Østergaard, N.B., Eklöw, A., Dalgaard, P. (2014). Modelling the effect of lactic acid bacteria from starter- and aroma culture on growth of Listeria monocytogenes in cottage cheese. Int. J. Food Microbiol. 188, 15-25.

This product specific set of models in FSSP is developed to predict the simultaneous growth of L. monocytogenes and lactic acid bacteria (LAB) in cottage cheese (Fig. 1). Potential growth of L. monocytogenes in cottage cheese is strongly influenced by the high concentration of mesophilic LAB originating from the starter and aroma cultures used in processing of thís fermented dairy product. FSSP includes specifiic models for LAB and L. monocytogenes in cottage cheese with fresh/un-cultured  cream dressing and separate models for LAB and L. monocytogenes in cottage cheese where the cream dressing is prepared by using an aroma culture (Lactococcus lactis subsp. lactis biovar. diacetylactis)  (See Fig. 1 below).

Information on the lag time of LAB and L. monocytogenes in cottage cheese is still limited. Therefore, these growth model in FSSP can be used with worst case lag times (fail safe predictions) or with average lag times (more realistic predictions for naturally contaminated products). 

FSSP predicts how high concentrations of LAB dampen and stops the growth of L. monocytogenes. This Jameson effect is important in order to accuratelty predict potential growth of L. monocytogenes and its influence on cottage cheese safety. As shown in Fig. 1 and Fig. 2 the FSSP software allow these predictions to be carried out conveniently for constant and variable temperature storage conditions.

Fig. 1. The graph above shows the predicted growth of L. monocytogenes and LAB in cottage cheese with aroma culture and stored at 5°C or 10°C. As shown FSSP predicts the time needed for the concentrations of L. monocytogenes and LAB to increase under the selected product characteristics and storage conditions. Importantly, FSSP predicts the inhibiting effect of high concentrations of LAB on growth of L. monocytogenes (The Jameson effect).

Fig. 2. Effect of two simple temperature profiles on the predicted simultaneous growth of L. monocytogenes and LAB in cottage cheese with cultured cream dressing..

Primary growth model

FSSP uses the Logistic model with delay and including the inhibiting effect of high concentrations of LAB on growth of L. monocytogenes as primary growth model (Eqn. 1). This primary growth model describes how the specific growth rates of L. monocytogenes ([dLm/dt]/Lm_t) is reduced when cell concentrations of LAB (LAB_t, cfu g^-1) approaches their maximum value (LAB_max, cfu g^-1). The model is an expansion of the differential form of the simple Logistic model and it includes the assumption that LAB inhibit growth of L. monocytogenes to the same extend that they inhibit their own growth and this has been confirmed for cottage cheese (Østergaard et al. 2014).

Eqn. 1. Primary model for simultaneous growth of Listeria monocytogenes and LAB according to the Jameson effect. t_lag is the lag time and other model parameters are described in the text above.

Secondary growth model:

FSSP uses eqn. 2 as secondary model for growth of LAB and L. monocytogenes in cottage cheese. This simplified cardinal parameter type model describe how the maximum specific growth rate  (µ_max, h^-1) at a reference temperature of 25°C (µ_ref) is reduced when environmental parameters become less favourable for growth. The term for each of the environmental parameters (temperature, pH, water activity (water phase salt), lactic acid and sorbic acid all has a value between 0 and 1. FSSP uses specifiic model parameter values  for growth of LAB and L. monocytogenes in cottage cheese with fresh/un-cultured  cream dressing and separate models for LAB and L. monocytogenes in cottage cheese where the cream dressing is prepared by using an aroma culture. The applied cardinal parameter values are shown in the table below.

Evaluation and validation of the Listeria monocytogenes and lactic acid bacteria models for cottage cheese: 

The models included in FSSP for prediction of the simultaneous growth of L. monocytogenes and LAB in cottage cheese have been evaluated by comparison of observed and predicted growth in inoculated products (challenge tests with L. monocytogenes) and in products storage trials (LAB) as shown in the Table below. The models performed well and bias factors for the four models were within the ranges suggested for acceptable validation of growth models for pathogenic and spoilage microorganism in food (Mejlholm and Dalgaard, 2013).