Biocides

Biocidal products are employed to achieve disinfection throughout the food (including dairy) and beverage sectors and form the backbone of the chemical methods for achieving this method of control, however other methods may also be employed:

• Thermal: for example, steam or very high temperature water
• Mechanical: high pressure being one example
• Physical: for example, UV-C, employing ultraviolet frequencies to trigger the generation of free radicals in cells, leading to their death.

When considering chemical biocides, we look to classify them by their mode of action on the microorganism:

• Oxidising - all organic material is oxidised leading to rapid, complete and effective destruction of all cells and most cellular material
• Non-oxidising - the product enters the cell either, through translocation or osmotic pressure, following a build-up on the exterior of the cell wall. Once inside the cell, the reproductive processes may be disrupted, as well as the interference with the mitochondria which effectively closes down the energy production capability of the organism, leading to death.

Oxidising Biocides

Common oxidising products, in use in food and beverage scenarios, are solutions of sodium hypochlorite (bleach), peracetic acid, hydrogen peroxide, chlorine dioxide and hypochlorous acid.  Whilst highly effective against a wide spectrum of microorganisms (including spores, yeasts and moulds) many of these compounds should be used with care, due to their corrosivity (particularly against soft metals), risk of taint due to odour, and their rapid degradation, especially in the presence of UV e.g. in sunlight, the exception is stabilised ionic silver hydrogen peroxide which is odourless, tasteless and of low corrosivity so can be used widely.  This latter compound is, however, expensive and has yet to find a place in general disinfection use. Oxidising biocides will often be employed in CIP (clean-in-place) in dairy and beverage processing operations where low cost, low foaming and lack of operative contact is considered a benefit, however it may be followed with a fresh water rinse to reduce the risk of taint, or chemical residue contamination.

Dosing of oxidising disinfectants into in-use concentrations should be undertaken with care. Where manual dosing (not recommended) is employed, then suitable measuring jugs and personal protective equipment (PPE) must be employed, automatic dosing can be achieved either through a diaphragm pump (Figure 2) or a suitable volumetric dosing unit, such as a Dosatron (Figure 3). Care should be taken with the latter, as not all units are suitable for dispensing oxidising disinfectants, and either a PVDF (polyvinylidene difluoride) construction or suitable bypass unit should also be used, to avoid damage to the unit which will lead to poor concentration control or complete failure.

Figure 2	Figure 3

Oxidising disinfectants do, however, suffer from an additional disadvantage in that they can quickly degrade or be consumed by physical debris, resulting in dosing at a higher concentration at the point of entry to that which is needed to actually achieve biocidal efficacy at the point of use.  This is particularly true of chlorine dioxide and non-stabilised hydrogen peroxide, where the efficacy can be reduced very quickly leading to bacterial survival, particularly in cases where a biofilm has been allowed to establish and mature.  When using a sodium hypochlorite solution, one must also consider the ageing effect whereby chlorate ions can be formed during storage, which can be considered a contaminant for food. Correct cleaning of containment vessels and frequent replenishment of solutions, as opposed to simply topping up, can assist in reducing this risk. Further mitigation practices can be found in the Food Biocides Industry Group (FBIG) guidance documents www.chilledfood.org/FBIG, for multicomponent foods, soft drinks and fruit juices, fresh and prepared produce and provisions (cured meat, butter, dried milk products) etc.

In situ, generation of disinfectant solutions has also become popular in recent times, with solutions of hypochlorous acid frequently employed in this field.  A brine solution (NaCl) is exposed to an electrolysis process, generating a weak sodium hydroxide solution at one electrode and hypochlorous acid at the other. This effect can also be achieved through dissolving sodium hypochlorite in water and slightly acidifying, using citric acid. This is commonly employed in salad and vegetable washing or decontamination,  and to maintain washwater hygiene.  Traditional chemistry wisdom dictates that sodium hypochlorite (NaClO) and acid are not to be mixed, and indeed this is generally true where conditions are not tightly controlled.  However, in this scenario the acid used (citric) is relatively weak and the pH is nudged to 6–6.5, where the dissolution of NaClO is into hypochlorous acid, and not fully dissolute into Cl₂ (readily lost as chlorine gas).

Non-oxidising Disinfectants

This class of products has formed the foundation of open plant disinfection processes for many years, as a result of their relative effectiveness, lower cost, low toxicity and corrosion profile.  The general mode of action is achieved by the positively charged disinfectant compound binding to the negatively charged bacterial cell wall, and effectively dissolving the phospholipid layer, resulting in the cell’s death. Other modes of action include the disruption of the mitochondria, as well as the cytoplasmic layer.

