Freeze-thaw stability of oilseed rape oleosome emulsions

This work investigated the stability of natural oleosome emulsions on freeze-thawing. Oleosomes were recovered from oilseed rape seeds following an aqueous extraction process using sodium bicarbonate (0.1 M). The final emulsions pH was adjusted to 9, 6 and 3 to achieve surface charge values of −50.3 ± 1.6, −20.0 ± 2.4, and +37.5 ± 0.5 mV, respectively. The emulsions with lipid mass fraction of 0.28 ± 0.02 were cooled to −20 °C for up to 24 h and thawed at 20 °C for 18 h, and their freeze-thaw stability assessed quantifying amount of released oil (oil yield) due to emulsion destabilisation. The destabilisation of the oleosome emulsions at pH 9 and 6 increased with isothermal holding time at −20 °C, whereas the emulsion at pH 3 destabilised more rapidly. Differential scanning calorimetry analysis of emulsions cooled from 20 °C to −20 °C at −10 °C/min, and held at −20 °C for 8h, revealed how the continuous phase rapidly crystallised on cooling, whereas lipid crystallisation started after 2 h at −20 °C and continued for the following 3 h. Oil yield data combined with differential scanning calorimetry curves suggest that the oleosome emulsions at pH 9 and 6 destabilised along with crystallisation of the dispersed lipid phase, whereas emulsions at pH 3 destabilised with the continuous phase crystallisation. It was hypothesised that oleosome emulsions at pH 9 and 6 ruptured by a mechanism of partial coalescence. At pH 3 the electrostatic interaction between phospholipids and oleosin molecules, the main components of oleosome interface, may be reduced resulting in a weaker interface which on cooling can be easily disrupted by ice crystals. Oil yield data for emulsions with increasing continuous phase mass fraction (0.50 and 0.70) suggest a lower extent of destabilisation than control (0.28 continuous phase). Increasing the number of freeze-thaw cycles from one to three did not cause significant increase in the oil yield.