The impact of structure on the outcome of pulsed electric fields (PEF) processing of plant organs

Abstract

The aim of this thesis was to understand how structure affects the distribution of pulsed electric field (PEF) induced changes in intact plant organs and to understand how the PEF induced structural modifications affect subsequent industrial applications. To achieve this aim, the impact of PEF processing on plant organs including and sweet potato (Ipomoea batatas cv. Owairaka), oca tuber (Oxalis tuberosa), carrot (Daucus carota, cv. White Belgium), and bunching onions (Allium fistulosum) were investigated. To ensure consistent PEF treatments, unless otherwise stated, whole plant organs including sweet potato, oca tuber and carrot roots were placed perpendicular to the electric field during PEF processing with electric field strength ranged from 0.3 kV/cm to 1.6 kV/cm, and this is referred to as a PEF (Perpendicular) treatment. The edible bases of bunching onion were treated parallel to the electric field to minimize interruption of the delivery of electric current by different layers and tissues. PEF induced structural changes in plant organs were examined using different approaches, including viability staining using the tetrazolium salt (2,3,5-triphenyltetrazolium chloride), microscopy (e.g., light microscopy, fluorescent microscopy, or scanning electron microscopy (Cryo-SEM)), enzymatic browning, electrolyte/carbohydrate leakage, and biomarkers of oxidative stress. PEF induced structural changes for each plant organ studied were heterogeneous. The structural properties of these organs are responsible for the distribution of PEF induced changes. The results have shown that the distribution of the PEF effect is associated not only with tissue type (i.e., epidermal, cortical or vascular tissues), but also with more general organ/tissue properties (i.e. the position of the tissue type within an organ and differences in the electrical conductivities of the various tissue types). In intact sweet potato tubers, PEF caused more cellular damage to the ground tissues than the dermal/subdermal tissues at the same electric field strength. However, the distribution of PEF induced cell death in sweet potato tubers cannot be easily distinguished due to the random pattern of vascularization throughout the ground tissues. Compared to sweet potato, oca tuber and carrot roots have more organised tissue patterns. The distribution of the PEF effect on the oca tubers and carrot roots was clearly associated with tissue/cell types. For instance, cells in the middle region of oca tubers are smaller and have thicker cell walls compared to the other cells, which makes the middle region more resistant to PEF induced damage. In addition, the middle region of the oca tuber has lower conductivity than the other region, which is also responsible for the its higher resistance to PEF induced damages. Besides the conductivity of the tissue, the presence of the endodermis, outside the central core, in carrot roots protected the central core from PEF induced damages at low and medium electric field strengths (< 1.2 kV/cm). While, at high electric field strengths (>1.2 kV/cm), the cells of endodermis might be damaged by PEF treatment, resulting in more oxidative damage in the central core than in the inner cortex. Moreover, the heterogeneous PEF effect also occurred in the same tissue region, the inner cortex of carrot roots. The heterogeneous conductivities of whole carrot roots might interrupt the delivery of the electric current to the same tissue regions, resulting in a non-uniform PEF effect in the same tissue region of the same plant organ. Sample rotation during PEF treatment improved the homogeneity of the PEF effect in the inner cortex and resulted in a more homogenous treatment. While the aforementioned plant models showed that different cell types/tissues have different sensitivities to PEF, we clearly showed that the same PEF treatment conditions had a greater effect on the epidermal cells of the outer scales of bunching onions compared to the inner scales. Hence, in a semi-intact, multilayered plant material the spatial location of the same cell type is also an important factor influencing PEF induced damage. This study also demonstrated that PEF induced changes in the structure of the plant organs affected their potential applications in food industries. Compared to untreated sweet potatoes, PEF pretreated sweet potatoes showed better frying quality, including less oil content in the fried chips and less energy required for frying. PEF also enhanced the removal of antinutrients from oca tubers, and enhanced the release of fructans from bunching onions. Since PEF induced softening occurred in all the plant organs studied, this effect could be used to save some of the energy required for mechanical disintegration of plant organs, particularly for the cutting process of carrots.