# Quantitative Characterization of _Geotrichum candidum_ Growth in Milk
## 3 Results
### Secondary Modelling of G. candidum Growth in Milk  
In general, temperature is the most significant external environmental factor that affects microbial growth. That is why the secondary modelling in terms of the quantitative evaluation of temperature's effect on _G. candidum_ growth was performed. As shown in Figure 3, the duration of lag phase decreased with increasing temperature and slightly increased in the area beyond the optimum towards the maximum values of the temperature. The qualitative growth study of [PERSON] and [PERSON] [44] demonstrated a similar pattern of surface growth responses to temperature in the _G. candidum_ lag phase duration. It was also obvious that the CM line representing the dependence of \(1/\lambda\) on temperature with 1% NaCl moved down to lower values of \(1/\lambda\) and toward a higher temperature range. Specifically, this means that the lag phase durations were prolonged, the differences between \(1/\lambda\) values increased gradually with the temperature, and the predicted \(T_{\text{opt}}\) and \(T_{\text{max}}\) were higher by 4.3 and 2.0 \({}^{\circ}\)C (Table 2), respectively, as a result of the addition of salt in the milk. On the other hand, the cardinal temperatures that resulted from CM when applied to the specific growth rate of the isolate in milk with and without added salt were very close to each other. Similar cardinal temperatures (\(T_{\text{min}}=1.18\)\({}^{\circ}\)C, \(T_{\text{opt}}=28.9\)\({}^{\circ}\)C, \(T_{\text{max}}=37.3\)\({}^{\circ}\)C) for _G. candidum_ growth in milk were also reported by [PERSON] et al. [50]. The optimal specific growth rate of the isolate slightly decreased in the milk medium with 1% NaCl addition in contrast to its optimal growth rate in milk. These results indicate the slower growth of _G. candidum_ at 1% NaCl, which is in accordance with the findings of [PERSON] et al. [7] and [PERSON] [51]. However, strain-dependent sensitivity of _G. candidum_ on salt concentration should also generally be considered [2,4,44]. Figure 3: Plots of the reciprocal lag phase duration of the _G. candidum_ isolate in relation to the temperature (6–37 \({}^{\circ}\)C) in milk (blue line) and milk with 1% NaCl (yellow line) according to the cardinal model (CM, solid line). The symbols indicate the experimental values of \(1/\lambda\) estimated from the primary model at each incubation temperature. Solid lines represent the fitted estimates according to the model. Dashed lines represent the 95% confidence intervals. According to [PERSON] et al. [7], _G. candidum_ species grow at temperatures ranging from 5 to 35 \({}^{\circ}\)C, which is in accordance with our observations, although we predicted a slightly higher maximum temperature for _G. candidum_ growth in milk at the level of 36 \({}^{\circ}\)C. The optimal temperature for _G. candidum_ growth is generally considered to be around 25 \({}^{\circ}\)C [2; 7]. However, based on our results, the optimal temperature was predicted to be close to 30 \({}^{\circ}\)C. These data illustrate the variability in the optimal temperature for growth, which can at least be partially explained through the variability of strains and culture conditions, as also reported by [PERSON] and [PERSON] [44]. Generally, within the experimental limits, the maximum specific growth rates of the isolate in milk with higher \(a_{w}\) level (0% NaCl) increased significantly (\(p<0.05\)) with the increase in temperature up to an optimum, and then decreased beyond the physiological limits for the isolate. The graphical evaluation of the fitted curves in Figure 4 at temperatures below 21 \({}^{\circ}\)C indicates that when \(a_{w}\) becomes more stressful (as a result of 1% NaCl addition), _G. candidum_ exhibits a slightly faster growth rate compared to the growth predictions in milk without salt. A previous qualitative study by [PERSON] et al. [52] showed a similar phenomenon. They investigated the effect of NaCl content on the radial growth of _G.