Sacha inchi oil encapsulation: Emulsion and alginate beads characterization

Food and Bioproducts Processing 1 1 6 ( 2 0 1 9 ) 118–129

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Sacha inchi oil encapsulation: Emulsion and alginate beads characterization Klycia Fidelis Cerqueira e Silva ∗ , Ana Gabriela da Silva Carvalho, Renata Santos Rabelo, Míriam Dupas Hubinger Department of Food Engineering, School of Food Engineering, University of Campinas (UNICAMP), 80, Monteiro Lobato Street, P.O. Box 6121, Campinas, SP, 13083-862 Brazil

a r t i c l e

i n f o

a b s t r a c t

Article history:

The sacha inchi oil (SIO) has about 82% polyunsaturated fatty acids and micronutrients, as

Received 15 November 2018

tocopherol and phenolic compounds. This work investigated the combination of encapsu-

Received in revised form 2 April

lation techniques (emulsification and ionic gelation) in order to produce food ingredients

2019

with SIO to the enrichment of foodstuffs, as well as to promote its protection against lipid

Accepted 1 May 2019

oxidation. For the SIO encapsulation processes, sodium alginate and nonionic surfactants

Available online 9 May 2019

(Tween 20 and 80) were used as encapsulating/gelation and stabilizers agents, respectively.

Keywords:

emulsions showed low kinetics stability with increased oil concentration. With polysorbates

Emulsions exhibited high electronegativity (≈ −80 mV) and pseudoplastic behavior. Control Polysorbate

addition, an increase in the stability of these emulsions and a significant decline in droplet

Interfacial tension

size, span and electronegativity values were observed. Systems with 1.0 wt% Tween 20 were

Polyunsaturated fatty acid

preferred for particles formation due to the low presence of drops agglomerates. The wet

Ionic gelation

Ca (II) - alginate beads showed characteristic results of ionic gelation technique for mois-

Hydrogel

ture content (>85 ± 1%) and water activity (>0.996 ± 0.001), with size ranging from 407 ± 11

Sacha inchi oil

to 448 ± 33 ␮m and high encapsulation efficiency (>99%). The combination of encapsulation methods allowed the improvement of oxidative stability of SIO from 132 ± 9 meq O2. kg−1 to values lower than 44 ± 18 meq O2. kg−1 . © 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1.

Introduction

Sacha inchi oil (SIO) is extracted from oil seeds of Plukenetia volubilis L. found in the Peruvian Amazon. This oil has a large amount of polyunsaturated fatty acids that represents about 82% of its total lipid content (Hamaker et al., 1992), with emphasis in alpha-linolenic (C18: 3) ranging from 44 to 50.8% and linoleic (C18: 2) fatty acids with 33.4–36%. (Fanali et al., 2011; Follegatti-Romero et al., 2009; Gutiérrez et al., 2011; Maurer et al., 2012). SIO has a singular proportion of unsaturated fatty acid (Saengsorn and Jimtaisong, 2017) and has been gaining international attention mainly in the pharmaceutical and cosmetic industries. In addition, SIO also presents micronutrients as phytosterols, tocopherols, ␤-carotenoids and phenolic compounds (Fanali et al., 2011). Polyunsaturated fatty acids consumption, especially alphalinolenic acid, presents benefits to the human organisms, mainly

∗

because of the high levels of precursor compounds of antiinflammatory mediators (Hong et al., 2003; Lavie et al., 2009). These compounds when consumed contribute positively to the decrease in obesity, diabetes, allergies, Alzheimer’s, coronary and neurodegenerative diseases (Molendi-Coste et al., 2011). Unfortunately, the high quantity of unsaturated fatty acids present in the SIO are easily degraded in the presence of catalysts, such as oxygen, light, humidity and high temperatures, leading to the formation of primary and secondary oxidation products (Cisneros et al., 2014). In this context, encapsulation processes are interesting to carry and protect the oil against to lipid oxidation. Emulsion is an encapsulation technique widely used and it can be done at ambient temperature, which it is interesting for the thermosensitive constituents. This system is widely used by pharmaceutical, cosmetic and food industries as a way to establish a protective barrier to the encapsulated active compound, allowing its release at specific sites and greater stability during storage (Azizi et al., 2018). Polysorbates (Tweens) are nonionic surfactants commonly used to stabilize emulsions. These surfactants are of quick adsorption and have an

Corresponding author. E-mail address: klycia fi[email protected] (K.F.C.e. Silva). https://doi.org/10.1016/j.fbp.2019.05.001 0960-3085/© 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Food and Bioproducts Processing 1 1 6 ( 2 0 1 9 ) 118–129

