Natural-Based Stabilisers for Emulsion-Based Delivery Systems

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1		Júlia Teixé-Roig	--	1714	2023-04-19 16:25:22	\|
2	format correct	Catherine Yang	-3 word(s)	1711	2023-04-20 03:24:29	\|

This entry is adapted from the peer-reviewed paper 10.3390/foods12071502

Initially, emulsion-delivery systems were obtained mostly by using synthetic ingredients, leading to high stability of the resultant systems. However, consumers have become aware of the impact of synthetic ingredients on health and on the environment, increasing the demand for food products containing ingredients from natural sources. Synthetic emulsifiers have been the most used to produce emulsion-based delivery systems for bioactive compounds, as these molecules can be rapidly adsorbed at the interface, efficiently reduce the interfacial tension, and provide systems with high stability. Nevertheless, the consumption of these synthetic emulsifiers might induce health problems and may cause toxic symptoms after long administration periods. For that reason, researchers focus on emulsion stabiliser ingredients from natural sources, which can be classified depending on their chemical structure in proteins, phospholipids, polysaccharides, or saponins.

emulsions delivery systems bioactive compounds

1. Proteins

Most proteins from natural sources present an amphiphilic structure since they contain a mixture of polar and non-polar amino acids, which means that can be adsorbed into oil–water interfaces stabilising lipid droplets in emulsions. These emulsifiers tend to be bulkier and diffuse slower to the interface than small molecule emulsifiers, and higher concentrations are needed rather than with smaller molecular weight. However, once at the interface, they provide a strong viscoelastic film that resists mechanical stresses and provides electrostatic and steric stabilisation ^[1]. Nevertheless, these natural emulsifiers have been found to be highly affected by pH changes and high ionic strength, which can cause bridging flocculation of droplets ^[2]^[3]. Regarding natural proteins, whey proteins and caseins from bovine milk have been widely used as emulsifiers, as they are effective for the stabilisation of emulsion-based systems ^[4]^[5]^[6]. Recently, some researchers have focused on the use of plant-based proteins such as those from peas, lentils, or rice, to stabilise emulsion-based delivery systems since they are better for human health, the environment, and animal welfare ^[7]. As an example, some authors have reported that despite being a poorly soluble protein, pea protein can be used to stabilise vitamin-D-loaded nanoemulsions after a pH-shifting and sonication treatment ^[8]. In this work, the authors reported small particle sizes < 150 nm and high UV radiation stability of vitamin D3. This highlights that the functionality of these molecules as emulsifiers can be improved by treating them before incorporating them into the delivery systems. Alternatively, rice bran protein was used as an emulsifier of quercetin-loaded nanoemulsions achieving reduced particle sizes (200 nm) and showing relatively high stability ^[9]. In addition, recent studies have investigated the emulsifying capacity of proteins from algae such as Nannochloropsis gaditana, Tetraselmis impellucida, and Arthrospira platensis ^[10]^[11]^[12]. In these works, proteins extracted from algae were shown to form stable emulsions at similar concentrations to proteins from other sources such as dairy or legumes. Indeed, the minimum particle size that was achieved was observed to be similar when comparing algae proteins to those from milk ^[10]. Moreover, emulsions containing a protein-rich extract from Arthrospira platensis as an emulsifier were shown to present a good emulsifying capacity and provided emulsions with physical stability for up to 30 days. Thus, the use of protein-rich algae extracts as emulsifiers presents an added value since the proteins that they contain can act as emulsion stabilisers, but they also contain great amounts of bioactive compounds.

2. Phospholipids

Phospholipids have non-polar and polar regions within the same molecule, so they are amphiphilic molecules that can adsorb to oil–water interfaces and stabilise lipid droplets. Phospholipid-based emulsifiers used in the food industry are usually called lecithins. This emulsifier type, which is a major component of cell membranes, can be obtained from both vegetal and animal sources. However, most of the research focused on emulsion-based delivery systems has been performed by using lecithins from vegetal sources, mainly soybean, sunflower, and cottonseed. The HLB of lecithins can be different depending on the phospholipid composition, but the values are usually approximately 8. This means that these emulsifiers can stabilise both O/W and W/O interfaces. Moreover, lecithins stabilise emulsion-based systems via electrostatic repulsion, so when they are adsorbed at the interface, they provide highly negative charges. As an example, Gao et al. ^[13] observed extremely negative ζ-potential (−70 mV) and particle sizes < 250 nm when soy lecithin was used at concentrations higher than 2% in nanoemulsions that were based on fractionated coconut oil. Moreover, this emulsifier type has been found to be highly effective in reducing the interfacial tension. Indeed, soy lecithin has been found to be more effective than whey protein or gum Arabic in reducing the interfacial tension, showing the lowest particle size when preparing oil-in-water nanoemulsions that encapsulate paprika oleoresin (<140 nm) ^[4]. Moreover, these authors reported that lecithin nanoemulsions were highly stable when exposed to temperatures (40–80 °C) but were affected by the ionic strength, showing an increase in the particle size and loss of negative electrical charge. Lecithin nanoemulsions have been shown to be stable at a wide range of pH values, presenting no instability phenomena for 7 days at various studied pH values ^[14]. Indeed, some authors have reported that lecithin emulsions presented a low particle size (<200 nm) at a pH range of 3–8 and a negative ζ-potential, especially at a pH > 4, which was about −60 mV ^[3]. Moreover, by using this emulsifier over 1% w/w, long-term stable nanoemulsions (up to 86 days) were obtained, which were able to efficiently entrap curcumin within, preventing its autoxidation and, hence, maintaining the antioxidant capacity of the bioactive compound ^[15]. Soybean lecithin has been found to be also effective in stabilising the oil–water interface of double emulsions. Indeed, by using this emulsifier, emulsions with a particle size of about 4 µm and a phycocyanin encapsulation efficiency of 82% were achieved ^[16]. Therefore, lecithins seem to be a highly valuable emulsifier since they are highly efficient in reducing interfacial tension and providing systems with high stability over time. Moreover, emulsion-based delivery systems containing these emulsifiers seem to be more stable to external factors such as pH or temperature compared to others such as proteins.

