Abstract
Pulsed electric field (PEF) processing involves subjecting food, placed between two electrodes, to pulsed high voltage electric fields for a very short time (μs to ms), inducing pore formation and hence permeabilization of cell membranes. Few studies have investigated the effects of PEF on solid foods and an in-depth investigations on the mechanisms by which PEF influences the structural and functional properties of solid foods particularly with respect to cell viability at the time of processing has yet to be conducted. The current research therefore investigated the impact of PEF on the quality of solid food materials with an emphasis on microstructural and biochemical changes. Beef muscles and potato tubers were chosen as they represent dead or living tissues, respectively. In addition to the impact of PEF alone, the effects of pre-treatments, such as freezing and peeling, commonly practiced in meat and vegetable industries, investigated in combination with PEF was assessed.
PEF treatments significantly decreased the moisture content of beef muscles and significantly increased purge loss. In a study on beef longissimus thoracis (LT) muscles an increase in the porosity of the myofibril structure was seen using cryo-SEM after PEF (in the range of 0.2–0.6 kV/cm, 1–50 Hz, 20 μs) treatment. However, tenderness and cooking losses of beef muscles were unaffected by the PEF treatments applied. PEF treatments (in the range of 0.2-1.1 kV/cm, 1-10 kJ/kg) affected cell permeability and integrity of intact potato tubers as shown by an increase in ion leakage and the conductivity in the incubation medium as electric field strength and energy level increased. Potato cells located in the pith and outer medulla close to the vascular bundles of potato tubers showed more damage as indicated by a higher level of cell death compared to cells in the outer medulla indicating that electric fields applied to intact potato tubers leads to non-uniform and complex changes in the potato microstructure and cell viability. The orientation of the tuber within the treatment chamber also affected the impact of PEF on cell viability and the direction, which the electric current takes through the tissues. The distribution of potassium ions within the potato cells, as determined by FESEM-EDS analysis, related well with the viability and the integrity of the potato cells as evaluated by tetrazolium salt staining and cryo-SEM, respectively.
The quality related biochemical changes in beef muscles and potato tubers after PEF treatment were also investigated. pH, colour stability (L*, a* and R630/580 values) and protein profile of beef LT muscles were not affected by PEF treatments. A high PEF treatment (1.4 kV/cm, 50 Hz, 20μs) did not affect the ratios of polyunsaturated/saturated fatty acids (PUFA/SFA) and omega 6/omega 3 (n6/n3) nor the free fatty acid profiles in beef semitendinosus (ST) muscles. The number of bacteria associated with PEF treated or untreated beef muscles were similar over time indicating that PEF caused neither significant inactivation nor favourable conditions for the microbial proliferation post-processing. The biochemical responses to PEF treatments were more pronounced for potato (living tissue) than found for the beef (dead tissue). Evaluation of antioxidative enzymes (APOX, SOD, CAT, GPOX and GR) and non-enzymatic antioxidants (ascorbic acid and glutathione) in conjunction with a marker of oxidative damage (protein carbonyls) indicated that the PEF treatment at 0.5kV/cm significantly degraded the stability of biochemical parameters depending on the viability of the cells. In contrast, after PEF treatment at 0.3kV/cm the biochemical markers investigated were not significantly different from untreated tubers which was similar to the cell viability patterns observed after viability staining. These results suggest that upon exposure to PEF induced stress, living cells are capable of changing their metabolisms and preventing the oxidative damage to cellular components caused by reactive oxygen species.
The effect of either a freezing or a peeling pre-treatment prior to PEF on the quality of beef ST muscles or potato tubers, respectively was also investigated. Combined freezing–thawing and PEF (1.4 kV/cm, 20 μs, 50 Hz) resulted in improved tenderness as indicated by a reduced shear force. Microstructural changes in frozen–thawed PEF treated samples compared to an untreated control further confirmed by TEM micrographs, in which significantly less myofibril organization, Z-line fractures and degraded myofibril structure were found. An initial two log-unit increase in aerobic bacterial numbers for frozen-thawed PEF-treated samples was positively associated with increased purge loss. Freezing with or without PEF greatly increased the volatile profile of the meat. The oxidative stability of frozen–thawed beef was negatively affected by PEF treatments as indicated by enhanced lipid oxidation at the end of storage (18 days at 4°C).
For the potatoes, the plant skin or periderm was shown to have a protective effect against the electric fields applied. Peeled potato tubers showed a decrease in the proportion of viable cells compared to unpeeled potato tubers with this effect increasing as the treatment intensity increased. It appeared that when the skin was removed the current had more points of entry to the tissue and therefore the effect of PEF treatment was more pronounced compared to the unpeeled potato tubers. Hence the presence or absence of skin highly influenced the subsequent degree of cell membrane permeabilization, cell viability and rupture. Protein oxidation measured as protein carbonyls content increased significantly in peeled potato tubers treated at 0.5kV/cm compared to unpeeled potato tubers treated with the same conditions. This study revealed that the complex impacts of the PEF treatments are more pronounced in living cells of plant tissues compared to post rigor animal tissues indicating that living cells are capable of changing their metabolism in response to PEF induced stress. Moreover, the effectiveness of PEF treatment could be modified by applying different pre-treatments. Different combinations of traditional physical pre-treatments and PEF can result in supplementary synergetic effects useful for food processing. From the practical point of view, development of such combined technologies as well as understanding the complex structure of solid food materials is very important for improvement of industrial applications.