Abstract
This thesis aimed to investigate how pulsed electric field (PEF) at low electric field strengths (E ≤ 2 kV/cm) influenced the phytochemical contents of plant-based foods. Effects of PEF on cell electroporation to enhance the release or extraction of phytochemicals and on electrostimulation to induce the production of compounds involved in plant antioxidant metabolism were studied using different food matrices (i.e. wine grapes, carrot purées and wheatgrass shoots).
PEF-assisted extraction of anthocyanins and carotenoids was studied in Pinot Noir and Merlot wine grapes (Vitis vinifera L.) and the purées of coloured carrot cultivars (Daucus carota subsp. sativus). The phytochemical contents and 2,2-diphenyl-1-picrylhydrazyl (DPPH) scavenging activities of grape juices and carrot purées following PEF treatments were assessed. Their bioprotective capacities were evaluated based on the health and integrity of human intestinal epithelial (Caco-2) cells by using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide cell viability, lactate dehydrogenase leakage and nitric oxide production, against hydrogen peroxide induced oxidative stress, as markers.
PEF treatment at 70 kJ/kg (1.5 kV/cm, 50 Hz, 1033 pulses, 20 μs pulse width) produced grape juices with high total anthocyanin contents during cold maceration (0, 2, 4, 8 and 14 days) particularly for the Merlot cultivar where the release of malvidin-3-O-glucoside, malvidin-3-O-(6-acetyl)-glucoside, malvidin-3-O-6-(p-coumaroyl)-glucoside, cyanidin-3-O-glucoside was greatly accelerated. PEF-treated Merlot and Pinot Noir mashes produced grape juices with increased bioprotective capacities after maceration for 2 and 8 days, respectively, compared to untreated juices. PEF treatment at 0.8 kV/cm (15 Hz, 461 pulses, 20 μs pulse width, 35 kJ/kg) was sufficient to increase the extractability of carotenoids from Yellow Solar and Nantes greater than in Nutri Red and Purple Haze carrot cultivars. These purées exhibited better bioprotective capacities after PEF treatment at 303 kJ/kg (0.5 kV/cm, 300 Hz, 8649 pulses, 20 μs pulse width). While PEF processing at low electric field strengths was effective in extracting more carotenoids from carrot purées and improving their bioprotective capacities, the applied processing parameters modified the catalytic reaction and thermal stability of ascorbic acid oxidase (AAO). Carrot AAO became more thermolabile following PEF processing. PEF at 0.8 kV/cm (15 Hz, 461 pulses, 20 μs pulse width, 35 kJ/kg) reduced variations in the thermal stability of AAO between carrot cultivars. Hence, this enzyme can then be completely thermally inactivated in a narrow range of processing temperature and time combinations after PEF.
The electrostimulation effect of PEF was investigated in imbibed wheatgrass seeds (Triticum aestivum L.). The seed water content at the time of PEF treatment was critical when manipulating the antioxidant metabolism of the resultant 7-day old wheatgrass shoots. Evaluation of glutathione and selected antioxidant enzymes (superoxide dismutase, catalase, glutathione reductase, glutathione peroxidase and ascorbic acid peroxidase) in conjunction with Caco-2 cell assay indicated that appropriate seeds imbibition at a water content of 45% (w/w) or greater followed by a PEF treatment of 2 kV/cm (5 Hz, 100 pulses, 20 μs pulse width, 1.5 kJ/kg) significantly stimulated the antioxidant metabolism of the wheatgrass shoots, which ultimately led to a greater bioprotective capacity than unstimulated shoots.
This study demonstrated that PEF at low electric field strengths is a very promising tool to increase the phytochemical contents of plant-based foods. Evaluation of bioprotective capacities using Caco-2 cell assay makes a unique contribution to the current knowledge regarding the health-promoting properties of PEF-treated plant-based foods.