Complex coacervation is a microencapsulation technology which has the benefits over other forms of encapsulation of allowing very high ‘payloads’ which are achievable up to 99%. The possibilities for controlled release are very good too, being based on mechanical stress, temperature and sustained release.
The technique is ideal for encapsulating flavour and essential oils, fish oils, nutrients and vitamins, preservatives and enzymes (Arneodo, 1996; Bakan, 1980). The physical and polymer chemistries underpinning the technology are extremely well understood.
The complex coacervates show good heat resistance and water insolubility and are especially good for unstable polyphenolic compounds (Munin & Edwards-Levy, 2011).
The Process Of Complex Coacervation
The phenomenon of coacervation occurs in colloidal solutions. It is rightly regarded as the first known example of encapsulation. Green & Scheicher (1955) first produced pressure-sensitive dye microcapsules for carbonless copying paper.
The process involves separation of a polymeric solution into two liquid phases. One phase is based on colloid particles from solution which then agglomerate into a separate, polymer-rich liquid phase known as the coacervate. The other phase is a diluted phase called the equilibrium solution. The core material used in coacervation must be compatible with the recipient polymer and be insoluble or sparingly soluble in the coacervation medium.
The encapsulating material is always hydrophilic and the core material is usually hydrophobic (Millqvist-Fureby, 2014).
The formation of the two distinct phases, the coacervate and the equilibrium solution, is induced by changes in the media brought about by altering pH, ionic strength and polyion concentrations.
Coacervation can be simple or complex. A simple coacervation involves one type of polymer with the addition of strongly hydrophilic agents to the colloidal solution.
In complex coacervation, two or more types of polymers are used. The forces causing complex coacervation are based on electrostatic interactions between two oppositely charged polyions in aqueous solution. The core material to be encapsulated like a flavour or oil has to be present in the mixture as coacervation proceeds. It must always be in a dispersed form when it is added to the polymer solution. This mixture is then suspended in an aqueous solution phase containing a surface-active agent. The coacervate nuclei are then adsorbed onto the surface of the volatile compounds.
The material to be encapsulated may also be added during or after phase separation. In all cases, the coacervation mixture must be continually stirred. Sometimes a droplet stabiliser i.e. an emulsifier is added to avoid coagulation of the resulting microcapsules.
Solidification of the coating is achieved through a variety of mechanisms: thermal, cross-linking or removal of solvent. The microcapsules tend to be freeze-dried (lyophilised) and can have a variety of textures.
A closely related technique is organic-phase separation which is a form of reversed simple coacervation. Here the polymer phase separates and deposits on a core that is suspended in an organic solvent rather than in water.
Examples And Applications Of Coacervated Materials
Gelatin (gelatine) which is by-product of collagen hydrolysis is often used for complex coacervation (Ducel et al., 2004). Some coacervate polymers have used proteins and polymers. A good example of this is the encapsulation of peppermint oil in gelatin gum and gum Arabic with transglutaminase as a cross-linking/hardening agent (Dong et al., 2011). They examined the release kinetics of the oil from this complex coacervate to demonstrate just how sturdy and stable the encapsulate was. The oil was released with hot water.
A number of products have all been successfully microencapsulated through complex coacervation. In a couple of examples worth examining, oregano, Cassia and Red Thyme essential oils were encapsulated in corn zein (Parris et al., 2005). The oil release was designed to be controlled using the method.
We earlier mentioned gelatine. In one good example, camphor oils were encapsulated in a gelatine and gum Arabic blend with a nearly 100% encapsulation efficiency. These were intended for use in medicinal and clinical products and were shown to have tremendous therapeutic potential because of the process (Chang et al., 2006). The latest example used a coencapsulation of echium seed oil with beta-sitosterol to examine various cross-linkers. Encapsulation reduced oxidation – a common issue as already mentioned. Here the fatty acids and phytosterol were encapsuled with gelatin-gum Arabic. The best cross-linking was achieved with a mix of sinapic acid and transglutaminase (Comunian et al., 2018).
