Drying Technology An International Journal
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Encapsulation of bioactive compounds through electrospinning/electrospraying and spray drying: A comparative assessment of food-related applications Christina G. Drosou, Magdalini K. Krokida & Costas G. Biliaderis To cite this article: Christina G. Drosou, Magdalini K. Krokida & Costas G. Biliaderis (2017) Encapsulation of bioactive compounds through electrospinning/electrospraying and spray drying: A comparative assessment of food-related applications, Drying Technology, 35:2, 139-162, DOI: 10.1080/07373937.2016.1162797 To link to this article: https://doi.org/10.1080/07373937.2016.1162797
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DRYING TECHNOLOGY 2017, VOL. 35, NO. 2, 139–162 http://dx.doi.org/10.1080/07373937.2016.1162797
REVIEW ARTICLE
Encapsulation of bioactive compounds through electrospinning/electrospraying and spray drying: A comparative assessment of food-related applications Christina G. Drosoua, Magdalini K. Krokidaa, and Costas G. Biliaderisb a
School of Chemical Engineering, National Technical University of Athens, Athens, Greece; bLaboratory of Food Chemistry and Biochemistry, Department of Food Science and Technology, School of Agriculture, Aristotle University of Thessaloniki, Thessaloniki, Greece ABSTRACT
KEYWORDS
Spray drying and electrohydrodynamic processes, namely, electrospinning and electrospraying, are the most promising encapsulation technologies for entrapping and effectively delivering bioactive compounds. Encapsulation is used by the food industry to incorporate such compounds into different food matrices, protect them from adverse environmental conditions, and thereby increase the product shelf life and maintain the health-promoting properties of the composite formulation. This review provides a succinct discussion on the potential of food ingredient-based applications of spray drying and electrohydrodynamic processes on encapsulation as well as the principles and the parameters that affect the structure–morphology of the carrier matrix and the encapsulation efficiency of the process.
Bioactives; electrospinning; electrospraying; encapsulation; food application; spray drying
Introduction Due to current consumer demand for more natural foods with fewer synthetic additives, together with a wide range of available bioactive compounds in various forms (isolates, concentrates, etc.), their incorporation in various food systems as value-added ingredients is a growing area of research. In this regard, the range of applications for encapsulation technologies in the food industry has been growing due to many advantages the encapsulation offers to the preservation of bioactive components instead of their direct incorporation in a food system.[1] Specifically, encapsulation is defined as a technology to “package” materials in the form of micro and nanostructures through entrapment (physical or molecular) of one active agent (core material) within another substance (wall material).[2,3] In the food industry, particularly, encapsulation is used to deliver a range of food ingredients within small capsules when direct addition of the food ingredient compromises the quality of the manufactured food product.[4] The main objective of encapsulation is to protect the core material from adverse environmental conditions, such as undesirable effects of light, moisture, and oxygen, thereby improving the stability of the functional ingredient and at the same time promoting its controlled or targeted release.[5] Moreover, the use of encapsulation can augment the nutritional quality of
food, enhance the solubility or dispersibility of lipophilic compounds, mask off-flavors without unfavorably affecting the taste, aroma, or texture as well as increase the stability and extend the shelf life of the product.[4] There are different methods for encapsulation of food ingredients. Spray drying is one of the oldest and most commonly used encapsulation techniques used in the food industry for several decades.[6] The process is economical and flexible, uses equipment that is readily available, and produces powder particles of fairly good quality. During spray drying, the active material is dispersed in a carrier polymer solution which is then atomized into small droplets. The solvent, usually water, is evaporated using a hot air current with controlled temperature and relative humidity to instantaneously obtain a powder.[7] However, the use of relatively high working temperatures during the drying process may cause heat degradation and thus affect stability of thermally labile ingredients.[8] As a result, over the last few years, electrospraying and electrospinning have attracted widespread interest and found applications in the food industry for the production of micro and nanosized particles and fibers, respectively. These electrohydrodynamic processes have also been used for the encapsulation of various food and bioactive compounds, including pharmaceuticals.[9] Electrohydrodynamic processes make use of high-voltage electric fields to produce
CONTACT Christina G. Drosou
[email protected] Laboratory of Process Analysis and Design, School of Chemical Engineering, National Technical University of Athens, 9 Iroon Polytechneiou Street, Zografou Campus, Athens 15780, Greece. Color versions of one or more figures in the article can be found online at www.tandfonline.com/ldrt. © 2017 Taylor & Francis
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electrically charged jets from viscoelastic polymer solutions which on drying, by evaporation of the solvent, produce ultrathin polymeric structures of varying morphologies.[10] The most interesting advantage of electrohydrodynamic processes is that they can achieve high encapsulation efficiencies without application of heat treatment. The absence of heat in any electrohydrodynamic process is a key advantage over spray drying and is important for preserving the structure and efficacy of the bioactive substances upon processing storage and delivery.[11] However, its potential in the field of food science is less explored thus the operating conditions for encapsulation of food bioactives as well as the properties and efficiency of the structures obtained have to be analyzed to improve and broaden the application range of these emerging technologies in the food industry. As for every encapsulation technique, there is an optimal combination of process parameters, including polymer solution characteristics, to obtain composite structures with desirable characteristics and high encapsulation efficiency. Hence, the current review aims to present the principles and the parameters that affect the particle or fiber morphology as well as the encapsulation efficiency obtained from both spray drying and electrohydrodynamic techniques with an emphasis on food ingredient applications. Moreover, it includes the state of the art of encapsulation of food bioactives by using each encapsulation method.
Principles of spray drying Spray drying is the oldest and the most widely used encapsulation technique in the food industry sector. It is a flexible, continuous, and economical operation that produces particles in the range from a few microns to tens of microns with narrow size distribution.[7,12] Encapsulation by spray drying protects, stabilizes, and enhances the solubility and controlled release of the bioactive compounds which are delivered in a powder form.[13] In
addition, spray drying ensures microbiological stability of the products, reduces the storage and transport costs, and minimizes the risk of chemical and/or biological degradations by decreasing water content and water activity.[7] Although spray dryers are widespread in the food industry, there are two limitations of this technique. Specifically, the carrier material should be film forming and soluble in water and the active substance should be heat resistant at the temperatures usually applied for this drying method during the relatively short exposure to the hot air stream.[14] Encapsulation by spray drying involves several steps as shown in Fig. 1. First, the active substance to be encapsulated is homogenized with the carrier material at a given ratio in a liquid medium, usually an aqueous phase. The mixture or emulsion that constitutes the feed solution is then pumped into the drying chamber through a nozzle. Upon exit from the nozzle tip, the droplets are atomized and come into contact with the drying fluid, i.e., hot gas (often air) inside the drying chamber. Subsequently, water is evaporated by the stream of hot air and when the droplet water content reaches a critical value, a dry crust is formed on the droplet surface and the wall material covers the microparticles of the active material. If the droplet shrinks very slowly, the dry crust can also develop at inner regions of the droplet and possibly leads to the formation of a full particle. In the case of faster water evaporation, a crust is rapidly formed on the surface of the droplet, decreasing subsequently the rate of the water evaporation process. Finally, the dry microcapsules in the form of free-flowing powder are collected at the bottom of the dryer or in the powder collector of the cyclone.[14–17]
Parameters influencing particle characteristics and encapsulation efficiency in the spray drying process Spray drying, being the most common method of encapsulating food ingredients, is used for coating or
Figure 1. Schematic of emulsion preparation (A) and spray drying apparatus (B).
