## 3 Results and Discussion
### Effect of Thermal and High-Pressure Pasteurization  
#### 3.3.1 Immunoglobulins  
* IgG  
As can be seen in Figure 1a, TP caused an approximately 79% decrease in the IgG content compared to the untreated samples. In the literature, information regarding the effect of temperature on IgG present in colostrum is very scarce, but in human colostrum, [PERSON] et al. (2005) [28] reported a similar reduction (\(\approx\)72%), while [PERSON] et al. (2014) [7] observed only a 23% reduction in IgG caused by Holder pasteurization. These decrements are lower than those verified for purified bovine milk IgG in other studies [29; 30] pasteurized by TP, suggesting that several colostrum components, such as fat, casein, salts and sugars, such as lactose, may protect IgG during TP [31]. Regarding HPP, the IgG content at 400 MPa was preserved (\(p>0.05\)) (Figure 1a), similar to results in the literature for human colostrum treated at low and mild pressures (200 and 400 MPa) [7]. On the other hand, for the pressure/holding times conditions of 550 MPa/2.5 min, 625 MPa/2.5 and 10 min, a significant increase (\(p<0.05\)) in the IgG content was observed, reaching a maximum of about 4-fold for 625 MPa/2.5 min and about 2.5-fold for 625 MPa/10 min. The reason for this can be due to the release of IgG that might be bound to other components, such as lactoferrin [32]. On the contrary, an increase in the holding time to 30 min, at 625 MPa, caused a reduction in the IgG content to about 50% compared to untreated colostrum, possibly due to inactivation of IgG after release from other components for longer pressure holding times [33]. [PERSON] et al. (2016) [12] verified that the IgG content in bovine colostrum decreased by about 5% at 400 MPa with the increase in the holding time from 10 to 20 min, but no significant differences were obtained at 300 MPa. * IgM  
TP and HPP caused a decrease (\(p<0.05\)) in IgM content (Figure 1b) but to a much lower extent in the case of TP, resulting in a reduction of 17%, while HPP caused decrements from 55% (400 MPa/10 min) to below the detection limit (in this case, particularly for 625 MPa). IgM seemed to be more heat stable than IgG, similar to the findings reported by  
Figure 1: IgG (**a**) and IgM (**b**) content (%) in untreated donkey colostrum and after thermal (TP) and high-pressure pasteurization (HPP). Data are presented as a percentage considering 100% as the value obtained for colostrum before any treatment (untreated colostrum). Different letters denote significant differences (\(p<0.05\)) between the different conditions (a–f). [PERSON] et al. (2014) [7] for human colostrum. At the same time, IgM in donkey colostrum seemed to be more pressure sensitive than in human colostrum for which no reduction effect at 400/30 min was verified, and an IgM retention of about 40% at 600 MPa/30 min was discovered [7]. * IgA content in donkey colostrum was found to be reduced to levels below the detection limit after TP and HPP. [PERSON] et al. (2014) [7] observed IgA retention on human colostrum for several pressure treatments ranging from 200 to 600 MPa (with holding times between 2.5 and 30 min), with a 74% retention even at 600 MPa (30 min, 8 \({}^{\circ}\)C), similar to that observed after TP. In the present study, and compared to IgG and IgM, IgA could be considered both more heat and pressure sensitive in human colostrum. * Kinetics analysis for immunoglobulins  
The inactivation kinetic parameters \(k\) (reaction rate constant, min\({}^{-1}\)) and D-values (min) were calculated for cases where enough data were available and are presented in Table 2. To the authors' best knowledge, there is no work reporting \(k\) and D-values for Igs losses caused by TP and HPP treatments in donkey colostrum. The D-value of IgG for 550 MPa was 2.6-fold higher than IgM, thus confirming that, at this pressure, IgG is more pressure resistant than IgM in donkey colostrum, while for IgG, the D-value at 550 MPa is about 3.3-fold higher than the one at 625 MPa. For human colostrum, [PERSON] et al.