Equipment hygiene and risk assessment measures as tools in the prevention of Listeria monocytogenes -contamination in food processes: Dissertation

Kaarina Aarnisalo

Research output: ThesisDissertationCollection of Articles

1 Citation (Scopus)


Several factors affect on the hygiene level of food processing equipment. A problematic pathogen occurring in food processing is Listeria monocytogenes, causing listeriosis with high mortality (20-30%) especially for individuals with reduced immunity. This bacterium is very tolerant to different stress factors and as it can be present in most of the raw materials of food processes, its total elimination is almost impossible. Efficient control of L. monocytogenes at the processing plant level requires good equipment hygiene, including functioning good manufacturing and hygiene practices used by all employees, effective means of decontamination and rapid detection of contamination sources, as well as hazard analysis systems supported by risk assessment procedures. The present thesis focuses on deficiencies and improvements in these equipment hygiene and risk assessment practices with the aim of elucidating and developing the most efficient practices against L. monocytogenes. The hygienically most problematic types of equipment in the Finnish food industry were investigated by using a mail-survey. These were identified as the packaging machines, conveyers, dispensers, slicing machines and cooling machines. The main reason for the equipment being considered as problematic was poor hygienic design. The results show clearly that equipment designers must focus their performance on more suitable equipment design. Additionally, an investigation based on a mail-survey and microbiological sampling was made concerning hygiene performance of the maintenance personnel in food processing plants. Clear deficiencies were found e.g. in use of protective clothing, washing of hands and tools as well as avoiding foreign bodies left on the production lines. The results of these studies also indicate that L. monocytogenes may be transferred through maintenance work. Training of maintenance personnel with reference to hygienic practices must be increased. Topics connected to the maintenance operations which have received only minor attention in previous studies include the growth and survival of L. monocytogenes in lubricants used in the equipment as well as control of the bacterium with disinfectants at cold temperatures. In the current thesis the survival, growth and transfer of the bacterium in lubricants used in food processing equipment was studied. The results showed that lubricants used in maintaining the equipment may act as contamination vehicles of L. monocytogenes. As the temperatures in food processing premises are usually low, an investigation of the efficiency of eight commonly used commercial disinfectants against L. monocytogenes strains at +5 °C was performed. The tested agents were generally efficient at the recommended concentrations and effect times. Thus they appear to be suitable for control of L. monocytogenes at the plant level, with only a few exceptions. Rapid, reliable and easy-to-use methods are needed at the processing plant level. Consequently the suitability of automated ribotyping was compared with the traditionally accepted and successfully used pulsed-field gel electrophoresis (PFGE) to discriminate L. monocytogenes isolates and thus trace contamination sources in food plants. PFGE had a higher discriminatory power for L. monocytogenes isolates than automated ribotyping. However, based on its automation and rapidity automated ribotyping can be considered a good method for control purposes, although in epidemiological studies identical results must be confirmed with PFGE. Additionally, in this thesis risk assessment practices were developed by investigating and modelling recontamination of a product and by a plant-level quantitative risk assessment. Transfer of L. monocytogenes from slicing blade to slices of cold-salted salmon was investigated and modelled. Transfer with a progressive exponential reduction in the quantity of bacteria (log CFU/g) in slices was detected. The results provide an example to food processors of how limited data from microbiological analysis can be used to assess the level of recontamination for risk assessment purposes. The principles of microbiological risk assessment can be used at the processing plant level to assist in developing Hazard Analysis Critical Control Point (HACCP)- systems in order to provide a more scientific and comprehensive approach to the control of L. monocytogenes and other microbiological hazards. As a concluding example, a practical approach to quantitative risk assessment of L. monocytogenes for one product at the plant level is presented. This approach helps food processors in illustrating the risks caused by the products for consumers and thus rationalizing risk management actions against L. monocytogenes
Original languageEnglish
QualificationDoctor Degree
Awarding Institution
  • Aalto University
  • Raaska, Laura, Supervisor, External person
  • Wirtanen, Gun, Supervisor, External person
Award date25 Jan 2008
Place of PublicationEspoo
Print ISBNs978-951-38-7069-0
Electronic ISBNs978-951-38-7070-6
Publication statusPublished - 2007
MoE publication typeG5 Doctoral dissertation (article)


  • food hygiene
  • Listeria monocytogenes
  • risk assessment
  • hygienic design
  • disinfectant
  • lubricant
  • maintenance
  • microbial modelling
  • transfer of bacteria
  • automated ribotyping


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