SYSTEMATIC REVIEW OF FACTORS AFFECTING THE ADSORPTION IN VARIOUS OIL PHASES IN OIL WATER EMULSION
Hydrocolloids are polymers that are water soluble that behave more like colloidal particles in a solution rather than structural polymers. The origination of the hydrocolloids can come from a variety of sources in nature like animals, algae, trees and plants or microbes. Gum Arabic is another common hydrocolloid which has found use in a lot of industrial practices most prominently in the food processing industry. The author independently identified published articles of oil in water emulsions, adsorption and factors affecting adsorption in emulsions and gum Arabic by the use of electronic and in some cases manual search engines. Results revealed that, hydrocolloids like gum Arabic are themselves composed of several distinct colloidal entities, each possessing a multi-scale structure and capable to self-associate in solution. The composition of interfaces in equilibrium with aqueous solutions is expected to drastically differ from bulk composition in complex mixtures. The bulk/interface relationship may be either thermodynamically or kinetically controlled, which associates to an important impact of formulation conditions. Emulsions are non-equilibrium systems the metastability of which is controlled by the dynamic interfacial structure, which relates to but differ from the static structure.
Hydrocolloids are polymers that are water soluble that behave more like colloidal particles in a solution rather than structural polymers. Hydrocolloids are comprised of a large variation of bio – polymers most generally polysaccharides and proteins which can be and are being used for a large number of industrial situations so as to extract specific functions (Saha and Bhattacharya, 2010). The various functions that they serve include processes like stabilising foams, thickening of aqueous solutions, gelling of aqueous solutions, emulsions and dispersions. The origination of the hydrocolloids can come from a variety of sources in nature like animals, algae, trees and plants or microbes. A lot of these materials are extensively used in the food industries around the world in order to change the texture of the food or the organoleptic properties of the products. Industrial products like agricultural products, inks, cosmetics and pharmaceuticals also utilise these materials for various purposes (Dickinson, 2003; Nussinovitch, 1977; Phillips and Williams, 2009).
The most commonly used hydrocolloid is starch, which is used for thickening, however, in recent times, there has been a growing use of the xanthan gum because of its particular rheological properties. Gelatin is another hydrocolloid that is extensively used as a gelling agent. In the current times there has been a shift away from animal originating products towards non – animal originating products and such hydrocolloids can be found from carrageenan.
Gum Arabic is another common hydrocolloid which has found use in a lot of industrial practices most prominently in the food processing industry. There it is used as for the emulsification of the “flavour oil.” However, owing to the unsteady and large fluctuations that Gum Arabic typically experiences, a lot of starch based substitute to the material has also emerged along with protein – polysaccharides conjugates (Atgie, 2018). Protein – polysaccharides conjugates are developed through electrostatic interaction or the maillard reaction in order to replicate the essential properties of gum arabic. Even so, there has not yet been developed any such products that can effectively replicate all the desired qualities of the gum Arabic as an emulsion stabiliser. In order to proceed further in mimicking or replicating the functionalities of the gum Arabic there needs to be a better understanding of the interfacial properties and the structural properties of the gum Arabic and the factors that affect its functioning (Phillips and Williams, 2009; Idris, et al., 1998).
The study presented in this paper studies the various academic literature to scour the properties, structure and the interactions of gum Arabia in order to arrive at a consensus regarding the factors that affect its adsorption in oil water emulsions.
The origination of Gum Arabia is from the Acacia tress which releases them as sticky exudates when they are subjected to pressure or stress (Nussinovitch, 1977; Schmitt and Turgeon, 2011). They are then collected from the trees in the form of granular solids that range from pale amber to white in colour. Most commonly, the acacia tress can be found in the semi – arid stretch of land in the sub Saharan Africa with the largest producer of gum Arabic being Sudan (Islam, 1997). It is also one of the oldest known industrial gums which have been in use for millenniums in the South and the North East Africa in tool manufacturing for its adhesive properties (Zipkin, et al., 2014). Today gum Arabic is used in a variety of industrial uses. It is being used in ceramics as a flocculating agent, in microencapsulating process as an additive, in the cosmetic industry as an adhesive and also in the food processing industry for its emulsifying properties (Chevaioler, 1924; Saha and Bhattacharya, 2010; Dickinson, 2003; Nussinovitch, 1977).
