Wheat germ, which is part of the wheat grain, a by-product derived from the wheat milling industry, containing about 10–15% of oil. Wheat germ oil (WGO) is widely recognized as a good vegetable oil with high nutritional value (Brandolini and Hidalgo 2012). WGO contains numerous beneficial bioactive compounds that are good for health. WGO has a higher tocopherol content than other vegetable oils, up to about 2,500 mg/kg, with α-tocopherol is predominant accounting for 60% of the total content (Ghafoor et al. 2017). In particular, it is worth noting that vitamin E has up to 500 ppm (Türkoğlu et al. 2021). WGO contains a high concentration of unsaturated fatty acids, specifically linoleic and linolenic acids, which are important compounds in human metabolism but cannot be produced by organisms’ method. WGO is rich in high-nutritional-value compounds, including sterols, unsaponifiable matter, pigments, phytosterols, policosanol, octacosanol, ceramide, etc. WGO is also a valuable source of micronutrients, which are necessary for many different bodily functions, such as A, B complex vitamins, and many minerals and fibers (Brandolini and Hidalgo 2012; Šramková et al. 2009). Due to its high concentration of bioactive compounds, WGO has the potential to provide numerous health benefits to humans when used on a regular basis. It functions as an antioxidant, preventing lipid oxidation and thus assisting in the cell protection from free radicals, as well as lowering plasma and blood cholesterol levels (Ghafoor et al. 2017; Jha et al. 2013; Zhu et al. 2011). It increases the efficiency of microcirculation in venous and arterial, blood flow in vessels. Therefore, it has the potential to prevent cardiovascular disease (Ghafoor et al. 2017). WGO contains linoleic and linolenic acids, which are precursors to prostaglandins, a group of hormones involved in muscle contraction and anti-inflammatory activity (Harrabi et al. 2021; Köse 2021). WGO also helps to lower cholesterol levels in the liver, provide precursor of cell membrane phospholipids, improve physical endurance, decongest the body, and slow the effects of aging (Koba and Yanagita 2014). It is considered a good food for athletes because it helps to improve exercise performance. Besides, it has the ability to improve platelet aggregation, and plasma cholesterol levels, as well as reduce the risk of obesity (Brandolini and Hidalgo 2012; Ghafoor et al. 2017). WGO can reduce free radicals and the immunosuppressive effects of lipid peroxidation in UV-exposed skin, as well as slow skin aging by moisturizing and smoothing skin (Dunford 2009). WGO also has strong antimicrobial properties against a variety of pathogenic bacteria (Al-Rimawi et al. 2020; B.-S. Choi and Kang 2009). Because WGO has a wide range of potential health benefits, it is becoming increasingly popular to develope in food production, formulation products, pharmaceuticals, cosmetics and even agriculture (Boukid et al. 2018; B.-S. Choi and Kang 2009; Türkoğlu et al. 2021; Wang et al. 2021).
WGO-in-water emulsion is a temporarily stable mixture of two immiscible liquids, the dispersed phase has WGO droplets and the dispersion medium is an aqueous phase. WGO-in-water emulsion are formed when two non-soluble (i.e. WGO and water) liquids with surfactant additions, are agitated together to disperse one liquid into the other, in the form of drops. WGO-in-water emulsion assists in overcoming difficulties in WGO incorporation in some products, particularly liquid products, caused by differences in rheological properties and solubility. The use of simple delivery systems such as oil-in-water, or water-in-oil emulsions is one of the most effective and widely used methods for combining oil into foods (S. J. Choi and McClements 2020; Galanakis 2019). It enables the mixing of components with different rheological properties, particularly solubility, and forms a physical barrier (i.e. interfacial layer) that helps prevent fat oxidation due to contact between oil and oxygen or prooxidants. Furthermore, because water soluble digestive/lipolytic enzymes are more readily active at the interface between oil/water and the surface area of the emulsion is increased, WGO-in-water emulsion improves fat absorption efficiency in the human body (Bodewes et al. 2015). WGO-in-water emulsions also combine with wall materials more effectively during spray drying, leading to greater encapsulation efficiency and easier WGO use and maintenance (Karadeniz et al. 2018). Therefore, WGO-in-water emulsion facilitates the addition of WGO into products such as food, cosmetic, and pharmaceutical. WGO-in-water emulsions are now used to product spray drying products, micro and nano WGO capsules; as well as to preserve the oil stability of cooked fish fillets; and use in cosmetics; or substitute animal fat in beef burgers (Barros et al. 2021; Ceylan et al. 2020; Dunford 2009).