DRYING TECHNOLOGY
entrapping a functional component within a suitable inert carrier matrix.[18] Actually, great interest is found in encapsulation of flavors, lipids, phenolic compounds, and colorants, including carotenoids. Successful encapsulation of functional ingredients should result in a powder with both minimum surface oil or other entrapped constituent and maximum retention of the core material inside the particle’s structure.[19] Quality of the obtained product and the powder efficiency are dependent on the operating conditions such as inlet and outlet air temperature, feed temperature, and flow rate as well as on the emulsion properties such as the nature of oil phase, types of wall materials, ratio of wall to core ingredients in the liquid dispersion, total solids content, viscosity of the atomizing fluid, stability, and droplet size.[20] Feed temperature Initially, feed temperature is one of the main factors that affect particle morphology and encapsulation efficiency. In fact, feed temperature modifies the viscosity (fluidity) of the feed, its fluidity, and its capacity to be homogeneously sprayed. When the feed temperature increases to a certain level, viscosity and droplet size should decrease but excessively high temperatures can cause volatilization or degradation of some heat-sensitive ingredients.[7] Shu et al.[21] who encapsulated lycopene using a gelatin–sucrose system, reported that encapsulation efficiency increased with an increasing feed temperature from 35 to 55°C, whereas when the temperature was further increased to 65°C, the encapsulation efficiency decreased. Similarly, the encapsulation efficiency of lycopene using maltodextrin as wall material increased with an increasing feed temperature from 20 to 45°C, whereas when the temperature increased to 70°C, the efficiency decreased significantly.[22] This observation may be attributed to the effect of feed temperature on emulsion viscosity. Brückner et al.[23] reported that the ideal feed temperature for microencapsulation of food ingredients is 40°C due to the lower feed viscosity and the higher drying rate, which leads to a quicker formation of the dry layer of carrier material around the droplet. On the other hand, a feed temperature of 60°C was found to be disadvantageous, because it lowers the viscosity of the carrier material, allowing for internal mixing inside the particle during the drying process, thus retarding the desired formation of a semipermeable crust around the particle.[23] Furthermore, Yamamoto et al. investigated the effect of feed temperature on encapsulation efficiency of D-limonene using a mixture of gum arabic and maltodextrin. The results showed that the powder
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prepared at a higher temperature had higher stability against the release and oxidation of D-limonene than that of the powder obtained at lower temperature. The findings may be partly attributed to an increase in shell thickness of the particle at higher temperatures. The thicker shell wall poses a stronger barrier against the flavor release as well as to the diffusion of oxygen and moisture from the surrounding environment.[24] On the other hand, Paramita et al.[25] reported that the encapsulation efficiency of D-limonene is not significantly dependent on the feed temperature. Inlet temperature An appropriate adjustment of the air inlet temperature and flow rate is important through spray drying process.[26] Specifically, the flow of the feed material should be adjusted so that the liquid sprayed evaporates before it comes into contact with the walls of the drying chamber. In a recent study, powders with higher content of bioactive compounds and higher antioxidant activity were produced at lower inlet drying temperatures and higher feed rate.[27] The influence of dryer inlet air temperatures has received considerable attention.[28] Generally, it is desirable as the use of a high inlet air temperature to allow rapid formation of a semipermeable membrane on the droplet surface but yet not so high as to cause heat damage to the dry product or excessive bubble growth and surface disruption, which increases losses during drying[29] or causes the particles to become sticky and cake.[30] For instance, the encapsulation efficiency of lycopene using maltodextrin as wall material increased with an increasing inlet air temperature from 110 to 150°C, whereas when the temperature increased to 160°C, the efficiency remained almost constant.[22] Similarly, increasing inlet temperature from 170 to 210°C, the encapsulation efficiency of lycopene increased with temperature at first, but then decreased dramatically when temperature reached the upper limit of 210°C.[21] In addition, microencapsulation of vitamin C by spray drying, using gum arabic as a microencapsulating agent, was studied at different air temperatures (140–200°C) and wall material concentrations (10–20% aqueous solutions). The highest concentrations of vitamin C were obtained on higher ranges of inlet air temperature, 152°C, and gum arabic concentration, 18%.[31] Similarly, de Souza et al.[32] reported that with increased temperature and lower carrier agent concentration (10%), there was a decrease in anthocyanin retention due to the degradation of these molecules. Therefore, the high air temperatures should accompanied with high solid concentration of solution to achieve high encapsulation efficiency and
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preservation of the bioactives. Moreover, the influence of inlet temperatures (160–200°C) on spray drying of mandarin oil was evaluated and an increase in inlet air temperature favored the retention of the volatile oil.[33] Similar behavior was observed by Botrel et al.[8] and Fernandez et al.[34] in the microencapsulation of rosemary essential oil and fish oil by spray drying. It is well known that an increase in inlet air temperature (within limits) leads to the rapid formation of a semipermeable membrane around the emulsion droplet, thereby providing better retention of volatiles.[35] On the other hand, Paramita et al.[25] reported that increasing inlet air temperature from 120 to 150°C results in lower flavor retention on spray-dried powder. These authors also reported that higher inlet air temperature promoted the vaporization of flavor during drying process. Outlet temperature The influence of dryer outlet air temperature on bioactive retention is not well documented. Actually for a given spray dryer system, the outlet temperature depends on the combination of inlet temperature and feed rate. Lower outlet temperatures are reached with higher feed rate since the amount of liquid in contact with the drying gas is higher and therefore higher quantities of heat are involved for moisture evaporation. Although higher temperatures promote faster evaporation and a rapid crust formation, it is important to take into consideration the temperature of degradation of the materials to minimize losses of the bioactive compounds.[36–38] A recent work on the encapsulation of lemon myrtle oil reported that the outlet air temperature of the spray dryer had little effect on oil retention but the surface oil increased significantly with the increasing outlet air temperatures, presumably due to the temperature-mediated enhancement of oil diffusion.[39] Another study demonstrated that a higher flavonoid content is obtained in the powder at an outlet drying temperature of 100°C than at 160°C.[27] In addition, encapsulation of bitter melon was conducted by spray drying to optimize the inlet (125.6, 130, 140, 150, and 154.1°C) and outlet (72.9, 75, 80, 85, and 87.1°C) temperatures of the process; optimal inlet and outlet temperatures were 140 and 80°C, respectively.[40] Table 1 summarizes the experimental conditions (inlet and outlet temperature) for the encapsulation of different food ingredients by spray drying. Core–wall material concentration There is an optimum concentration of core material that could be encapsulated efficiently depending on
the physicochemical properties of both wall and core materials used in a particular spray drying process. Higher core loads resulted in poorer retention or lower encapsulation efficiency and a higher core material content at the surface of the powder particles.[56–58] This trend may be attributed to greater proportions of core materials close to the drying surface, thereby shortening the diffusion path length at the air–particle interface. For instance, the encapsulation efficiency of lycopene increased with an increasing core-to-wall material ratio from 1:19 to 1:3, whereas when the ratio increased further to 1:1.5, the efficiency decreased. Minemoto et al.[59] also observed that the encapsulation efficiency of linoleic acid decreased when the weight ratio of core to wall material increased. According to these authors, at higher ratios, the amount of wall material is not enough to fully cover the oil droplets and this insufficiency may result in a decrease in encapsulation efficiency. Moreover, Soottitantawat et al.[60] reported a significant increase in the surface oil of microencapsulated L-menthol when the mass ratio of L-menthol:wall material was increased from 1:9 to 3:7, using gum arabic and two types of modified starch (Hi-Cap 100 and Capsul) as wall materials. In most of the published works, a typical core-to-wall material ratio of 1:4 was adopted and identified as being optimal for various wall materials, such as gum arabic and modified starches.[19,61] In another study, the encapsulation efficiency of peppermint oil decreased with an increase in the initial oil concentration from 70.6% for 10% oil to 57.2% for 30% oil, using maltodextrin as wall material.[46] Similar behavior was noted by Huynh et al.[39] and Tan et al.[57] in the microencapsulation of lemon myrtle oil and fish oil by spray drying. In another study, the influence of wall material concentration on the properties of rosemary oil microencapsulated by spray drying, with gum arabic as carrier was investigated. The results indicated that oil retention was affected significantly (p < 0.05) by the wall material concentration and higher oil retention values were observed when there was a high solid content in the liquid dispersion compared with its low solid content counterpart.[34] Emulsion characteristics Another important factor in the microencapsulation of oils is the physical stability of the emulsions from which particles are produced and is related to some emulsion properties such as droplet size, droplet surface charge and other interfacial properties, and viscosity. In general, emulsions with weaker stability result in encapsulated powders with higher surface oil contents and a lower retention of volatiles.[62,63] Many studies have
143
150 180 150 165 160 190 147 200 150 152 110
Maltodextrin
Maltodextrin/gum arabic or WPC or modified starch Blend of WPC and gum arabic Gelatin, gum arabic, and maltodextrin Gum arabic, maltodextrin
Gelatin/sucrose Maltodextrin Gum arabic/maltodextrin Maltodextrin Gum arabic Maltodextrin, gum arabic, starch sodium octenyl succinate, WPC, and egg albumin Maltodextrin
Maltodextrin/soya protein isolate
Soy protein isolate/gum arabic
Pomegranate juice
Tamarind pulp
Tomato oleoresin
NR, not reported; WPI, whey protein isolate; β-CD, β-cyclodextrin; WPC, whey protein concentrate.