The acacia gum is comprised of a mixture including magnesium, calcium and potassium salts associated with polyelectrolyte. The hybrid polyelectrolyte thus formed has both the polysaccharides subunits and proteins. Mostly, it is comprised of six different carbohydrate moieties namely, arabinopyranose, glucuropyranosyluronic acid, rhamnopyranose, galactopyranose and 4-O methyl glucuropyranosyluronic acid along with a relatively smaller proportion of proteins. The main chain of the polymer is comprised of a 1,3-linked β-D-galactopyranosyl units which are further composed of side chains linked to the main chain through the 1,6 – links. The end units of this polymer are mostly uronic acid moieties, however, the main and the side chains additionally contain carbohydrate moieties units (Schmitt and Turgeon, 2011; Islam, 1997; Phillips and Williams, 2009; Idris, et al., 1998).
Structurally, the Gum Arabic is comprised of moisture (12 – 16 per cent), protein moieties (1.5 to 2.4 per cent), glucoronic acid units (15 – 16 per cent), rhamnose units (12 – 16 per cent), arabinose units (24 to 27 per cent) and glactose units (39-42 per cent).These percentages have variation s based on the ages of the trees that they are collected freom and also the locations that the trees grow in (Schmitt and Turgeon, 2011).
This composition of the polysaccaharide will impact the solubility of the gum and also its electrophoretic mobility which is mostly due to the percentage presence of the glucuronic acid moieties. Further, the percentage of protein moieties is also another key parameter in defining the adsorption characteristics of the gum Arabic in oil water interfaces.
The distribution of the polysaccharide in terms of the mass or the size and the breakdown of the proteins in the given mixture is not represented by the chemical composition of the gum Arabic. Ever since the 1960s the structural composition of the gum Arabia has been extensively studied. Electrophoresis has helped to reveal the heterogeneous composition of the gum component (Schmitt and Turgeon, 2011) or chromatographic techniques (Islam, 1997). Several other techniques including the improved fractionation techniques like that of the size exclusion chromatography has been in use for the separation of the of the gum Arabic moieties as a function of its molecular weight (Daoub, et al., 2016; Zipkin, et al., 2014).
Rheology of gum Arabic solutions
The thickening property of the gum Arabic molecules remains relatively lesser than that of other hydrocolloids even though it is comprised of molecules with high molecular weights. In fact, a 50 w/w % gum Arabic solution shows a similar parent viscosity to that of a mere 1.5 w/w % Xanthane gum solution (Islam, 1997). According to the observations the rum Arabic solutions behave more as a “shear thinning fluid” with the Newtonian plateau at the larger shear rates. Therefore, the apparent viscosity of the gum Arabic solution is a function that grows exponentially based on the concentration of the gum Arabic in the solution with a rate of growth that is directly related to the nature of the gum that is in use (Nussinovitch, 1977; Saha and Bhattacharya, 2010). In addition to being shear thinning, the gum Arabic solutions are thixotropic in nature and the nature of their deformation is dependent on the amount of time. Sanchez, et al. had reported that the the apparent viscosity and its increase is a function of shearing time more than that of the shear rates which are significantly low (Nussinovitch, 1977). Apparent viscosity could also be shown to increase with increase in resting times prior to the measurements of the stress – strain. This could be attributed to the contribution of the surface rheology of the solution to the bulk strain response as proven by the authors (ibid.). The contributions were found to arise from the adsorption process relating to the proteinaceous species of the gum which builds up over time a “viscoelastic network at the sample-air interface” (Atgie, 2018). The time dependence of the rheologicial behaviour increases when the water – air interface is replaced with an oil – water interface or if a larger surface to volume ratio is available. In other studies concerning the rheological measurements that were performed by Li, et al. (2009; 2011), a PDMS film was deposited on the surface of the sample to avoid evaporation (Phillips and Williams, 2009; Idris, et al., 1998). In the first paper (Li, et al., 2009), the authors hypothesise the existence of an association- dissociation equilibrium of the AG molecules which depends on the imposed shear rate of the resting time of the solution. It was observed that the longer the resting time was, the more compact the aggregation of AFG was and the faster the deformation rate was the looser the aggregates were which tended more toward a Newtonian behaviour at larger shear rates. The below figure schematises this structuration mechanism. This proves that the “history of the gum solution” does have a direct impact on the solutions mesoscopic structure and the associated rheological behaviour.