Intestinal fluid often triggers release in the gut. Chui et al., (2007) looked at a system which would lend itself extremely well to complex coacervation. Here, lycopene was encapsulated within gelatin and poly(γ-glutamic acid) (γ-PGA)
A couple of pest control agents rely on complex coacervated polymers. Rosemary oil from Rosmarinus officinalis and an oil from Thyme (Thymus herbabarona) have been encapsulated in gelatin 120 Bloom with cross-linking from glutaraldehyde. Freeze drying was coupled here with coacervation to help retain the delicate oils with 98% complex efficiency (Moretti et al., 2002). Lemongrass for the same application was encapsulated with poly(vinyl alcohol) also cross-linked with glutaraldehyde (Leimann et al., 2009).
Sodium alginate has been used to encapsulate the oil extracted from Artemisia arborescens with a minimum encapsulation efficiency of 86% at the least. Again, designed for release of the oil for clinical therapies (Lai et al., 2007).
When it comes to other protein-carbohydrate encapsulates, soybean protein isolate (SPI) coupled with gum Arabic was used for encapsulating sweet orange oil (Jun-Xia et al., 2011). The SPI was subjected to ultrasound to improve dispersion. Addition of sucrose increased microencapsulation efficiency.
Vanilla oil was encapsulated in chitosan with gum Arabic (Yang et al., 2014). This was intended for flavour and spice systems. Cross-linking was achieved with genipin. In this process, they prepared a 1% w/v chitosan solution in 1% acetic acid which was heated to 50 Centigrade. The vanilla oil was added to this solution. Then, they added a 2% w/v gum Arabic solution with span-83 as an emulsifier. The whole mixture was vigorously stirred to keep all the material dispersed. To start the complex coacervation process, they reduced the stirring speed and raised the pH from 3 to 6 using 10% sodium hydroxide solution. With continuous stirring throughout, the whole mixture was rapidly cooled to below 5 Centigrade and the pH made neutral with more sodium hydroxide solution. To produce microcapsules, a small amount of genipin ethanol solution (25% w/w) was added to the whole mix and the microcapsules formed were collected by centrifugation. Freeze-drying completed the process.
Chitosan and sodium carboxymethylcellulose was used to encapsulate palm oil with beta-carotene. Ionic gelation was the process of creating the coacervate (Rutz et al., 2016). Ionic gelation uses a multivalent anionic salt as the cross-linker to prmote complexation with a positively charged polymer. This particular cross-linking technique has been used to encapsulate ascorbyl palmitate (Yoksan et al., 2010).
Since that study with chitosan, soy protein and chitosan has been used to generate a marine algal oil product with exceptional stability against oxidation (Yuan et al., 2017). Chitosan is also coupled with xanthan gum or with pectin gum for releasing carotenoids in yoghurt from palm oil. (Rutz et al., 2017).
Impregnated tea bags with complex coacervates have been developed. One example used encapsulated green tea and orange peel extracts using a double emulsion followed by complex coacervation. The researchers produced a primary emulsion of 2:1 v/v green tea to orange peel extract including its essential oil. PGPR 90 (polyglycerol polyricinoleate) was the lipophlic emulsifier. They re-emulsified the whole mixture in a gelatin solution at 50 Centigrade and pH 8 to create a w/o/w double emulsion with homogenisation. Gum arabic was slowly added and in this case the pH was dropped from alkaline to acid (pH 4). Precipitation occurred on cooling and the microparticles were freeze-dried before impregnated onto black tea bags to create this novel functional drink (Rasouli Ghahroudi et al., 2017).
References
Arneodo, C. J. F.(1996). Microencapsulation by complex coacervation at ambient temperature. FR 2732240 A1
Bakan, J. A. (1980). Microencapsulation using coacervation/phase separation techniques. In A. F. Kydonieux (Ed.), Controlled release technologies (vol 2) Boca Raton, FL, USA: CRC Press (chapters 4 and 5)
Chang, C.P., Leung, T.K., Lin, S.M., Lin, C.C. (2006) Release properties on gelatin-gum Arabic microcapsules containing camphor oil with added polystyrene. Colloid. Surf. B: Biointerfaces, 50, pp. 136-140
Comunian, T. A., Nogueira, M., Scolaro, B., Thomazini, M., Ferro-Furtado, R., & Favaro-Trindade, C. S. (2018). Enhancing stability of echium seed oil and beta-sitosterol by their coencapsulation by complex coacervation using different combinations of wall materials and crosslinkers. Food Chemistry, 252, pp. 277-284.
