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Positive test for Listeria – what next?


“Your apples have tested positive for Listeria”

What happens when you answer the phone to find your fruit has tested presumptive positive for Listeria monocytogenes? A quality assurance consultant for an apple producer who took just such a call tells his story.

How were you notified?

Our wholesale market agent advised that fruit had tested presumptive positive for L. monocytogenes during routine Quality Assurance testing, but we would have to wait for enumeration testing to determine the level of infection.

What were your first steps?

Immediately we identified where any fruit was still available for sale and instructed those market agents to hold ALL fruit subject to further testing.

Product trace determined that over 1000 packages of various types were still available on three wholesale and one pre-packing operation.  ALL product holders were asked to halt sales and hold fruit in isolation pending further investigation.

Samples were taken from each consignment and packaging type for further testing to determine the extent of the infection.

A complete day’s packing was stopped on farm from transport.

Clean up started immediately with draining water flume, scrubbing and super chlorination (+150 ppm), followed by dumping the next day and then allowed to dry until recommencing production.

What was response of buyers and health authorities?

The market agent notified their main buyer who felt apples were unlikely to have high enumeration numbers. No recall to retail was deemed necessary.

The results were notified by the testing laboratory to their state Department of Public Health, and from there to our state Public Health dept, which contacted the local government authority to manage the outbreak.

On day two the Local Government Health Officer contacted us regarding our actions to date and our plans.

By day three or four a series of swabs from the apple grader were sent for testing. Water samples were taken from the water flume and rinse water for testing.

Secondary fruit testing results showed not all fruit was affected. This was explained when the source of infection was located in the washing brushes on the fruit grader, and as some consignments had been held in cold store under ozone-rich conditions longer than others.

Public Health did not consider a recall necessary, or practical, given the time lag between first test and the halt and confirmation; the low level of detection and the product’s low ability to grow L. mono.

What was the source?

Initial tests of fruit samples taken at market found two of ten samples to be presumptive positive.

Enumeration tests showed levels on the fruit were satisfactory (ie <100 Colony Forming Units per gram, (CFU/g)), at <10 CFU/g. Although unchlorinated, the rinse water was clear in tests.

A number of sites on the grader were initially swabbed, with the brushes considered the most likely point of infection given the design of brushes and the fact they stay wet for long periods of time.

Swabbing deep into the base of the brushes showed a degree of debris jammed into the brushes thatwas not removed with normal cleaning.

Levels on the swab from the brushes were extremely high.

We can only speculate on the origin– a cloddy dirty or rotten apple or a hand brush that fell on the floor and was picked up and used to scrub the brushes? Source is irrelevant. What matters is making sure we have a process that ensures it doesn’t survive in equipment to transfer to fruit.

What was required for the release of fruit for sale?

Public Health accepted our proposal that, as the fruit was within safety levels, all implicated fruit be consolidated to one cold store and treated with ozone for 24 hours. This treatment enabled us to disinfest the fruit, the packaging, (thus saving it from being dumped), and the cold room; save a transport and re-pack of product following sanitising on the packing line, and ensure no wastage of product.

Packed fruit in the grower’s cold store was also treated as above. New tubes were placed into the ozone generators prior to treatment.

All lines were re-tested and found clear before Public Health released the fruit for sale.

A deep cleaning programme of the entire grader, including washing brushes/grader occurred including lifting all errant fruit stickers on the cups and belts using steam and high-pressure treatment of the brushes followed by three chlorine soakings (100 ppm) into the brushes and all were re-swabbed and found clear.

Public Health allowed grading to re-commence given these test results.

How long after the initial detection were you cleared to trade?

Three weeks. It could have been earlier but for a wait for new ozone tubes and the thoroughness of the machine steam-cleaning.


No loss of fruit, but around $2000 cost of testing and four to five days’ work cleaning, plus no marketing for almost two weeks. Incidentals such as new cleaning materials, new ozone tubes etc. would add another $500 at least. Some changes to incoming water treatment and ongoing disinfection will also add to the overall exercise but will hopefully prevent any repeat. The major cost was to mental health. It is a very stressful position to be in.

What was helpful?

My background, and knowledgeable colleagues allowed for a measured approach. Our documented Recall/Withdrawal procedure was implemented immediately to halt further fruit movement.

The Local Government Health Officer was both helpful and practical, while the Public Health Department were pragmatic in their approach provided we supplied the scientific data in the form of papers (on ozone) or testing results to prove effective control.

What have you learnt about listeria testing process?

“A hell of a learning curve” (Owner)

Have you changed anything in the way you handle fruit as a result of this experience?

The source of infection was located in the washing brushes on the fruit grader.

Water flume was always chlorinated but only at 30ppm, which we subsequently discovered was not enough to kill L. mono. We have now raised that level to +80ppm in addition to chlorinating the rinse water at the same level. A deep sanitation of brushes is carried out at the end of 12 packing days with high pressure chlorine and every night after work a non-rinse organic sanitiser Citran is applied to the grader.

As an additional control measure, we no longer load out on the day of packing, but leave the fruit for a night in the cold room under ozone rich conditions.

Any tips for growers who find themselves in the same boat?

Very quickly, get professional support and be honest with your customers. Because we took the issue very seriously and did everything we could to rectify the situation, all our customers were very supportive and understanding.

Don’t be complacent – just because you have been growing and packing fruit for decades doesn’t mean you will never have an issue like this.

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