2010/705 Evaluation of a PCB bifilar capacitance sensor to measure the concentration of sodium metabisulphite in sea water

By John Fielke, Muhammad Ashraf, Michael Tonkin and Andrew Burge

Black spots (melanosis) form on prawns and other shellfish within a few hours after harvest. Refrigeration alone does not prevent but only slows this process. The pigments themselves do not pose a risk for human consumption, but affected crustaceans are rejected by consumers due to an unsightly appearance. The current practice in preventing black spot formation is to treat prawns with sulphite based chemicals, most commonly with a dipping solution of sodium metabisulphite. In this practice, immediately after catching and sorting, the prawns are dipped in a solution of sea water and sodium metabisulphite for a pre-determined length of time. The aim is to prolong the shelf life of the prawns by achieving the highest possible sulphur dioxide residual without breaking regulatory limits. The current limit being: 100ppm for frozen whole raw prawns.
This project evaluated a prototype sensor to measure sodium metabisulphite in sea water. The sensor was originally developed as a water salinity sensor as salt and sodium metabisulphite exhibit some similar properties in water based solutions and hence, it was envisaged that the sensor could also respond to sodium metabisulphite. Extensive experiments were conducted using the existing prototype sensor to detect increasing levels of sodium metabisulphite in salt water. Although this sensor did distinguish varying levels of sodium metabisulphite, the signal noise from the existing sensor at the required frequency limited the sensitivity and hence the accuracy of the results. Therefore this design as it is cannot be used to develop a commercially useable device. The test results showed that there is a potential for the technology to be further developed to reduce signal noise. In order to confirm the technology, a purpose built sensor will need to be designed, built and evaluated.