Practical considerations in the application of ORP as a water quality metric

The main article of the April Postharvest Technology Center newsletter is by the Director, Trevor Suslow, and deals about the practical considerations in the application of Oxidation Redution Potential, ORP, as a water quality metric. Follows the original text. The higher the ORP value, read in millivolts (mV), the greater the oxidizing action and the shorter the microbial kill time in water. In general, various market-access standards specify a redox potential critical limit of 650 millivolts (higher operating limits; often ~ 725 to 850mV) and a pH window of operation (6.5 to 7.5) to reduce the risk of cross-contamination within and among treated lots. The combined redox and pH metric are commonly referred to as an Oxidation-Reduction Potential (ORP) standard measured by in-line or handheld sensors. Technically, specifying a pH range is less critical and, when operating properly in our experience, the mV value is the sole determining criterion. Unfortunately, our extensive experience in evaluations of commercial wash water systems has demonstrated that this level of control and reliability of ORP measurements, against microbiological quality objectives, is often elusive in daily operations. Postharvest systems can be optimized to accommodate ORP as a single value control and operating standard in some but not all systems. In commercial settings, recirculated water with substantial, progressive accumulation of suspended solids and combined chlorine appear to be the most challenging to execute good process control. The result is in-line sensor saturation and diminished response time to correct for periodic deficiencies in dosing. The current questions consider chlorine and hypochlorites in relation to an ORP standard. Therefore, this note will restrict comments to commonly used oxidizers. For brevity, operational challenges with ORP with treatments such as chlorine dioxide, ozone, and peroxyacetic acid are not discussed. .. complete article below