140
170
100
171.8 80
Gelatin, lactose, pullulan, and their mixtures Gum arabic, maltodextrin, isolated soya protein, soya protein powder, soya milk powder
110 160 150, 110 200
Egg powder Elderberry (Sambucus nigra L.) juice Elemi and peppermint oils Flaxseed oil Gac oil Juçara pulp Lippia sidoides essential oil Lycopene Lycopene Mandarin oil Peppermint oil Pequi pulp extract Persimmon pulp
Wall material
Casein sodium salt Gum arabic/maltodextrin/modified starch WPI β-CD
β-Carotene Cinnamon oleoresin Curcumin D-Limonene
Inlet temperature (°C)
Spray drying for the encapsulation of bioactive compounds.
Encapsulating ingredient
Table 1.
55–60
NR
NR
52 NR 80 80 NR 85
110 95 NR NR
80
72.5 NR
80 120 NR NR
Outlet temperature (°C) Purpose
Reduction of the stickiness of the products, retention of anthocyanin and phenolic content Reduction of maltodextrin required for the production of tamarind pulp powder using soya protein isolate, enhancement of quality of tamarind pulp Control release, increase stability of lycopene and maintenance of color
Storage stability to avoid the damage from oxygen and light during storage Enhance storage stability of lycopene Retention of volatiles Characterization of water vapor sorption properties Retention of high vitamin C and carotenoid content Efficient powder recovery and retention of phenolic content
Protection against lipid oxidation Preservation of color and carotenoid, lycopene content Retention of anthocyanin content Improve antifungal activity of essential oil
Analysis of essential oils components
Improve storage stability of β-carotene Improve stability of volatiles during storage Improve solubility and bioavailability of curcumin Evaluate release profile of D-limonene under different humidity and temperature conditions Improve oxidation stability during storage Enhance stability of the phenolic content and color of the powder
[55]
[54]
[53]
[21] [22] [51] [14] [31] [52]
[47] [48] [49] [50]
[46]
[44] [45]
[41] [42] [43] [6]
Reference
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demonstrated that the reduction in size of the emulsion droplets results in greater retention of volatiles and lower surface oil.[64–66] According to Jafari et al.,[19] the higher surface oil in the particles produced from emulsions with larger droplets can be attributed to droplet breakdown during atomization. The reduction in size can be achieved, for example, using high-pressure homogenization or by increasing the emulsion viscosity or decreasing the oil load. For instance, in a microencapsulation process of flaxseed oil by spray drying, lower oil content resulted in higher emulsion viscosity.[67] Many authors reported that an increase in emulsion viscosity generally reduces the time needed to form a semipermeable membrane or an outside shell in the atomized droplets at the early stages of the drying process.[19,39,60] According to Huynh et al.[39] and Tonon et al.,[67] the membrane formed around the droplet surface becomes impermeable to larger compounds such as flavors and oils, thus reducing the oil migration to the particle surface.
Encapsulation of bioactive compounds by spray drying Spray drying is the most commonly used encapsulation technology in the food industry and is considered as a cost-effective way to produce microcapsules in a relatively simple and continuous processing operation.[68] Since the main purposes of microencapsulation are to protect sensitive components in solid carriers, to reduce volatility, to promote easier handling, and to control the release of the encapsulated material during storage and in applications,[69] the selection of an appropriate wall material is critical to the encapsulation efficiency by spray drying.[70] Depending on the core material and the characteristics desired in the final product, wall materials can be selected from a wide variety of natural and synthetic polymers. Among the available ingredients, the major wall materials used for spray drying applications are carbohydrates, including modified and hydrolyzed starches, cellulose derivatives, gums (exudates, extracts, etc.), and cyclodextrins as well as proteins including whey proteins, caseinates, and gelatin.[19] The wall material must be soluble in water as almost all spray drying processes in the food industry are conducted using an aqueous feed formulation as well as should have film forming and good emulsification properties, and finally the wall concentrated solutions should be of low viscosity.[70,71] For instance, carbohydrates, with the exception of gum arabic, lack active surface properties and must be used in conjunction with emulsifying agents to encapsulate hydrophobic core materials;[72] among the polysaccharides
used, gum arabic solutions are of relatively low viscosity even at high polymer concentrations On the other hand, proteins are surface active and have the ability to form films at the interface and stabilize emulsion droplets; as a result, in many studies, blends of carbohydrate and proteins are being used.[8] In addition, the incorporation of hydrolyzed carbohydrates (maltodextrins, oligosaccharides) into the wall system has been shown to improve the drying properties of the wall matrix, probably by enhancing the formation of a dry crust around the drying droplets and increasing the oxidative stability due to reduced oxygen permeability.[73] In recent decades, lipophilic bioactive components, such as oils and carotenoids, have been gaining great public interest globally due to their health benefits. However, oils, containing a high amount of omega-3 fatty acids, are highly susceptible to oxidation; oxidation of omega-3 fatty acids leads to peroxide formation and finally to volatile compounds, some of which produce off-flavors and ultimately decrease the nutritional value of omega-3 fatty acids.[74] Therefore, the encapsulation of lipophilic bioactive components in an edible form and their incorporation into food systems or in food supplements has been extensively investigated by the food industries.[75] For instance, flaxseed oil, one of the richest sources of plant-derived omega-3 fatty acids, has been stabilized using microencapsulation by spray drying and the results showed that whey proteins and sodium caseinate improved the oxidative stability of flaxseed oil during storage.[76] In another study, the encapsulation of flaxseed oil using a mixture of maltodextrin with gum arabic or whey protein concentrate (WPC) or two types of modified starch (Hi-Cap 100TM and Capsul TA) at a 25:75 ratio was investigated; with respect to encapsulation efficiency, the blend of maltodextrin:Hi-Cap showed the best performance, while the mixture of maltodextrin:WPC enhanced the protection of the active material against oxidation upon storage.[47] In addition, flaxseed oil containing crawfish astaxanthin powder was successfully encapsulated by spray drying using a wall system consisting of sodium caseinate and lactose. The microencapsulation efficiency for crawfish astaxanthin was 86.06%, which indicated that more oil was encapsulated than displaced on the particles’ surfaces.[77] Furthermore, Botrel et al. investigated the encaspulation efficiency of fish oil using different blends of whey protein. The use of maltodextrin or inulin together with whey protein is a good alternative in the spray drying process of fish oil as the surface oil content was 5.6, 6.5, and 7.7% for the particles produced using whey protein:inulin, whey protein:maltodextrin, and whey protein alone, respectively.[70] Moreover, spray drying has been
DRYING TECHNOLOGY
