Mathematical modelling and in situ determination of pH in complex aqueous solutions during high-pressure carbon dioxide treatment

High-pressure carbon dioxide (HPCD) treatment is currently considered as an attractive non-thermal process for preserving food. Since the first level of interaction between HPCD and the bacterial cells is lowering of the pH, knowledge of the pH of a food product in contact with CO₂ at high-pressure conditions is essential for a better understanding of the inactivation mechanism of HPCD. Therefore, a mathematical model was developed to predict the pH in complex aqueous food systems in contact with CO₂ at high-pressure conditions as function of pressure, temperature and buffer capacity. In addition, a spectrofluorometric method using calcein as fluorescent pH indicator was designed for the in situ measurement of the pH of complex aqueous systems in contact with pressurized CO₂ as function of pressure (10.5-18.0 MPa), temperature (25-35 °C), initial pH (4.0-8.0), working volume ratio (41.6-70.0%) and broth composition (0.1 M citrate buffer, 0.1 M phosphate buffer and 10% whey protein). To mimic a complex matrix, the bacterial Brain Heart Infusion (BHI) broth was used.In general, there was a good agreement between the measured and predicted pH values. Only for the lowest initial pH of the broth, a discrepancy between the measured and calculated pH values was noticed, implying that the proposed model needs some further refinement to properly take into account non-ideality of the liquid phase. Furthermore, the pH of the broth was significantly influenced by the initial pH of the solution but was not dependent on pressure, temperature and working volume ratio. The addition of complementary buffers to the broth also influenced the pH drop and this effect was dependent on the initial pH of the solution.