Scientific papers on BIA and its use in our industry
The physical device
The bioelectrical impedance physical device is described here. All models are equipped with a 4-electrode compression stainless steel array’s that are salt-water and corrosion proof. The array consists of two signal electrodes and two detecting electrodes that introduce an 800μA, 50kHz, AC current that is capable of voltage changes between 3.75-10.60V.
When the 4-electrode array touches fish, the outside signal electrodes introduce the current and the inside two measure it. A 4-electrode array is used as they help the applied current approximate parallel field lines within the fish, negate skin electrode interfaces, and approximate a cylindrical shape. From this, the two electrical measures that are vectors of the current, are sensitive to changes in the cellular and interstitial volumes of the tissue, and are representative of the impedance of flow.
What is the electricity measuring and why it is instant?
The electricity is measuring
1) non-conductive intact cell membranes and
2) conductive extracellular material.
We use electricity to measure both non-conductive and conductive parameters at the same time. Specifically, the small low frequency current is not strong enough to penetrate cell membranes and therefore is carried by electrolytic ions located within interstitial spaces but at the same time, the current is bumping the non-conductive intact cell membranes. Another way to look at it is the conductive stuff (electrolytes and ions) are outside the cell and that is where the current flows while getting interrupted by the non-conductive bilayers.
An example using the second term to show sensitivity to interstitial volumes (conductive extracellular material). Lets look at Ohms law, I = ∆V*R-2 , where V=voltage (volts), I=current (amps), and R=resistance (ohms). Our method of bioimpedance holds the current (I) steady and measures the changes in voltage (V) to calculate resistance (R). Theoretically, both R and V are negatively correlated so an increase in R is a result of an increase of interstitial electrolytes. The second measured value is sensitive to non-conductive materials because it is relative to the capacitance of the system.
It is instant because electricity moves really fast~ approximately 93,000 miles per second.
How does it measure grade?
As a fish de-grades, cells release their salty watery insides into the interstitial spaces. As more time passes, more enzymatic processes break down protein and cellular structures. Many of these are conductive, but cell membranes are not (see above for description). So as the fish de-grades, the tissue is losing non-conductive membranes and gaining conductive fluids (from the inside of the cells), and therefore changing both values.
We know heat speeds all this up and icing slows it down.
A freshly caught fish will have a high amount of non-conductive cell membrane material compared to one that has completely decomposed or has not been properly taken care of. Bruising, microbial growth, open sores, cuts and scale loss can increase the loss of non-conductive cell membranes with resulting affects of loss of grade, soft flesh and poor quality. We measure that electronically.
How does it measure shelf life?
Shelf life depends on the rate at which the tissue degrades. Icing slows this process down and temperature speeds it up. Because this is predictable (if you know the starting point – which is what we measure) once a fish is iced, and you know the starting point, predictions of shelf life can be predicted quite easily. For example, a fish that is high quality and immediately placed on ice will measure a longer shelf life than one that was poorer quality and then placed on ice. Maybe here, the coho graph with a dot on the curve, vertical line straight down to the days, and a horizontal arrow pointing to the right showing shelf life.
How does it measure if a fish has been frozen?
After mechanical freezing and subsequent thawing, tissue cells break apart and there are no non-conductive living cell membranes intact within the fish. All the fluids inside the cell are flushed outside and both numbers that are sensitive to these changes reflect that.