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Model Morganella morganii and Morganella psychrotolerans - growth and histamine formation at different storage temperatures
References 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.
Primary growth model Expanded Logistic model with delay
Secondary growth model Square-root growth rate model
Environmental parameters in model Temperature
Product validation studies M. morganii: Bluefish, canned tuna, tuna, mackerel (Emborg & Dalgaard, 2008b)

M. psychrotolerans: Fresh garfish, fresh tuna, canned tuna and cold-smoked tuna (Emborg & Dalgaard, 2008b)

Range of applicability Temperature (0 - 37C)
 

The combined Morganella morganii and Morganella psychrotolerans growth and histamine formation model included in FSSP has been developed to predict the effect of a wide range of storage temperatures on histamine formation in fresh marine finfish products. At high storage temperatures the mesophilic bacterium Morganella morganii grow and produce histamine rapidly wherease at low storage temperature growth and histamine formation by the psychrotolerant bacterium Morganella psychrotolerans is more important. At temperature above ca. 15 C Morganella morganii grow faster than Morganella psychrotolerans (Fig. 1). However, the relative importance for histamine formation by the two bacteria also depends on their initial concentrations and this effect can be predicted by FSSP. Importantly, FSSP predicts the effect of different chilling scenarios. As shown in Fig. 2, FSSP predicts that 25C in 17 hours followed by storage at 5C result in critical histamine formation after ca. 7 days and due to activity of Morganella psychrotolerans. In comparison, 25C in 22 hours and then storage at 5C results in critical histamine formation within less than one day and due to activity of Morganella morganii.

 

 

Fig. 1. Effect of storage temperature on the maximum specific growth rate (max, h-1) of Morganella psychrotolerans and Morganella morganii.

 

 

Fig. 2. Predicted effect of two delayed chilling scenarios on growth and histamine formation. 25C in 17 hours and then 5C results in critical histamine formation after ca. 7 days whereas 25C in 22 hours and then 5C results in critical histamine formation within less than one day.

 

Secondary growth and histamine formation models:
Eqn. 1 below shows the secondary growth rate models for Morganella morganii (Eqn. 1a) and Morganella psychrotolerans (Eqn. 1b). Simulation of these growth rate models are shown in Fig. 1. The predicted histamine formation by both bacteria is directly related to growth so that a constant amount of histamin is produced when the cell concentration is doubled (Emborg & Dalgaard, 2008b).

 

Eqn 1a. Square-root model for the effect of temperature on growth rate of Morganella morganii

Eqn. 1b. Square-root model for the effect of temperature on growth rate of Morganella psychrotolerans

Eqn. 1. Secondary models for the effect of temperature on growth rates of Morganella morganii and Morganella psychrotolerans (Emborg & Dalgaard, 2006b)

 

Evaluation of the Morganella morganii growth and histamine formation model: 
The Morganella morganii growth and histamine formation models in FSSP have been evaluated by comparison of observed and predicted data from challenge tests with inoculated foods and broth (See Table below). max-values obtained from the literature were similar for fresh fish and broth and other sources of variation must be responsible for the relatively high accuracy factor of 2.44. On average predicted growth rates were 41% faster than the observed growth rates (Bias factor 1.41). These 'fail-safe' predictions of growth corresponds to the predicted histamine formation which on average was faster than the obsereved histamine formation. The histamine formation model is not highly accurate and in this situation we find it reasonable to use a model that on average predicts somewhat too short times to formation of critical concentrations of histamine ('fail-safe' predictions).
Model Data used for evaluation and validation of the model Indices of performance
Morganella morganii  growth rate model (Emborg & Dalgaard, 2008b)

93 growth curves for Morganella morganii in inoculated fresh fish (n=60) and broth (n=33). Storage conditions included temperatures in the range 7-37C(Emborg & Dalgaard, 2008b).

Bias factor            = 1.41

Accuracy factor    = 2.44

Morganella morganii histamine formation model (Emborg & Dalgaard, 2008b)

13 histamine formation curves determined for Morganella morganii in inoculated bluefish, canned tuna, tuna and mackerel stored at temperatures in the range 7-37C (Emborg & Dalgaard, 2008b).

% difference between observed and predicted times to formation of histamine:

  100 ppm =   -42 39%

  500 ppm =   -21 40%

2000 ppm =   -42 39%