Oleofoams: Properties of Crystal-Coated Bubbles from Whipped Oleogels—Evidence for Pickering Stabilization

Abstract

Interfacially stabilized nonaqueous lipid-based foams, which we name here oleofoams, are rarely encountered as opposed to the large number of aqueous foams stabilized by molecular or particulate emulsifiers. There is no case well described in the literature with a convincing characterization of the interfacial contribution to oleofoam stability. Methods for filling this gap are described here, which reach out to a large part of the lipid phase diagram. We bring here complete evidence that lipidic crystals made of a high fraction of fully soluble monoglyceride (MG) in oil do not only adsorb at the oil-air interface but also can easily form a jammed, closely packed layer of crystals around the bubbles of a foam produced by whipping (Pickering effect). Very fine bubbles, soft textures, or firmer ones such as for shaving foams could be obtained, with a high air fraction (up to 75%), which is unprecedented. A thin, jammed layer of crystals on bubbles can cause bubbles to retain nonspherical shapes in the absence of bulk effects for times much longer than the characteristic capillary relaxation time for bare bubbles, which is actual evidence for Pickering-type interfacial stabilization. By comparing to foams obtained by depressurization, we show that whipping is necessary for bubble wrapping with a layer of crystals. The origin of high stability against Ostwald ripening at long times is also discussed. Furthermore, we show that these Pickering whipped foams have rheological properties dominated by interfacial or film contributions, which is of high interest for food and cosmetics applications because of their high moduli. This system can be considered to be a model of the crystallization behavior of MG in oil, which is similar to that in many fats. Our methods are very general in the context of lipid-based foaming, in particular, from food materials, and were used in patent applications.

Arresting dissolution by interfacial rheology design

“…In this work, we apply this paradigm to interfacial materials, specifically particle-stabilized drops and bubbles. These systems with high interfacial area have broad applicability from food formulation and processing (1,2), encapsulation (3,4), ultrasound medical technologies (5), to lowweight/high-strength materials (6). One of the key challenges in using solid stabilized emulsions and foams in applications is curtailing Ostwald ripening, which causes the growth/shrinkage of large/small bubbles and increased size heterogeneity (7).…”

Synthesis, physicochemical properties, and health aspects of structured lipids: A review

“…Gunes et al. () have studied the interfacial contribution of crystalline particles to the foam stability in a MAG‐high oleic sunflower oil system, where the adsorbed MAG crystals were found to form a jammed, closely packed layer that could stabilize the air bubbles against a dilution step.…”

“…Oil foams, also called oleofoams, non-aqueous foams, or whipped oils, refer to a colloidal dispersion where air bubbles are dispersed in oils (Figure 3d; Heymans et al, 2018), which can be used as low-calorie food products (Gunes et al, 2017) and lubricating oil (Binks, Davies, Fletcher, & Sharp, 2010). Crystalline particles including MAGs (Gunes et al, 2017;Heymans et al, 2018), DAGs (Shrestha, Shrestha, Sharma, & Aramaki, 2008;Shrestha, Shrestha, Solans, Gonzalez, & Aramaki, 2010), TAGs (Binks & Marinopoulos, 2017;Mishima, Suzuki, Sato, & Ueno, 2016), fatty acids (Binks, Garvey, & Vieira, 2016), fatty alcohol (Fameau et al, 2015), and a combination of sucrose ester and lecithin (Patel, 2017d), as well as solid particles such as fluorinated particles (Binks, Johnston, Sekine, & Tyowua, 2015), have been reported to be capable of stabilizing air-oil interface in oil foams. Here, we mainly focus on edible oil foams stabilized by crystalline MAGs and DAGs.…”

“…The obtained samples had overrun percentages ranging from 47% to 52% and showed excellent storage stability that could maintain the initial foam volume as long as 2 months. It is believed that the foamability and foamstability of oil foams are determined by the phase behavior (solubility boundary) of crystalline stabilizers in the bulk oil (Binks et al, 2016;Chen, Van Damme, & Terentjev, 2009;Gunes et al, 2017;Shrestha et al, 2006b;Shrestha et al, 2006c;Shrestha, Aramaki, Kato, Takase, & Kunieda, 2006a), which can be controlled by altering types and concentrations of stabilizers, foaming temperature, storage temperature, and bulk oil types (Fameau & Saint-Jalmes, 2017). For example, when foaming temperature is well below the solubility boundary, MAGs or DAGs crystallize into particles to stabilize the air bubbles formed upon whipping.…”

“…For example, Heymans et al (2018) have endeavored to relate the crystalline structure of oleogels formed by tuning tempering protocols to the foamability and foam stability of the resultant oil foams. Gunes et al (2017) have studied the interfacial contribution of crystalline particles to the foam stability in a MAG-high oleic sunflower oil system, where the adsorbed MAG crystals were found to form a jammed, closely packed layer that could stabilize the air bubbles against a dilution step.…”