Objective
The aims are to identify and characterize the specific spoilage organisms producing changes in the various fish products and to provide a scientific basis for understanding the mechanisms of microbial spoilage. Growth response of the specific spoilage organisms to combinations of preservative factors will be used to construct mathematical models for growth and spoilage helping the industry to do rapid prediction of realistic shelf lives and risk assessment of various products. The ultimate aim is to reduce the number of rejections, minimize microbiological hazards and provide the basis for much more cost efficient quality assurance programmes.
Microbiological changes and spoilage of chilled fish products have been studied and the results utilized to construct mathematical models for predicting the shelf life of various products. Vacuum packed (VP) and modified atmosphere packed (MAP) cod fillets, VP salmon steaks and air packed smoked salmon were studied. As a tool in identifying the specific spoilage organism (SSO), the yield factor Y (ie the amount of metabolite produced per bacterial cell) was introduced. In a series of comparative studies on cod fillets it was demonstrated that Y for Photobacterium phosphoreum was 25-30 times larger than for Shewanella putrefaciens.
Findings suggested that Photobacterium phosphoreum is the SSO for VP and MAP cod fillets. The salmon steaks were spoiled mainly by Aeromonas species and Pseudomonas fluorescens and Shewanella putrefaciens may also have important roles. In smoked salmon, salt inhibits the bacterial flora and in the case of dry salting, spoilage delayed to over 40 days was not accompanied by high bacterial numbers. Products prepared by brining had shorter shelflives and at spoilage had moderate microbial counts. The contribution of yeasts and moulds was studied but was inconclusive.
A number of mathematical models have been produced within the project including the kinetics of microbial growth and the effect of storage conditions (carbon dioxide, temperature, salt content). A mathematical model for the effect of carbon dioxide on Photobacterium phosphoreum is validated in product studies and within the spoilage domain the model predicts shelf life accurately. It is concluded that prediction of shelf life is perfectly possible once the SSO and the spoilage domain door for the particular SSO has been identified.
In phase 1, a series of experiments will be conducted to determine the spoilage potential of Shewanella putrefaciens in relation to extrinsic and intrinsic factors.
In phase 2, all data relating to Shewanella putrefaciens will be used to construct models to accurately predict growth and time to reach minimum spoilage level (MSL).
Phase 3 is model validation.
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2800 Lyngby
Denmark