Process-structure-property relationship of edible oil-based foams investigated with Process Analytical Technologies (PAT) tools and X-ray tomography

Oil-based foams, also called oleofoams, are a novel type of soft matter material with great potential in the field of foods, cosmetic and pharmaceuticals. Oleofoams comprise a liquid oil phase, and dispersed gas bubbles which are stabilized by surfactant crystals through a Pickering mechanism. Oleofoams are routinely prepared through the aeration of a dispersion of fat crystals in oil, also called oleogels. Compared to their aqueous counterparts, however, limited literature is available on oleofoams, due to the narrow range of suitable surfactants able to stabilize the air-oil interface. In order to promote the use of oleofoams in the manufacturing industry, further research is required, both fundamental and applied. The physical properties of oleofoams are affected by the properties of the stabilizing crystals (size, shape, polymorphism and concentration), the processing conditions (i.e., crystallization and aeration), as well as by the microstructure, that is, the three-dimensional arrangement of the gas bubbles, fat crystals and oil.
In this doctoral project, the complex process-structure-property relationship of cocoa butter–based oleofoams was investigated using online Process Analytical Technologies (PAT) tools, synchrotron radiation X-ray tomography and a wide range of traditional offline characterization techniques such as differential scanning calorimetry, polarized optical microscopy and small angle X-ray scattering. The effect of the crystallization conditions on the properties of the oleofoams were studied by preparing cocoa butter (CB) and high oleic sunflower oil (HOSO) oleogels by varying the CB concentration and cooling rate. The crystallization was carried out in a small-scale vessel and monitored in-situ with light turbidimetry and pulsed acoustic spectroscopy. The oleogels were then aerated with a planetary mixer, and the air incorporation was measured gravimetrically with a cup of fixed volume. Both the oleogels and oleofoams were characterized ex-situ using optical and electron microscopy, X-ray diffraction, differential scanning calorimetry, oscillatory rheology and nuclear magnetic resonance. Results showed that CB crystallized as spherical aggregates of crystalline nanoplatelets (CNPs) in the β(V) form. The nucleation was detected by light turbidimetry, whereas crystal growth could be monitored also by a novel acoustic probe, which was also used to measure the solid fat content (SFC%) through a novel predictive machine learning model. The aeration process broke the spherical aggregates to individual CNPs of similar size, which stabilized the entrained air bubbles by adsorbing at the air/oil interface. Therefore, the main parameter affecting the oleofoam aeration was the total amount of solid cocoa butter crystals (e.g., SFC%). The rearrangement of the crystals around the air bubbles caused an increase in the G’ and G” moduli by an order of magnitude for 15% CB samples, while for 22% and 30% CB samples the viscoelasticity was not significantly affected. Nevertheless, all oleofoams samples had a density reduced by a factor of 3 to 1.6, and displayed stability against oil drainage for a least 3 months.
Due to the soft and opaque appearance of oleofoams, a novel methodology to characterize their internal microstructure was developed. The method was based on the use of synchrotron radiation X-ray tomography and radiography, combined with cryogenic conditions to prevent sample deformation and melting. Temperature control on the sample also enabled the study of the effects of heating on the microstructure in real time, through radiography images. The methodology was applied to the study of two reference samples, one with low and one with high CB %, in relation to the aeration time, storage conditions and heating. The aeration resulted in similar bubble size distributions for both samples, whereas the thickness of the continuous phase decreased in favour of air incorporation, which was higher for samples with lower CB%. Samples with higher CB% displayed higher stability against air loss and bubble size disproportionation after 3 months and 15 months of storage. For both samples, an increase in the bubble sphericity was observed, resulting from the dissolution of smaller CNPs in favour of larger crystal aggregates. Finally, the heating caused bubble coalescence in the oleofoam microstructure; samples with low % of CB were affected more significantly, due to the lower amount of stabilizing crystals. The bubble size distribution increased, as well as the bubble sphericity, due to the dissolution of the smaller CNPs adsorbed at the interface, and due to the less dense packing of air bubbles.
The results gathered during this doctoral project aim at expanding the fundamental knowledge about oleofoams, both regarding the effect of the processing conditions, also via the development of characterization techniques that can probe the internal microstructure of oleofoams and how it evolved during manufacturing, storage and external stimuli.