Dong, Z., Ma, Y., Hayat, K., Jia, C., Xia, S., & Zhang, X. (2011). Morphology and release profile of microcapsules encapsulating peppermint oil by complex coacervation. Journal of Food Engineering, 104(3), 455-460. Article
Eratte, D., Dowling, K., Barrow, C. J. and Adhikari, B. (2018) Recent advances in the microencapsulation of omega-3 oil and probiotic bacteria through complex coacervation: A review. Trends in Food Science & Technology, 71, (121)
Hussain, M.R. and Maji. T.K. (2008) Preparation of genipin cross-linked chitosan-gelatin microcapsules for encapsulation of Zanthoxylum limonella oil (ZLO)using salting-out method. J. Microencapsulation, 25, (6) pp. 414-420
Jun-xia, X., Hai-yan, Y., & Jian, Y. (2011). Microencapsulation of sweet orange oil by complex coacervation with soybean protein isolate/gum Arabic. Food Chemistry, 125(4), pp. 1267-1272 Article
Lai, F., Loy, G., Manconi, M., Manca, M.L. and Maria Fadda, A. (2007) Artemisia arborescens L. essential oil loaded beads: preparation and characterization. AAPS PharmSciTech 8, (3), Article 67, pp. E1-E7
Leimann, F.V., Gonçalves, O.H., Machado, R.A.F., and Bolzan, A. (2009) Antimicrobial activity of microencapsulated lemongrass essential oil and the effect of experimental parameters on microcapsules size and morphology. Mater. Sci. Eng. C, 29, (2) pp. 430-436
Millqvist-Fureby, A. (2014). Aqueous two-phase systems for microencapsulation in food applications. In: Microencapsulation in Food Industry. (edited by A.G. Gaonkar, N. Vasisht, A.R. Khare & R. Sobel) Pp. 157–169. Boca Raton, FL: CRC Press
Moretti, M. D.L., Sanna-Passino, G., Demontis, S., Bazzoni, E. (2002) Essential oil formulations useful as a new tool for insect pest control. AAPS PharmSciTech 3, (2), Article 13, pp. 1-11
Munin, A. & Edwards-Levy, F. (2011). Encapsulation of natural polyphenolic compounds; A Review. Pharmaceutics, 3, pp. 793–829
Parris, N., Cooke, P.H., Hicks, K.B. (2005) Encapsulation of Essential oils in Zein Nanospherical Particles. J. Agric. Food Chem., vol. 53, pp. 4788-4792
Rasouli Ghahroudi, F., Mizani, M., Rezaei, K., & Bameni Moghadam, M. (2017). Mixed extracts of green tea and orange peel encapsulated and impregnated on black tea bag paper to be used as a functional drink. International Journal of Food Science & Technology, 52(7), 1534-1542.
Rutz, J.K., Borges, C.D., Zambiazi, R.C., da Rosa, C.G., da Silva, M.M. (2016) Elaboration of microparticles of carotenoids from natural and synthetic sources for applications in food. 202 pp. 324-333 Article
Rutz, J.K., Borges, C.D., Zambiazi, R.C., Crizel-Cardoso, M.M., Kuck, L.S., Norena, C.P.Z,. (2017) Microencapsulation of palm oil by complex coacervation for application in food systems. Food Chem., 220 pp. 59-66 Article
Yang, Z., Peng, Z., Li, J., Li, S., Kong, L., Li, P., & Wang, Q. (2014). Development and evaluation of novel flavour microcapsules containing vanilla oil using complex coacervation approach. Food chemistry, 145, pp. 272-277 Article
Yoksan, R., Jirawutthiwongchai, J., Arpo, K. (2010) Encapsulation of ascorbyl palmitate in chitosan nanoparticles by oil-in-water emulsion and ionic gelation processes. Colloids and Surfaces B: Biointerfaces 76(1) pp. 292-297 Article
Yuan, Y., Kong, Z-Y., Sun, Y-E., Zeng, Q-Z, Yang, X-Q. (2017) Complex coacervation of soy protein with chitosan: Constructing antioxidant microcapsule for algal oil delivery. LWT-Food Sci. Technol. 75 pp. 171-179 Article
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