Pulsed electric fields in combination with vacuum impregnation for improving freezing tolerance of vegetables

Abstract

Freezing is a widely used method of preserving food products. Efforts are currently being directed towards improving the quality of sensitive tissues of plant foods such as leaves, after freezing and thawing. One of the methods under investigation is the combination of vacuum impregnation (VI) with cryoprotectants and the application of a pulsed electric field (PEF) to the plant tissue prior to freezing. In this chapter were identify mechanisms for the efficient introduction of a cryoprotectant molecule into the heterogeneous structure of leaf tissue and improve our understanding of the consequences of the introduction of this foreign molecule into the tissue regarding cell metabolism, freezing point, and ice propagation rate. To obtain precise information on the electroporation of internally located cells, a three-dimensional numerical model of the cross section of a leaf was developed. Validation of the models showed the importance of the wax layer and stomata for the successful electroporation of all cells in the tissue. VI, and the subsequent application of PEF, increased the metabolic activity of the tissue. The increase in metabolic activity after VI was accompanied by the accumulation of trehalose-6-phosphate in the cells. Leaves impregnated with trehalose, sucrose, glucose, and mannitol exhibited significantly lower ice propagation rates and higher freezing temperatures than untreated controls. Leaves subjected to PEF also showed higher freezing temperatures than untreated leaves; however, the ice propagation rate was not influenced by PEF.