times smaller than that of ABF samples, on average. The PSF process reduced thawing drip compared with air-blast freezing. Conversely to this classical freezing process, the storage time did not adversely influence the thawing drip of PSF samples. In addition, PSF appeared to reduce cooking drip after 45 days of storage at −20°C. Differential scanning calorimetry analysis showed a significant reduction of the total enthalpy of denaturation for the pressure shift frozen samples compared to fresh and conventional frozen samples. Besides, a new melting transition appeared on the thermogram of PSF samples at approximately +40°C.

Freezing and ice crystals formed in a cylindrical food model: part II. Comparison between freezing at atmospheric pressure and pressure-shift freezing

Cylindrical gelatin gels were pressure-shift frozen at different pressure levels (100, 150 and 200 MPa). Temperature and pressure profiles were compared and the maximum supercooling obtained after pressure release was evaluated. A comparison between the freezing steps at atmospheric pressure and those of pressure-shift freezing was carried out to compare the time steps during the processes. The degree of supercooling increased with the level of pressure. For pressure-shift freezing, the size of ice crystals appeared to be more homogeneous in the whole sample and independent of the location. The mean representative ice crystal diameter decreased as the pressure level increased. This may be due to the degree of supercooling achieved during pressure-shift freezing. The ice crystal size for pressure-shift frozen sample was smaller than those obtained with classical freezing methods. A regression between the supercooling level and the mean ice crystal size was proposed.

High Pressure Calorimetry. Comparison of two systems (differential vs. single cell). Application to the phase change of water under pressure

In high pressure calorimetry the pressure change is used to obtain the desired phenomenon (i.e. phase change) at constant temperature. Two high pressure calorimeters have been developed to measure the latent heat of fusion of pure water (hexagonal ice-type I) at subzero temperature. Both calorimeters used a constant pressurisation rate produced with a high pressure pump driven by a step motor. The first calorimeter was a single cell calorimeter where mercury acted as the pressurisation fluid, while the second one was differential (two cells) and was pressurised with pentane. Both calorimeters gave high accuracy data of latent heat of fusion of pure water, which were determined taking into account that either the fluid used to pressurise or the pressurisation rate affected the calorimetric signal.

Phase diagram of aqueous solution at high pressure and low temperature

Abstract This paper presents a constant mass high pressure isothermal calorimeter and describes the range of applications that this type of equipment can deal with. As an example, the thermophysical properties of an aqueous solution of MgSO4 at low temperature and at high pressure have been investigated. The main results showed that the phase change heat of the solution decreased with increasing pressure and the phase change temperature of the eutectic concentration was depressed under high pressure. Those changes could be related to the MgSO4 solubility and to water latent heat changes.

Evaluation of the ice ratio formed during quasi-adiabatic pressure shift freezing

Abstract Pressure shift freezing consists in cooling a biological substance (mainly containing water) under pressure without phase change followed by a sudden release of the pressure. The high supercooling obtained during the quasi adiabatic depressurisation permits to achieve a rapid and uniform ice nucleation. The ice fraction formed during the pressure release of a sample of pure water has been calculated using a mathematical model. In addition, this fraction was experimentally evaluated by isothermal calorimetry. The calculations and measurements were carried out at 3 different initial points of the ice I melting curve. A relatively good agreement is observed between the experimental and calculated ice ratio which were between 0.117 and 0.402 (kg ice/kg ice-water mixture) for an initial temperature-pressure values of -10°C/1 15 MPa and -21°C/210 MPa respectively.

High pressure calorimetry; application to phase change under pressure

Calorimetry is a very versatile technique used to study phase change or reactions of any material. This paper presents a calorimeter which has been designed for studying phase change at elevated pressure. Its originality lies in that a pressure scan is realised at constant temperature. A differential calorimeter head was used with two cells made of stainless steel. Pentane was used as a pressurisation fluid. A high pressure pump driven by a step motor permits to obtain constant pressurisation or depressurisation rates. Data on the latent heat of pure water at −5°C, −10°C and −15°C are proposed. Some statements on the behaviour of the sample of water during the peak are presented and are discussed. Results were within 3% of the existing data from the literature.

Pressure shift freezing of turbot (Scophthalmus maximus) and carp (Cyprinus carpio): effect on ice crystals and drip volumes

Turbot and carp fillets were frozen either by Pressure Shift Freezing (PSF, 140MPa, −14°C) or Air-Blast Freezing (ABF), and then stored at −20°C for 75 days. PSF resulted in smaller and more regular ice crystals compared to ABF samples whatever the fish species was. Ice crystals area in PSF samples was about ten and seven times smaller than in ABF ones for turbot and carp, respectively. In addition, PSF induced less thawing and cooking drips than ABF. The storage time did not adversely influence the ice crystal size, thawing drip and cooking drip for turbot and carp for PSF samples.