effective action in the interfacial tension reduction of oil in water (O/W) emulsions (Gomes et al., 2018). O/W emulsions are commonly used together with other encapsulation methods, mainly physical and physico-chemical, such as spray drying (Jiménez-Martín et al., 2015; Turchiuli et al., 2005), spray chilling (de Matos-Jr et al., 2017; Park et al., 2014), and ionic gelation (Benavides et al., 2016; Riquelme et al., 2017; Tello et al., 2015), because it allows a pre-immobilization of active in a thermodynamically stable two-phase system. In addition to emulsion technique, the ionic gelation is another encapsulation method widely used, which comprises particle formation by ionic interaction between a biopolymer, as sodium alginate, and an electrolytic solution, such as the calcium chloride solution (Ca2+ ). The Ca (II) - alginate beads are explained by the egg-box model, based on the capacity of exchange between of di- or trivalent cations, such as Ca2+ , Mn2+ , Al3+ , and the side chains of the carboxylic acid (−COOH) present in the sodium alginate (Traffano-Schiffo et al., 2018). This encapsulation system is interesting for thermosensitive substances, as polyunsaturated fatty acids, since it allows the protection of the active compound as well as its application in food systems. Moreover, ionic gelation enables a higher oil load compared to spray drying, where 10–30% in respect to total solids is the usual load (Menin et al., 2018). The production of hydrogel microparticles with high oil load may be justified for the development of functional foods, where the high active content allows achieving the daily minimum intake recommended for polyunsaturated fatty acids (200 mg/day) while reducing the volume of particles in the final product (Chan, 2011; Peniche et al., 2004). In this way, encapsulation through the combined methods of emulsification and ionic gelation is an interesting alternative to produce food ingredients with oils rich in polyunsaturated fatty acids. For SIO, few studies were conducted using the combination between the encapsulation techniques, such as the developed work by Sanchez-Reinoso and Gutiérrez (2017) and Fadini et al. (2018). Sanchez-Reinoso and Gutiérrez (2017) developed SIO microparticles by spray drying using maltodextrin and Hi-Cap 100 as wall materials, aiming at increasing oxidative stability. Fadini et al. (2018) encapsulated the SIO through three combined techniques (emulsification process, spray drying and chilling) in order to decrease the particles solubility, which, consequently, changed the release trigger in food application. For the best of our knowledge, it is the first time that the SIO encapsulation by the association of emulsification and ionic gelation process is described, using the sodium alginate as continuous phase and polysorbates as emulsifiers. Thus, the goal of this work is to producing emulsions of SIO with different concentrations of nonionic surfactants (Tween 20 and 80) for the production of Ca (II) - alginate beads by ionic gelation method with high capacity of oil loading.

2.

Material and methods

2.1.

Material

Emulsions were prepared using SIO purchased from American Calix (Lima, Peru) with the following fatty acid composition: 0.06% 14:0 (tetradecanoic acid); 0.02% 15:0 (pentadecanoic acid); 6.62% 16:0 (lauric acid); 0.06% 16:1 (palmitoleic acid); 0.12% 17:0 (heptadecanoic acid); 3.81% 18:0 (stearic acid); 14.65% 18:1 (oleic acid); 42.5% 18:2 (linoleic acid); 30.75% 18:3 (linolenic acid); 0.21% 20:0 (eicosanoic acid); 0.24% 20:1 (gondoic acid); 0.19% 22:0 (docosanoic acid) and 0.08% 24:0 (tetracosanoic acid). Surfactants polyoxyethylene sorbitan monolaurate (Tween 20) and polyoxyethylene sorbitan monooleate (Tween 80) were obtained from Dinâmica Química Contemporânea Ltda (Diadema, Brazil) and Vetec (São Paulo, Brazil) with a high purity degree, respectively. Sodium alginate was characterized according to the number (Mn = 66 ± 6 kg/mol) or weight (Mw = 78 ± 1 kg/mol) of molar mass averages and the polydispersity index (PDI (Mw/Mn) = 1.2 ± 0.1) through size exclusion chromatography

119

with inline multi-angle light scattering (SEC-MALS). The ratio between guluronic and mannuronic acids (M/G) = 40/60 was determined by circular analysis in the Jasco J-810 Spectropolarimeter according to Morris et al. (1980), being donated by Danisco Brazil Ltda (São Paulo, Brazil). Calcium chloride (CaCl2 ) with 99% ACS grade was purchased from Anidrol (São Paulo, Brazil).

2.2.

Methods

2.2.1.

Emulsion formation

Sodium alginate solution (SAS) was prepared by the dispersion of 2 wt% of the polymer in distilled water at room temperature. The solution was maintained under agitation overnight to ensure the sodium alginate (SA) hydration. Oil concentration and conditions of the emulsification process, such as temperature, time and homogenization speed, used in the O/W emulsions production were defined after preliminary assays. Emulsion formulations were produced with different concentrations of SIO and surfactant, with the percentage of SAS kept at 2 wt%, according to Table 1. Emulsions were prepared using the surfactant (Tween 20 or 80) in a concentration range from 0.0 to 1.0%. For the emulsion formation, SAS, surfactant and SIO were homogenized in a rotor-stator (ULTRA-TURRAX, Silverson Machines, USA) for 3 min at 5600 rpm. Double-walled beaker used in emulsion preparation was connected in a thermal bath at 3 ◦ C to control the temperature during the process. Emulsions formulation were performed in duplicate. Control emulsions (no SAS addition) were prepared to compare with the ones composed by SAS in the continuous phase for the evaluation of kinetics stability, zeta potential and pH. The formulations of control emulsions were: Emulsions I (no polysorbates addition): 1.64 wt% SIO and 98.36 wt% water; 3 wt% SIO and 97 wt% water; 6 wt% SIO and 94 wt% water. Emulsions II (0.5 wt% Tween 20 or 80): 2.05 wt% SIO, 0.5 wt% polysorbates and 97.45 wt% water; 3.75 wt% SIO, 0.5 wt% polysorbates and 95.75 wt% water; 7.5 wt% SIO, 0.5 wt% polysorbates and 92 wt% water. Emulsions III (1 wt% Tween 20 or 80): 2.46 wt% SIO, 1 wt% polysorbates and 96.54 wt% water; 4.5 wt% SIO, 1 wt% polysorbates and 94.5 wt% water; 9 wt% SIO, 1 wt% polysorbates and 90 wt% water. For the preparation of control emulsions, the same conditions of homogenization were used (5600 rpm for 3 min at 3 ◦ C).

