Factors Influencing Mass Transfer During Immersion Cold Storage of Apples in NaCl/Sucrose Solutions

Immersion chilling and freezing (ICF) consists of dipping a food material in an aqueous freezant (AF). During ICF, mass transfer (solute, water) arise at the solid/liquid interface, before and after thermal equilibrium (denoted respectively the primary and secondary stages). The aim of this work was to assess the evolution of mass transfer against time during the secondary stage. Apple cylinders (height 3 cm, diameter 2 cm, initial temperature 5°C) were soaked in four different AF: a binary monophasic NaCl-water liquid (m1=4.55 mol.kg-1), a biphasic mixture (m1=4.25 mol.kg-1) obtained from a binary NaClwater solution, and two ternary NaCl-sucrose-water liquids (m1=4.55 mol.kg-1, m2=0.5 or 2 mol.kg-1). The effect of temperature (-17.8, -17.4, -5°C), composition and physical state (monophasic/biphasic) of the freezant, and of time duration (6 h, 21 d) on mass transfer during the secondary stage were assessed. Results showed that mass transfer was lower over the whole process (stages I and II) when the AF temperature was lower, for example, after 6 h WL and SG were less than 2% of initial material (i.m.) at TAF=-17.8°C, against more than 7% i.m. at TAF=-5°C. The addition of sucrose in the AF led to a subsequent increase in WL (which could be as high as 29% i.m. after 21 d), but did not affect significantly SG levels over 21 d storage. Only the use of a biphasic mixture could limit mass transfer (less than 3.5% i.m. after 21 d).