Introduction:
In the ever-increasing need and awareness for food safety and HACCP, Water has become a focus point for proper disinfection treatment as pathogens and harmful microorganisms can become a danger for contamination of fresh fruits and vegetables during growing and post harvest contact. Water is a critical control point of contact where proper treatment can greatly reduce these microbes, thereby reducing the chances of food borne illness.

Typical water applications include:
Field and growing water including irrigation water
Post Harvest Pre-cooling operations

Hydrovac
Ice Injection
Hydro-cooling

Wash and Dip Tanks
Flume wash systems
Spray wash systems

Sanitizers:
Many water treatment sanitizers have been introduced in the market such as Chlorine, Bromine, Ozone, Peroxyacetic Acid, Hydrogen Peroxide, Ultraviolet light (UV), as well as many derivatives of Chlorine such as Chlorine gas, Sodium Hypochlorite, Calcium Hypochlorite, Chlorine Dioxide, Dichlor, Trichlor, Hypochlorous Acid.

The Problem:
With so many chemical choices and food commodities, it becomes very difficult to standardize upon an adequate treatment level. How much chemical (ppm) of each type to use for which product under varying conditions of organic matter, dirt load, wash contact time, temperature, pH and pressure or vacuum?

ORP the Solution:
The objective of using any sanitizer is to disinfect the water and destroy pathogens. All mentioned sanitizers except UV are oxidizers and they all create oxidation. The stronger the oxidation, the faster the microbe is killed. By measuring this oxidation level, we can directly measure the rate at which these microbes are killed. ORP (Oxidation Reduction Potential) is a measure of the oxidation level in the water and is measured in mV (millivolts) and thereby measures the activity of the sanitizer rather than its concentration levels (ppm). The choice of sanitizer may be left up to the user bearing in consideration, their own process requirements.

Standard for Water Disinfection:
Research has shown that at a level of 650 mV of ORP, bacteria such as E. Coli are killed on contact or within a few seconds. Yeasts and molds may require typically 750 mV or higher in order to be killed. Regardless of any type of oxidizer or any combination of sanitizer, and regardless of other varying water conditions, if there is adequate ORP, there will be proper disinfection. Balancing pH and keeping the organic and dirt load low helps in the strain and demand of the water, therefore it is recommended that clean water quality be maintained as best possible which will also reduce the amount of sanitizer needed to reach effective ORP.

Measurement of ORP:
There are several types of ORP measurement meters that vary depending on the process requirements and design. Regardless of their style and design they all have a sensor (electrode) that measures the ORP made of Platinum, and an instrument that reads the ORP electrode value.

Notes:
All ORP meters and electrodes need to be kept moist and require periodic cleaning and calibration check with standards of known value to ensure reliability and accuracy. Measurement is made by simply dipping the electrode in liquid and observing the display. A stable reading is normally reached within a couple of minutes.

Pocket Meters:
Typically low cost and small in size to fit in the pocket or in a small case and very portable. Both meter and electrode are integrated together in to a compact design. These are low cost disposable meters and very affordable with expected useful life of approx. one year. Readings are reasonably reliable and provide a good indication of the water ORP. Calibration test and cleaning is recommended at least once a week to ensure ongoing reliability.

Hand Held Meters:
Portable hand held meter offers a high degree of accuracy and reliability. Electrodes may be either built-in or external on a cable and are replaceable. Useful life of the electrode may be 1 to 2 years. Most hand held meter also provide a pH and temperature measurement which may be useful in many applications. Calibration check and cleaning is recommended at least once a week.

Process Meters:
Used in a fixed location for continuous monitoring of ORP and provides a highly reliable up to date measurement as conditions continually change in most process water applications. These instruments may also be used as a source of reading only and manually dosing chemicals, however continuous monitoring instruments are commonly used to automatically inject sanitizer on demand, and provide a reliable method of process control as well as recording for automated record keeping.

Maintenance:
All ORP and pH electrodes need periodic maintenance and cleaning. A simple cleaning procedure of dipping the electrode tip in dilute (1:100) acidic solution for two minutes and rinsing with clean water. Then dip in electrode checking solution to verify accurate readings. Some ORP meters allow calibration adjustment but most do not. Inaccurate ORP readings are usually a result of electrode contamination, which may be corrected by cleaning, or depletion of useful life, which needs electrode replacement. Some ORP meters allow slight slope offset adjustments to accommodate these errors.

Unlike a pH measurement that follows a logarithmic curve and therefore requires more calibration adjustments, ORP follows a linear relationship and does not need instrument adjustment as much as it needs electrode maintenance.

Conclusion:
ORP has proven to be a reliable method of measuring water disinfection and provides the operator with a single value of measurement regardless of which product, commodity, operation, treatment or sanitizer is used, and regardless of varying field conditions or method by which chemical is applied.