Michel van Schaik, September 2009
Introduction
Over the past few years, the public has been bombarded with advice on healthy eating in lifestyle features on television, radio and in the magazines and newspapers. Government health agencies and school catering services have also started to focus on the same topics,encouraging the consumption of more fruit, vegetables and salads. But whilst this has been happening there has been a parallel bombardment regarding poor hygiene, food scares and the hazards of chemical residues in these self-same healthy foods.
The public is in a dilemma and the perception is that they feel that cannot fully trust many of the so-called healthy foods. This is particularly heightened when food is imported from other parts of the world where standards may not be as rigorous or the risks may be greater. There is also an increased awareness of the lack of hygiene integrity of some supply chains. The produce may change hands many times between farm and consumer and become exposed to potential contamination at each step. Processors and providers try to anticipate the many hazards in this chain and remove, reduce or manage them to the best of their ability without altering the taste, texture or quality of the foodstuffs. In attempting to do this, many are now regarded to be adding yet more hazards where chemical disinfection methods or microbistatic additives are used. A dilemma exists for the processors in weighing up the provision of microbiologically safe food on the one hand yet not is perceived to add chemicals or cause formation of by-products in the food on the other.
The hypochlorite issue
This is particularly an issue where sodium hypochlorite or ‘bleach’ is used. So great is the concern generated by many reports, particularly in the US, of by-product residues that the food industry is steering away from this traditional disinfection method although it has provided good protection in the past. The sensitivity of detection of residues has also increased by orders of magnitude in the last 10 years; however, the fact that one can detect something does not mean that it is a major risk. For example, to achieve amounts of trihalomethane residues approaching toxicological limit values in typical hypochlorite-treated lettuce, one would have to consume many kilograms of such lettuce per day. Some, mainly foreign, production practices (irrigation using ‘grey’ unpurified water, fertilization of crops using poorly treated sewage sludge or manures or the application of pesticides, herbicides and other chemicals) are of course increasing the risks of by-product formation on encounter with hypochlorite. Similarly, high concentrations of hypochlorite and poor quality hypochlorite can result in the same. Detection of significant levels of residues from any of these incidences fuel the public’s concern and skepticism. The search is on now for some time for a safe and reliable method of reducing the risk of food poisoning which will not affect the quality parameters of fruit, vegetables or salads and which will be acceptable to the consumer.
Chemical disinfection methods to date have used a wide range of chemicals. These include:
- sodium or calcium hypochlorite
- chlorine dioxide
- acidified calcium chlorite
- organic acids
- ozone
- hydrogen peroxide
- acetic acid
- ethanol
- calcium hydroxide
Back to basics
As researchers approach the problem of improving the safety of fresh produce, some of the basic concepts have altered a little whilst others remain the same. While the goal for most is the complete removal of microorganisms from the fresh product, there has been a renaissance in the idea that there is a need to retain the natural microbiota in order that it may challenge and educate our immune systems. This has arisen within the last years following the discovery by a Japanese team of immunologists that our immune system is controlled by a heretofore unrecognized set of cells and may in fact work in reverse of the currently held model. As we remove the challenges, we may increase the risk of allergies and other malfunctions.
One of the basic concepts of disinfection remains unchanged and still gets ignored by many. It is of course that decontamination is the fundamental key to successful disinfection. If extraneous soil and detritus are not first removed from the product, disinfectant cannot totally access the product effectively. Also, the disinfectant may be absorbed or inactivated by residual soil, which reduces its availability. This may seem a simple concept however it still causes problems, even in the realms of clinical disinfection where its omission may be life threatening yet still happens. The following principles therefore apply to microbes found on vegetables:
• They originate from many sources and consist mainly of bacteria and yeasts in close association with the surface. Fungi are usually more loosely associated as part of the surrounding rhizosphere in the soil.
• Microbes pathogenic to humans are not generally commensals on vegetables. They arrive as contaminants from a pollution source and are rarely very firmly attached.
• Commensals on vegetables normally form adherent biofilms and may entrap pathogens coincidentally. This can be on stems, leaves, roots or fruits.
• The microbes which occupy the surface of vegetables normally contribute to the healthy stimulus of the immune system
• Biofilms protect most microbes within them from dessication and from a wide range of chemical treatments. Hydration followed by physically agitated removal are the most effective means of removing the biofilms.
• Root vegetables and sprouting seeds chemically communicate with microbes and allow them to partially invade as part of their growth cycle. These become symbiotic and are impossible to remove e.g. alfalfa. The molecular mechanisms by which the microbes signal to be taken in by the plant are now known to be similar if not the same as those by which pathogens signal our gut to take them in. Experiments have shown that Salmonella and E.coli can easily be taken into sprouting beans from contaminated water from a very early stage.