In the second paper (Li, et al., 2011), the existence of the “elastic contribution in the limit of low shear rate (<10s-1) is demonstrated due to the self-association mechanism of hydrocolloids, as well as a viscous contribution which takes over at larger shear rates, once the aggregates are dissociated” (Atgie, 2018; Li, et al., 2011). The stress relaxation curve obtained from the study after a sudden decrease of the shear rate clearly depicted there existed a rapid development of the elastic network from the molecular association from the gum.
According to research (Islam, 1997), for the gum Arabic in solution “a value of approximately -1.5 μmcm/VS for the electrophoretic mobility was found above pH 4.5 and the value decreased close to zero as the pH was decreased to 2. This electrophoretic behaviour was assigned to the carboxylic groups present on the polysaccharides glucuronic acid moieties.” Further (Daoub, et al., 2016) showed that the pKa of the glucuronic acid moieties is of 3.2 which has been in accordance with the electrophoretic mobility that had been measured as a function of the pH.
It is now well admitted that the proteinaceous materials of the gum are responsible for its emulsifying properties. The treatment of the gum with a protease indeed inhibits its emulsifying activity [18*]. Comparison of size exclusion chromatogram for gum Arabic solutions before and after emulsification has shown that a significant proportion of the high molecular mass species adsorbed at the interface as illustrated in Figure I-16 [18*], [49*]. It was also suggested that the proteinaceous moieties adsorbed at the interface and that carbohydrate groups extended into water to provide steric repulsion [24*]. Chikamai et al. demonstrated that heating a solution of gum Arabic at 100°C for 6h importantlydestroyed the emulsifying properties of the gum whereas a temperature of 65°C did not reallyimpact this property. [42*]
Preparative size exclusion or hydrophobic interaction chromatography were used by Ray andcoworkers to recover fractions of gum Arabic with different molecular masses or hydrophobicities[26*]. It was found that the higher molecular mass fraction possessed the higher protein rate. The SECrecovered fractions were then used to stabilize oil-in-water emulsions at constant nitrogenconcentration. Emulsions stabilized with fractions possessing the higher molecular weight and higherprotein rate appeared to be the more stable. Also, it was observed that emulsion stability wasincreased when using a mixture of low and high molecular weight fractions. Finally, authors observedno differences in the zeta potential of the different emulsions and concluded that differences instability were not originating from electrostatic effects.
Emulsifying properties of acacia gum were investigated by Buffo et al.. Using viscosity measurementson the oil-in-water emulsions they developed a model to estimate the thickness of the adsorbedspecies layer at the interface [50*]. Layer thicknesses ranging from 190nm to 430nm were estimatedaccording to the different gums studied (Senegal and Seyal). Gum processing such as pasteurizationand demineralization were shown to enhance emulsion stability, due to an increase of molecularmobility and unfolding of the protein moieties at the interface.
Another emulsification stability study was carried out by H. Aoki and coworkers in 2007 [51*].Stabilities of oil in water emulsions stabilized by acacia senegal gum and mature gum Arabic werecompared.According to accelerated stress conditions it was observed that emulsions stabilized withless than 20% of gum Arabic were not stable whereas when using a matured gum the stability wasimproved even with a concentration of 5%. Moreover when using a same concentration of native gum or matured gum to stabilized an oil in water emulsion the authors observed that a matured gum provided emulsions with a finer droplet size. The same trend was observed in 2010 by O. Castellani et al. [52*].