2.2.2. Emulsion characterization 2.2.2.1. Interfacial tension. Interfacial tension between SIO and water with polysorbates (Tween 80 and Tween 20) in concentration at 0.0, 0.5 and 1.0 wt% was measured at 25 ◦ C using a Tracker-S (Teclis, Longessaigne, France) tensiometer by the rising drop method. For the SIO and the SAS (2 wt%) with the same polysorbates concentrations the measurement was by pendant drop method. Polysorbates were dissolved into water and SAS at room temperature until their complete dissolution. The analyses were made in duplicate.

2.2.2.2. Emulsion stability. Immediately after the emulsion preparation, 25 ml aliquots of each emulsion were poured into a cylindrical graduated glass tube, sealed and stored at 25 ◦ C for a period of one day. The emulsion stability was analyzed by the volume of the upper phase measured after 24 h. The phase separation was calculated according to the Eq. (1). CI (%) =

V V0

x 100

(1)

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Food and Bioproducts Processing 1 1 6 ( 2 0 1 9 ) 118–129

Table 1 – Experimental conditions for emulsion formation using 2 wt% sodium alginate solution (SAS), surfactant (T20 (Tween 20) and T80 (Tween 80)) and SIO. Nonionic surfactant (wt%)

Emulsion I (control)

Emulsion II

Emulsion III

Emulsion IV

Emulsion V

T20

T80

0.00 0.00 0.00 0.50 0.50 0.50 0.00 0.00 0.00 1.00 1.00 1.00 0.00 0.00 0.00

0.00 0.00 0.00 0.00 0.00 0.00 0.50 0.50 0.50 0.00 0.00 0.00 1.00 1.00 1.00

SIO (wt%)

SAS (wt%)

1.64 3.00 6.00 2.05 3.75 7.50 2.05 3.75 7.50 2.46 4.50 9.00 2.46 4.50 9.00

98.36 97.00 94.00 97.45 95.75 92.00 97.45 95.75 92.00 96.54 94.50 90.00 96.54 94.50 90.00

where CI is the creaming index, V0 represents the initial emulsion volume and V is the upper phase volume.

where ˙ is the shear rate (Pa), n is the flow behavior index,  is the shear stress and k is the consistency index (Pa sn ).

2.2.2.3. Droplet size distribution. Droplet size distribution was

2.2.3.

measured by a laser light diffraction method using a Mastersizer 2000 (Malvern Instrument Ltd., UK). The emulsion droplet size was expressed as the volume-surface mean diameter (D32 ) and the dispersion index (Span), according to the Eqs. (2) and (3).

For the particles production, the emulsions formulated at fixed conditions of surfactant and three oil concentration (45, 60 and 75% of SIO in relation to total solids) were selected based on the results of kinetic stability, droplet size distribution, optical microscopy and ␨-potential. Particles were produced by atomization process of the emulsions selected from the adapted system of spraying (mini spray dryer B-291 – nozzle 0.7 mm), under a cross-linking solution of CaCl2 (2% m/v). The experimental parameters were adjusted, being the feeding rate of 0.6 kg/h, air pressure of 0.15 bar and distance of 15 cm between the nozzle and crosslinking solution. After atomization, the particles were held in the CaCl2 solution and left to harden for 30 min. The particles were produced in two independent lots. A fraction of moist particles was freeze-dried in a lyophilizer LS 3000 (Terroni Equipamentos Científicos, São Carlos, Brazil) at −40 ◦ C and 100 ␮mHg for 48 h. The wet and lyophilized particles were separated and kept under refrigeration at 5 ◦ C for future analyses.

D[32]

 3 ni di =  2

Span =

ni di

(d90 − d10 ) d50

(2)

(3)

where ni is the number of the droplets, diameter di , d10 , d50 and d90 are the equivalent volume diameters at 10%, 50% and 90% of cumulative volume, respectively.

2.2.2.4. -Potential measurements. To determine the zetapotential of droplets, the emulsions were diluted at concentration of 0.001% (v/v) in natural pH (ranging from 6 to 7 pH). The measurements were performed in quintuplicate in the chamber of a microelectrophoresis instrument (Nano ZS Zetasizer, Malvern Instruments Ltd., Worcestershire UK).

2.2.2.5. Optical

microscopy. The O/W emulsions optical microscopy was performed immediately after their preparation. The samples were poured onto microscopes slides, covered with glass coverslips and observed using a Carl Zeiss Model Axio Scope.A1 optical microscope (Zeiss, Germany) with 40× and 100− objective lenses.

2.2.2.6. Rheological

behavior. Emulsion flow curve was obtained using a cone-plate geometry (Ø =45 mm) in a stresscontrolled rheometer AR1500ex (TA Instruments, Elstree, UK). The analysis was done in the range between 0 and 300 s−1 in a three-step sequence: up-down-up. The third curve data was fitted to the power Law model, according to Eq. (4). The emulsions were evaluated at 25 ◦ C, 24 h after their preparation. ˙ n  = k* ()

Production of Ca (II) - alginate beads

2.2.4. Particles characterization 2.2.4.1. Encapsulation efficiency (EE). Encapsulation efficiency (EE) was performed according to the method described by Vasile et al. (2016) with minor modification. 5 mL of hexane was added to 1 g of wet Ca (II) - alginate beads shaken for 1 min at room temperature for oil extraction of the particle surface. The supernatant, hexane, was transferred to a receptacle previously weighed. Then, the amounts of unencapsulated oil of the wet particles was determined gravimetrically by the mass difference. For solvent evaporation, the receptacle was conditioned in the oven at 70 ◦ C until achieving constant weight, as SIO is not volatile, the total oil was assumed to be equal to the emulsion oil. Encapsulation efficiency (EE%) was calculated from Eq. (5). All measurements were made in triplicate and expressed as g oil/100 g of wet particles. EE (%) =

