THERMODYNAMIC MODELLING OF pH IN CITRUS SIMULATION SYSTEMS UNDER HIGH PRESSURE CARBON DIOXIDE
Thermodynamic models were developed to predict the pH of liquid citrus simulation systems as functions of high pressure carbon dioxide process conditions. Such processing holds promise as an alternative to heat pasteurization for inactivating pectic enzymes through temporarily lowering pH instead of using heat. Predicted pH values were compared with those measured experimentally at different process conditions with the aid of a pH probe especially designed to withstand high process pressures. The four simulation systems included pure water-carbon dioxide, ascorbic acid-water-carbon dioxide, citric acid-water-carbon dioxide, and ascorbic acid-citric acid-water-carbon dioxide. Gage process pressures ranged from 0 to 34.5 Mpa at temperatrues of 32, 37 and 42 C. Acid concentrations ranged from 0 to 1.17 10^-3 M and 0 to 6.08 10^-4 M for ascorbic and citric acids, respectively. For all the simulation systems, the predicted pH value closely followed the measured pH values. In the pure water-carbon dioxide system, the pH was a strong function of process pressure but a weak function of process temperature. In the ascorbic acid-water-carbon dioxide, citric acid-water-carbon dioxide, and ascorbic acid-citric acid-water-carbon dioxide systems, however, the pH was relatively insensitive to pressure while remaining a strong function of acid concentrations.
The pH in the ternary and quaternary simulation systems showed a very small variation over the experimental pressure range, indicating that the low pH alone can not totally account for pectinesterase inactivation in citrus juice treated with supercritical carbon dioxide. Since the pH variation was greater in the water-carbon dioxide system than the other three systems, more carbon dioxide was dissolved in the binary system, resulting in the generation of greater quantities of carbonic acid in the solution. The reduction in the solubility of carbon dioxide in the solution was believed to be caused by the presence of other ions that formed as a result of dissociation of ascorbic and citric acids in the ternary and quaternary systems.