It was shown by Galazka and co-workers that protein unfolding is promoted under pressure [53*].Gum Arabic conjugates possess a small hydrodynamic radius with respect to their molecular masssuggesting a random coil structure unlike pectin for instance that is a semi-flexible hydrocolloid. Thusa high pressure could help the conjugates moieties to unfold and cover a greater surface at oil
Gum Arabic is a natural hydrocolloid and is highly soluble in water. The substance is tremendously used for the purpose of providing meta-stability to water or oil emulsion. a complex mixture of proteins and polysaccharides is used to create the gum Arabic substance which demonstrate a multiple structure and concentration dependent solution. the species that have absorbed gum Arabic have the ability to adopt conformational changes when water or oil interface is absorbed. It is basically a natural product that is issued in sub – Saharan countries from acacia trees. For the inter facial properties of this hydrocolloid such as the stabilization and binding, the substance is immensely used for different purposes like the oil – in – water emulsion stabilization in the food industry. Because of this, it has become an extensively studied and researched material in the literature, in regards to its structure and composition especially. Gum Arabic is defined as a complex mixture of polysaccharides and proteinmacromolecules with its protein rate, hydrophobicity and size. The composition of the substance is notably studied with the use of chromatographic separation process like hydrophobic interaction and size exclusion. With the use of the hydrophobic interaction chromatography, the gum has been described as a composition of three fractions like: a glycoprotein fraction of 2 per cent, a polysaccharide – protein conjugate fraction of 10 per cent and an arabinogalactan polysaccharide fraction of 88 per cent. In the previous studies, the solution of gum Arabic has been described as a multiple structure having two population of peptide – arabinogalactan in different protein content and size. It also has a number of glycoproteins along with a varied content of hydrophobic amino – acids.
the use of gum Arabic in the stabilization of water or oil emulsion has also been studied extensively where the protein content was displayed as a key factor to stabilize the gum properties. It leads to the conclusion that for the absorption of properties, the polypeptide moieties are found to be responsible. Furthermore, for the emulsifying gum properties, the high fraction of molecular weight conjugate is considered as responsible, with the polypeptide chain and the carbohydrate moieties absorbing at the interfaces and providing strong steric repulsions respectively. The best emulsion metastability is provided by the combination of both high and small weighed molecular species – the fact appears to the knowledge of mankind from the emulsifications with the use of recovered gum Arabic fractions from the protein content or size (hydrophobic interaction or size exclusion chromatography). This observation also suggests that gum Arabic emulsifying properties develops from a macromolecules combination and not merely the conjugates of the high weighed molecular. It is also found that all the size distribution of gum species could have been in absorption in varying range of proportion. Moreover, in contrary to the interfacial composition, the surface concentration is highly dependent on the formulation condition nad most importantly, it increased with the developing gum concentration of reducing ionic repulsion with the help of decreasing pH or increasing ionic strength.
Many research groups have also performed interfacial measurements on the gum Arabic. It is found that the decreasing gum Arabic interfacial measurements were slow because of the high weight of the gum species molecular. It requires sufficient time to spread to the interface, rearrange and absorb. in addition to that, the adsorbed gums species and their interfacial rheology behaviour is also studied and discussed by different authors. In comparison to the other systems, comparatively high surface shear viscosity presented by the gum adsorbed species’ rheological behaviour. All together, many studies have also reported that upon drying, gum Arabic gets the ability to create cohesive interfacial films.
