Development and application of Geobacillus stearothermophilus growth model for predicting spoilage of evaporated milk

• Geobacillus stearothermophilus constitutes one of the most important quality problem for evaporated milk.
• A cardinal type model was used to model the effect of the temperature on microbial growth.
• The model adequately predicted the growth in milk under dynamic conditions.
• The developed model was used to predict the time-to-spoilage of evaporated milk.
• The proposed model can be applied for the development of a risk assessment approach for assuring evaporated milk quality.

The presence of Geobacillus stearothermophilus spores in evaporated milk constitutes an important quality problem for the milk industry. This study was undertaken to provide an approach in modelling the effect of temperature on G. stearothermophilus ATCC 7953 growth and in predicting spoilage of evaporated milk. The growth of G. stearothermophilus was monitored in tryptone soy broth at isothermal conditions (35–67 °C). The data derived were used to model the effect of temperature on G. stearothermophilus growth with a cardinal type model. The cardinal values of the model for the maximum specific growth rate were Tmin = 33.76 °C, Tmax = 68.14 °C, Topt = 61.82 °C and μopt = 2.068/h. The growth of G. stearothermophilus was assessed in evaporated milk at Topt in order to adjust the model to milk. The efficiency of the model in predicting G. stearothermophilus growth at non-isothermal conditions was evaluated by comparing predictions with observed growth under dynamic conditions and the results showed a good performance of the model. The model was further used to predict the time-to-spoilage (tts) of evaporated milk. The spoilage of this product caused by acid coagulation when the pH approached a level around 5.2, eight generations after G. stearothermophilus reached the maximum population density (Nmax). Based on the above, the tts   was predicted from the growth model as the sum of the time required for the microorganism to multiply from the initial to the maximum level (tNmaxtNmax), plus the time required after the tNmaxtNmax to complete eight generations. The observed tts was very close to the predicted one indicating that the model is able to describe satisfactorily the growth of G. stearothermophilus and to provide realistic predictions for evaporated milk spoilage.