• If the normal microflora is removed from vegetables it becomes easier to contaminate them with pathogens.
• Post-decontamination treatments aimed developed to prevent this include coating with lactobacilli and bacteriocins. Poor adherence by lactobacilli is currently a major problem. Advances in understanding adherence mechanisms may soon improve this situation.
The following principles apply to disinfection of vegetables:
• Complete removal/disinfection of microbes may not be possible from all types of vegetable. (Killing of microbes in situ may only be possible by irradiation, cooking or high pressure treatment in some situations.)
• Adequate decontamination must always precede disinfection for efficacy
• Physical removal of soils and biofilm is essential for access by chemical disinfectants
• Many disinfectants are neutralized by soils and biofilm
• Many vegetable types produce catalase or peroxidase. If cut or damaged, these exude and can neutralize ozone or hydrogen peroxide treatments for example
• Where adequate removal of soils and biofilm has not occurred, the propensity for higher residual by-products occurs
New directions with Electrolyzed Water
Sodium hypochlorite has, as was mentioned earlier, for a long time, been the chemical disinfectant most frequently used with vegetables. Due to increasing concerns over physical safety, chemical safety and organoleptic changes associated with it, an alternative is required. Electrolyzed Water has excellent disinfectant properties with few disadvantages. However, unlike the other alternative disinfectants Electrolyzed Water can be produced in a number of different forms, each having different effects. In many cases, this has not fully been appreciated and has led to inaccurate comparisons and conclusions.
Neutral Electrolyzed Water (NEW)
Electrolyzed Water for the purposes of vegetable disinfection is produced at neutral pH, has consistent, fixed production parameters, is referred to as Neutral Electrolyzed Water (NEW) and is as produced by a particular patented electrolytic flow cells.
To make NEW, one starts with 99.8% water containing 0.2% sodium chloride. Assuming purity of both of these components, the only elements present are hydrogen, oxygen, sodium and chlorine. The solution contains molecules, ions and free radicals derived from these. The process then raises this solution to a highly activated and reactive state in a catalytic flow chamber through insertion of electrical energy. This is a complex process involving both electrochemical activation and physicochemical activation. The latter involves formation of microbubbles and plasma events close to the anode surface which impart a range of properties beyond those of standard chemical disinfectants. The resulting product stream, NEW, consists of the following dilute mix of short-acting oxidants in water.
NEW kills all accessible microbes including spores and Cryptosporidium within seconds. This weak but activate HOCLE solution, returns to its former ground state in immediately on use. Because of this and also because the user produces the product on-site it is not currently classified as a chemical disinfectant under the Biocides Directive. Free molecular chlorine is not a feature of this product.
The activated hypochlorous acid (HOCL) in NEW is over 400% more effective than that formed chemically in for example, bleach. NEW carries a redox charge of +900mV which directly and irrepariably damages the microbial cell wall. In addition, the arrangement of water molecules is electrochemically altered which allows better penetrability and interaction of the microbicidal ions. This is another feature not found in conventional disinfectants.
The process of HOCL formation is almost identical to that found in phagocytes in all warm-blooded animals and man where it is effected by catalytic enzymes and energy from the electron transport chain. There, HOCL is a natural (organic, if you like) microbicidal process which to date has not seen evolution of microbial resistance to it, yet is harmless to ourselves. We produce our own antidotes or anti-oxidants.
In this electrochemically HOCL production process, electro-positive ions are removed from an aqueous saline solutions into a totally separate flow stream which is referred to as Alkaline Water (AW) or Catholyte. This stream, in turn, can be used for a range of purposes utilizing its unique detergent-like physicochemical properties. This can be used as the preliminary decontamination wash for vegetables and can be used as a general cleaning agent anywhere.
This HOCL production process by electrochemical activation is therefore unlike the dissociation process in hypochlorite where all components still remain in the disinfectant solution and become involved in any ensuing reactions such as the creation of trihalomethanes, chloroforms etc. The high activation state of NEW destroys such substances if they arise and in fact it can be used to reduce or remove such contaminants from surfaces or solutions. Use of HOCL is also a recognized method of breaking down phenols,taints and odoriferous substances.
The use of HOCL as a disinfectant for vegetables is very attractive from many standpoints in terms of effectiveness, variety of forms of use, cost, safety to workers and consumers, ease of use, lack of disposal problems with the added bonus of co-production of a detergent-like Alkaline Water (AW) or Catholyte stream.
Numerous studies have shown HOCL to outperform hypochlorite for vegetable disinfection and extension of shelf life and also to outperform the other short acting oxidants on their own in terms of speed and safety.