Results and discussion
When gum Arabic is dissolved in water, an outstanding metastability is displayed from the oil – in – water emulsion. It is also found in some studies that amphiphilic species are contained by the gum Arabic which consume irreversibly at water / oil interfaces. In a good formulation condition, a similar behavioural pattern is suggested between the particle-stabilized emulsions and gum stabilized-emulsions. The limited-coalescence regime is an important characteristic of the particle-stabilized emulsions. In terms of the irreversibly adsorbed stabilizers, this regime is significantly found and is compared with a default stabilizer to the surface area that is available and is created at the time of emulsification. The droplets of emulsion will consolidate the stabilizer completely covers the interface. Typically within minutes this mechanism occurs in the particle – stabilized emulsions. The limited amalgamated regime is quantitatively equivalent to a linear variation droplet Sauter diameter contradiction. it is also found that the preparation of emulsion at a low concentration of gum Arabic can never be reached by the finite mean droplet size. As of now, it can be concluded that a limited coalescence regime is not exhibited by gum Arabic emulsions. However, from several experiments done in the previous period, it is also found that gum Arabic either behaves as a stabilizer with high effectiveness or can act as a poor one. Moreover, through a chromatographic analysis, it was also displayed that the gum concentration drastically varied from the oil droplets of interfacial coverage despite of the upsurge of the stabilizing species in solution, where as the composition of the interfacial coverage remained almost unchanged. When the ionic repulsion magnitude is being decreased i.e., at the time of decreasing pH or increasing ionic strength, a similar kind of trend has come into notice. Thus the structure can now firmly be related with the coverage density – the fact is led by the prominent role played by the interfacial structure on the emulsion metastability.
Gum Arabic adsorbed species recovery from interfaces
Gum Arabic is a complex mixture of protein or polysaccharides portraying a large hydrophobicity and size range. Due to this, the interface is also composed in a complex manner and with the help of an analysis technique called double chromatographic, it has been examined. a method that supervise the process of emulsion metastability has been produced in order to recover the gum species that have been consumed at the droplet interface. The separation method that is mediated by this emulsion is consisted of preparing the pentane-in-water emulsion with the help of gum Arabic in the form of a stabilizer. The emulsions are destitute to cream either in a centrifuged or in a separating funnel. the concentrated emulsion that is resulted into was rinsed three times with the help of distilled water. Ultimately, the entire emulsion was dried and frozen removing both water and pentane. After free – drying, the collection of absorbed species is done in the form of a powder in a vial. In a surprising note, a cloudy solution is yielded once the powder is being dissolved in water which goes in contrary to a gum Arabic solution that is completely transparent. There are chances that the cloudiness generates from the colloidal aggregates that does not usually remain present in the gum solutions when in their original form but is created at the interface of oil and water. All these findings can be considered as a sign of the change in structure that takes place in the oil and water interface.
The cloudiness formed in the solution is predominantly dependent on the separation and formulation condition that range from the milky solutions to the gum – like transparent solutions. With the use of a spectrophotometer, the solution transmittance has been measured, at different separation condition and two gum concentrations under centrifugation or under gravity in a separate funnel. It is also notable to record here that an increase in the droplet diameter can result into the centrifugation which indicates an all-round decrease in the interfacial area. The film – forming properties of gum Arabic is extensively used in this literature.
Composition of aggregates
The identification of the aggregates composition is done by analysing both absorbed and non – absorbed with the dual chromatographic with the use of hydrophobic interaction chromatograph and size exclusion. A comparison of the chromatograms of recovered adsorbed species and chromatograms was done where these are filtrated at 200 nm and re-dissolved in water. Two parameters are taken into account for the investigation, in solutionthe native gum concentration (5 and 1.6 w / w %) and the centrifugal acceleration (104 and 1 g) which are applied at the time of separation of phase. According to a usual trend, the recovered species chromatograms area is of lower intensity as compared to the chromatograms that is calculated. This distinction in regards to the mass loss results from the aggregates filtration that is formed at the interface. Therefore, this method is able to provide a pertinent tool to solve the aggregates composition establishing at the water oil interface. On the basis of colloidal nature of the gum Arabic species, it can be stated that small – angle scattering methods are found to be very appropriate to the non – intrusive characterization. With the help of the neutron small-angle scattering and X – ray (SANS and SAXS) the studies on the colloidal length scales can be done. At the low concentration of gum, a solution structure consisting of rich protein cluster and co – existing with protein – poor regions is proposed while going for the study and inside the cluster, the chains would be extended locally yielding the q-1 slope. The gum Arabic is extensively used for the purpose of stabilizing properties of water – in – oil emulsion. The gum Arabicstructure is echoed by the network of the structure at the corresponding concentration in the large scale of length but displays remarkable distinctions in the small scale of length. It is also found that these composites are aggregated in the form of a function of the different conditions and showed that they were of glycoproteins and at low surface densitiesa small hydrophobic population of Arabinogalactan peptide. Large Arabinogalactanprotein conjugates at the higher surface densities also enters into the aggregate composition. in order to decrease the interfacial area, the centrifugating emulsions results into the small Arabinogalactan peptides partial desorption as well as a an aggregation that is drastically extended of all the species compose the film. Hence, the study suggests that the metastability of the stabilized emulsions of the gum Arabic which can be varied from time to time i,e, it can be very poor or can be excellent, for the most of the time remains rooted to the formation and generation of the network and not to the ionic or steric repulsions between the polysaccharides.
