Experimental investigation on the ice formation mechanism in an isochoric freezing system

• The proportion of unfrozen water in an isochoric system was quantified. • Unfrozen water proportions deviate from hexagonal ice equilibrium predictions. • At –6°C, water remained completely in a stable supercooled state. • Experimentally calibrated free energy analysis to delineate nucleation threshold. Isochoric freezing has recently been gaining significant attention as an innovative preservation technique. This approach allows food to remain non-frozen at freezing temperatures, maintaining structural integrity and nutritional values. The process relies on freezing water at constant volume, which creates some portion of ice in the system while the remainder of the system is maintained at the supercooled state. For the first time, we measured unfrozen water fractions of 22.00% to 48.00%, significantly lower than the 57.53% to 73.22% predicted by current thermodynamic theory. At the higher temperature of –6°C, for the specific system investigated, the isochoric freezing technique was also capable to maintain water completely in a stable supercooled state. This corroborates with the isochoric supercooling phenomenon described by Rubinsky and co-workers. The proposed experimentally-calibrated free energy analysis elucidates this phenomenon. The analysis suggests the existence of a potential “nucleation threshold” in isochoric freezing systems, delineated by a small pressure jump in the current work, that influences the thermodynamics of isochoric freezing. These deviations are observed under the specific configuration (100 mL) and slow cooling rate employed, suggesting that system geometry, cooling rate, and other experimental factors may influence the measured behaviour, although the exact threshold conditions remain uncertain.