Abstract
Background
Opuntia cactus, or prickly pear, is a good source of bioactive compounds, especially betalains that have high antioxidant activity and play an important role in human health. However, despite their nutritional and functional benefits, these compounds are highly sensitive to environmental conditions, leading to reduced stability and bioactivity during processing and digestion.
Scope
To overcome the stability challenges, double emulsions (W1/O/W2) have emerged as an innovative encapsulation and delivery system for bioactive compounds. This review explores the structural characteristics of double emulsions, their encapsulation mechanisms, and models describing release kinetics under digestive conditions. Factors influencing emulsion stability—including emulsifier type, droplet size, phase ratio, and processing conditions—are critically discussed. Studies demonstrate that double emulsions can significantly enhance the stability, bioaccessibility, and controlled release of sensitive compounds such as betalains, while maintaining desirable sensory and functional properties. Their successful incorporation into various food matrices, including beverages, dairy, and bakery products, has resulted in improved nutritional profiles, antioxidant retention, and extended shelf life. Additionally, industrial scalability and formulation challenges are examined, emphasizing the importance of optimizing processing parameters and ensuring consumer acceptability.
Conclusion
Double emulsions provide a novel approach to improve the stability and delivery of bioactive ingredients in functional foods. Future research and development should focus on optimizing their design and functionality, enabling widespread application across the food industry and healthier food products.
Introduction
Recent advancements in colloidal delivery systems have intensified interest in multi-compartment emulsions due to their ability to encapsulate both hydrophilic and lipophilic compounds within a single droplet structure [1, 2]. Among these systems, water-in-oil-in-water (W/O/W) and oil-in-water-in-oil (O/W/O) emulsions are the most widely studied configurations [3]. In W/O/W emulsions, internal aqueous droplets are encapsulated within oil globules, which are subsequently dispersed in an external aqueous phase, creating two interfacial boundaries [4].
This layered structure permits the simultaneous loading and controlled delivery of both water-soluble and fat-soluble bioactives, making such emulsions highly versatile for applications in food, pharmaceuticals, and cosmetics [5, 6]. Despite their functional benefits, the production of DEs via conventional two-step high-shear homogenization techniques often results in polydisperse droplet sizes, structural instability, and low encapsulation efficiencies, thereby hindering reproducibility and scalability [3, 7]. To counteract these constraints, newer fabrication processes like membrane emulsification and microfluidic systems have come into focus, which provide better control of droplet uniformity and interfacial stability [8]. These developments are complemented by the application of natural stabilizers such as biopolymers, proteins, and polysaccharides that improve emulsion strength and biocompatibility [9, 10]. Parallel to such technological developments, plant bioactive compounds have been of interest for their therapeutic and functional properties in preparing health-benefiting foods. Among these, Opuntia extract (OE) is particularly rich in a variety of bioactive compounds such as betalains, flavonoids, quercetin and kaempferol, phenolic acids, vitamins, and dietary fibers—all contributing to antioxidant, anti-inflammatory, antimicrobial, and hypoglycemic effects [11, 12]. Betalains, the pigments that cause the intense red-violet and yellow color of Opuntia fruits, have been shown to possess potent free radical scavenging activity, thus exerting a protective effect against oxidative stress—a significant causative factor of chronic diseases such as cardiovascular disease and cancer [13]. Flavonoids like quercetin and kaempferol augment these effects by regulating inflammatory processes and maintaining metabolic homeostasis [14].
Although they hold great nutritional potential, in practice the application of Opuntia bioactives to food systems is hampered by multiple stability and bioavailability issues. Betalains, for example, are extremely light, oxygen, temperature, and pH-sensitive and thus might be subject to pigment breakdown and loss of antioxidant activity throughout food processing and storage [15]. In addition, their low aqueous solubility and sensitivity to enzymatic hydrolysis during digestion decrease their effectiveness when administered as free from [16, 17]. This twofold susceptibility—during food processing and gastrointestinal transit—demands protective measures to maintain their functionality. In this regard, double emulsions (DEs) have been an innovative encapsulation system capable of improving the stability and targeted delivery of Opuntia bioactives. The W/O/W structure, in turn, facilitates the localization of water-soluble molecules such as betalains in the internal aqueous phase that is protected by an oil layer and held suspended in an external aqueous phase [2, 6]. Such compartmentalization greatly promotes physical protection against oxidative, enzymatic, and thermal degradation and also allows spatial and temporal regulation over release during digestion [3, 7]. Additionally, application of biocompatible stabilizers or natural emulsifiers and wall materials like whey protein isolate gum arabic and starch-based nanoparticles supports better encapsulation efficiency and emulsion stability [18]. Supporting these developments is mathematical models that characterize release kinetics using diffusion, degradation, and swelling mechanisms to optimize DEs performance. These models facilitate the prediction of bioactive release behavior under conditions of the gastrointestinal tract and guide formulation approaches for the optimization of functional efficacy [9, 19]. Combined, Opuntia bioactives incorporation into DEs systems offers a stable platform for functional food development. These emulsions have been effectively formulated into various food systems including functional beverages, dairy foods (e.g., yoghurt and cheese), and baked products, where they impart natural coloring, antioxidant protection, and nutritional fortification [20]. Future studies should target the development of sustainable formulation techniques employing natural emulsifiers, enhanced modeling strategies, and scale-up processing technologies to enable the industrial use of DE-based delivery systems. This technological advancement is in line with the growing interest globally in functional foods capable of preventing lifestyle disorders like obesity, type 2 diabetes, and cardiovascular