Commonly used actives are: quaternary ammonium compounds [e.g.  didecyldimethylammonium chloride (DDAC), benzalkonium chloride (BAC)], alkyl amine and biguanides. Unintended consequences of pesticide residue legislation have curtailed the application and usability of two of these actives, which we will return to later.

Dosing can be relatively easily achieved with many of these products being used at a 1-2% v/v (volume concentration) dose rate, often via venturi blocks (Figure 4) or a standard Dosatron (Figure 5).  Once again, manual dosing is not recommended due to the inconsistencies in concentration that will be achieved.

Figure 4	Figure 5

Relative Efficacy of Biocidal Actives

When choosing a disinfectant product to be utilised, consideration must not only be given to the material the contact surface is constructed from, but also the target microbiological contaminant of concern, as different biocidal actives have different efficacies against classes of bacteria, fungi, yeasts and moulds, or spores.  The relative resilience can be best visualised in the accompanying graphic (Figure 6) which illustrates the susceptibility of different classes from enveloped virus (such as SARS-CoV-2) at the ‘weakest’ end, to prions at the ‘most resilient’ end.

 Ref: - McDonell, G.E. Antisepsis, Disinfection, and Sterilization: Types, Action, and Resistance; 2nd ed.; American Society for Microbiology Press: Washington DC, 2017

Each formulator will incorporate, a mixture of a biocidal active and additional components, such as surfactants and other adjuncts, into their blend, which will enhance the efficacy of the solution, by providing a physical action on bacterial populations.  Table 1 below, provides an illustrative example of the generic efficacies of the commercially available formulations, typically used in food and beverage operations.

Table 1: Comparison of relative biocidal active efficacy

***   Effective

**     Moderately effective

*       Partially effective

Ref: Campden BRI. Cleaning and disinfection of food factories - a practical guide, Second Edition: 2020 (Guideline G55)

As this table clearly demonstrates, when selecting a disinfectant product, consideration should be given not only to the material of construction of the food contact surface under treatment, but critically to the anticipated target contamination, or microorganism involved.  The adage ’know thy enemy’ is certainly true in the realm of disinfectant biocidal active selection.

When considering physical methods for reducing microorganisms, the most commonly used method, at least in processing scenarios, is the application of heat.  Whether this is cooking, pasteurisation, HTST (high-temperature, short-time), UHT (ultra-high temperature) or canning processes, the thermal destruction of pathogens and spoilage organisms is well understood and applied, however in food contact surface disinfection this technique is not widely used, mainly due to the possible damage to the surface, or the health and safety risk that would be posed to the operative undertaking the decontamination procedure.  The exception to this is the use of steam, in particular dry steam, in electrically sensitive equipment, or hard-to-reach areas of processing machinery, and in CIP systems where a final rinse water temperature of around 80^OC is often employed, Of course, in this latter application, the operative is never in direct contact with the solution, thereby reducing the risks.

Other physical methods of bacterial decontamination have yet to be commercialised and remain at the  development stage, however these methods show promise, with various research being undertaken to quantify applicability (although mainly focussed on food product studies rather than food contact surfaces):

• Cold Plasma: this technology may lend itself in time to food contact surface decontamination, in particular conveyor belts and other flat surfaces have been considered, where ’shadowing’ is not an inherent problem as the energised plasma can only decontaminate where it touches.
• UV-C: as with cold plasma, UV-C has been demonstrated to be highly effective at microorganism decontamination on surfaces where shadowing is not a feature.  Items such as conveyor belts could benefit from this technology, in particular in dry manufacturing scenarios, or areas where water usage is to be avoided, for example, sandwich operations where Listeria species could thrive in the presence of water.
• HPP (High Pressure Processing) and PEF (Pulsed Electric Field): whilst both of these have been demonstrated to provide good decontamination characteristics in foods, their application to food contact surfaces is difficult to conceive at this point in time. HPP is ineffective on bacterial spores, used in isolation, and its effect is very organism group dependent.

The debate surrounding the need to rinse any disinfectant residue from food contact surfaces, following a suitable contact time, often focusses on the potential for chemical contamination of the subsequent food, and in many countries rinsing is a common practice that may even be required by law, as is the case in Denmark.  However, for many years in the UK, this has not been common practice for the majority of the industry, unless a tainting biocidal product, such as sodium hypochlorite, had been employed.  With the advent of approved organic products, such as through schemes administered by the Soil Association, rinsing has become commonplace in order to comply with the requirement that products must not be exposed to chemical residue. Developments in ionic silver stabilised hydrogen peroxide (Huwa-San), has led to this product being acknowledged as complying with that requirement, without the need for a rinse step.

