Ozone and the treatment of superficial scald

Why ozone?

Ozone is a powerful oxidant that has several advantages. Most notably, it leaves no detectable residue on treated products. After its recognition as generally regarded as safe (GRAS) in the US, the use of ozone in the food industry has rocketed. In addition to being an effective sanitizer, ozone is also used to prolong the postharvest life of horticultural products by helping to oxidate ethylene.

Currently, ozone does not form part of postharvest treatments used by the South African pome-fruit industry. Therefore, its potential effect on superficial scald on "Granny Smith" apples is yet to be determined.

To fill this knowledge gap, Dr Asanda Mditshwa, an associate professor at the University of KwaZulu-Natal, embarked on a study to i) determine the concentration at which ozone should be applied to inhibit superficial scald, and ii) investigate the effect of ozone levels on superficial scald-associated biochemical precursors during cold storage. The latter was done through a detailed biochemical analysis of major metabolites regulated by ozone treatment.

The research methodology

The study was carried out over two seasons. Owing to the findings of the first season, the objectives of the study had to be slightly modified. Its first objective was to identify the best ozone concentration for inhibiting superficial scald. However, challenges in terms of controlling the gaseous ozone concentration resulted in the objective being altered to assessing the suitable ozone exposure time for controlling superficial scald.

For each treatment and sampling interval, uniformly sized "Granny Smith" fruit was randomly divided into four replications, each comprising 40 apples that were packed in a ventilated plastic carton. The cartons were placed in three cold chambers controlled at 0 °C and 95% relative humidity.

To determine the best ozone exposure time to control scalding, the fruit was exposed to continuous 0.3 ppm ozone for 24 or 48 hours.

At each sampling interval, 10 randomly selected apples per replicate were assessed for fruit colour and firmness, as well as total soluble solids (TSS) and titratable acidity (TA). Scald incidence was recorded as the percentage of fruit with superficial scald symptoms.

The biochemical analyses focused on the levels of ethylene, ascorbic acid, antioxidants, -farnesene and conjugated trienols, all of which influence the development of superficial scald.

What the research found

The first season's findings showed that an ozonated atmosphere can inhibit superficial scald development in "Granny Smith" apples. After 15 weeks of storage, 80% of the fruit in the control group had superficial scald compared to zero incidence in the ozonated atmospheres.

However, although the ozone inhibited scalding, fruit stored under ozone had necrotic spots that worsened with storage time. Similar spots did not develop on the control fruit, indicating that the continuous 0.3 ppm ozone was too high for "Granny Smith" apples.

The second trial found that the ozone exposure time has a significant effect on superficial scald incidence. Throughout the storage, fruit exposed for 24 hours to gaseous ozone had much lower scalding compared to the untreated fruit and those exposed for 48 hours. After 21 weeks of storage, scald incidence in untreated fruit was 70%, compared to 45% in fruit ozonated for 24 hours and 65% for fruit that spent 48 hours in the ozonated atmosphere.

Although not conclusive, the results also showed that ethylene, -farnesene and conjugated trienols are significantly suppressed by gaseous ozone. As these compounds are all involved in superficial scald development, existing postharvest treatments generally aim to suppress them.

What next?

The study delivered a mixed bag of positives and negatives. As far as positives go, exposing apples to ozone for an uninterrupted 24 hours before long-term storage is partly effective in controlling superficial scald. Plus, the laboratory tests showed that gaseous ozone has an enormous effect on the physicochemical and biochemical attributes of "Granny Smith" apples. The reduced ethylene production and conjugated trienols under ozonated atmospheres are irrefutable evidence of the potential inherent in ozone treatments.

However, the 45% incidence of the disorder after 21 weeks of storage is a major negative, along with the loss of ascorbic acid in the ozonated fruit. Fortunately, the research team is of the opinion that combining currently used technologies such as controlled atmosphere, with gaseous ozone applications could mitigate the problem. Therefore, further research is warranted.

The case for on-going research is strengthened furthermore, given the high cost of some of the treatments currently used by the industry. Not only could gaseous ozone be a far more affordable alternative for producers of deciduous fruit, its anti-microbial attributes might also reduce the costs of quarantine treatments.