(Oil total − Oil surface) *100 Oil total

(5)

2.2.4.2. Moisture content and water activity. The moisture con(4)

tent of the microparticles was determined gravimetrically

Food and Bioproducts Processing 1 1 6 ( 2 0 1 9 ) 118–129

121

Fig. 1 – (A) Interfacial tension curve of SIO – water (control system). (B) Interfacial tension curves of control systems composed by SIO – water in two concentrations 0.5 and 1.0 wt% for the Tween 20 (T20) or Tween 80 (T80). (C) Interfacial tension curve of SIO – 2.0 wt% sodium alginate solution (SAS). (D) Interfacial tension curves of systems composed by SIO – SAS (2.0 wt%) in two concentrations (0.5 and 1.0 wt%) of polysorbates (T20 or T80). Data of interfacial tension curves presented variance coefficient ≤0.05. using a moisture balance analyzer (Shimadzu, Brazil) at 105 ◦ C until achieving constant mass. Water content was expressed in wet basis. Water activity (aw ) was measured using an AquaLab instrument (Decagon Devices Inc., Pullman, USA) at 25 ◦ C.

2.2.4.3. Particle size distribution. Particles size distribution was measured by a laser light diffraction method using a Mastersizer 2000 (Malvern Instrument Ltd., UK). The sample was dispersed in distilled water and the particle size was expressed as D43 , defined by Eq. (6). The Span was calculated from Eq. (3), as presented in Section 2.2.2.3.

D[43] =

 4 nd  i i3 ni di

(6)

where ni is the particles numbers and di is particles diameter, diameter di , d10 , d50 and d90 are the equivalent volume diameters at 10%, 50% and 90% of cumulative volume, respectively.

of gold in a Sputter Coater EMITECH K450 model (Kent, United Kingdom) with a thickness of 200 Å. After metallization, the samples were observed with 500× and 2000× magnification.

2.2.4.5. Lipid oxidation. Test of lipid oxidation in the alginate microparticles were performed through evaluation of peroxides values (PV) at the weeks 0 and 4 (two points) at 45 ◦ C. Briefly, in 0.5 g of wet beads 8 mL of 4% (w/v) sodium citrate solution were added to break the initial structure for the oil release, which was extracted by the method of Bligh and Dyer (1959). 0.1 g of the extracted oil sample was diluted in 4 mL of chloroform/methanol solution (7: 3), where 200 ␮L of the aliquot were taken for PV measurement. The PV determination was performed spectrophotometrically according to the IDF standard method 74A:1991 using the Unico 2800UV/VIS spectrophotometer (United Products & Instruments Inc., New Jersey, USA) absorbance at 500 nm. The PV was determined using a standard curve for Fe3+ , method described by Shantha and Decker (1994).

2.3.

Statistical analyses

2.2.4.4. Morphology and microstructure. The wet Ca (II) - alginate beads morphology was evaluated according to the optical using an optical microscope Carl Zeiss Axio Scope A1 (Gottingen, Germany) with 10× magnifications. Confocal microscopy was used in order to observe the oil distribution under the wet beads. The SIO was tagged with the Nile Red dye at the concentration of 1 mg/mL of oil. Samples were examined using a Zeiss LSM 780-NLO confocal on an Axio Observer Z.1 microscope (Carl Zeiss AG, Germany) using 10x of magnification. The images were measured at a wavelength of 528 nm. The freeze-dried microparticles were observed in a Scanning Electron Detector microscope with Energy Dispersive X-ray, LEO 440i — 6070 (LEO Electron Microscopy/Oxford, Cambridge, England), operating at 15 kV and electron beam current of 150 pA. The samples were fixed directly on door-metallic specimens (stubs) of 12 mm diameter and 10 mm height and then subjected to metallization (sputtering) with a thin layer

Results were statistically analyzed by one-way analysis of variance, using the software Minitab® 16.0 (Pennsylvania, USA). Mean analysis was performed using Tukey’s procedure at p ≤ 0.05.

3.

Result and discussion

3.1.

Emulsion characterization

3.1.1.

Interfacial tension

Interfacial tensions of all the studied systems decreased with time as shown in Fig. 1. The interfacial tension of the SIO-pure water system presented initial and equilibrium point around 20.00 ± 0.03 and 17.0 ± 0.2 mN/m (Fig. 1A), respectively. After sodium alginate addition, the initial interfacial tension of this system was reduced to 13.5 ± 0.1 mN/m and 4.40 ± 0.01 mN/m for equilibrium point, as can be seen in Fig. 1C. Then it can