successfully applied for carotenoid stability in oils such as gac oil which is rich in β-carotene and lycopene. Specifically, gac oil was encapsulated using a mixture of WPC and gum arabic at a ratio 7/3 (w/w), as optimized in a previous study,[48] and the encapsulation efficiency of the oil, β-carotene, lycopene was 87.22, 82.76, and 84.29%, respectively under optimal conditions of inlet, 154°C, and outlet temperatures, 80°C.[78] In another study, maltodextrin was used to produce gac powder by spray drying and the results showed that total carotenoid content and antioxidant activity were preserved at inlet temperature of 120°C and maltodextrin concentration of 10%.[79] Additionally, encapsulation by spray drying is applied to oleoresins which are susceptible to degradation in the presence of air, light, moisture, and high temperatures and have short storage life if not stored properly.[42] For instance, microencapsulation of cinnamon oleoresin by spray drying helps to stabilize oleoresin by preventing against volatile losses. Gum arabic is considered to be a better wall material for encapsulation of cinnamon oleoresin as compared to maltodextrin, and modified starch, although the blend of gum arabic:maltodextrin: modified starch (4:1:1) proved to be more efficient.[42] The microencapsulation of garlic oleoresin by spray drying was investigated and the optimum conditions were found to be 10% garlic oleoresin, 60% maltodextrin concentration, and 200°C inlet air temperature, with maximum encapsulation efficiency of 81.9%.[80] Furthermore, zein was used as coating material to encapsulate tomato oleoresin by spray drying and the results showed that zein particles could protect most of the lycopene from being released in the stomach.[81] Similarly, essential oils are quite susceptible to oxidation and volatilization and therefore are encapsulated to increase their shelf life and enable their controlled release and the conversion of liquid flavorants into solids.[56] The microencapsulation of the essential oil from the fruits of Pterodon emarginatus by spray drying using gum arabic and maltodextrin was studied. A blend of 1:3:3.6 of oil:gum arabic:maltodextrin offered the best protection, with 98.63% of the essential oil being retained.[82] Spray drying microencapsulation of Lippia sidoides essential oil was investigated using maltodextrin and gum arabic at different ratios. The best encapsulation efficiency was achieved at maltodextrin:gum arabic ratio 0:1 and a carrier:essential oil ratio of 4:1.[50] Recently, there is an increased demand for food colorants from natural sources to substitute synthetic dyes. For example, anthocyanins are considered as potential replacements for synthetic colors because of their bright, attractive hue, and water solubility, which
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facilitates their incorporation into aqueous food systems as well as of possibly exerting health benefits (antioxidant and anti-inflammatory effects). However, the food industry requires effective technologies which protect the natural pigments, due to their instability under different environmental stresses (light, air, humidity, and high temperatures) and interactions with other food constituents.[3] Currently, to accomplish this protection, microencapsulation by spray drying is used by applying different biopolymers as wall materials. For instance, microencapsulation of anthocyanin pigments present in Garcinia indica Choisy was performed with maltodextrins of various dextrose equivalents (DE 06, 19, 21, and 33) and other additives such as gum arabic and tricalcium phosphate to enhance the stability of the pigment. The microencapsulated pigment using dispersions containing 5.0% maltodextrin DE 21, 0.25% gum arabic, and 0.25% tricalcium phosphate was found to have the highest antioxidant activity (69.90%) and anthocyanin content (485 mg/100 g).[83] In another study, the stability of anthocyanins and antioxidant activity of blackberry powder, obtained by spray drying, using maltodextrin, gum arabic, or a blend of both carriers was studied. Maltodextrin provided greater stability for spray-dried blackberry powder, because the particles produced with this wall material exhibited the longest half-life and the lowest anthocyanin degradation rate at 25°C.[84] Phenolic compounds have been also attracting considerable interest because of their positive effects on human health. However, polyphenols show low water solubility and low stability to different environmental conditions (exposure to light, oxygen, temperature, and enzymatic activities). For these reasons, several microencapsulation systems have been examined to preserve the biochemical functionalities of these components.[85,86] For instance, Paini et al.[87] investigated the encapsulation efficiency of olive pomace polyphenols and the highest total phenolic content was obtained at inlet air temperature 130°C, maltodextrin concentration 100 g/mL, and feed flow rate 10 mL/ min. In addition, peanut sprout extract, rich in resveratrol, was successfully encapsulated using WPC or maltodextrin emulsions and the stability of capsules was investigated. The powdered peanut sprout extract microcapsules coated with WPC exhibited high stability during storage, over 80% at 4 and 20°C during 10-day storage.[88] In another study, cinnamon polyphenols were encapsulated using maltodextrin as a wall material and the highest total phenolic content and antioxidant activity were observed at inlet temperatures of 160 and 180°C and flow rate of 10 mL/min.[89] Finally, retention of elderberry juice polyphenols was enhanced
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by encapsulation through spray drying using as coating materials soya milk powder, soya protein powder, isolated soya protein, gum arabic, and maltodextrin. Among these wall materials, gum arabic and maltodextrin gave better results at the inlet temperature of 80°C and a feed flow rate of 180 mL/h.[45]
Principles of electrohydrodynamic techniques The electrohydrodynamic processes use electrostatic forces to produce electrically charged jets from viscoelastic polymer solutions which on drying, by the evaporation of the solvent, produce ultrathin structures.[90] In particular, the technique is referred to as electrospinning when ultrathin continuous fibers are generated, whereas when size-reduced capsules are produced, the technique is called electrospraying. The typical setup for electrospinning and electrospraying consists of four main components: (i) a high-voltage source (1–30 kV) usually operated in direct current mode, though alternating current mode is also possible, (ii) a blunt-ended stainless steel needle or capillary, (iii) a syringe pump, and (iv) a grounded collector, either in the form of a flat plate or a rotating drum (Fig. 2a and 2b). The apparatus of the process could be vertical or horizontal.[91] Both techniques work on the same principle with very minor differences. Concerning the electrospinning process, it involves the application of a strong electrostatic field by high voltage across a conductive capillary, attached to a reservoir containing a polymer liquid, and a screen collector. Upon increasing the electrostatic field strength up to a critical value, charges on the surface of a pendant drop destabilize the shape of the solution stream at the exit side from partially spherical into conical, i.e., the so-called Taylor cone effect. Two electrostatic forces play an antagonist role in this context: The electrostatic repulsion between the surface charges generated by the applied voltage that tends to break the polymer solution drop at the spinneret and the
attractive forces originating from the surface tension. Beyond a critical voltage, the repulsive forces overcome the surface tension of the solution and an electrically charged liquid jet erupts from the tip of the spinneret. As the charged jet accelerates toward regions of lower potential, the solvent evaporates, while the molecular entanglements (interchain associations) among the polymer chains prevent the jet from breaking up. This results in ultrathin fiber formation. Generally, a grounded or an oppositely charged plate is used to guide to the collector the spinning jet.[92–94] The difference between electrospinning and electrospraying which may be considered as “sister” technologies is based on concentration and viscosity of the polymer solution.[9,95] Generally, the viscosity increases with the concentration and chain stiffness of the polymer in solution. If the viscosity is high enough, a stable elongated jet can be obtained. An increase in the viscosity of the solution promotes a high cohesion and entanglements between the polymeric chains which prevent the liquid from breaking up into droplets, and thus a transition from electrospray to electrospinning occurs. In other words, particles will be formed if the polymer solution has a low viscosity or low polymer concentration, but fibers will be formed if a higher viscosity or a higher concentration solution is being used.[96] Recently, electrohydrodynamic processes are used for the encapsulation of bioactive compounds for food applications. Similarly with the spray drying process, encapsulation is achieved by dissolving, emulsifying, or dispersing the core substance in an aqueous solution of the wall material, followed by spraying or spinning of the entire solution. The novelty of the electrohydrodynamic processes is that they can be performed in two different ways, either through a coaxial electrohydrodynamic process or through direct incorporation of the material within the polymeric solution (cosolubilization of core and wall materials in the solvent) depending on the nature of the materials used (molecular compatibility). Specifically, in
Figure 2. Schematic of setup of electrospinning (A) and electrospraying (B) apparatus.
