Surface Steam Pasteurisation of Fish Products: Thermal Characterisation and Microbial Selection/Resistance (Oppervlakte-stoompasteurisatie van visproducten: Thermische karakterisatie en microbiële selectie/resistentie)

With the increasing market demands for minimally processed foods, processes that optimise bacterial reduction while minimising the thermal load are of continued interest. Fish, in particular, is very sensitive to thermal processing and often becomes tough and dry when exposed to excess heat. In several stages of food production, microbiological contamination is most likely to occur on the food surface. Hence, optimal heat processes, should target the surface contamination without causing unwanted thermal degradation. Short time, high temperature surface treatments using steam is potentially an efficient process that combines these factors.Listeria monocytogenes remains a challenging pathogenic organism for the seafood industry. It is omnipresent and it can grow at refrigerated temperatures. It is also the most heat resistant of the non-spore forming pathogens. Hence, the inactivation of L. monocytogenes during processing, as well as the growth conditions for potential survivors during storage, are important factors to be taken into account in order to ensure food safety. The objectives of this work were: (i) to identify an appropriate surrogate of Listeria monocytogenes, (ii) to develop a test system for artificial inoculation with realistic levels of the bacteria on fish products, (iii) to characterise a steam surface pasteurisation process and finally (iv) to quantify the microbial recovery post-treatment for assessing the efficacy of this decontamination process.The growth Listeria monocytogenes strains isolated from fish processing environment and two reference (non pathogenic) Listeria innocua strains, was investigated in liquid broth at refrigerated and close to optimal growth temperatures. Estimates for the growth rates and the lag phase duration of single strains and cocktails were obtained from optical density measurements by using different growth parameter estimation methods. The serial dilution method was preferred it as yielded growth rate (μmax) estimates close to the biological value, and in addition, lag time (λ) estimates. To discriminate between different strains, λ appeared to be a more suitable parameter than μmax. This effect was most prominent for L. innocua, which showed significantly longer lag times as well as lower growth rates than L. monocytogenes at 4 and 8 ºC. In order to accurately describe a thermal process such as steam pasteurisation thermal characterisation of the system is required. Heat transfer modelling can be used to estimate the surface temperature in a steam environment, or alternatively thermal load can be estimated from recorded surface changes. In this work, reflectance spectroscopy was investigated with respect to monitoring thermal load. A well-defined water bath treatment procedure was compared to a surface steam pasteurisation process, both for the heat transfer modelling and the spectroscopic analyses. The spectroscopic measurements showed that visible spectroscopy (400-550 nm) has potential for assessing the thermal load of a fish product (surimi) surface heated in water in the temperature range between 70 and 95 ºC. Heat transfer modelling confirmed the thermal load indicated by the measurements, and demonstrated the large temperature gradients that occur with short time, high temperature treatments. The accurate thermal characterisation of the system could then serve as an input for its antimicrobial assessment that follows in this Thesis.Antimicrobial efficacy assessments were performed by first investigating the effect of the inoculation technique and the initial inoculation level for Listeria innocua on cooked surimi. High and low inoculation levels were subjected to the steam surface pasteurisation process previously characterised. Inoculation with high levels confirmed findings in the literature, showing an initial 1-2 log reduction with no further reductions with increasing processing time (up to 20 s). Inoculation with more realistic, low levels, and analysing the treated samples using a novel contact plating approach could serve as a reliable method to assess the efficacy of the treatments. Using logistic regression, results from this method were used to model the binary responses of Listeria. The low level inoculation represents a more realistic scenario, and as such the binary responses derived from this method are more in line with industrial processing assessments.The results reported here have contributed to improving the understanding of the mechanisms involved in the steam surface pasteurisation of real foods and therefore, assisted in improving food safety assurance under specific operation conditions.nrpages: 149status: publishe