The Effect of Crystallization Conditions on the Structural Properties of Oleofoams Made of Cocoa Butter Crystals and High Oleic Sunflower Oil

Abstract

Edible air-in-oil systems, also referred to as oleofoams, constitute a novel promising material for healthier, low-calorie fat replacers in confectionary products. Oleofoams can be formed by whipping oleogels, which are dispersions of fat crystals in an oil phase. Understanding how the properties of the fat crystals (i.e. size, shape, polymorphism) contained in oleogels affect the microstructure and stability of oleofoams is essential for both the efficient design and manufacture of novel food products. In this work, cocoa butterone of the main fat phases present in confectionary productions, which is responsible for pleasant texture and mouthfeel propertieswas mixed with high oleic sunflower oil and crystallized to obtain an oleogel. This was subsequently whipped to yield a stable, highly aerated oleofoam. The effect of the crystallization conditions (oleogel composition and cooling rate) on the properties of the oleogels and related oleofoams was investigated with a multi-technique characterization approach, featuring polarized light microscopy, cryogenic scanning electron microscopy, X-Ray diffraction, differential scanning calorimetry and oscillatory rheology. Oleogel crystallization was performed in a labscale vessel, and was monitored using light turbidimetry as an in situ technique. Results showed that the concentration of cocoa butter in sunflower oil was the parameter that affected most strongly the foamability and rheology of oleofoam samples. The size and shape of cocoa butter crystals within the oleogel was found to have a less significant effect, since crystals were broken or partially melted during the aeration process. Oleofoams whipped from oleogels containing 15% w/w and 22% w/w cocoa butter displayed an overrun of 200%corresponding to a calorific density reduction to one-thirdand an increase in both the elastic and viscous moduli compared to their oleogel precursor. This was explained by a structuring effect caused by the aeration process, where cocoa butter β(V) crystal nanoplatelets (CNPs) in the oleogel rearranged to stabilize 3 the air bubbles via a Pickering mechanism. Oleofoams prepared from 30% w/w cocoa butter oleogels, on the other hand, incorporated less air (overrun between 150% and 180%) and displayed a similar viscoelastic profile to their un-whipped precursors, potentially due to air incorporation being limited by the relatively high elastic modulus of the parent oleogels. Nevertheless, the calorific density of these samples was reduced by a factor of 1.6 to 2.5 compared to their full-fat analogues.

Application of X-ray Microcomputed Tomography for the Static and Dynamic Characterization of the Microstructure of Oleofoams

“…Slices of the reconstructed volumes, represented in a 2D plane, showed a clear distinction between the gas phase (dark gray pixels) and the continuous phase (light gray pixels) for all samples. The continuous phase in the oleofoam is the CB-HOSO oleogel, 17 which comprises both the fat crystal network (CB) and the entrapped oil (HOSO). The thickness of the oleogel, calculated from the tomography data, is shown as a volume distribution for the samples.…”

“…It contributes, together with the air phase, to the viscoelastic profile of the oleofoams; furthermore, the continuous lipid phase has a significant role in stabilizing the air bubbles against coalescence, as demonstrated by several authors. 9 , 15 , 17 , 29 Furthermore, the fat crystal network present in the oleogel phase also affects the oil binding capacity of oleofoams, preventing liquid drainage from the structure. 33 The evolution of the continuous phase thickness during storage is also of great importance, as lipid crystals dispersed in the oleogels are subjected to Ostwald ripening and sintering (formation of crystal bridges).…”

“…The oleogels were whipped for a total time of 30 min, collecting samples every 5 min to study the effect of aeration time. 17 At each step, the overrun (i.e., the increase in the sample volume) was calculated with the technique most commonly known as the cup method, where the sample is weighed in a cup of known volume. The overrun is then determined through eq 1 where w oleogel and w oleofoam are the weight of the un-whipped oleogel and the weight of the oleofoam, respectively.…”

“… 9 Most of the research on oleofoams has focused on understanding the relationship between the properties of crystals within the oleogel (size, shape, polymorph) and the resulting foamability and foam stability in the whipped state. 1 , 17 , 20 , 24 …”

Buckling versus crystal expulsion controlled by deformation rate of particle-coated air bubbles in oil

“…11,12 Much progress has been made in understanding the formation and stability of oil foams stabilised by crystallising agents, yet our knowledge of the mechanical properties and dynamic behaviour of such systems remains limited, despite its importance for product shelf-life, as well as optimal processing conditions and performance in applications. In particular, while there is significant literature available on the rheology of oleogels formed by the crystal network in the bulk oil phase, 1,2 and on the link between crystal formation and network properties, 13,14 less is understood about the microstructure and properties of the crystal-stabilized interfaces. 15 The stability, dynamics and mechanics of particle-stabilized (or "armoured") bubbles 16 have been studied extensively for model systems stabilized by micron-scale colloids; these studies provided important fundamental insights thanks to the ability to directly visualise the interface microstructure and its evolution.…”