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the coaxial methodology, the polymer and the core material are introduced into the electrohydrodynamic equipment from separated solutions; thus, the component immiscibility problem is alleviated.[97]
Parameters influencing fiber characteristics in the electrospinning process The electrohydrodynamic process has been attracting considerable attention for the production of polymer fibers or particles as it can generate them with diameters in the range from several micrometers down to tens of nanometers.[98] It is well known that fiber or particle diameter and morphology are strongly dependent on a number of parameters.[90] These include intrinsic properties of the polymer solution such as type of polymer, conformation of the polymer chain, viscosity, elasticity, electrical conductivity, polarity, and surface tension of the liquid dispersion. Moreover, operational conditions such as the strength of the applied electric field, the distance between spinneret–collector and the feeding rate of the polymer solution are known to influence the fiber or particle characteristics. Other variables, such as the humidity and temperature of the surroundings may also play an important role in determining the morphology and the diameter of the electrospun structures as well as their physical state.[92,99–103] The processing parameters of electrohydrodynamic process are given in Table 2. Polymer concentration and solution viscosity Initially, the solution viscosity is a critical factor that influences solution spinnability and morphology of the electrospun fibers. Viscosity is directly related to concentration, molecular weight, and the structure and conformation of the polymeric chains as well as solvent type.[104,105] Generally, an increase in viscosity increases fiber diameter and uniformity.[100,106] The effects of zein solution’s concentration and viscosity on electrospun fiber morphology and size were investigated and the authors report that beadless and uniform fibers were Table 2. Processing parameters in electrohydrodynamic processes. Solution properties
Processing conditions Ambient conditions
Viscosity Polymer concentration Molecular weight of polymer Surface tension Electrical conductivity Applied voltage Flow rate Tip to collector distance Temperature Humidity
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noticed at higher concentrations of zein, while the average diameter of the electrospun fibers increased.[92,104,107,108] In addition, b-cyclodextrin (b-CD)/poly(vinyl alcohol) (PVA) blend fibers were produced by the electrospinning process and the effect of b-CD concentration on fiber morphology was examined. The viscosity of b-CD/PVA solution increased with the concentration of b-CD and it was noted that the high viscosity of b-CD/PVA solution led to formation of more uniform nanofibers with larger average diameter. The same trend was observed for PVA/ chitosan blend nanofibers.[109] As far as molecular weight of the solubilized polymeric chains, Deitzel et al.[100] and Geng et al.[110] showed that very lowmolecular weight polymers have a tendency to form beads rather than fibers, whereas high-molecular weight polymers give fibers with larger average diameter if the concentration of the polymer is adequate, presumably due to more extensive interchain associations. Surface tension The surface tension of a liquid dispersion, being a function of composition (solvent and dissolved constituents) and temperature, plays a critical role in the electrospinning process.[99] It is the primary counteracting force to the charge effects generated by the applied voltage during the electrospinning process and determines the overall electrospinnability of the liquid medium.[111] Surfactants are often used for electrospinning fibers to improve the electrospinnability, to reduce the electrospun fiber diameter, to eliminate the formation of beaded fibers, and to form core–shell fibers by emulsion electrospinning.[112] The influences of surfactants on the diameter size and uniformity of electrospun poly(L-lactic acid) fibers were examined by adding various surfactants (cationic, anionic, and nonionic), and significant diameter reduction and uniformity improvement were achieved in this way.[113] In addition, blend solutions of chitosan and poly(ethylene oxide) (PEO) at different acidities and in the presence of surfactant Tween 20 were electrospun to produce membranes of nanofibers with different diameters. It was noted that solutions at the highest acidity yield less beads and beaded fibers than the others. This was attributed to an increase in acetic acid concentration that gave viscous solutions with lower surface tension and an adequate polymer chain entanglement.[114] On the other hand, the effect of Tween 20 was neither a significant factor on fiber diameter, while Wang et al.[112] have reported that the average diameter of the fibers decreased as the concentration of the nonionic surfactant, TritonX-100, increased.
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Conductivity The electrical conductivity of solution also plays an important role on fiber morphology and size;[115] this property is related to the charge density on a jet (liquid stream), thereby affecting the extent of elongation of the jet by the applied electrical force. Under the same operating voltage and spinning distance, a solution with a higher electrical conductivity may cause higher elongation of a jet along its axis and thus yield electrospinning fibers of smaller diameter.[116] For instance, Tan et al.[116] investigated the effect of solvent electrical conductivity on morphology of electrospun poly (L-lactide-co-caprolactone) fibers using dichloromethane (DCM), N,N-dimethylformamide, and pyridine as solvents. The results showed that electrospun polymer nanofibers with the smallest fiber diameter and highest uniformity were obtained from the DCM/ pyridine (50/50 wt%) solution, having the highest electrical conductivity. The same trend was also noted for poly(acrylic acid)[117] and polystyrene[118] fibers. Applied voltage The applied voltage is the critical element of any electrospinning process because it provides surface charge on the electrospinning jet, and thereby influences the nanofiber diameter. Generally, increasing applied voltages lead to decreasing diameters of nanofibers due to increasing electrostatic repulsive forces on the fluid jet,[99,119] e.g., for chitosan nanofibers produced by electrospinning it was noted that by increasing the voltage, the diameter of nanofibers decreased.[120] However, very high voltages may facilitate nanofibers of larger diameters as increasing the voltage above a certain level increases the electrostatic stresses, which in turn, may draw more material out of the syringe.[103,121] For instance, zein solutions were electrospun at different applied voltages and the results showed that the low voltages gave small fiber diameters. Specifically, an applied voltage of 13 kV gave fiber diameters ranging from 250 to 300 nm, while a voltage 18 kV resulted in fiber diameters ranging from 250 to 350 nm at different solution feed rates.[122] In addition, bead formations can occur at high voltages.[100,121] Flow rate The flow rate of the polymer from the syringe is another important process parameter as it influences the jet velocity and the material transfer rate. A minimum value of solution volume suspended at the end of the needle should be maintained to form a stable Taylor
cone and facilitate solvent evaporation.[99,123] The feed rate therefore determines the amount of solution available for the electrospinning process. Typically, when the feed rate increases, a corresponding increase in the fiber diameter is observed.[124] Few studies have systematically investigated the relationship between solution flow rate on fiber morphology and size. For instance, Okutan et al.[111] investigated the influence of feed rate during electrospinning on properties of electrospun gelatin and reported that with the increasing feed rate, there were larger fiber diameters and bead formations. Moreover, zein fibers were produced at 1.0 and 2.0 mL/h solution feed rate and the fiber diameters ranged from 250 to 275 nm and from 250 to 350 nm, respectively.[122] Similarly, Henriques et al.[125] observed that the average diameter of electrospun PEO fibers increases linearly with solution feed rate. Tip to collector distance Another influential parameter is the distance between the needle tip and the collector device, and this is correlated with the flight time of the electrospinning jet. Longer flight time may produce a thinner fiber as the fiber gets more time to stretch and elongate before it is accumulated on the collector.[126] Consequently, the fiber diameter generally decreases with larger distance between needle tip and collector.[127] Nevertheless, Lee et al.[128] have proposed that the fiber diameter increases at a longer distance due to reduction of the effective voltage. Similarly, Sencadas et al.[120] instigated the influence of the distance between the needle tip to the grounded collector on the fiber average diameter of a 7% (w/v) chitosan in 70/30 trifluoracetic acid (TFA)/DCM solution. It was observed that the fibers with the smallest average diameter, ∼260 nm, were obtained for samples with 5 cm in distance between the needle tip and the collector and that the mean fiber diameter increased by increasing the distance between the needle tip and the collector. A maximum average fiber diameter of ∼500 nm was obtained for a traveling distance of 20 cm.[120] Humidity and temperature The humidity and temperature of the process can also affect the morphology and diameter of the electrospun fibers, although no systematic study was conducted on the influence of evaporation and solidification of the jet. Most investigations have been performed in an open atmosphere without controlling the vapor concentration of the solvent. Tripatanasuwan et al.[129] investigated the effect of different relative humidity (5.1–63.5%) environments on PEO electrospun fiber morphology and
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size. The average diameter of PEO nanofibers gradually decreased as the relative humidity increased. The evaporation rate from the fluid jet decreased at higher relative humidity, which allowed the charged jet to continue to elongate. On the other hand, more beads appeared at higher humidities.
