Pressure-shift freezing of o/w emulsions: influence of fructose and sodium alginate on undercooling, nucleation, freezing kinetics and ice crystal size distribution

Abstract

Oil-in-water emulsions (50 g samples in flexible tubing) stabilised by sodium caseinate and containing fructose (10, 18 or 25% w/w), or sodium alginate (0.25 or 0.5% w/w), or 18% fructose+0.25% w/w sodium alginate, were frozen by fast pressure release in 1–3 s (pressure-shift freezing (PSF)) from 200 MPa and −18 °C to 0.1 MPa and −1/−9 °C (depending on fructose concentration), followed by freezing completion at 0.1 MPa. Freezing kinetics were measured with T-thermocouples inserted at the sample centre or 1–2 mm below sample surface. The extent of undercooling was followed on pressure–temperature coordinates with a fast data acquisition system (10 Hz). The imprints of ice crystal clusters were observed after freeze substitution of PSF samples, using light microscopy and image analysis. PSF induced a high extent of sample undercooling and initiated massive ice nucleation and crystallisation throughout sample depth. In the control emulsion (without fructose or alginate) subjected to PSF, ice crystal clusters displayed irregular shapes without specific orientation. Image analysis suggested that these clusters probably formed during crystal growth at 0.1 MPa, each one resulting from about five nuclei. The presence of fructose decreased the rate of temperature increase ( V i) during pressure release, the extent of undercooling (because freezing temperature at 0.1 MPa was lower) and the duration of the freezing plateau at 0.1 MPa (due to a lower water content). Ice crystal clusters were more numerous, smaller and more spherical, resulting from a smaller number of nuclei per crystal cluster, when the fructose concentration increased, suggesting that nucleation predominated over ice crystal growth during pressure release, in spite of the lower extent of undercooling. The presence of sodium alginate (0.25 or 0.5%) did not significantly affect the freezing kinetics ( V i, extent of undercooling, duration of the freezing plateau at 0.1 MPa), increased the smoothness of ice crystal clusters but did not reduce their size, as compared to the control PSF emulsion. At 0.5% alginate, high pressure–low temperature conditions appeared to induce protein/polysaccharide phase separation, leading to the aggregation of ice crystal clusters. The addition of 0.25% sodium alginate did not modify the small size of ice crystal clusters obtained with 18% fructose alone. The presence of sodium alginate did not enhance nucleation nor reduce crystal growth in PSF emulsions.