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be observed that sodium alginate presents poor emulsifying capacity due to its low surface activity, as has already been reported by Gaonkar (1991). The systems with the surfactants Tween 20 or 80 in both concentrations (0.5 and 1.0 wt%) exhibited bigger reduction for the initial and equilibrium tension values, as well as for the equilibrium time values (Fig. 1B and D). The interfacial tension between the SIO-Water and SIO-SAS, with different concentrations of surfactants was evaluated separately to better understand the behavior of the components on emulsions stability. The addition of 0.5 wt% Tween 20 at the SIO-Water system reduced the initial tension in 73% (5.4 ± 0.1 mN/m) while increasing the concentration to 1.0 wt% allowed a decrease of 77% (4.59 ± 0.04 mN/m). For both concentrations, the equilibrium tension was reached after 3000 s. The Tween 80 addition at the SIO-Water lessened the initial tension by 65 and 68% to 0.5 (6.9 ± 0.1 mN/m) and 1.0 wt% (6.37 ± 0.02 mN/m), respectively, and arrived at the equilibrium tension after 4000 s. From these results, among the systems added of polysorbates, the Tween 20 (SIO-Water + T20) showed the lowest values of tension at the initial and equilibrium points for both concentrations. The addition of polysorbates in the two concentrations to the SIO-SAS systems (Fig. 1D) showed similar behavior in relation to the values of initial tension with respect to SIO-Water systems (Fig. 1B). However, the time to arrive in the tension at the equilibrium point for these systems (SIOSAS + polysorbates) was different. The system with 0.5 wt% Tween 20 did not reach equilibrium up to 800 s of observation. The system with the same concentration of Tween 80 showed almost constant values after 600 s. For the concentration of 1.0 wt% systems with Tween 20 and 80 did not show a significant difference for the equilibrium tension values (Fig. 1D). This time reduction to reach the equilibrium tension is due to the ability of steric stabilization by sodium alginate added to the quick diffusion of polysorbate molecules. Both polysorbates, Tween 20 and 80 present similarities, as they show low molecular weight (1.310 and 1.228 kg/mol for Tween 80 and 20, respectively) and the presence of hydrophilic groups characterized by polyethoxylated sorbitan, differing only in hydrophobic groups. However, Tween 20 is composed of linear hydrocarbon chain of lauric acid, whereas Tween 80 ˛ and Podgórska, 2016). The samples that has oleic acid (Bak contained the Tween 20 exhibited higher interfacial effectiveness in tension reduction, compared with the ones with Tween 80. We assume that the more condensed molecular structure of Tween 20 may have contributed to rapid saturation of the droplet surface. Surfactants are effective when quickly adsorbed on the droplet surface. Tween 80 required a little higher time to diffuse and rearrange onto the droplet surface due to its unsaturated hydrophobic chain. When polysorbates and SAS coexisted in the same system (SIOSAS + polysorbates), surfactants dominated the drop interface, since they presented a more active surface compared to sodium alginate. This behavior also was noted by Zhang and Wang (2016), which evaluated a series of nonionic surfactant to emulsion destabilization composed by peanut oil.

3.1.2. Emulsions droplet mean diameter, size distribution and microstructure The optical microscopy evaluation showed the effective performance of both surfactants Tween 20 and 80, in the reduction of droplet size when compared to control emulsions, as can be

clearly seen on micrographs in Fig. 2. The images of the control emulsions (SAS; no surfactant addition) with 45 and 60% SIO (Fig. 2A and B) exhibited smaller droplet sizes and similar behavior when compared to the control emulsion, prepared with 75% SIO (Fig. 2C), which presented heterogeneous droplet sizes. Emulsions with SAS plus Tween 80 showed groups of flocculated droplets for both concentrations (0.5 and 1.0 wt%) (Fig. 2D,E,F,G, H and I). On the other hand, emulsions prepared with SAS plus Tween 20 presented smaller droplets diameters and more dispersed ones at the continuous phase, according to Fig. 2J,K,L,M,N and O. The O/W emulsions showed the mean droplet diameter ranging from 1.7 ± 0.1 to 3.6 ± 0.1 ␮m, as demonstrated by results in Table 2. Control emulsions exhibited the highest droplets sizes ranging from 2.4 ± 0.1 to 3.6 ± 0.2 ␮m. In general, all emulsions presented unimodal behavior of droplets size distributions (Fig. 2), except for control emulsion with 75% SIO concentration, which showed bimodal behavior. The droplet size distribution of control emulsion with 75% SIO (Fig. 2C) shows one minor peak with a lower volume (<2%) and smaller droplets diameters (approximately 1 ␮m), one predominant peak with larger volume (around 6%) and larger droplets sizes around 10 ␮m. The minor peak represents initial or individual droplets of the emulsion, whereas the second peak expresses the droplets formed after the coalescing or flocculation process (Sato et al., 2015). All emulsions with addition of surfactants, in both concentrations (0.5 and 1.0 wt%), showed reduced droplets dimensions and Span values. The smaller diameters of droplet can be associated with the high surface activity of the emulsifier at the droplets interface during the emulsification process (Dickinson, 2003). The increase of surfactant concentration from 0.5 to 1.0 wt% promoted a further significant reduction in the droplet diameters for the emulsions with SAS plus Tween 20 (Fig. 2J,K and L for 0.5 wt%; M, N and O for 1.0 wt%). For the systems with SAS plus Tween 80, the same effect cannot be observed, according to the Fig. 2D,E and F for 0.5 wt%; G, H and I for 1.0 wt%. Chung et al. (2012) observed similar results, where the emulsions after the emulsifiers addition showed a significant decrease in droplets size. The higher reduction in oil droplets sizes after incorporation of 1 wt% Tween 20 indicates better retention of SIO in the dispersed phase that, consequently, resulted in improved kinetics stability.

3.1.3.

