Metastable phases during high-pressure–low-temperature processing of potatoes and their impact on quality-related parameters

The existence of liquid and solid metastable phases within the phase diagram of water and water-containing food products has been proved to open a new range of possibilities in pressure-shift freezing (PSF) and pressure-induced thawing (PIT) processes. A reduction of processing time was obtained in a metastable region (for pressures above 200 MPa and temperatures below −20 °C) thanks to the increment of temperature gradients (in food material, before and after pressure release, in the case of PSF, and between sample and heating medium for PIT). 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. Additionally, key quality-related parameters (colour, drip loss, texture and microstructure) were evaluated at pilot scale, showing better responses for PSF and PIT than for atmospheric freezing and thawing.