The author independently identified published articles of oil in water emulsions, adsorption and factors affecting adsorption in emulsions and gum Arabic by the use of electronic and in some cases manual search engines. An initial screen of the identified abstracts was done for each of these articles. Following that a full text screen of all thse articles had to be performed. This search was supplemented by scanning the bibliographies of all the thus recovered articles. The comprehensive search was undertaken in August, 2019.
The databases, CQLibrary, PubMed, Academia, Research gate and NDL were systematically searched using the keywords: Hydrocolloids, Emulsifiers, oil in water, Emulsion. Three comprehensive search themes had to be designed. The first was performed by using the Boolean operator ‘OR’ with the following headings: Emulsions, Hydrocolloids, Gum Arabia. The second used the Boolean operator “OR” with the terms “temperature, molecular weight, viscosity, hydrophobicity, surface tension, ph, gum Arabic”. The third method used was to combine the two searches through the Boolean function “AND” to further continue the electronic search.
The author independently evaluated all the identified studies and articles for the study based on the following four criteria:
- The articles report original data from primary publications.
- the articles had to be experimental studies.
- the articles make specific mention of the adsorption in oil water emulsions.
- the articles clearly depict the experimental models and procedures.
Despite identification of a large number of eligible articles and papers, there was a significant amount of heterogeneity across the various studies that precluded the conduct of a valid pooled quantitative analysis. The data, therefore, has been synthesised qualitatively for the purposes of this study.
The search revealed 69 unique citations, of which 31 were identified to be potentially relevant and were reviewed further for secondary screening. Finally a total of 26 percent of the papers had been included which had been found to have fulfilled all inclusion criteria. The agreement for inclusion was relatively high with an observed agreement of 99 %.
Understanding the stabilization of emulsions by amphiphilic hydrocolloids combines several challenges:
- Hydrocolloids like gum Arabic are themselves composed of several distinct colloidal entities, each possessing a multi-scale structure and capable to self-associate in solution.
- The composition of interfaces in equilibrium with aqueous solutions is expected to drastically differ from bulk composition in complex mixtures. The bulk/interface relationship may be either thermodynamically or kinetically controlled, which associates to an important impact of formulation conditions.
- Emulsions are non-equilibrium systems the metastability of which is controlled by the dynamic interfacial structure, which relates to but differ from the static structure.