3. Polysaccharides

Some polysaccharides from natural sources can also be useful as emulsifiers since they present an amphiphilic structure that can adsorb at the water-in-oil interface and help to stabilise the system ^[17]. Moreover, most of them are of vegetal origin, so they can be used in plant-based products. This type of emulsifiers generally present good pH, salt concentration, and temperature stability, but they need to be used in higher amounts to stabilise emulsion-based systems and produce small particles due to their large molecular weight and dimensions ^[18]. When polysaccharides are adsorbed at the interface, they form relatively thick layers that provide steric repulsion, so they are less affected by changes in pH and ionic strength than proteins ^[19]. Among them, Arabic gum has been widely used and has been shown to reduce interfacial tension, providing emulsions with particle sizes < 1 µm. However, this polysaccharide seems to be less effective in reducing the particle size and preventing the degradation of the encapsulated carotenoids than others such as whey protein or lecithin ^[4]. Nevertheless, it provides emulsions with better flocculation stability at different pH values, high ionic strength, and high temperatures than those containing whey protein as an emulsifier due to their steric stabilising mechanism ^[20]. Therefore, it seems that polysaccharides such as Arabic gum can be potential emulsifiers to obtain stable systems against external factors but present some disadvantages, such as the low stability of the encapsulated compound and higher particle sizes when compared with proteins or phospholipids. Moreover, a natural hydrocolloid exudated by the bark of Cercidium praecox tree (Brea gum) has been found to produce emulsions with even more stability than Arabic gum at the same concentration, which was attributed mainly to its higher viscosity ^[21].

Another polysaccharide that is widely used in the food industry is pectin, which has been reported to present emulsifying properties, although the particle sizes that were achieved were not in the range of nanoemulsion ^[22]. However, a recent work has reported that extracts from avocado residues (from peel and seeds) that are rich in phenolic compounds presented a higher interfacial activity than that of low-methoxyl pectin ^[23]. Thus, this work demonstrated the advantages of agrifood residues as a source of polysaccharides with emulsifying properties but with added value due to the high content of bioactive compounds that reduced lipid oxidation. In the same way, polysaccharides isolated from seaweed have also been tested as emulsifiers that are rich in bioactive compounds. As an example, polysaccharides from alga Ulva fasciata have been tested as emulsifiers in β-carotene-loaded emulsions, showing particle sizes of about 0.8 µm and <10% of encapsulated compound degradation for 4 days at 4 °C ^[24]. Other algae polysaccharides such as fucoidan have been found to have a good emulsifying capacity, especially when isolated by using microwaves, presenting also antioxidant activity ^[25]^[26]. This polysaccharide has shown to form emulsions with higher stability and fucoxanthin encapsulation efficiency than Arabic gum ^[27]. Moreover, it has been used in combination with other biopolymer, forming complexes. As an example, Jamshidi et al. ^[28] used whey protein–inulin–fucoidan complexes to stabilise double emulsions and concluded that the presence of fucoidan had a significant influence on the nutritional quality and oxidative stability.

4. Saponins

Saponins are relatively small amphiphilic molecules that are mostly obtained from plants and that consist of a hydrophobic aglycone and a hydrophilic sugar moiety ^[29]. These plant-based emulsifiers appear to be highly effective at forming small droplets that are stable over a wide range of conditions (pH, ionic strength, and temperature) ^[3]. These emulsifiers, which have been shown to provide steric and electrostatic stabilisation, can form interfacial layers with a high dilatational elasticity, inhibiting droplet deformation and coalescence. Among them, saponins obtained from the bark of the Quillaja saponaria tree have been shown to reduce the interfacial tension in the oil–water interface faster and to a higher extent than other emulsifiers such as lecithin, whey protein, or Arabic gum, rendering to emulsions with a smaller particle size ^[30]. The use of this emulsifier has been compared with saponins extracted from other plants: Tribulus terrestris, Trigonella foenum-graecum, and Ruscus aculeatus ^[31]. These authors reported the best results by using the Tribulus terrestris extract and highlighted the use of saponin-rich extracts as potential emulsifiers due to their similar or even additional functional properties than saponin pure forms, avoiding complex extraction and purification treatments. In another work, by using tea saponin extract from Camellia lutchuensis (51.8 wt% saponin content) stable emulsions were obtained in a pH range of 3–9 and thermal processing from 30 °C to 90 °C ^[32].

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Subjects: Food Science & Technology

Contributors MDPI registered users' name will be linked to their SciProfiles pages. To register with us, please refer to https://encyclopedia.pub/register :

Júlia Teixé-Roig

Gemma Oms-Oliu

Isabel Odriozola-Serrano

Olga Martín-Belloso

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Update Date: 20 Apr 2023

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