The last issue of Australian Citrus News reviewed the use of postharvest fungicides to control decay during storage. This article summarises the use of sanitisers which are used in conjunction with these fungicides in the packinghouse.
Sanitisers are essential for food safety and kill pathogens on contact in water, but once they are inactivated there is no residual protection. This is a fundamental distinction to postharvest fungicides, where sanitisers only kill fungi in water suspensions and do not penetrate the skin with little or no control of established disease causing infections.
However, some sanitisers such as peracetic acid (PAA) based sanitisers do have a minor benefit in slowing postharvest decay and their use is important in organic decay management.
Figure 1 shows the incidence of postharvest decay was lower in oranges infected with green mould following PAA treatment (although blue mould was less effected).
However it is critical to note the postharvest decay control was only possible by dipping the infected fruit with the postharvest fungicide, in this case imazalil. Fungicides are essential for postharvest decay control. As mentioned in the previous Australian Citrus News articles, the continued use of the same postharvest fungicide over time leads to the development of resistance to that fungicide and loss of value.
Fortunately, this is not a problem with the use of sanitisers, as sanitisers have a broad mode of action to kill pathogens and the development of resistance is not possible.
When choosing the type of sanitiser for your packinghouse there are a number of factors to consider, where the best sanitiser is the one that works best for your situation.
Each sanitiser has its own merits and issues of adoption. Factors that influence which sanitiser to choose include price, water source (channel, ground or council), water quality (pH, hardness), packing line machinery (corrosiveness), buyer’s requirement, method of application and monitoring (manual vs. automatic), etc.
But it is always important to follow the label guidelines and manufacturers’ instructions to ensure correct and safe use of sanitisers at recommended concentrations and conditions.
In addition, measuring and monitoring the concentration of sanitisers is a crucial for quality assurance and the successful use of sanitisers.
A number of sanitisers with different chemistries are currently available in the market and are registered with the APVMA.
Chlorine has been a very popular sanitiser for many years across many industries. Chlorine-based sanitisers include (a) sodium hypochlorite, calcium hypochlorite, (b) bromochloro-dimethylhydantoins and (c) chlorine dioxide.
Chlorine-based sanitisers – hypocholorite
The use of chlorine such as sodium and calcium hypochlorites as sanitisers in wash water requires active management to monitor the pH, chlorine concentration and sanitation potential of water.
The effectiveness of chlorine as a sanitiser is determined in large part by pH. It is critical to ensure that the pH of the wash water after addition of chlorine should be maintained between pH 6.5 and 7.5.
In addition to the pH, organic load also affects the efficacy of chlorine because active chlorine is neutralised by debris and dirt present in water.
Chlorine concentration and pH can be monitored using paper test strips available from chemical suppliers, but digital equipment is more accurate and objective.
Another indirect measure of the sanitising capacity of water is oxidation-reduction potential (ORP), which can be measured using an ORP probe.
An ORP reading above 700 mV suggests acceptable levels of sanitation potential. The impacts of chlorine by-products have been subject to a number of health and environmental reviews but chlorine is a well-used and accepted sanitiser.
Bromo-chloro-dimethyl hydantoins (BCDH) are another chlorine-based option as a sanitiser registered with APVMA.
These sanitisers also provide broad spectrum control of microbial pathogens. They represent a unique blend of chlorine and bromine chemistries which provide sanitising activity in a broad pH range especially towards alkaline conditions (pH 7.0 to 8.5).
It is also less affected by organic load compared to chlorine and relatively lower concentrations are required (5 – 10 ppm). Bromo chlorine products include Nylate®.
Chlorine-based sanitisers – chlorine dioxide
Chlorine dioxide is another form of chlorine sanitiser which is currently available in different formulations with a stabilised active ingredient.
Chlorine dioxide gas itself has been used a sanitiser which is dissolved into water but it can de-gas to cause WHS issues.
Stabilised chlorine dioxide is safe to use and because of its very high reactivity and low concentration (up to 10 ppm), less contact time is required for wash water treatment.
It is effective against bacteria, moulds and yeast over a pH range up to 8.5. Organic matter in water does not affect the efficacy of chlorine dioxide and it does not corrode metal. Stabilised chlorine dioxide products include Vibrex®.
Peracetic acid (PAA)
There has been a shift towards peracetic acid (PAA) based sanitisers which has been driven by the management issues associated with chlorine and the environmental impacts of chlorine by products.
PAA is sold in a mixture with acetic acid and hydrogen peroxide to maintain its stability and is reactive and quickly decomposes to oxygen, water and acetic acid (the acid in vinegar). It is very effective against a broad spectrum of fungi and bacteria.
The use of PAA at the recommended concentration (up to 80 parts per million) is effective in reducing microbial load and achieving product sanitation.
Unlike chlorine, it is effective over a wide range of pH and requires less management. However, the effective monitoring and recording of PAA concentrations in wash water is critical as for any other sanitiser and can be done using test strips.
PAA is corrosive and measures to protect machinery (such as drive chains and gears) and workers should be taken if it is used. There are a range of PAA sanitisers such as Tsunami®, Adoxysan® etc.
Iodine is an effective sanitiser for water treatment. It can be used over a wide range of pH and the water’s organic load does not affect it greatly. Iodine has limited availability for this type of use, is less corrosive to metals and is expensive.
Fully automated iodine dosing and recovery systems are commercially available For example the Isan® system is equipped with a fully automated computerised control, monitoring and logging system.
It is a closed loop process whereby the process captures all by-products and converts the captured byproducts back into the original biocide, providing a clean closed loop process. The main issues are cost and availability however it is used by some citrus packers. Providers of iodine sanitation system include the Isan® system.
Ozone is a gaseous sanitiser which can be dissolved in wash water. Ozone is very unstable and quickly decomposes into oxygen and must be generated continuously using an ozone generator.
Ozone is very effective at killing microbial pathogens over a wide range of pH but very reactive and can corrode metal and other surfaces (including packingline machinery).
High levels of organic matter will significantly decrease its activity. In addition it has a pungent smell which can cause throat, eye and nose irritation even over short exposures and has significant WHS issues if not properly managed. However it has successfully been used in some packinghouses.
A relatively new sanitation system uses electrolysed water and is currently used in other industries, including vegetable washing, waste water treatment and cooling tower water treatment.
This was discussed in a recent ACN article (Summer 2021, pages 31-33). Electrolysed water is produced by running the wash water through a series of cells and applying an electrical current (electrolysis).
If required, the water is automatically dosed with small amounts of dissolved table salt to increase electrical conductivity. This process produces a range of highly reactive but shorted lived chemicals.
EW is safe and has been shown to destroy pathogens and chemicals without compromising safety or the environment. While there is a high initial installation cost, a benefit for the use electrolysed water is the ease of use and efficacy where it has been shown to have low maintenance and running costs.
There are a number of sanitisers that are used, mainly in the organic industry. These are generally not as effective as traditional sanitisers but are useful in markets which do not use or permit traditional sanitisers.
It is important that when assessing the efficacy of these sanitisers, to note the contact times required for treatment as often these sanitisers require significantly longer treatment times to reach similar efficacy.
This is article is a contribution from the Citrus Postharvest Program (CT19003) funded by Horticulture Innovation and NSW Department of Primary Industries.
Dr John Golding is a research horticulturist with NSW DPI.