Ozone treatment could also be an effective and safe replacement for post-harvest pesticides as it leaves no residues on foods, said lead researcher Dr Ian Singleton, of Newcastle University, who recently presented his findings at the Society for General Microbiology's Spring Conference.
The scientists have further found that exposing fruit such as tomatoes to ozone prior to storage acts as a vaccination against infection, boosting their ability to resist fungal spores.
Fungal contamination is the most common cause of spoilage of stored fruit, salads and vegetables and the risk of microbial contamination increases with longer storage periods. Previous studies have estimated that microbial spoilage can cause the loss of up to 30 per cent of fresh produce, said the senior lecturer at the university’s School of Biology.
However, storing fresh produce such as strawberries, tomatoes, grapes and plums in low levels of gaseous ozone could radically reduce the formation of the microbes that lead to fungal spoilage. Ozone was employed because of its powerful anti-microbial properties and its ease of application to fresh produce in gaseous form.
95 per cent reduction
His research has shown that enriching the storage environment with ozone not only substantially cuts fungal spore formation but also reduces the appearance of visible lesions on fruit that are already infected. This is important as the lesions act as a point of infection through which fungal spores are spread.
“Our results show that ozone gas at low concentrations of around 200 ppb (v/v) and lower, can inhibit fungal spoilage such as Botrytis cinerea on a variety of fruit, and reduces fungal spore production,” he said. “This reduction in spore production would act to reduce the spread of disease during post harvest transit/storage.”
Fruit stored at low levels of ozone for up to eight days prevented almost 95 per cent of disease from developing depending on the fruit and levels of fungal infection compared to a conventional non-ozone storage environment of 4°C , said Dr Singleton.
From the 1950s onwards, heat treatment was replaced with cheap and effective synthetic fungicides, often used in combination with pre-pack sanitation treatment containing chlorine or bromine.
"There are public concerns over pesticide residues on fresh produce,” added Dr Singleton. “Ozone is a viable alternative to pesticides as it is safe to use and effective against a wide spectrum of micro-organisms. Importantly, it leaves no detectable residues in contrast to traditional methods of preserving fresh produce."
The team has discovered that exposing tomatoes to ozone before infecting them with fungus also reduced spoilage.
"This suggests that ozone treatment exerts a 'memory' or 'vaccination' effect that protects fruit from damage. It is unclear how this phenomenon works, but is certainly worthy of further, detailed investigation," said Dr Singleton.
The group said the need to optimize levels of ozone and length of exposure for each variety of produce was key.
"Different fruits have been shown to have different tolerances for ozone,” he said. “We need to look carefully at how we control the atmospheric concentration of the gas in stores and transit containers, since levels of ozone that are too high can damage produce, causing financial losses.”