For much of the processed food industry, a non-rinse disinfectant product remains preferable, to act as both an ongoing microbiological hurdle, and to avoid additional application of water during rinsing, that may encourage bacterial growth and spread as well as cross-contamination from floors and drains, throughout droplet spread transmission.  Those disinfectant formulations which make non-rinse claims are required to undergo both toxicological and taint test evaluations, to ensure that at the recommended in-use concentrations, the risk of contamination is low to none.  Previously the majority of non-oxidising disinfectant use was based on QAC compounds (DDAC and/or BAC), however as will be described later, the enforcement of legislation intended to control pesticide residues led to this compound requiring rinsing or switching to an alternative active substance, such as the alkylamine compounds which do not, at the time of writing, have a maximum residue level (MRL) applied.

Disinfectant Tolerance

Much discussion, often misunderstood, has taken place surrounding the ability of microorganisms, particularly bacteria, to become tolerant or resistant to the commonly used active substances employed in food hygiene.  Whilst there is significant evidence to show that sub-lethal concentrations [minimum inhibitory concentrations (MIC)] of the pure active substance, for example, DDAC or BAC, can show tolerance by some species of bacteria, when trained in a laboratory by growing in gradually increasing, but sub-MIC amounts of the active component] there is little evidence to support the argument that a properly formulated and applied disinfectant product, based on the biocidal active substance suffers in the same way This is primarily due to the inclusion, within the formulation, of other constituents that have a detrimental effect on the survivability of the bacteria concerned – for example, surfactants which weaken the bacterial cell wall, enabling the biocide to more effectively complete its mission. Further details can be found in the GFSI paper referenced at the end of this statement. 

Where bacterial populations have been demonstrated to survive cleaning and disinfection regimes, this is more commonly a result of:

• Poor cleaning practices leaving debris on surfaces resulting in occlusion
• Detergent residue remaining following an ineffective rinse stage, which inactivates the disinfectant
• Application of a 1% v/v solution onto a wet surface, leading to dilution to sub-lethal concentrations whereby there is simply not enough biocide to deal with any bacteria present.

It is worth noting that there is little to no documented evidence of bacterial resilience to oxidising disinfectants, although some studies in the US have suggested that Listeria monocytogenes may be rendered viable, but non-culturable, when exposed to a chlorine solution. More research is underway to confirm this. It is worth noting that Listeria, in particular, hides in the rough surfaces of poor welds.

Current biocidal testing protocols involve exposing known concentrations of bacteria to the formulation under examination and assessing the level of microbial reduction.  For example, the basic entry level test would be BS EN 1276 whereby bacteria are suspended in planktonic form in a solution of the disinfectant formulation, and the level of reduction is measured.  A more stringent test is BS EN 13697 whereby bacteria are dried onto stainless steel coupons and the disinfectant under test is sprayed onto the surface. This test is more rigorous due to the nature of the disinfectant only having one plane of attack to the bacterial cells, as well as the bacteria undergoing slower metabolic processes, once desiccated onto a surface.

For viruses, yeasts, moulds and certain specific pathogens or spores, different Euronorms exist which set out specific time, temperature, concentration and other test parameters, such as atmospheric gases present.

Testing is undertaken in specialist laboratories and should always be conducted against ISO methods to ensure validity and reproducibility, enabling the results to be relied upon.

In-use scenarios require the users to assure themselves that the concentration deployed to food contact surfaces is sufficient and that recommended by the formulation manufacturer.  Commonly this is achieved through the use of titration testing, either using laboratory equipment or more commonly with dropper test bottles, whereby a defined colour change is used to calculate the concentration of product present, using a conversion factor determined by the product itself. 

A simpler but cruder assessment may be undertaken using test strips, which reveal a colour change depending on the concentration of product present.  These results can be variable and should only be relied upon as an indicator of the concentration range present.  In all instances, the concentration present should be documented and recorded as part of the food safety management system, whereupon the information can be recalled, should the need for a due diligence defence be required.

Where concern is centred around a particular pathogen, many laboratories may offer a service whereby a suitable surrogate is utilised to avoid having to expose operations and equipment to unnecessary contaminants.  For example during the SARS-CoV-2 pandemic, the bacteriophage F6  (phi 6) was identified as a suitable surrogate which could be handled without risk and ‘added’ to a surface, prior to a cleaning and disinfection regime, thereby enabling the process to be fully validated as suitable to remove the organism.

With the changes brought about by the BPR outlined earlier, little research and development is now taking place to bring new liquid disinfectants to the marketplace, and indeed there is concern that further implementation of MRL’s on active substances in foods may further reduce the choices of food, and beverage manufacturers. Commercialisation of some of the physical methods described earlier may assist with fulfilling the needs of sections of the industry, however it is advisable that processors engage with their hygiene support company, and the wider industry stakeholders to ensure that effective microbiological control can be maintained to ensure consumer safety.