Bioimpedance: A Review for Food Processing
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Electrical measurement is a simple innocuous tool for material characterization. Unlike for instance milk or dairy products, the majority of naturally grown food is composed of cells. The cells of meat or vegetables are surrounded by an insulating membrane, while the cytosol and the extracellular fluids are electrolytes. Despite the high permittivity of water, electrolytes behave like ohmic resistors up to hundreds of MHz. In contrast, membranes form capacitive elements due to their high resistance. The typical time constant for charging cell membranes is of the order of a microsecond. Thus, cells influence the impedance in a frequency range up to several MHz. At higher frequencies, the cytosolic content, i.e., macromolecules, gives rise to characteristic relaxation processes. Using the impedance in the microwave range where water dipoles show a distinct dispersion, humidity of dried matter can be addressed. Moreover, with sensitive measurement setup and proper models, one can determine the dry content in mashes and slurries as well. Several quality standards correlate well with the permeability of the membranes or the total water content. Because of the comparatively simple measurement of the electrical impedance together with advanced mathematical modeling, it is often a good approach in quality assessment of agricultural products. The change in conductivity of a culture medium contains information about the metabolism of incubated cells. Using specific culture media and time-lapse conductivity monitoring allows a high sensitivity and selectivity in microbial detection. Because the electrical impedance is very sensitive to the permeability of cell membranes, it is a great choice for the assessment of changes due to high voltage application. Today, many attempts to use impedance measurement in food technology show fast success in research but fail in practice. The reason is often an overestimation especially of the selectivity while underestimating the uncertainties in a harsh environment of a food-processing plant. Established, however, is the use of robust process measurement systems and the limitation of impedance measurement to applications with highly significant outcome or as supplemental measurement in a multiparameter approach. This review introduces the basics of bioimpedance measurement, points to sources of uncertainty, and presents successful applications in food industry.