-potential and emulsion stability

The Supplementary File (Table S1) shows information about pH and zeta potential values for O/W emulsions without SAS addition. The emulsions composed of water and SIO, presented ␨-potential with electronegative values ranging from −39 ± 1 to −34 ± 2 mV with a pH of 5.4 ± 0.1 to 6.7 ± 0.4. These negative values of ␨-potential at original pH indicate that SIO droplets carried electronegative charges on the surface. The electronegativity of SIO used in this work is a consequence of the presence of phospholipid molecules since the oil was obtained by cold press extraction and have not suffered any refining process (Xie and Dunford, 2016). Emulsions after polysorbates addition (no SAS addition) exhibited a small increase in the electronegativity of ␨-potential (−61 ± 1 to −32 ± 3 mV) and a decrease at pH values (5.3 ± 0.1 to 6.2 ± 0.1). The decrease of zeta potential values in the emulsions added of nonionic surfactant was due to the polarization of droplet interface caused by the presence of ethoxylated groups (Hsu and Nacu, 2003). Moreover, all emulsions formulated with-

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Fig. 2 – Curves of droplets size distribution were obtained by light scattering and optical micrographs with 25 ␮m scale to O/W emulsions. The images A, B, C, F, J, K, L, and M were used objective lenses of 40× while D, E, G, H, I, N, and O were used objective lenses of 100 × . *Percentage in relation to total solids of the final emulsion. out the SAS presence should be considered stable because of their results of zeta potential demonstrated a good electrostatic repulsion (>|30|mV) (Cano-Sarmiento et al., 2018), though these systems presented phase separation after 24 h of observation. The 2 wt% SAS used in emulsions production exhibited a ␨potential of −81.73 ± 0.01 mV at pH 5.7 ± 0.1 (data not shown). The ␨-potential values of control emulsions (SAS; no surfactant addition) varied from −80 ± 4 to −85 ± 3 mV with pH range from 7.6 ± 0.2 to 6.43 ± 0.04, as can be seen in Supplementary File (Table S2). Both, SAS and control emulsions (SAS;

no surfactant addition) presented ␨-potential values with high electronegativity, it is attributed to the adsorption of the alginate molecules on the drop interface (Acevedo-Fani et al., 2015; Pongsawatmanit et al., 2006). The adsorption of polymer on the interface of the droplets and the decrease in interfacial tension after addition of alginate in the system (Section 3.1.1) indicate an improvement in the emulsions stability, promoted by an increase electrostatic repulsion and steric stabilization (Su et al., 2018). After the surfactant addition to emulsions, it was observed a significant increase in ␨-potential values that ranged from −73 ± 2 to −62 ± 2 mV in a pH range from

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Table 2 – Mean droplet diameter (D [32] ) and Span of O/W emulsions. SIO concentration*

Control

0.5% T80

0.5% T20

D[32] (␮m)

45% 60% 75%

2.4 ± 0.1a;C 3.00 ± 0.03a; B 3.6 ± 0.2a;A

1.9 ± 0.1b;B 1.95 ± 0.04c; B 2.05 ± 0.01 b;A

1.9 ± 0.1b;B 2.1 ± 0.1b;A 1.95 ± 0.04c;B

Span

45% 60% 75%

1.6 ± 0.1a;C 1.9 ± 0.1a;B 2.2 ± 0.1a;A

1.0 ± 0.3b;A 1.00 ± 0.02c; A 1.0 ± 0.1b;A

1.0 ± 0.1c;B 1.2 ± 0.2b;A 1.0 ± 0.1b;AB

SIO Concentration* 45% 60% 75%

Control 2.4 ± 0.1a; C 3.00 ± 0.03a;B 3.6 ± 0.2a;A

1.0% T80 2.00 ± 0.01 b; B 2.0 ± 0.1b; B 2.1 ± 0.1b;A

1.0% T20 1.74 ± 0.01c; C 1.78 ± 0.03c;B 1.80 ± 0.02c;A

45% 60% 75%

1.6 ± 0.1a;C 1.9 ± 0.1a;B 2.2 ± 0.1a;A

0.96 ± 0.03b;B 1.02 ± 0.04b;A 0.9 ± 0.1b;B

0.85 ± 0.04c;A 0.83 ± 0.03c;A 0.8 ± 0.1c;A

D[32] (␮m)

Span

Results are presented as the mean ± standard deviation. Different letters indicate significant difference at p < 0.05. Small letters indicate significant differences in the same line, while the capital letters in the same column. ∗

Percentage in relation to total solids of the final emulsion.

Fig. 3 – Emulsions that exhibited phase separation. A (Control emulsion with 75% of SIO); B (0.5 wt% Tween 80 with 75% of SIO). 7.5 ± 0.2 to 6.1 ± 0.1. This result was observed due to interactions between alginate and emulsifier molecules adsorbed onto droplet surfaces (Fioramonti et al., 2015). The interaction between sodium alginate and polysorbates promoted an increase in repulsive forces when compared to systems composed by only water, SIO, and surfactant. Then, the ␨-potential values reached by these systems >|60| mV should be enough to provide a high stability for emulsions. After 24 h, all emulsions have been shown to be kinetically stable. Despite of that two emulsions A (SAS; no surfactant addition) and B (SAS plus 0.5 wt% Tween 80) both with 75% of SIO exhibited phase separation (Fig. 3), being that these emulsions distinguish only by the presence of nonionic surfactant, as described in Table 1. Emulsion A (creaming index (CI) = 17 ± 2%) showed different instability mechanisms in relation to emulsion B (CI = 92.1 ± 0.6%), as can be seen in the micrographs of the Fig. 2 C and F. The instability mechanism of emulsion A is gravity creaming, where the larger droplets migrate more easily to the top of glass tubes, stimulating creaming. The low surface activity of the sodium alginate

