Testing free chlorine in produce washing

Chlorine is commonly used as a sanitizer for fresh produce. Like all sanitizers, the levels of chlorine need to be monitored to ensure food safety and effective disinfection. In a number of applications, including the manufacturing of fresh produce, best practice suggests that free chlorine must be applied to the water. The concentrations will vary depending on the organic load, wash water source, contact time, and produce being washed, but this is usually over 10 mg/L and can be as high as 200 mg/L.


Why free chlorine?

The process of adding chlorine in large quantities to overcome nitrogenous materials in water and achieve ‘breakpoint’ is vital to managing microbiological cross-contamination in fresh produce washing. In order to understand ‘breakpoint’, we need to be able to differentiate between free and combined chlorine. Free chlorine, usually exists in water as hypochlorous acid (HOCl) or the hypochlorite ion (OCl), with the proportion of each dependent upon the pH of the sample. When nitrogenous material such as ammonia is present in solution, chloramines will form. There are 3 types of chloramines; monochloramine (NH2Cl), dichloramine (NHCl2) and trichloramine [also called nitrogen trichloride] (NCl3). The amount of free chlorine in a sample, plus the amount of chloramines in a sample is equal to the total chlorine:-

Free Chlorine + Combined Chlorine (or ‘Chloramines’) = Total Chlorine

Distinguishing between free and combined chlorine below the breakpoint is of vital importance in ensuring adequate disinfection. Chloramines are weaker disinfectants than free chlorine so often a longer contact time or higher doses are required when used as a disinfectant. If a process uses free chlorine to disinfect a source of water containing ammonia, organic nitrogenous material or chloramines, then an understanding of chlorine chemistry and some knowledge of the water matrix is required.


What is breakpoint?

Chlorine is added to the water sample and dissolves to form hypochlorous acid and the hypochlorite ion:

Cl+  H2O  à HOCl + OCl

Zone 1

On addition of the chlorine, the water will consume some of this chlorine.  The amount it consumes is known as the chlorine demand of the water.

Zone 2

After this initial period of consumption of chlorine by ammonia, the chlorine reacts with the free ammonia and forms monochloramines and a small amount of organic chlorine compounds:

HOCl + NH3 ↔ NH2Cl + H2O

Zone 3

Once all the ammonia in solution has been consumed to form chloramines, dichloramines and nitrogen trichloride begin to form. The amount of monochloramine and organic chlorine compounds are reduced:

HOCl + NH2Cl ↔ NHCl2 + H2O

HOCl + NHCl2↔ NCl3 + H2O

Zone 4

As more and more chlorine is added, the free chlorine begins to oxidise the chloramines.

The point at which all the dichloramine/nitrogen trichloride is oxidised into nitrogen is the ‘breakpoint’ and is the point most super-chlorination techniques are seeking to achieve as from this point, any additional chlorine added exists as free chlorine and therefore has high germicidal properties.

In general, the ratio of Chlorine:Nitrogen ratio required to achieve breakpoint is 7:6 and in some applications such as fresh produce manufacturing, may be higher if there are high concentrations of organic nitrogen.

This description is a simplistic representation to explain the theory of breakpoint chlorination.  In reality, there are many complicated side reactions that occur depending on various conditions such as pH, and it is hard to predict exactly what is formed and at what levels as the chlorine is dosed.  Likewise, the nuisance residual can be unpredictable and differ between water samples.

What is clear is that it is important to maintain a free chlorine residual in wash water, which should be regularly monitored and validated to ensure food safety protocols are met at Critical Control Points. Find out more information about how our technology can support effective validation of your chlorine sanitization system by clicking here. 

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