Membrane emulsification, also known as membrane homogenization, a relatively new emulsification technology (Spyropoulos et al. 2014a). Membrane emulsification employs low energy inputs to press coarse emulsion through membrane pores, resulting in the formation of droplets at the membrane. Droplets form at pore openings and detach when reaching a certain size (Alliod et al. 2018). This is the result of a balance between four major forces that govern the membrane emulsification process: shear, pressure/inertia, interfacial tension, and buoyancy forces (Joscelyne and Trägårdh 2000). Membrane emulsification enables the development of emulsions with a uniform droplet size distribution over a wide range of mean droplet sizes ranging from less than 1 to more than 100µm (Vladisavljević 2019a). The advantage of this method over other emulsification methods is that it is low-pressure and does not significantly raise temperature or shear stresses during the process, which can prevent damage to emulsion components such as proteins, starches, etc. (C Charcosset et al. 2004; Vladisavljević 2019a). At low energy inputs, it still can be efficient in preparing droplets with very narrow particle size distributions. Even so, it easily can produce emulsions with higher droplet concentration (Vladisavljević 2019b). Beside that, the technique is highly appealing due to its simplicity, low surfactant requirement, suitability for large-scale production, and continuous or semicontinuous operation (Emma Piacentini and Giorno 2016). It has been used to produce monodispersed multiple emulsions (oil-in-water (o/w) and water-in-oil (w/o) emulsions), as well as o/w/o and w/o/w emulsions, especially those with shear sensitive components (Catherine Charcosset 2021; Consoli et al. 2020; E Piacentini et al. 2020). Applications contribute to the creation of high value in pharmaceuticals, chromatography beads, luxury cosmetics, and food industry (Jiang et al. 2020; Spyropoulos et al. 2014b; Vladisavljević 2019b).
In conventional direct ME, fine droplets are formed on the interface between membrane and continuous phase by pressing pure the dispersed phase through the membrane (Aserin 2007). However, there are several drawbacks to direct ME, including a relatively low dispersed phase flux, low productivity, a long production time, suitability for emulsions with a dispersed phase of up to 30%, and difficulty in preparation and operation. Premix ME was developed to overcome the drawbacks of direct ME by forcing a preliminarily emulsified coarse emulsion (rather than a single pure dispersed phase) through the membrane (Vladisavljević and Williams 2005). This is accomplished by first mixing the two immiscible liquids together with a conventional stirrer mixer, then passing the preliminarily emulsified emulsion through the membrane (SUZUKI et al. 1996). Premix ME was used to homogenize an oil-in-water emulsion in food. There are several studies that have evaluated the influence of membrane and emulsifier type and demonstrated the effectiveness of premix ME compared to other homogenization methods (Berendsen et al. 2014; Ramakrishnan et al. 2013). However, the most important factors influencing membrane emulsification include not only membrane and emulsifier parameters, but also dispersed phase and process parameters. It influences filtration process efficiency as well as properties of the emulsion system, such as particle size distribution and emulsion stability (Catherine Charcosset 2009; Jiang et al. 2020). So far, there are no studies on using premix ME to emulsify WGO-in-water emulsion. Therefore, the present study aims to investigate the feasibility of production of WGO-in-water emulsion by premix membrane emulsification, using ultrafiltration membrane, specifically, polyether sulfone (PESU) membrane with cellulose acetate active layer. The influences of operating pressure, content of WGO phase, and lecithin ratio on the droplet mean diameter, particle size distribution, emulsion stability, and permeate flux were investigated. The study's findings will reveal the feasibility of using premix ME in WGO-in-water emulsification in particular, and other emulsions in general.