solution led to the formation of larger droplets, which can favor the creaming process (Gaonkar, 1991), whereas, for emulsion B the droplet flocculation occurred. Flocculation is the main instability mechanism to interactions between polysaccharides and surfactants, which consists in droplet-droplet interplay promoted by attractive forces between polysaccharide bridges (Blijdenstein et al., 2004). The flocculation of emulsion B can be attributed to two factors: (1) the Tween 80 concentration present in the emulsion was not enough to completely envelop the drop surface and, (2) hydrophobic interactions between the Tween 80 and sodium alginate chains through self-association (Blijdenstein et al., 2004; Picone and Cunha, 2013). In the first case, the insufficient amount of Tween 80 molecules available for promoting film production at droplet interface can be the reason for the reduction of the adsorption speed. In the second case, the mixture of nonionic surfactants with a polysaccharide solution electrostatically charged can led to formation of complexes by hydrophobic interactions among the chain (Grant et al., 2006; Picone and Cunha, 2013). The increase of these interactions decreases the electrostatic repulsion at the interface of the droplet, leading to agglomeration. For systems with Tween 20 this effect is less pronounced due to the smaller size of hydrophobic chain. Therefore, for mixtures of ionic biopolymers and polysorbates, the Tween 20 has shown to be more appropriate.

3.1.4.

Rheological behavior

The rheological behavior of the systems is presented in Supplementary File (Table S3). All systems showed flow curves with Non-Newtonian and shear thinning behavior with flow behavior index (n) of 0.70 ± 0.01 with R2 < 0.986, while the 2 wt% SAS presented n of 0.5 ± 0.1 with R2 < 0.992 (data not show). The increase in the flow behavior index can be associated with the SIO incorporation into the SAS since oil is, generally, classified as a Newtonian fluid (n = 1). In relation to consistency index (K), emulsions and SAS presented values ranging from 2.5 ± 0.3 to 2.0 ± 0.1 Pa sn and 10.6 ± 0.1 Pa sn (data not shown), respectively. This significant reduction in the consistency index can be attributed to the shear rate used for emulsions production as well as to the incorporation of the SIO. Emulsions added of nonionic surfactant showed a high decrease in the consistency index and in the apparent viscosity when compared to

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Table 3 – Characterization of the hydrogel microparticles containing SIO. SIO Concentration*

Moisture content (%)

Water activity

Encapsulation efficiency(%)

45% 60% 75%

93.70 ± 0.24 90 ± 2b 85 ± 1c

0.996 ± 0.001 0.998 ± 0.001a 0.997 ± 0.001a

99.95 ± 0.02a 99.97 ± 0.01a 99.97 ± 0.03a

a

a

D [43] (␮m)

45% 60% 75%

Span

Wet particles

Dried particles

Wet particles

Dried particles

448 ± 33 455 ± 7a; A 407 ± 11b; A

309 ± 15 307 ± 16b; B 356 ± 13a; B

0.67 ± 0.02 0.65 ± 0.01b; B 0.66 ± 0.01a; B

1.08 ± 0.02a; A 1.0 ± 0.1a; A 1.05 ± 0.04a; A

a; A

b; B

a; B

Results are presented as the mean ± standard deviation. Different letters indicate significant difference at p < 0.05. Small letters indicate significant differences in the same column, while the capital letters in the same line. ∗

Percentage in relation to total solids of the final emulsion.

the control emulsions. This decrease can be attributed to the lower fraction of suspended droplets, which provide less interaction between droplets that easily break under high shear rates (Gomes et al., 2016).

3.2.

process of the microparticles, as well as along the extrusion by the atomizer nozzle and crosslinking bath. Apart from that, the large number of G-blocks contributed to the formation of a dense layer of gel on the particles surface (Chan, 2011), which made the oil release to exterior difficult.

Particles characterization 3.2.2.

From the results of emulsions characterization, in relation to the size and droplets distribution, kinetic stability, and optical microscopy images, three formulations with different oil concentrations (45, 60 and 75% of SIO in relation to total solids) and stabilized with Tween 20 at 1% (g/100 g) were selected for particles production by the ionic gelation technique.

3.2.1. Moisture content, water activity and encapsulation efficiency Table 3 shows the results of the characterization of the hydrogel microparticles. All particle’s formulations presented high moisture content ranging from 93.6 ± 0.2 to 85 ± 1% w b. and water activity values higher than 0.996 ± 0.001. The microparticles showed a significant decline in the moisture values with the increase in oil concentration, inversely proportional to the rise of solids concentration in the system. The high moisture content and water activity of the particles are characteristic of the external ionic gelation technique since the crosslinking process occurs in aqueous solution. These results were close to that observed by other authors, which used sodium alginate and pectin as a gelling material for encapsulation (Aguilar ˇ et al., 2016; Heck et al., 2017; et al., 2015; Belˇscak-Cvitanovic Tello et al., 2015). The wet microparticles displayed encapsulation efficiency above of 99% and without significant differences among them (Table 3). It was not possible to observe the impact of the increase in oil concentration in the encapsulation efficiency. The high encapsulation efficiency observed may be attributed to two facts: (1) the surfactant addition associated with the condition of the emulsification process, which led to the dimension reduction of the oil droplets size. (2) the fraction of homopolymer blocks of guluronic acid (G-blocks) present in sodium alginate. In the first case, the emulsification process with 1 wt% Tween 20 allowed the formation of small oil droplets with size ranging from 1.74 ± 0.01 to 1.80 ± 0.02 ␮m (Table 2). The reduced droplet size provided better entrapment of oil in the alginate hydrogel structure, thus resulting in high encapsulation efficiency values. The smaller droplets of SIO in the emulsion may lead to greater stability of the emulsified system, thereby avoiding oil losses during the manufacturing

