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Listeria: a persistent threat

Demi Mardyri talks about Listeria which remains a big concern in the food industry, especially among the Ready-to-Eat food manufactures.

Listeria became a hot topic in recent years and regardless the many industry guidelines and new legislation requirements, it still remains a big concern in the food industry, especially among the Ready-to-Eat food manufactures.

According to data obtained from FSA, in 2016 there were 11 product recalls relating to elevated Listeria counts, more than any other bacteria [1]. All these recalls were related to fish and dairy products, which suggests that this number could be considerably higher, if products like sandwiches and salads with much shorter shelf life and equally vulnerable to Listeria, could be included. Also, an average of 180 cases of Listeriosis are reported annually [2]. Despite the advances in food technology and the industry’s efforts, these figures have remained unchanged for the last decade.

The missing link in the chain
Different control measures have been established by the manufactures, some of which include, zoning, intensive cleaning and disinfection procedures, cooling systems, heating steps and pre-treatment of raw materials. However, what most companies fail at, is to effectively monitor these activities. Monitoring activities should always include: (a) measuring of product and processes parameters (b) testing those parameters against established tolerances/limits (c) Regulating any necessary changes and (d) Taking corrective actions [3].

Product monitoring
Listeria concentration in raw materials is a key factor on whether a lot should be accepted or not, but also for the evaluation of suppliers’ performance. Since in many RTE products, like salads and sandwiches, raw materials are minimally processed, the final products quality is highly determined by the raw materials quality [4]. Every company should have a sampling programme and a positive release procedure for the most sensitive and prone to Listeria raw materials. Also, when raw materials with high Listeria counts are introduced in a factory, the risk for the pathogen to enter and contaminate the processing environment is higher [5].

Listeria counts in finished RTE products, other than those intended for infants or for medical purposes, should not exceed 100CFU/g throughout their shelf-life [6]. Indeed, it was found that products containing more than 100 CFU/g Listeria counts were responsible for more than 99% of Listeriosis illnesses [7]. In order to evaluate the safety of foods, a company must perform regular testing. Regulation indicates typical sampling plans and methods, however it is highlighted the liability of food business operators to decide on the appropriate sampling frequencies [6,8].

Product characteristics can determine how easily Listeria can grow in a product. RTE foods considered to be more resistant against Listeria growth are those with either pH ≤ 4.4, or aw ≤ 0.92, or pH ≤ 5 and aw ≤ 0.94 [8]. Product monitoring during production will guarantee that Listeria counts will not exceed the established limits during the shelf-life. The risk of Listeriosis is increased in long shelf-life products where Listeria has adequate time to proliferate. Predictive microbiological modelling based on key physicochemical factors like pH, aw and salt concentration can confirm if the assigned shelf life is the appropriate one, showing the potential Listeria growth. The European Union has published protocols for durability and challenge test that can be performed to guarantee the shelf-life and Listeria safety of RTE foods [9].
Consumers complains on the final products are also part of the product monitoring activities and should be well documented, since they can provide information for possible malfunctions and changes needed in the production line [3]. Experts from various departments (commercial, technical, production and planning) should analyse complains, identify trends and agree on corrective actions.

Process monitoring
Process monitoring activities verify whether the food production is under control and products meet the safety and quality requirements. Every process should be stable and repeatable, in order to provide consistent results, so every company must set requirements on process and equipment parameters.

Instruments used for measuring process parameters (e.g. temperature, pH meters) must be selected to provide accurate and precise results. Considerations during the selection of a measuring instrument should be the material, easiness of cleaning and calibration [10]. Calibration must follow a written standardised procedure. Display units, cool rooms, freezers and vehicle temperature gauges should be reviewed frequently depending on the safety risk and/or previous documentation. Inadequate instrument calibration has been mentioned as a cause of safety problems. There have been found big divergence between the temperatures indicated in the cooling storages and the temperatures measured by calibrated thermometers.