diseases [11, 12]. Among the ingredients rich in bioactives gaining focus in this regard are plant-derived bioactives, which are preferred due to their natural origin, safety, and multiple health-improving attributes. Among these is Opuntia ficus indica, also popularly known as prickly pear cactus. Extracts derived from this species in total as Opuntia extract (OE) are very rich in a diverse array of bioactive molecules. These range from betalains, flavonoids like quercetin and kaempferol, phenolic acids, vitamins, and dietary fiber—each of which is responsible for the well-documented antioxidant, anti-inflammatory, antimicrobial, and hypoglycemic activities of the extract [21]. Betalains, the pigments imbuing Opuntia fruit in its bright red-violet and yellow hues, have shown excellent free radical scavenging capacity, hence a protective function against oxidative stress—a paramount contributor to diseases of long duration such as cardiovascular disease and cancer [13]. Flavonoids quercetin and kaempferol augment these actions by modulating inflammatory mechanisms and maintaining metabolic homeostasis [22]. The synergy between these phytochemicals amplifies the therapeutic potential of OE, making it a potential candidate for developing functional foods and nutraceuticals with the purpose of enhancing public health results [23, 24]. The synergy between these phytochemicals amplifies the therapeutic of OE, making it a potential candidate for developing functional foods and nutraceuticals with the purpose of enhancing public health results [25]. Nevertheless, while having their nutritional benefits, the functional use of Opuntia bioactives is greatly hindered by stability concerns during processing and digestion. Stability is a vital parameter in formulation and commercial success of functional food products, especially when using sensitive bioactive compounds. To preserve nutritional activity and provide desired health effects, bioactives should be chemically and physically stable during processing, storage, and gastrointestinal digestion [16]. Instability may result in the breakdown of crucial functional ingredients, undermining both product quality and consumer confidence. Thus, stability of bioactives is vital to attain extended shelf life, sustained release, and uniform therapeutic responses in functional foods. OE, though rich in beneficial phytochemicals, poses significant challenges as it is prone to degradation under environmental and physiological stresses. Bioactive compounds like betalains, flavonoids, and phenolic acids are highly vulnerable to external stress factors like heat, light, oxygen, and pH variations, which may happen in food processing or storage [15, 17]. For example, betalains—although useful as antioxidants—are hydrolytically and oxidatively labile, resulting in rapid pigment degradation and a concomitant decrease in biological activity.
Besides environmental instability, the gastrointestinal bioavailability of OE constituents is generally low. Most of these compounds have poor water solubility and are subject to enzymatic degradation in the gut, further lowering their effectiveness upon oral ingestion in unprotected state [16]. This twin susceptibility—both during product manipulation and upon ingestion—constitutes a major hindrance to the functional implementation of OE in food products. Consequently, the establishment of stable encapsulation techniques is imperative. These technologies do not just need to protect OE bioactives from inactivation but must also facilitate their selective and prolonged release at the point of absorption. Without such protective mechanisms, the full therapeutic potential of Opuntia bioactives cannot be realized within functional food applications. To overcome these limitations, DEs have gained attention as an improving their functional performance. The W/O/W emulsion formulation, for example, encapsulates water-soluble compounds within inner water droplets that are, in turn, distributed in an oil medium and ultimately suspended in an outer aqueous medium. This multi-compartmentalization provides a physical defense mechanism that secures sensitive compounds from environmental degradation as well as enzymatic attack during digestion [2]. Encapsulation is the process of choosing suitable emulsifiers and wall materials to stabilize the phase interfaces. Naturally occurring biopolymers like starch-based nanoparticles, proteins (e.g., whey protein isolate), and polysaccharides (e.g., pectin, gum arabic) are very common because they are biodegradable, biocompatible, and can easily form stable films around the droplets [6]. Methods like complex coacervation, membrane emulsification, and high-pressure homogenization control the distribution of droplet size, encapsulation efficiency, and release profiles of DEs [3].
Significantly, mathematical modeling of release kinetics is crucial to predict and understand the release of bioactives from DEs under different physiological conditions. Diffusion, degradation, and swelling mechanism-based models enable formulation parameters to be optimized to obtain controlled and sustained release, optimizing bioavailability and functional efficacy [7]. The practical application of DEs in food systems offers substantial industrial potential. By encapsulating OE within DEs, manufacturers can enhance the stability and bioavailability of sensitive bioactives while also improving product functionality. Encapsulated OE has been successfully incorporated into various food matrices, including functional beverages, dairy products like yogurt and cheese, and baked goods, where it contributes natural pigmentation, antioxidant activity, and added nutritional value [1]. These benefits not only enhance consumer appeal but also support the growing demand for clean-label and health-promoting food products. However, transitioning DEs from laboratory-scale formulations to commercial-scale production presents ongoing challenges. Maintaining long-term emulsion stability, avoiding phase separation during processing, and ensuring cost-effective manufacturing within regulatory frameworks are critical considerations [9, 20]. To address these limitations, current research emphasizes the use of natural emulsifiers, biocompatible wall materials, and green processing technologies, along with advances in modeling and characterization techniques. Collectively, these innovations position DEs as a promising platform for delivering plant-based bioactives like OE in next-generation functional foods.
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Soni, Y., Bashir, O., Morya, S. et al. Stability, release mechanisms, and applications of double emulsions loaded with Opuntia extracts: a comprehensive review. Beni-Suef Univ J Basic Appl Sci 14, 129 (2025). https://doi.org/10.1186/s43088-025-00711-0
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