Application of pulsed electric fields and vacuum impregnation to improve the freezing tolerance of fresh Origanum vulgare leaves

Abstract: One of most ancient food preservation methods is freezing because there are not caused significant changes the sensory characteristics and nutritional value of foods. Last decade, have been made great steps for the optimization of freezing tolerance of fresh vegetables, fruits and species. Cold injury occurs when the cells are exposed to temperatures lower than 0oC. Cold injury is a consequence of the ice formation in the plant tissue and cell membrane injury . Freezing tolerance of organic fresh oregano leaves is optimized by applying two pre-treatments: vacuum impregnation and pulsed electric fields using a trehalose as cryoprotectant. Firstly, it was applied vacuum impregnation. An essential part of the vacuum impregnation system was the chamber in which the pressure was automatically regulated to increase or decrease. The fresh oregano leaves were vacuum impregnated while they were immersed in isotonic trehalose solution 10 % (w/v). The ideal conditions for the vacuum impregnation on fresh oregano leaves were determined by applying different combinations of vacuum-atmospheric pressures and durations. To decide the most suitable the leaves were weighted. The fresh oregano gained 24% weight in isotonic solution. Secondly, it was applied the process of pulsed electric fields. The conditions of PEF were chosen according to the descriptions of Phoon et. al., 2008. These conditions (field strength, pulse width, number of trains, space between trains, number of pulses in a train, space between trains) allowed the homogenous and reversible electroporation of fresh oregano leaves. Next, it was followed resting time of 1, 2 and 3 days at 4oC. The leaves were frozen and immediately thawn. The combination of vacuum impregnation and pulsed electric fields treatment optimized the freezing tolerance but not for a very long time. When the leaves were frozen and thawn, they started to die gradually. The rate of dying of the pretreated leaves that were rested for 1,2 and 3 days was different. The longer the resting time the slower the dying rate of the leaves. Dying of pretreated leaves presented a pattern. Small dark brown spots started to form on the leaves. These spots were agglomerated latter until the whole surface of the leaves was covered. The problem was not solved, thus it is needed further research.