Parameters influencing particle characteristics in the electrospraying process The size and morphology of the particles prepared by electrospraying influence the release kinetics of the entrapped bioactive compounds. Spherically shaped particles are considered more suitable for delivery of bioactives than irregularly shaped particles, mainly because of polymer dissolution and thus the bioactive release might be inconsistent from within the irregularly shaped particles.[130] Therefore, all the parameters that have an impact on particle morphology and size are discussed below.
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increase in particle size with higher polymer concentration and viscosity has been also noted for other polymers, such as polycaprolactone (PCL),[133] alginate,[134] chitosan,[135] and elastin.[136] Surface tension Surface tension plays a role in the electrospraying process, since it affects the ease of initial drop formation as the solution gets out of the nozzle. It is worth mentioning that the initial drops are formed when the electrostatic forces overcome the surface tension of the emerging liquid jet, and then these initial drops explode to smaller drops as they travel in the electrostatic field between the nozzle (usually positively charged) and the collector (usually negatively charged). A lower surface tension leads to smaller initial drops and hence smaller final nanoparticles. The effect of higher surface tension on increased particle size has been reported for solutions of amylose and amylopectin[137,138] as well as PCL.[139]
Polymer concentration, molecular weight, and viscosity
Conductivity
Polymer type, molecular weight, and concentration are very influential factors in the electrospraying process; i.e., changing polymer concentration and molecular weight largely affect the viscosity and surface tension of the solution.[131] With greater length of polymer chains, intermolecular associations are favored, particularly as the solution concentration is raised, leading to larger size microcapsules. For low-molecular weight polymers, a higher concentration is required for the formation of particles. In contrast, high-molecular weight materials display a great number of entanglements, allowing particle formation even with polymer solutions of low concentration.[1] To have a stable process, it is important to take into consideration the viscosity of the polymer solution; with an increasing concentration, the increased viscosity leads to the production of capsules with bigger size. For instance, Gomez-Estaca et al.[1] investigated the effect of zein concentration on the morphology of the resulting zein structures for polymer concentrations ranging from 1 to 20% (w/w) in aqueous ethanol solutions. It was found that 1% of zein in the solution was too low for particle formation, while a zein concentration of 20% gave rise to the transition from particles to fibers, a finding which was in accordance with observations made in earlier work.[132] With zein concentrations of 2.5 and 5% (w/w), the generated particles were round in shape and had relatively smooth surfaces, whereas much larger particles in size were obtained by increasing the polymer concentration. An
The electrical conductivity of the polymer and solvent is an important parameter when optimizing the electrospraying process as it affects the electrostatic attraction of the charged particles to a grounded or oppositely charged collector. With higher electrical conductivity, the Coulombian repulsion forces are enhanced and compete with the viscoelastic forces of the solution; as a result, disentanglements in the polymer chain network occur during electrospraying, producing smaller particles. Gañán-Calvo et al.[140] showed that a decrease in particle size can be obtained with an increasing solution conductivity. Nevertheless, if only conductivity had been the parameter controlling the particle size, higher conductivity values would have produced larger particle dispersity due to Coulomb fission forces, which overcome chain entanglement forces, and thereby produce secondary droplets that are ejected from primary droplets, resulting in broader particle size distribution.[141] Correlating with viscosity, stable electrospraying can be achieved only when viscosity is high or conductivity is low as reported for chitosan particles.[142] Changes in electrical conductivity can be obtained by changing the electrospraying solvent or using cosolvents, although this latter case may be detrimental to size distribution and morphology of the particles. For instance, electrospraying was used to prepare poly(lactic-coglycolic acid) (PLGA) microparticles using two different solvents, DCM and trifluoroethanol. Trifluoroethanol enabled the preparation of even smaller sizes of PLGA
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particles, which can be attributed to the high conductivity of trifluoroethanol (TFE) (1.07 μS/cm) compared to DCM (0 μS/cm).[143] In addition, PLGA particles were produced using a mixture of DCM with chloroform to increase the conductivity of the solution. The results showed that the addition of a solvent with high conductivity destroys electrospraying stability and produces smaller and polydisperse particles. In another study, the effect of conductivity on particle formation was studied by adding the organic salt didodecyldimethylammonium bromide (DDAB) in the polymer solution. The conductivity of a 5% (w/v) PLGA solution in acetonitrile containing 10% (w/w) paclitaxel was shown to increase from 0.51 to 116.5 μS/cm by the addition of 2 mM DDAB. This led to a particle size decrease from around 1.2 µm to 355 nm.[133]
Applied voltage The applied voltage is a key parameter in achieving a stable cone-jet mode for obtaining monodispersed nanoparticles.[1] Within the single cone-jet mode, size is not significantly affected by voltage, and only a slight decrease in size is observed when voltage is increased.[134,144–146] Morphology, however, will be changed as stated by Shenoy et al.,[147] since as the voltage is increased, the particle morphology is largely modified from a spherical shape to elongated particles or beaded fibers to eventually only fibers if the polymer concentration is sufficiently high. This is due to more charge acting on droplets with increased voltage, leading to stretching and elongation of droplets. It is therefore recommended to use moderate voltages that allow for the single cone-jet mode to take place while maintaining the spherical morphology of the particles.[130]
Flow rate The flow rate of the liquid stream in the electrospraying process influences the size of the particles. Higher flow rates could lead to a larger diameter of capsules as the jet speed increases and higher concentration of polymer solution drips out of the nozzle. Gomez-Estaca et al.[1] noticed that the size of the particles decreased with the zein solution flow rate and the nanoparticle diameters were observed to lie between 80 and 130 nm and 130 and 175 nm for the 0.05 and 0.10 mL/h flow rate, respectively. The effect of the flow rate on the diameter of several polymeric nanoparticles generated by electrospraying has been reported by several authors using polymers such as PLGA,[144,146] chitosan,[135] elastin,[136] and PCL.[133,139]
Tip to collector distance Electrospraying distance can also affect capsules’ size and have an impact on the final product morphology. In the electrospray process, once the compound jet is formed, droplets are largely discharged by a needle connected to positive voltage. The position of the spray nozzle does affect particle size and particle size distribution. The investigation of this parameter revealed that increasing the tip–collector distance decreases the average size of particles as mentioned for amylose and amylopectin[148] as well as for PCL.[139] This is due to the fact that a longer flight of droplets in the electric field, as a result of longer tip–collector distance, provides more time for further coulomb explosions, and as a result smaller particles are being formed. In addition, increasing the distance leads to more spherical morphologies since polymer chains have sufficient time to diffuse within the droplet and thus also reduced polydispersity.[130] It must be pointed out, however, that increasing the tip–collector distance to more than a threshold will lead to disruption of drop breakage because of too weak electrical field.[148]
