# Effects of Isochoric Freezing on the Quality Characteristics of Raw Bovine Milk
## 3 Results and Discussion
### pH and Titratable Acidity  
The pH and titratable acidity of the different milk samples under different storage conditions are shown in Table 2. The initial pH value of raw milk was 6.84 \(\pm\) 0.04, which was not significantly different from the initial value of 6.82 \(\pm\) 0.01 following HTST treatment. These values are similar to values reported in the literature [29]. After two weeks of storage, the pH value of the RF sample decreased significantly to 6.57 \(\pm\) 0.03, while the pH value of the HTST treated milk remained relatively stable at 6.80 \(\pm\) 0.01. The decrease in the pH value of the milk sample was probably caused by the accumulation of lactic acid produced by lactic acid bacteria. A previous study found that a decrease in pH value only occurred when microbial levels reached >6.5 log CFU mL\({}^{-1}\)[29], which was consistent with the microbial numbers reported in Table 1. For IF treated samples, the pH values after 2 weeks at \(-5\)\({}^{\circ}\)C and \(-10\)\({}^{\circ}\)C showed no significant changes, with pH values of 6.77 \(\pm\) 0.01 and 6.79 \(\pm\) 0.02, respectively. [PERSON] et al. [30] obtained comparable results with high pressure (200 MPa) and low temperature (\(-4\)\({}^{\circ}\)C) treatments of raw milk for 10, 20, and 30 min. The authors found the treatments had no significant effects on the pH levels of the milk samples. After 5 weeks, the pH value of the HTST treated milk had decreased significantly to 6.63 \(\pm\) 0.0, which could be explained by the microbial count of >6.5 log CFU mL\({}^{-1}\). For IF treatments, the pH values after 5 weeks at \(-5\)\({}^{\circ}\)C (6.77 \(\pm\) 0.01) and \(-10\)\({}^{\circ}\)C (6.79 \(\pm\) 0.02) showed no significant changes compared to the initial values. These results were consistent with the microbial count results, with both samples having microbial levels well below 6.5 log CFU mL\({}^{-1}\). \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline  & **Initial** & **RF** & **HTST** & & **IF (\(-5\)\({}^{\circ}\)C)** & & **IF (\(-10\)\({}^{\circ}\)C)** \\ \cline{2-10}  & **0 Weeks** & **2 Weeks** & **0 Weeks** & **2 Weeks** & **5 Weeks** & **2 Weeks** & **5 Weeks** & **2 Weeks** & **5 Weeks** \\ \hline TPC & 6.98 & 8.26 & & & & & & & & \\ (Log CFU mL\({}^{-1}\)) & (0.05)\({}^{\circ}\) & (0.19)\({}^{\mathrm{b}}\) & \(<\)0.01 & 5.28 & 9.59 & 3.60 & 1.33 & 1.26 & \\  & & (0.05)\({}^{\mathrm{d}}\) & (0.05)\({}^{\mathrm{d}}\) & (0.06)\({}^{\mathrm{a}}\) & (0.12)\({}^{\mathrm{e}}\) & (0.28)\({}^{\mathrm{f}}\) & (0.24)\({}^{\mathrm{f}}\) & \(<\)0.01 \\ \hline PS & 7.22 & 8.07 & & & & & & & & \\ (Log CFU mL\({}^{-1}\)) & (0.13)\({}^{\mathrm{c}}\) & (0.09)\({}^{\mathrm{b}}\) & \(<\)0.01 & 5.31 & 9.80 & 4.85 & 1.80 & 3.45 & \\  & & (0.09)\({}^{\mathrm{b}}\) & \(<\)0.01 & (0.20)\({}^{\mathrm{d}}\) & (0.13)\({}^{\mathrm{a}}\) & (0.26)\({}^{\mathrm{e}}\) & (0.040)\({}^{\mathrm{g}}\) & (0.04)\({}^{\mathrm{f}}\) & \(<\)0.01 \\ \hline \end{tabular} Results presented as mean and standard deviation in parenthesis. Different letters across each row indicate significant differences.  
\end{table} 
Table 1: Effects of microbial inactivation treatment and storage conditions on total aerobic mesophilic bacteria (TPC), and _Pseudomonas_ spp. (PS) in raw bovine milk. The initial tittatable acidity (TA) value for the raw milk was 1.27 \(\pm\) 0.08 g/L lactic acid, which was similar to the initial value of 1.36 \(\pm\) 0.08 for HTST treated milk. After 2 weeks, the TA value for the RF sample showed a significant increase to 2.06 \(\pm\) 0.03 g/L, whereas the TA value for the HTST treated sample remained at 1.35 \(\pm\) 0.05 g/L. These values were consistent with those found in the literature [11]. In addition, the TA values after 2-week IF treatment at \(-\)5 \({}^{\circ}\)C (1.35 \(\pm\) 0.05 g/L) and \(-\)10 \({}^{\circ}\)C (1.37 \(\pm\) 0.02) showed no significant changes compared with the initial value. After 5 weeks, the TA values for all milk samples increased significantly (\(p\) < 0.05), albeit at different rates.