Particles size distribution

According to Table 3, the mean diameter of wet particles ranged from 407 ± 11 to 448 ± 33 ␮m, whereas the dried particles were from 307 ± 16 to 356 ± 13 ␮m. The reduction of particle sizes was approximately 30% after the freeze-drying process for the formulations containing 45 and 60% of SIO, while for 75% of SIO the reduction was 12%. This significant reduction of the mean diameter (D43 ) can be associated with the high water-holding capacity of three-dimensional networks of hydrogels. In the case of wet particles, the diameter of the atomizer nozzle has great influence on the particle diameter. However, the size of the dried particles is related to the internal variables of the process along the formation of the particles, such as oil content and G-blocks. Fig. 4 showed monomodal distribution for the wet and dried particles, although two formulations of dry particles (60 and 75% of SIO) presented population peaks with a volume inferior to 2%. Another point observed through the size distribution curves was the polydispersity index (Span). The span for the dry particles (≤0.67 ± 0.02) was significantly higher than in the wet particles (≤1.08 ± 0.02). This increase may be attributed to size diversity or the agglomerates formation.

3.2.3.

Microstructure

The light microscopy images (Fig. 5A, E and I) showed wet particles with spherical and slightly oval shape. After the freeze-drying process, the Ca (II) - alginate beads preserved their original sphericity with small surface irregularities. The preservation of the particles three-dimensional structure of particles after water loss may be attributed to the high content of G-blocks. The G-blocks connect more easily with the Ca2+ ions, due to their higher affinity along the crosslinking, compared to the blocks of mannuronic acid (M-blocks) (Ramos et al., 2018). Therefore, the G-blocks content is one of the process variables that allows stronger structural networks in gel formation. The confocal microscopy allowed appraising the oil distribution in the Ca (II) - alginate beads. In the Fig. 5D,H and L observed the homogeneous SIO distribution by alginate matrix, being that this fact is possibly related to the addition of Tween 20 to emulsions. Other authors obtained the

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Fig. 4 – Wet (A) and dry (B) particle size distributions.

Fig. 5 – External and surface morphology of the microparticles by optical microscopy (A, E, and I), scanning electron microscopy (B, C, F, G, J, and K) and confocal microscopy (D, H, and L). *Percentage in relation to total solids of the final emulsion. results similar using substances with emulsifying properties for external ionic gelation, as the found at the present work (Li et al., 2012; Morales et al., 2017; Zhang et al., 2016).

3.2.4.

Lipid oxidation

Fig. 6 exhibit results of the oxidative stability in relation to PV of the hydrogel particles and unencapsulated SIO at the times 0 and after 4 weeks at 45 ◦ C. At week 0, pure SIO presented low PV with 4.43 ± 0.03 meq O2. kg−1 , as well as the encapsulated SIO ranging from 5.8 ± 0.4 to 6.01 ± 0.01 meq O2. kg−1 . This significant difference between the PV at time zero may be assigned to the high shear rate employed during the emulsion and microparticles production process, which led to the production of levels peroxides. After 4 weeks at 45 ◦ C, the free SIO differed expressively from the encapsulated SIO with 132 ± 9 meq O2. kg−1 of PV, while the encapsulated SIO showed variations between 44 ± 18 and 28 ± 1 meq O2. kg−1 . Tello et al (2015) reported that their alginate microparticles without coated showed high PV to the oil model, being these results superior to found for PV of free

Fig. 6 – Lipid oxidation of encapsulated SIO evaluated by peroxides values during storage at 45 ◦ C. Different letters in each block indicates the significant difference between samples (p ≤ 0.05).

Food and Bioproducts Processing 1 1 6 ( 2 0 1 9 ) 118–129

oil. They attributed this behavior to the characteristic pores of the Ca (II) - alginate beads, which enabled easy access and increase the contact surface between oxygen and encapsulated oil. The microparticles in this work not displayed the same conclusions possibly due to the oil distribution through the polymeric matrix (Fig. 5D,H, and L) and the composition of sodium alginate in relation to G-Blocks. The preliminary results of oxidative stability allow us to conclude that the encapsulation method by ionic gelation was effective in SIO protection, since that the PV indicate the formation of primary products of lipid oxidation.

4.

Conclusion

The results demonstrate that the combined encapsulation method, emulsification and ionic gelation process, for SIO protection was efficient, where the nonionic surfactant addition contributed to high encapsulation efficiency. In general, the added emulsions of nonionic surfactants showed an improvement in their characteristics (droplets size and creaming index) and the effects of oil bulk raise were attenuated compared to the control systems. The concentration increase (0.5–1.0 wt%) and the size of the hydrophobic chain of surfactants influenced the development of the drop agglomerates, and, thus, the 1.0 wt% Tween 20 exhibited the most appropriate behavior for alginate interactions. Hydrogel microparticles containing SIO showed high moisture content and water activity which make them interesting to be applied in foods with these same properties, like yogurts, cheeses, and candy. They can allow enrichment of food products with the polyunsaturated fatty acids that are essential for the metabolic reactions.

Acknowledgements The authors are grateful to FAPESP (2015/11984-7), CNPq (304475/2013-0) and CAPES with Finance Code 001 for the financial support, and for CNPq (132420/2016-3; 150132/2018-2) by fellowships support. We thank National Institute of Science and Technology on Photonics Applied to Cell Biology (INFABIC) at the State University of Campinas by use of the Confocal Laser Scanning Microscopy, which it is co-funded by FAPESP (08/57906-3) and CNPq (573913/2008-0).

Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/ j.fbp.2019.05.001.

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