Freezing Kinetics of a Model Oil-in-Water Emulsion under High Pressure or by Pressure Release. Impact on Ice Crystals and Oil Droplets

Abstract

Oil-in-water emulsions stabilized by sodium caseinate were frozen by the following processes: (a) Cooling to −18°C at 207 MPa, followed by slow (approximately 9 min) pressure release (SPR) or fast (approximately 3 s) pressure release (FPR), then by a freezing plateau at −0.6°C; (b) Freezing under a constant pressure of 100 MPa, at −9°C (freezing under high pressure, FUP); (c) Control freezing at atmospheric pressure and −0.6°C (freezing at atmospheric pressure, FAP). In all cases, emulsion samples were frozen in a cylindrical tubing immersed in a propanediol/water mix at −29°C. Freezing kinetics, measured with T thermocouples inserted at the sample centre or the surface, indicated very slow freezing by FUP (due to a ΔT of only 20°C). Freezing by FPR produced the shortest freezing plateau. The pressure–temperature coordinates of the sample on the phase diagram of water, determined with a fast data acquisition system, revealed moderate or extensive supercooling before nucleation for SPR or FPR, respectively. Samples were fixed by freeze substitution and the imprints of ice crystals at different sample depths were examined by light microscopy. Ice crystals resulting from FPR were small, irregularly shaped and without specific orientation, while those from FUP were larger, needle-shaped, radially oriented and with a marked size gradient from sample surface to centre. Results are consistent with enhanced supercooling and uniform ice nucleation assumed to occur upon FPR. Pressure release from 300 MPa and −16.5°C gave similar results as from 207 MPa and −18°C, while FPR from 100 MPa and −8°C resulted in less nucleation, a longer freezing plateau and some needle-shaped crystals. None of the freezing processes significantly modified the size distribution of oil droplets, as determined on thawed emulsions.