Maintenance (or replacement) of the processing equipment should also be a schedule procedure, to prevent an equipment of being source of contamination and prevent a breakdown during production. In case of emergency maintenance, production must be stopped, products must be appropriately stored and after the repair has complete a thorough cleaning step is essential. Moreover, periodical inspection for cracked or worn parts and places where pathogens can developed should take place.

Environment monitoring
Environment monitoring activities aim to identify the presence of L. monocytogenes in the processing environment and consequently to verify the effectiveness of control measures, e.g. the sanitation programme. By testing food contact surfaces (FCS), a company can evaluate the sanitation in the processing environment. Listeria control can be part of a company’s HACCP plan, sanitation SOP or prerequisite program. It is argued that Listeria control is determined by the design of the environment monitoring program and the response to possible positive findings [11]. Food companies are expected to have a routine and an enhanced sampling program when positive samples for Listeria are found. Based on the outcome of the environment monitoring, it is determined if adjustments or additional preventive or intervention measures are needed [10].

The frequency of sampling program must guarantee that the processing environment is under control. Samples collected must be analysed immediately in order to act rapidly in case of positive findings. Follow-up sampling in case of positive samples (enhanced sampling program) will ensure that the source of contamination has been detected and eliminated. Short-term (the last 8 to 10 samples) and long-term (quarterly, annually) assessment to detect growth trends and patterns,
niches and overall progress towards continuous improvement [10,11]. When positive samples have not been obtain for long periods of time then the efficiency of the monitoring program should be re-evaluated. Contrary, occasional positive results should not be considered as a failure but as a confirmation of the program’s effectiveness. Monitoring programs which cannot detect any source of Listeria contamination may cause a false belief that there is no safety risk.

Initially samples must be collected at random, so that the different food conduct surfaces have the same probability of been tested. The company must keep records so that all food contact surfaces are tested for Listeria over a period of time. At a second step and when enough data have been collected, the company can adopt a risk-based sampling program. Like Tompkin [11] described “environmental sampling programs should reflect previous experience in each facility and the sampling sites should include areas that have been found to be good indicators of control”. Food contact surfaces include conveyor belts, slicers, utensils, tubs, trays, racks, even employees’ hands and gloves if they come into direct contact with the products.
Listeria control is definitely a multi- dimensional issue, hard to eliminate, but small interventions and shift from a reactive to a proactive approach can bring back control.

Demi Mardyri, PhD. Cand, MSc., MIFST
Food Technologist FMCG


3. Luning, P. A. and W. J. Marcelis (2009). Food quality management: technological and managerial principles and practices, Wageningen Academic Publishers.
4. European Chilled Food Federation (2006). "Recommendations for the production of prepackaged chilled food " ECFF Recomendations.
5. Gilbert, S., R. Lake, A. Hudson and P. Cressey (2009). "Risk profile: Listeria monocytogenes in processed ready-to-eat meats." Institute of Environmental Science & Research Limited Christchurch Science Centre: 1-90.
6. (EC), C. R. (2005). "Regulation (EC) No. 2073/2005 of 15 November 2005 on Microbiological Criteria for Foodstuffs." Official Journal of European Union 338: 1-26.
7. Gombas, D. E., Y. Chen, R. S. Clavero and V. N. Scott (2003). "Survey of Listeria monocytogenes in ready-to-eat foods." Journal of Food Protection® 66(4): 559-569.
8. Chilled Food Association (2010). "Shelf life of ready to eat food in relation to L. monocytogenes - Guidance for food business operators." first edition.
9. (EC), E. C. (2008). "Guidance document on Listeria monocytogenes shelf-life studies for ready-to-eat foods, under Regulation (EC) No 2073/2005 of 15 November 2005 on microbiological criteria for foodstuffs." Commision staff working document (SANCO/1628/2008 ver. 9.3 (26112008)).
10. Food Safety Authority of Ireland. (2005). "The control and management of Listeria monocytogenes, contamination of food." from
11. Tompkin, R. (2002). "Control of Listeria monocytogenes in the food-processing environment." Journal of Food Protection 65(4): 709-725.

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