Wspólnotowy Serwis Informacyjny Badan i Rozwoju - CORDIS

Study of the kinetics of stability related aspects of enzymes in model solutions, model foods and real foods

To prevent enzyme related quality degradation during frozen storage of fruits and vegetables, blanching is traditionally used to inactivate such enzymes prior to freezing. At the same time this heat treatment can cause undesirable changes in different quality parameters including colour aspects and nutrients.

Therefore, the influence of high pressure - low temperature treatments succeeding a regular freezing unit operation on the inactivation of quality related enzymes including pectinmethylesterase, polygalacturonase, lipoxygenase and peroxidase and polyphenoloxidase in model systems, crude enzymes extracts and real food products was evaluated. To inactivate the enzymes in the conventional freezing (air-blasting) a thermal pre-treatment (blanching) is necessary before the freezing treatment. In the PSF treatment is also necessary to have a good flavour and odour.

In general vegetable-frozen products by B-PSF) have a good odour, flavour, colour and texture. The texture in PSF products is a parameter that could compete with the conventional frozen products (air blasting). The existence of metastable phases of ice I and liquid in the domain of ice III (as explained by Schlüter et al., 2004), permits the use of optimized paths for the processes PSF and PIT in the domain of ice III without actual crystallization of ice III. Pressure-shift freezing was performed at 240 MPa and around -24°C. At these conditions, although the stable phase is ice III, liquid water was still present in the samples before the pressure release. Lowering the temperature before pressure release means increasing the temperature gradient in the samples for the instantaneous nucleation, that is, the jump from this temperature level of -24°C to the plateau (at -1°C for potato, approximately).

If the equipment is able to provide lower temperatures, a lowest level can be also used, until around -28°C, without nucleation of ice III, due to the existence of a metastable liquid phase in this region (the cause of the supercooling phenomenon of around 15K when freezing to ice III). Pressure-induced thawing was performed at 290 MPa. After placing samples in the high pressure vessel, pressure was increased until 290 MPa, and at this pressure level, the temperature profile “followed” the extension of the phase transition line of ice I into the domain of ice III. At this pressure level, the area of nucleation of ice III is not reached and, therefore, a metastable mixture of ice I and the partially melted water in the surface during pressurization is present in the samples. In this way, a higher driving force due to a higher temperature gradient was reached, being the difference between sample (around -35°C at 290 MPa) and the surrounding medium (+11°C) temperatures of 46K. Compared to the classical PAT, in which the temperature gradient is around 32K (from -21°C from the sample to +11°C from surrounding medium), the gradient is increased in 14K using this process, leading to a shorter processing time.

The enzymatic activity of polyphenoloxidase (PPO) was chosen to evaluate the effectiveness of PSF and PIT processes for food preservation, since it is dependent on the cell disruption, which is caused by ice nucleation during freezing, and crystal growth during storing and thawing. Results showed at laboratory and pilot scale that the activity of PPO was not increased after freezing and thawing processes when pressure was applied, being even slightly reduced in the metastable region.

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