O. Schlüter

Effect of combined application of high pressure treatment and modified atmospheres on the shelf life of fresh Atlantic salmon

Abstract High pressure (HP) processing at low temperatures combined with modified atmosphere packaging (MA) was used for the preservation of salmon. A shelf life extension of 2 days was obtained after a HP treatment of 150 MPa for 10 min at 5°C compared to unpressurised, vacuum-packed salmon. MA storage (50% O 2 +50% CO 2 ) alone extended the shelf life of salmon for 4 days at 5°C. When salmon had been subjected to HP treatment in the presence of 50% O 2 +50% CO 2, the threshold value for microbial spoilage of salmon (7.0–7.2 log CFU/g) was not reached for at least 18 days at 5°C. Spoilage microorganisms (lactic acid bacteria, Shewanella putrefaciens ) as well as pathogens ( Listeria monocytogenes Scott A, Salmonella typhimurium ) spiked on salmon prior to the treatment, were more susceptible to HP in the presence of 50% O 2 +50% CO 2 . The mode of action of compressed gases is probably related to intracellular formation of reactive oxygen species as well as to phase transition phenomena. Although microbial growth on salmon was retarded, the combined HP and MA treatments, at the settings used in this study, promoted a detrimental effect on colour and changes in the balance of oxidative rancidity.

Kinetic studies on high-pressure inactivation of Bacillus stearothermophilus spores suspended in food matrices

Abstract Bacillus stearothermophilus spores ATCC 7953 can effectively be inactivated by high-pressure treatment, but only if it is applied at elevated temperatures; however, these temperatures are much lower compared to the temperature level used in heat inactivation under atmospheric pressure. Temperature and pressure in a range between 60 and 120°C and 50–600 MPa were applied to inactivate spores suspended in mashed broccoli and in cocoa mass. Utilizing an empirical mathematical model, derived from nth order kinetics, the survival curves of the spore strain investigated could be described accurately. The model can predict the impact of combined action of pressure and temperature on spore reduction. It was demonstrated that the inactivation of B. stearothermophilus spores ATCC 7953 improved with increasing treatment intensity. Beside intrinsic microbial inactivation mechanisms, the role of the pressure-induced shift in crystallization temperature of fat on spore inactivation in cocoa mass is discussed.

Impact of high pressure assisted thawing on the quality of fillets from various fish species

A comparison between the influence of pressure assisted thawing at 200 MPa and that of conventional thawing in 15 8C water at ambient pressure on the quality of thawed fish fillets has been carried out. Data showed that pressure assisted thawed and water thawed fillets exhibit visual differences. Organoleptic characteristics of the high pressure treated fillets were better compared to the conventional water thawed samples before cooking. Further differences were observed, in the total viable count of micro- organisms, pH value, water binding ability, instrumentally evaluated texture parameters and in thermoanalytical behaviour. Also, the effects of the selected treatments differed according to the fish species. Consequently, the specific properties of samples must be taken into consideration when designing pressure supported processes. 2003 Elsevier Science Ireland Ltd. All rights reserved. Industrial relevance: This paper is of high industrial relevance as it deals with an approach to solve the commercially important problem of thawing frozen fish. The use of high hydrostatic pressure enables the freezing and melting point to be controlled. The melting point of pure water can decrease to a temperature of y22 8C at a pressue of 209 MPa. Thus, a higher absolute temperature difference between the product and the surrounding medium can be achieved at moderate temperature levles. The necessary heat of fusion decreases likewise to a minimum at 209 MPa as compared to atmospheric pressure. Together these two effects result in an acceleration of the thawing process. A further target of the application of high pressure assisted thawing in the processing of fish is a reduction of the amount of tape water currently used as thawing medium.

Metastable States of Water and Ice during Pressure-Supported Freezing of Potato Tissue

Different ice modifications were obtained during freezing processes at several pressure levels from atmospheric pressure up to 300 MPa. In the pressure range between 210 and 240 MPa, a metastable ice I modification area was observed, as the nucleation of ice I crystals in the thermodynamically stable region of ice III was reached. A significant degree of supercooling was obtained before freezing the tissue water to ice III, which has to be considered when designing pressure‐supported freezing processes. The effect of supercooling phenomenon on the phase transition time is discussed using a mathematical model based on the solution of the heat transfer governing differential equations. Phase transition and freezing times for the different freezing paths experimented are compared for the processes: freezing at atmospheric pressure, pressure‐assisted freezing, and pressure‐shift freezing. Different metastable states of liquid water are defined according to their process‐dependent stability.

Pressure Assisted Thawing of Potato Cylinders

Water as the major constituent of most food materials undergoes phase transition in a way which can be influenced by increased hydrostatic pressure. Besides a depression of the melting point a reduction of the required latent heat can be observed. Therefore, in a range up to 200 MPa thawing times of pure water were markedly reduced without an increase of the surrounding temperature. Plant tissue like potato cylinders showed a similar behaviour. Using a multi-thermocouple and a finite difference method, thawing times at different pressure levels can be predicted.

Freezing Kinetics Due to Ice III Formation in Potato Tissue

Freezing of food samples at high hydrostatic pressure (e.g. 300 MPa) results in crystal formation of ice III type. Compared to the conventional freezing process at ambient pressure the volume changes are low and in opposite direction due to the increasing specific density. Consequently minor damaging effects on the food matrix are assumed by inducing less mechanical stress. However, the quantity of investigations concerning the formation of denser ice polymorphs in foods is low and no models for predicting the phase transition times are available. In this study typical temperature profiles during freezing of potato tissue were measured at different pressure levels (0.1 MPa, 180 MPa, 300 MPa). The experimental data show a significant degree of undercooling (> 17 K) before nucleation to ice III. Taking the often neglected undercooling into consideration a two step model based on a finite difference scheme was suggested to calculate the freezing times for different ice polymorphs (ice I, ice III). Calculated curves were fitted to experimental data using modified thermophysical properties from literature.

High pressure treatment of liquid whole egg and advantages of low temperature application

Abstract High hydrostatic pressure in combination with controlled temperature could be an alternative process for prolonging the shelf life of liquid whole egg (LWE). As a first step of process optimisation, the equivalent lines of pressure-induced coagulation are provided at three different temperatures (5°C, 25°C and 45°C). The excess work of structure breakdown (ΔW) was calculated from rheological measurements and used as a parameter for the degree of coagulation. Below the line of ΔW = 5.0 kj/m3, the rheological properties of processed LWE were comparable to those of fresh LWE, considered as a critical value. Pressure treatment at higher temperature caused increased coagulation and allowed only a limited possibility of performing high pressure treatment. Furthermore, the inactivation kinetics of E. coli in LWE showed that temperature optima for inactivation could also be found in the low temperature region.

Comparison of Structural Cell Membrane Changes Induced by Different High Pressure Processes at Low Temperatures

To design and optimize high pressure (HP) processes at low temperatures a quantification of the effects of different processing steps on the food structure is required. In this study an impedance analysis was used to evaluate the process-dependent effect on the cell membrane of potato tissue. The obtained data correspond to the mechanical stresses due to specific volume changes induced by the relevant processes. An increase in the cell membrane damaging effects of the potato tissue results after HP treatment in the following order: long time (24 h) HP treatment without phase change, freezing to ice III, freezing to ice V, pressure shift freezing (PSF), freezing to ice I.