Blends of polymers used in electrohydrodynamic process There has been growing interest in the development of natural polymer-based nanofibers. Compared to nanofibers prepared from synthetic polymer solutions by electrospinning, bionanofibers have remarkable advantages in terms of high crystallinity, uniformity, biodegradability, and biocompatibility.[149] However, electrospinning of nanofibers from biopolymers has proven to be quite challenging because these materials have limited solubility in most organic solvents, are often polyelectrolytes when dissolved, have poor molecular flexibilities, readily form three-dimensional networks through hydrogen bonding, and most importantly are insufficiently entangled to facilitate electrospinning.[150] Therefore, researchers have focused on the use of composite blends of biopolymers with polymers that are compatible for formation of electrospun fibers with enhanced material properties such as higher tensile strength.[151] Synthetic polymers such as PEO and PVA, when combined with biopolymers, improve the fiber forming ability of the blending solution.[152] For instance, chitosan-based nanofibers have been successfully electrospun from chitosan solutions blended with PEO,[153] PVA,[109,149] and silk fibroin.[154] Similarly, sodium alginate fibers have been fabricated by electrospinning of aqueous mixtures in the presence of PVA[155–157] and PEO.[158,159] In addition, collagen was blended with PEO to produce
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electrospun fibers.[160] In addition to synthetic polymers, there has been a lot of interest in the study of polymer blends, especially focused on biodegradable and sustainable materials. Increasing attention has been given in recent years to natural polymers, such as polysaccharides, due to their abundance in nature, unique structures, and characteristics with respect to synthetic polymers.[161] This renaissance of polysaccharide materials is influenced by their useful properties including nontoxicity, biocompatibility, biodegradability, and water solubility, making these biopolymers suitable for different applications. Several polysaccharides, including starch, pectin, cellulose, chitin, and chitosan, have been finding new potential uses in the food field. Chitosan is the second most abundant biological polysaccharide (after cellulose) derived from nature and this material finds many applications due to its unique properties, such as antibacterial performance under certain specific conditions.[162] Nevertheless, fiber formation from chitosan by electrospinning on its own is rather difficult because of its limited solubility and polycationic nature in solution. Therefore, Torres-Giner et al.[163] used blends of zein with chitosan to develop novel antimicrobial materials which have potential for applications in different fields such as active and bioactive packaging, antimicrobial food coatings, etc. Furthermore, zein nanofibers containing cyclodextrins (α-CD, b-CD, c-CD) were produced through electrospinning and the results showed that the addition of cyclodextrin in the polymer solutions improves the electrospinnability of the zein nanofibers at lower polymer concentration. Thermal analyses showed that zein/ b-CD nanofibers have higher glass transition temperatures and higher degradation temperature with an increasing cyclodextrin content. In another study, novel ultrathin electrospun fibers from different blends of amaranth protein isolate and the microbial polysaccharide pullulan were developed. The presence of pullulan (neutral polysaccharide with flexible conformation in aqueous solutions) in the blends resulted in increased viscosity and lower conductivity of the solutions; these led to improved chain entanglements and weakening of the polyelectrolyte character of the protein component, both favoring fiber formation.[164] Another work described the electrospinning of zein/hyaluronic acid blend fibrous membranes, where, to enhance compatibility between the protein and the polysaccharide components, poly(vinyl pyrrolidone) was introduced into the aqueous ethanol solutions of the blend.[165] The use of food hydrocolloids complicates the electrospraying process, since these materials may consist of low-molecular weight polymers which do not generate sufficient viscosity and they generally exhibit strong
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intermolecular and intramolecular forces, which need to be somehow counteracted to promote capsule formation. In addition, the use of aqueous solutions further complicates the process due to the ionization of water molecules at high voltages in an air environment, which may cause corona discharge. Moreover, aqueous solutions present high-surface tension values which hinder the formation of stable liquid fluid jets during the process.[166,167] One way to reduce the high surface tension of aqueous solutions to stabilize the electrospraying process is the use of surfactants. Surfactants are used in a wide array of applications because of their potential to lower surface or interfacial tension of the medium in which they are dissolved.[168] For instance, capsules of low-molecular weight carbohydrates (e.g., maltodextrin, resistant starch) were obtained by use of the surfactants Tween 20, Span 20, and lecithin.[169] In another study, gums and surfactants were used to modify the aqueous hydrocolloid dispersion properties allowing capsule formation through electrospraying. The results showed that the use of nonionic surfactants was the most interesting strategy for improving the sprayability of these materials, since gums retained too much solvent that led to aggregated and wrinkled particles.[168]
Encapsulation of bioactive compounds by electrohydrodynamic process The growing consumer’s interest for the promotion of health and disease prevention through improved nutrition has led to numerous attempts to develop food-grade delivery systems, including active packaging, to encapsulate, protect, and controlled release of bioactive components.[170–172] More specifically, natural compounds with antimicrobial features are of great interest for the active packaging industry and their efficient encapsulation and controlled release represent a major challenge considering their sensitivity to heat, oxygen, and light. Recently, electrospinning has been receiving great attention in functional food and active food packaging systems.[173,174] This relatively simple technique can be applied for the production of nanofibrous polymeric membranes (coating films) formed by polymer fibers. Electrospun nanofibers exhibit properties such as high porosity, large surface area per unit mass, high gas permeability, and small interfibrous pore size, with most of these properties being quite important when these materials are used as carriers for delivery of bioactive compounds.[175,176] In the literature, few works are focused on electrospinning of edible biopolymers,[132,177] and even less on edible polysaccharides for controlled release of bioactives. In general, polysaccharides do not need toxic solvents to be
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electrospun[167] and are commonly used in food applications as coating and thickening agents, or additives for technological aims. Common polysaccharide-based wall materials used in electrospinning and electrospraying are pullulan, chitosan, alginates, dextrans, cellulose, and its derivatives. For instance, a blend of edible carbohydrate polymers (pullulan and cyclodextrin) was used to encapsulate antimicrobial aroma compounds for enhancing microbial safety of food products. The release of aroma compound was negligible at ambient conditions (23°C and 55% RH), even at high temperatures (up to 230°C), but it occurs beyond a given relative humidity threshold (90%),[178] presumably due to water plasticization of the carbohydrate matrix and the underlying enhancement in molecular mobility of the encapsulated components. In addition, folic acid has been encapsulated through electrospinning in amaranth protein isolate:pullulan ultrathin fibers with very high encapsulation efficiency (>95%). Encapsulation within the amaranth protein isolate:pullulan structures increased the thermal stability of folic acid and no degradation of it was observed after 2 h of UV exposure.[179] Similarly, amaranth protein isolate:pullulan ultrathin fibers were used to improve the antioxidant characteristics of two bioactive phenolic compounds, quercetin and ferulic acid, during an in vitro digestion protocol.[10] The authors reported that the encapsulated bioactive compounds kept their antioxidant capacity to a greater extent in comparison with the nonencapsulated ones during in vitro digestion.[180] Moreover, other researchers have focused on the use of proteins as wall materials for the encapsulation of active compounds. Common proteins used as encapsulating materials in electrospinning are WPC, whey protein isolate (WPI), soy protein isolate, zein, gelatin, and casein. In this context, electrospinning of zein (prolamine) shows an excellent outlook for its application in the stabilization of light and oxygen-sensitive food components;[181] zein is a hydrophobic protein (prolamin) extracted from corngrains, and is known for its high thermal resistance and great oxygen barrier properties. Fernandez et al. (2009) encapsulated β-carotene, a good source of provitamin A with antioxidant properties, in ultrafine zein fibers through electrospinning and showed that the encapsulated β-carotene was stable and well dispersed within the zein fiber matrix. The electrospun β-carotene had significantly higher light stability than the nonencapsulated control.[182] In addition, gallic acid had retained its antioxidant activity after incorporation in zein electrospun fibers.[183] Li et al.[132] encapsulated ( )epigallocatechin gallate in electrospun zein fiber to stabilize the polyphenol during simulated food processing conditions. The authors report that successful
