A bulletin for the Australian Food Industry September 2001
Contents: Chloropropanols in food | Irradiation of herbs, spices and certain nuts in Australia | Surface decontamination of melons | Antibiotic use in food producing animals | 75 years of CSIRO Research
Surface decontamination of melons
Fresh cut melon products have been associated with foodborne illness on a number of occasions (Dairy, Food and Environmental Sanitation 17 1997 284). Australia has not been exempt from this problem (Food Safety & Hygiene November 1999).
The latest reports come from the United States where there have been numerous incidents of infection across at least 14 States by Salmonella poona. The outbreak has been associated with consumption of cantaloupes grown in Mexico and imported from two suppliers. Two deaths have been recorded and the Food and Drug Administration has halted importation from the suppliers (Promed Digest 117 2001; Microbiology Newsletter 5 2001).
A number of different melons are produced around the world for human consumption. All grow at ground level and most have very rough surfaces. The cantaloupe melons grown in Mexico have an uneven, scaly rind while the popular rock melons grown in Australia are characterized by a raised network of surface tissue. All are vulnerable to surface contamination and subsequent contamination of the flesh during slicing. This is of increased significance as melons have become a popular fresh cut item at the food service and retail level in this country.
Many researchers have evaluated the effectiveness of a wide range of chemical disinfectants in reducing the number of pathogens on raw fruit and vegetables. None has proved effective in any consistent manner.
One of the more recent investigations (Journal of Food Science 66 2001 345) has concentrated on fresh cut cantaloupe and is of special relevance following the outbreaks attributed to melons cited above.
These researchers note that in their previous attempts to improve the microbiological quality and shelf life of fresh cut cantaloupes, the results were erratic and the benefits small. These studies involved treatment of fresh cut cubes with various antimicrobial washes after an initial wash of the intact melons with 1000 mg/L chlorine at pH 6.5.
Because of chemical changes and residues in the fresh cut cubes, the researchers decided on this occasion to try to more effectively decontaminate the whole melons rather than the cut flesh. A vigorous protocol to avoid cross contamination was developed including physical isolation of rind removal steps from cube preparation and use of separate knives and cutting boards for the two operations. Separate work stations and technicians were also part of the protocol.
They found that fresh cut cubes prepared from control, chlorine washed (1000 mg/L) and hydrogen peroxide (5%, 50°C) washed melons showed minimal difference in total aerobic count at the beginning of storage. However during storage at 4ºC, the counts of melon cubes were consistently lower after the hydrogen peroxide treatment. They believe this effect was not due to residual peroxide transferred to the flesh during fresh cut preparation. No peroxide could be detected in the flesh after 20 minutes whether whole melons were rinsed after treatment or not.
The researchers conclude that variability in the response of bacterial contaminants of cantaloupes to antimicrobial treatments is a continuing problem. However a hydrogen peroxide wash applied to melons prior to cutting shows promise of improving the microbiological quality and shelf life of fresh cut cantaloupe.
As if to emphasise the problem of surface decontamination of melons, members of the same group from the US Department of Agriculture Eastern Regional Research Centre have also published a companion study (Journal of Food Safety 21 2001 31). In this study, they found that a similar hydrogen peroxide treatment was more effective than 1000 mg/L of chlorine in reducing the natural surface microflora of cantaloupe. However chlorine treatment was more effective against an inoculated non-pathogenic strain of Escherichia coli.
It should be noted that both groups were endeavouring to obtain as long a shelf life as possible of the fresh cut melon at 4ºC. Another important risk minimization step with fresh cut produce is to maintain the shelf-life within realistic boundaries especially if uncertainty exists about temperature control.
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