Characterization of mutual relations between Geotrichum candidum and Lactobacillus rhamnosus GG in milk: a quantitative approach

Modelling the Radial Growth of Geotrichum candidum: Effects of Temperature and Water Activity

Modelling the growth of microorganisms in relation to environmental factors provides quantitative knowledge that can be used to predict their behaviour in foods. For this reason, the effects of temperature and water activity (aw) adjusted with NaCl on the surface growth of two isolates and one culture strain of Geotrichum candidum were studied. A dataset of growth parameters obtained from almost 600 growth curves was employed for secondary modelling with cardinal models (CMs). The theoretical minimal temperature resulting from the modelling of the mycelium proliferation rate ranged from -5.2 to -0.4 °C. Optimal and maximal temperatures were calculated and found to have narrow ranges of 25.4 to 28.0 °C and 34.2 to 37.6 °C, respectively. Cardinal aw values associated with radial growth (awmin from 0.948-0.960 and awopt from 0.992-0.993) confirmed the salt sensitivity of the species. Model goodness-of-fit was evaluated by the coefficient of determination R2, which ranged from 0.954 to 0.985, and RMSE, which ranged from 0.28 to 0.42. Substantially higher variability accompanied the lag time for growth modelling than the radial growth rate modelling despite the square root transformation of the reciprocal lag phase data (R2 = 0.685 to 0.808). Nevertheless, the findings demonstrate that the outputs of growth modelling can be applied to the quantitative evaluation of the roles of G. candidum in fresh cheese spoilage as well as the ripening of Camembert-type cheeses or various artisanal cheeses. Along with validation, the interactions with lactic acid bacteria can be included to improve the predictions of G. candidum in the future.

Growth dynamics of lactic acid bacteria and dairy microscopic fungus Geotrichum candidum during their co-cultivation in milk

Production of high-quality and microbiologically safe fermented dairy products requires controlled growth and microbial interactions between lactic acid bacteria and microscopic fungi. For this purpose, detailed knowledge of their growth characteristics is needed. Therefore, the objective of this study was to analyse the growth dynamics of lactic acid bacteria of commercial DVS® FRESCO® 1000NG culture and dairy isolate of microscopic fungus Geotrichum candidum during their co-cultivation in milk. The growth dynamics of microorganisms was studied in dependence on their initial counts at 12, 15, 18, 21 and 30 °C. Growth parameters were calculated by two primary predictive models, model of Baranyi and Roberts and Huang's model. Both models showed good ability to describe the growth dynamics of studied microorganisms, as it was confirmed by low values of RMSE index. Both microbial cultures, Fresco culture and Geotrichum candidum, showed good growth ability in milk since they reached the average maximum density of 9.50 ± 0.13 log CFU/mL and 5.85 ± 0.69 log CFU/mL (n = 45), in order. Maximum density of studied microorganisms was not affected by their initial counts or incubation temperature. On the other hand, effect of mutual ratio of microbial initial counts and increasing temperature had a significant impact on growth dynamics.