immobilization of ( )-epigallocatechin gallate occurred when the fibers were aged for at least 24 h under dry conditions at ambient temperature. Moomand and Lim[184] encapsulated fish oil, rich in omega-3 fatty acids, in zein fibers and the results indicated that electrospun zein fibers provide a greater oxidative stability in comparison to nonencapsulated fish oil. These studies showed that electrostatic encapsulation processes are very versatile for preparing a zein-encapsulant polymeric matrix to protect bioactive compounds. Furthermore, electrospun soy proteins have been used for controlled release of allyl isothiocyanate.[185] On the other hand, electrospraying has been widely used to generate micro, submicro, and nanocapsules for several applications in the area of encapsulation of bioactive compounds.[186] Biopolymer-based particles are used to protect and deliver bioactive compounds in food systems or as fat replacers in foods by simulating the rheological, optical, and sensorial properties of lipid droplets. For food applications, capsules are generally preferred rather than fibers, since apart from facilitating better handling and subsequent incorporation into different products with minimal effects on sensorial attributes, they usually present greater surface/volume ratio and, thus are expected to have better release profiles than fibers.[146] Therefore, there has been considerable interest in the formation of biopolymer particles from proteins and/or polysaccharides.[187] Recent works have demonstrated that it is possible to obtain hydrocolloid-based encapsulation structures using electrospraying through proper adjustment of the process parameters and/or changing the solution properties through the addition of proper additives.[168,169] For instance, WPC micro, submicro, and nanocapsules were produced through electrospraying from aqueous solutions for the encapsulation of the antioxidant βcarotene. The results demonstrate that changes in pH of the solution have an impact on polymer conformation and capsule size, with larger capsules being obtained at pH 6.4. Moreover, the capsules were effective in stabilizing the β-carotene against photooxidation as very high encapsulation efficiencies were achieved.[10] Electrospraying was also evaluated for the encapsulation of folic acid using both a WPC matrix and a commercial preparation of resistant starch. Greater encapsulation efficiency was observed using WPC as wall material and the WPC protected the folic acid against degradation during storage, especially under dry conditions. In another study, curcumin, a food colorant with low solubility in water, was encapsulated in gelatin matrix, and the water solubility and bioaccessibility were significantly improved by encapsulation.[188] Similarly, lycopene was encapsulated
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through electrospraying within different edible biopolymeric matrices (dextran, WPC, and chitosan); it was seen that WPC presented the greatest encapsulation efficiency (around 75%) and the WPC capsules were also able to better protect lycopene against moisture and thermal degradation.[189] Torres-Giner et al.[190] encapsulated docosahexaenoic acid (DHA) in zein capsules using an electrospraying process; the encapsulated DHA showed enhanced chemical stability against degradation under different environmental conditions such as relative humidity and temperature and reduced off-flavor notes. In another study, gelatin, WPC, and soy protein isolate were compared as emulsion stabilizers and wall matrices for encapsulation of a-linolenic acid. The results showed that the WPC and soy protein isolate capsules significantly delay the degradation of alinolenic acid at 80°C.[191] Extensive lists of applications of electrospinning and electrospraying as techniques for the encapsulation of bioactive compounds are presented in Tables 3 and 4, respectively.
Future needs The food industry becomes more integrated with nutritional sciences due to current consumer demand for natural and healthy products; therefore, it guides and facilitates new processing approaches, including encapsulation technologies, for the enrichment of food matrices with bioactive compounds. Generally, encapsulation has been used to protect bioactive compounds in food systems from adverse environmental factors and to facilitate the control release at the targeted site. Spray drying is a well-established industrial process for the Table 3.
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continuous production of dry powders and has been widely used for the encapsulation over the past few decades. However, in spite of the recent developments, the process remains far from completely being controlled. In the spray drying process, the final properties of the particles (e.g., size, size distribution, and morphology) as well as the efficiency of encapsulation are affected by process parameters and solution properties; as a consequence, an extensive set of trial and error experiments are required for the production of microstructures with desirable characteristics. Therefore, future research needs to be focused on establishing a welldefined link between the solution properties, process parameters, and particle microstructure performance. Moreover, a great work remains to be made concerning the choice of encapsulating materials according to the chemical and physical nature of the core and wall material, the interaction (compatibility) between those as well as their relative proportion in the formulation of the capsules. In addition, the screening of new wall biopolymers, which could ensure both the effective preservation of functional components and the modulated release of the core ingredient, becomes essential to replace overly used materials such as gum arabic and maltodextrins. In recent years, electrospraying and electrospinning have attracted researchers’ interest due to their possible applications in food materials. These electrohydrodynamic processes have shown great potential for making micro and nanosized particles and fibers and lately are being used for encapsulating bioactive compounds. Nevertheless, the potentialities of fibers and particles have not been widely explored in food-related
Electrospinning technique for the encapsulation of bioactive compounds.
Encapsulating ingredient β-Carotene β-Carotene-loaded nanoliposomes Allyl isothiocyanate Cinnamaldehyde Curcumin
Wall material Zein prolamine PVA, PEO Soy protein isolate /PEO, poly(lactic acid) Chitosan/PEO Zein
Size range studied
Reference
Enhance stability against UV irradiation Enhance stability against UV irradiation
[182] [192]
Control release, enhance antimicrobial activity
[193]
∼50 nm 310 nm
Enhance antimicrobial activity Improved sustained release and effective free radical scavenging ability Increase oxidative stability of fish oil
[194] [195]
Evaluate thermal stability and retention of antioxidant activity Examine in vitro release characteristics
[183]
Fish oil rich in omega-3 fatty acids Gallic acid
Zein
190–500 nm
Zein
327–387 nm
Magnesium L-ascorbic acid 2-phosphate, α-tocopherol acetate Perillaldehyde Quercetin and ferulic acid
Polyacrylonitrile
200 � 15 nm
Pullulan/β-CD Amaranth protein isolate: Pullulan
∼250 nm 260.8 � 73.2 nm, 362.6 � 94.4 nm
R-(þ)-limonene Vanillin
Pullulan/β-cyclodextrin PVA/cyclodextrin
370 nm 120–230 nm
PVA, poly(vinyl alcohol); PEO, poly(ethylene oxide).
Purposes
1,140 nm 407.9 � 138.6 nm (PVA), 379.7 � 118.8 nm (PEO) 200 nm–2 µm
Control release of aroma compound Evaluate release characteristics and protection ability of antioxidant compounds during in vitro digestion, enhance thermal stability Increase storage stability, control release Prolong shelf life, enhance high temperature stability
[184]
[196] [178] [180] [197] [177]
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Table 4.
Electrospraying technique for the encapsulation of bioactive compounds.
Encapsulating ingredient α-Linolenic acid
Wall material
Size range studied
Gelatin, WPC, and soy protein isolate WPC
Nano-, submicro, and microcapsules were obtained