This thesis work addressed these different issues in order to gain a better comprehension of the emulsifying properties of gum Arabic. In the first manuscript displayed in chapter 2, we presented a study on the structural composition of gum Arabic species in solution. Combining new experimental results with relevant previously published studies, we came to important clarifications regarding the structure of gum Arabic in solution. A two dimensional separation of gum Arabic species, using size exclusion as the first dimension followed by hydrophobic interaction, was achieved and compared with another dual chromatographic separation method taken from the literature by Renard and coworkers. We concluded that neither size (molecular weight) nor hydrophobicity are truly efficient separation criteria. Nevertheless, combining both methods together with structural characterization by small-angle scattering, yields a more accurate overview of the various species composing gum Arabic and of their association in solution. This description is consistent with literature reports, but more specific. Two populations of arabinogalactan-peptide (AGp) were identified, rather than one. They differ in size, hydrophobicity and protein content but correspond to an equal number of species. Four populations of glycoproteins, rather than one, corresponding to different sizes and content in hydrophobic amino-acids were also identified. Larger gum species are made up of a combination of these two populations to form arabinogalactan-protein conjugates (AGP), underlying the great heterogeneity in size, even after enzymatic or chemical attacks. In solution, the structure is dominated at large length scales by polypeptide association, which drastically changes with gum concentration from a cluster regime to a semi-dilute regime. At smaller length scales, the signature of several porous structures is observed, one of them corresponding to hyper-branched and weakly charged polysaccharides present in the AGp populations. Smaller structures are detected, possible signature of secondary structures and hydrophobic clusters.
It is important to mention that right after emulsification all the emulsions displayed the same size distribution. After one hour, no noticeable change can be observed with the emulsion made with a solution concentrated in 3% w/w in gum at a pH of 3.5 (black curve). Dividing the gum concentration by 10 at same pH leads to a significant coarsening of the emulsion (green curve). Increasing the pH of the solution to 5.8 leads to a fast destabilization of the emulsion (red curve), while adding salt significantly compensate the pH increase, leading to a size distribution slightly unchanged compared to the initial state (orange curve). These primary observations further underline the importance of the physicochemical parameters employed in a gum Arabic stabilize emulsion formulation.
Secondly hexadecane-in-water emulsions were stabilized using recovered species from the interface of previous gum Arabic stabilized emulsions. Two samples of recovered species were used: non aggregated or partially aggregated. Evolution of the emulsions size distributions after one hour and one week of aging are presented in Figure 16. Using the nitrogen rate of the sample of recovered species as compared to the nitrogen rate of gum Arabic, the concentration of stabilizer in the formulation was calculated to obtain an equi-nitrogen ratio between each formulation.
As observed from the size distributions, emulsions stabilized in these conditions gave slightly finer drops (orange and red curves) than when using native gum Arabic (black curve). After one week of aging, without addition of salt, oil-in-water emulsions formed with the non-aggregated recovered species remained quite metastable, whereas those formed with partially aggregated species destabilized quickly.
This observation shows the importance of the interfacial structuration in the emulsion metastability.
Indeed species irreversibly aggregated probably can no longer adsorb or provide interfacial aggregation, leading to a poor stabilization of droplets interface. Whereas using an addition of salt provided better emulsion metastability in both case, probably through an increased amount of interfacial agent adsorbed at the interface.
In a third stage a default of recovered non-aggregated species was used to stabilize a hexadecane-inwater emulsion. The evolution of the emulsion size distribution over time is presented in Figure 17. Interestingly, over one week coalescence of the smaller droplets and a reduction of the emulsion
polydispersity were observed. This behavior looks similar to that of Pickering emulsions which present a limited coalescence mechanism. It was previously shown in this manuscript that gum Arabic stabilized emulsion do not present such a mechanism, but it seems the most amphiphilic adsorbed species of gum Arabic favor limited coalescence events. This observation confirms that within the gum almost all the species have the ability to adsorb at oil/water interfaces, but only part of them possesses good stabilization properties, under the action of interfacial aggregation and steric repulsion. Therefore when using only the most surface active species of gum arabic to stabilize an emulsion, with a default of stabilizer regarding the available interfacial area, limited coalescence is a possible mechanism of stabilization.
Another observation pointed out in this thesis is the fact that emulsions that were forced to partially coalesce through centrifugation, possessed interfacial films with a higher protein rate and a more aggregated network. Hence, forcing emulsion droplets to undergo slight coalescence events might be a promising method to enhance emulsions metastability. As a final conclusion, the detailed understanding of structures and phenomena at play in the emulsification and stabilizing properties of gum arabic shown in this thesis should prove itself to be a precious ally in the design of complex formulations.
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