Application of an iterative approach for development of a microbial model predicting the shelf-life of packed fish

An iterative approach was used to develop a microbial model for shelf-life prediction of cod fillets packed in modified atmospheres. The effect of temperature (0-15°C) and CO₂ (0-100%) on growth of the specific spoilage organism, Photobacterium phosphoreum, was studied in packed cod and in liquid media. P. phosphoreum was a dominant part of the spoilage microflora of packed cod stored at the extremes of the range of conditions studied. The organism is therefore likely to be important for spoilage and the development of a microbial model within this domain seems relevant. A liquid medium was developed to provide growth kinetics of P. phosphoreum similar to those observed in packed cod. Using this medium, the effect of temperature and CO₂ on the maximum specific growth rate of Photobacterium phosphoreum was determined by absorbance measurements and modelled by a square root equation and by a polynomial equation. Product validation studies were carried out during summer and winter using naturally contaminated packed cod fillets which were stored at constant and at changing temperatures. The shelf-life of the packed fillets was predicted on the basis of the initial numbers of P. phospholeum, product temperature profiles and the level of CO₂ in the modified atmosphere. The average deviations between shelf-life determined by sensory evaluation and shelf-life predicted by the square root equation and by the polynomial equation were 17% and 9%, respectively.