Inactivate enzymes using thermal-assisted high-pressure processing.

Pectin methylesterase is an enzyme found in all species of higher plants. This particular enzyme is imperative to fruit and vegetable processing, as it catalyzes the de-esterification of pectin. This activity destabilizes pectin materials in fruit juices and concentrates.

Inactivating this enzyme makes it possible to prevent cloud loss in juices and other food products. Scientists in New Zealand examined a combination of thermal and high-pressure processing techniques to see how efficient these may be in inactivating pectin methylesterase in orange juice. They developed kinetic models that predicted inactivation rates at low and intermediate pressures, and to a less extent, at high pressure.

Squeezed orange juice was subjected to range of processing conditions from 150 MPa to 800 MPa, with 5 to 30 minutes of holding time at 35 C to 60 C. Results from the inactivation experiments on orange pectin methylesterase indicated that there was a synergistic effect between pressure and temperature under the processing conditions investigated.

At the low pressure of 150 MPa and low temperature of 35 C, the scientists observed the activation of pectin methylesterase. They formulated a mathematical model describing the inactivation rate constant as a function of pressure and temperature. An Arrhenius model--used in accelerated life testing to establish a relationship between absolute temperature and reliability--described the pressure-temperature inactivation over the entire pressure-temperature range that was studied. An Eyring model, an accelerated life testing model based on quantum mechanics for use when temperature is the accelerating factor, was also tested for its ability to describe the pressure-temperature dependence of the inactivation rate.

Pectin methylesterase initiates a sequence of events that cause juice-cloud precipitation in improperly stabilized and handled citrus juices. Multiple forms of the enzyme in citrus juice, peel and rag have different effects on the stability of juice clouds. To determine its mode of action, each form must first be purified and characterized.

Further information. Mohammed Farid, The University of Auckland, Department of Chemical and Materials Engineering, Private Bag 92019, Auckland Mail Centre, Auckland 1142 New Zealand; phone: +64(9) 373-7599; fax: +64(9) 373-7463; email: m.farid@auckland.ac.nz.