Emborg, J. & Dalgaard, P. (2008b). Growth, inactivation and histamine formation of Morganella psychrotolerans and Morganella morganii – development and evaluation of predictive models. Int. J. Food Microbiol. 128, 234-243.

This model for Morganella psychrotolerans includes the effect of four environmental parameters (temperature, atmosphere (CO₂), water activity (water phase salt) and pH) on growth and histamine formation. As shown below SSSP can be used to evaluate how changes in storage conditions e.g. the chill storage temperature (Fig. 1) or product characteristics e.g. salt concentration (Fig. 2) influence growth and histamine formation by Morganella psychrotolerans.

Information on the lag time of Morganella psychrotolerans in naturally contaminated marine finfish products is limited. Therefore, the growth and histamine formatiuon model for Morganella psychrotolerans can be used without lag time (fail safe predictions) or with lag time (probably more realistic predictions for naturally contaminated products). SSSP uses a relative lag time of 2.55 for Morganella psychrotolerans (Emborg & Dalgaard, 2008a).

Fig. 1. Effect of chill storage temperature (2.0°C or 4.4°C) on the predicted growth and histamine formation by Morganella psychrotolerans in fresh vacuum packed tuna (with pH 5.9).

Fig. 2. Effect of salt concentration (as obsereved e.g. for cold-smoked tuna) on the predicted growth and histamine formation by Morganella psychrotolerans.

Secondary growth and histamine formation models:

Eqn. 1 below shows the secondary growth and histamine formation models for Morganella psychrotolerans. The cardinal parameter type growth rate model (Eqn. 1a) describes how the maximum specific growth rate  (µ_max, h^-1) at a reference temperature of 20°C (µ_max-ref) is reduced when environmental parameters become less favourable for growth. The term for each of the environmental parameters (temperature, water activity (water phase salt), pH and CO₂ (atmosphere) all has a value between 0 and 1.

The maximum cell concentration (N_max, cfu/g) for Morganella psychrotolerans is reduced by increasing concentrations of CO₂ in the atmosphere and by increasing concentrations of salt (corresponding to reduced water activity). A secondary model has been developed to describe these effects on Log (N_max) (Eqn. 1b) (Emborg & Dalgaard, 2006a).

Histamine formation by Morganella psychrotolerans is directly related to growth so that a constant amount of histamine is produced when the cell concentration is doubled under specific storage conditions and product characteristics. This is described by including a yield facor in the primary model for histamine formation. The value of this yield factor, however, depends on storage conditions and product characteristics as described by a simple secondary model (Eqn. 1c).

Eqn 1a. Secondary growth rate model

Eqn. 1b. Secondary model for the maximum cell concentration depending on storage conditions and product characteristics

Eqn. 1c. Secondary model for the effect of storage conditions and product characteristics on the yield factor for histamine formation by  Morganella psychrotolerans

Evaluation and validation of the growth and histamine formation model: 

The models included in SSSP for prediction of growth and histamine formation by Morganella psychrotolerans have been evaluated by comparison of observed and predicted data from challenge tests with inoculated seafood and broth as shown in the Table below. The growth model performed well with a bias factor within the range (0.75 < Bias factor < 1.25) suggested for acceptable model validation (Dalgaard, 2002). On average the predicted histamine formation also corresponded well with the measured (observed) histamine formation. The histamine formation model, however, is not highly accurate and this must be taken into account when predictions from the model are used.