Effect of Glyceryl Monoolein Addition on the Foaming Properties and Stability of Whipped Oleogels

Foams are colloidal systems containing air in the form of bubbles entrapped in a continuous phase [1]. An in-depth understanding of these systems is of paramount importance. In the cosmetics, food, and pharmaceutical industry, aqueous foams have been extensively used and studied. Foams are considered as metastable systems that are stabilized by low-molecular weight molecules such as surfactants, amphiphilic polymers, proteins, or, alternatively, by dispersed particles [1,2,3]. Oil-based foams (gas-in-oil systems) consisting of edible raw materials have gained a lot of attention due to their obvious potential in the oil structuring field, mainly as an alternative way of producing healthier food products [1]. The stabilizing compound used in such systems should be insoluble to the continuous phase, ensuring in this way the foamability of the system [1,4]. The process for preparing such foams typically involves heating so as to melt a mixture that contains the continuous oil phase (vegetable oil) along with the stabilizing agents (fatty acids, acylglycerols, etc.), until the latter is fully incorporated therein. Subsequently, the system is cooled down following a precise time–temperature protocol, allowing the stabilizing agent to crystallize. The foam is then formed by applying a whipping process that promotes air entrapment within the continuous phase [5]. Whipping process has been reported to be necessary so as to obtain stable foams, since through this process the formed bubbles are covered by a layer of crystals (Pickering monoglyceride crystals) [1,5,6,7,8,9,10,11,12]. Two important factors that strongly influence the final properties of crystal–oil mixtures, especially in the case of monoglycerides, are the duration and the temperature of the process [5].

The aim of the present study is to conduct an in-depth investigation of stable food-grade oleofoams as a viable, relatively fast, and scalable process which, in turn, could be applied to pharmaceutical products for humans or for veterinary applications. These applications include but are not limited to materials that can be used as internal teat sealants (ITSs) in dairy animals. Udder health is a key issue dictating the productivity and longevity of dairy animals as well as the financial viability of farms. In practice, prevention of clinical and sub-clinical intramammary infections (IMI) is a priority that focuses on pathogens entering through the teat canal. The latter is achieved using internal teat sealants (ITSs) at the commencement of the drying-off period. Here, we propose the use of designated stable food-grade oleofoams as components of ITS products for dairy animals [13,14,15]. Although the application of ITSs during the dry period is thoroughly studied, there is a growing demand for new environmentally friendly products that consider “green”, “natural”, and “sustainable” alternatives to existing ITS products. The notion is that these alternative products should have equal levels of IMI prevention without having components such as heavy metals and petroleum-based materials. An interesting resource of materials that could cater to the needs of ITSs are structured vegetable oils (SVOs).

The latter include all types of oleogels and oleofoams [16]. The application of SVOs as ITSs is described in a patent (PCT International Application No. PCT/GR2020/000024 filed on 15 May 2020 and published under No. WO2020/229851 on 19 November 2020; European Patent Application No. EP20742414.4 published under No. ΕΡ 3968954 on 23 March 2022) where vegetable oils are structured towards the formation of stable oleogels. These oleogels are subsequently homogenized to form thick pastes that are suitable for intramammary infusion as part of ITSs. The raw materials used in the current study were of natural origin, edible, and of pharmaceutical grade. Glyceryl Monostearate (GMS), Glyceryl Monooleate (GMO), and Medium Chain Triglyceride oil were chosen as raw materials because they are of plant origin, can be obtained at food- and pharmaceutical-grade, are relatively inexpensive, and have been studied extensively. Additionally, Medium Chain Triglycerides are reported to have antimicrobial properties [17], thus making them a very promising material for the creation of products with antimicrobial properties.

These materials were combined to produce oleogels by using different cooling rates, with a focus on the incorporation of GMO. The oleofoams were prepared in two steps, namely the preparation of stable oleogels and their subsequent aeration by whipping to produce stable oleofoams. The study of both oleogels and oleofoams is considered very important, as the physical and structural properties of the initial oleogels play a significant role in the final properties and stability of the produced oleofoams. Thus, in this study both oleogels and oleofoams comprised of GMS, GMO, and MCT oil mixtures were thoroughly studied. The effect of the cooling rate on the oleogel microstructure was investigated through rheometry, Differential Scanning Calorimetry (DSC), Polarized Light Microscopy (PLM), Confocal Laser Scanning Microscopy (CLSM), Fourier Transform Infrared Spectroscopy, and X-ray Diffractometry (XRD). The optimum conditions were determined for the preparation of oleofoams, with a focus on foamability and foam stability.

As shown in Table 1, predetermined amounts of the tested monoglycerides (Glycerol monostearate 40–55 (Type I, EP), GELEOL, Gattefossé, Saint-Priest, France; Glycerol monooleate (Type 40, EP), PECEOL, Gattefossé, Saint-Priest, France) were added to MCT oil (Medium Chain Triglycerides, EP, LABRAFAC LIPOPHILE WL 1349, Gattefossé, Saint-Priest, France) in a glass beaker and heated to 80 °C under continuous magnetic stirring until a clear homogeneous solution was obtained. The molten mixture was then transferred to 50 mL centrifuge tubes and cooled to room temperature before any further analysis.