A bulletin for the Australian Food Industry November 2001
Contents: Food allergies and food sensitivities | Pathogens in fresh fruit and vegetables | Decontamination of fresh fruit and vegetables | Preservation of vegetables in oil and vinegar
Decontamination of fresh fruit and vegetables
The difficulties inherent in decontaminating fresh fruit and vegetables, particularly cut or otherwise damaged, have been discussed previously (Food Safety & Hygiene September 2001; Food Safety & Hygiene May 2000). These difficulties were confirmed by a number of speakers at the Fresh conference hosted by Food Science Australia in Melbourne in October. The conference was sub-titled, The Future in Food Safety and Processing Technologies for Value-Added Horticultural Products.
The problems are multi-faceted. The effectiveness or otherwise of sanitizer treatments is dependent on the ability of the active sanitizing agent to contact the target organisms. Microorganisms embedded in tissue will be protected from chemicals in solution that have little penetrating power. Thus widely used and normally effective sanitizers such as chlorine, ozone, and hydrogen peroxide have had limited success in achieving significant reductions in microbial loads of cut fruit and vegetables.
US researchers using lettuce leaves as the test substrate have shown that the only penetration of the leaf by inoculated Escherichia coli O157:H7 occurred at cut edges (Journal of Food Protection 62 1999 3-9). E. coli associated with stomata on the leaf surfaces did not penetrate the leaf tissue but was also protected from inactivation by chlorine (20mg/L). These observations were made using sophisticated microscopic techniques.
These workers conclude that the safety of lettuce, and perhaps other leafy produce, could be enhanced by discarding damaged leaves and exposing undamaged leaves to effective levels of sanitizer before cutting, thus preventing unwanted contamination of damaged tissue.
Other US researchers have reached similar conclusions working with E. coli O157:H7 and injured and uninjured green pepper (capsicum) surfaces (Food Microbiology 17 2000 643-655). Again using advanced microscopy techniques, this group showed that more living bacteria were found at injured regions of pepper surfaces after a sanitizing treatment with chlorine dioxide gas.
They conclude that their results are important for minimally processed fruits and vegetables especially fresh cuts where the cut surfaces will largely protect bacteria from chemical treatments. This group of workers also suggest it is critical to sanitize uninjured surfaces before cutting or damage occurs. Once the cut or injured surfaces are contaminated by pathogens, it becomes very difficult to inactivate these attached or growing bacteria.
These workers also makes the point that intact surfaces will not provide nutrients for adhered bacterial growth and the bacteria are in worse living conditions prior to any sanitizing treatment. It is also of course important to maintain a clean processing environment after sanitation.
A notable feature of the work of this group from Purdue University was the success achieved with chlorine dioxide gas as sanitizer on both injured and uninjured pepper surfaces. On the uninjured surface in excess of a 7 log (10 million) reduction of E. coli O157:H7 was obtained with 0.60 mg/L chlorine dioxide gas treatment for 30 minutes at 20°C and 90-95 percent relative humidity. For injured surfaces, a greater than 6 log reduction (1 million) was achieved with 1.2 mg/L chlorine dioxide gas under the same conditions.
While noting their results as encouraging, the researchers caution that more studies on chlorine dioxide gas sanitation should be done including application technology on a commercial processing scale. The experiments reported were carried out in a 10 L glass cylinder with a stainless steel shelf where the green pepper samples were placed.
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