Characterising the concentration-dependent behaviour of heat-treated sunflower oleosomes at an air-water interface
Mohammad Amin Aliyari, Vincenzo di Bari, Liam P. D. Ratcliffe, Pallab Kumar Borah, Yuanzhang Dong, David A. Gray
Abstract
The behaviour of fresh and heat-treated sunflower oleosomes (oil bodies) at the air-water interface was studied using a force tensiometer with a conventional method and a new approach based on the fresh interface formation. Oleosomes were recovered using a wet milling process and washed to produce washed oil bodies (WOB), then dispersed as rinsed-washed oil bodies (RWOB) in ultra-pure water. Thermal treatment of oleosomes at 75 °C for 5 min did not alter the quality of oleosomes so it was used as the pasteurisation treatment. All oleosome preparations, including fresh RWOB, reduced surface tension (ST) at the air-water interface. Fresh RWOB exhibited faster, and more extensive ST reduction compared to heat-treated samples (HT-RWOB). Serum phase (a protein rich phase produced as part of the oleosome purification process) analysis indicated extraneous proteins contribute to ST reduction. ST measurements using HT-RWOB showed concentration-dependent ST reduction from 0.005% to 0.1% (w/v), with minimal changes at higher concentrations. Fitting a linear model on the concentration-dependent zone and concentration-independent zone showed that the cross over happens at the concentration of 0.1% (w/v) which is regarded as the critical packing concentration (CPC, the concentration by which the interface is saturated). Colloidal instability phenomena were observed at high concentrations. Particle size measurement and micrographs of emulsions before and after ST measurement indicate significant breakage and coalescence of oleosomes at high concentrations because of the formation of multiple layers at the interface after the CPC. This paper explores the impact of these changes on the ST and suggests possible explanations. • Fresh and heat-treated sunflower oleosomes behave differently at the air-water interface, particularly in equilibrium time. • Oleosomes particle size and surface charge remained stable after heat treatment. • Surface tension was measured by a new methodology based on formation of a fresh interface. • Thermally treated oleosomes showed concentration-dependent behaviour and ruptured at the interface beyond a certain level (>0.8% w/v). • Two modes of oleosome packing at the interface were identified as low concentration and high concentration zones.