Microstructural properties and distribution of components in microparticles obtained by spray-drying

Journal of Food Engineering 152 (2015) 105–112

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Microstructural properties and distribution of components in microparticles obtained by spray-drying J. Porras-Saavedra a, E. Palacios-González b, L. Lartundo-Rojas c, V. Garibay-Febles b, J. Yáñez-Fernández d, H. Hernández-Sánchez a, G. Gutiérrez-López a, L. Alamilla-Beltrán a,⇑ a Departamento de Graduados e Investigación en Alimentos, Escuela Nacional de Ciencias Biológicas, Instituto Politécnico Nacional, Carpio y Plan de Ayala s/n, CP 11340 México DF, Mexico b Laboratorio de Microscopía Electrónica de Ultra Alta Resolución (LAMEUAR), Instituto Mexicano del Petróleo, Eje Central Lázaro Cárdenas No 152, Edificio 33-11. Col. San Bartolo Atepehuacan, CP 07730 México DF, Mexico c Centro de Nanociencias y Micro Nanotecnologías del IPN, Calle Luis Enrique Erro s/n, Unidad Profesional Adolfo López Mateos, Col. Zacatenco, CP 07738 México DF, Mexico d Departamento de Bioingeniería de la Unidad Profesional Interdisciplinaria de Biotecnología, Instituto Politécnico Nacional, Av. Acueducto s/n, Barrio la Laguna Ticomán, CP 07340 México DF, Mexico

a r t i c l e

i n f o

Article history: Received 14 October 2014 Received in revised form 23 November 2014 Accepted 24 November 2014

Keywords: Wall materials Spray drying Components distribution Microstructure Focused ion beam

a b s t r a c t The aim of this work was to analyse the relationship between microstructural development, distribution of elemental components and characteristics related to functional properties of powders obtained by spray drying process. Blends of soy protein isolate (SPI), gum Arabic (AG) and maltodextrin 20 DE (MD) at 20% TS were prepared and spray-dried at 250 °C, 200 °C, 175 °C and 150 °C as inlet temperatures of drying air, and the outlet temperature at 70 °C was kept constant. Bulk density, hygroscopicity and wetting time were affected by concentration of wall material, drying air temperature did not suggest effect on these characteristics. Using combined microscopy techniques, the internal microstructure of particles obtained at 150 °C/70° as inlet/outlet drying air temperatures were analysed, results revealed that 26% was the highest percentage of hollow microparticles and the shell thickness from 264 nm to 290 nm. In these microparticles, the elemental analysis revealed that nitrogen was present from 2.93% to 15.37% in particles surface. Additional, a descriptive model to explain the possible assemble of wall material of a microparticle was proposed. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction The encapsulation process is widely used in the chemical, pharmaceutical and food industry as a method to conserve sensible materials that can be degraded by environmental agents. Some encapsulated products are oils, fats, aromatic compounds, oleoresins, vitamins, minerals, dyes, enzymes, and microorganisms (Madene et al., 2006; Jafari et al., 2008). In encapsulation processes, a compact and continuous polymeric film encloses the core material, forming a microcapsule. The microencapsulation by spray drying is widely used in the food industry (Wang et al., 2009), due to low cost of the process, effectiveness of the protection of materials, high stability of the final product, and mass production (Madene et al., 2006; Gharsallaoui et al., 2007). Although the principal ⇑ Corresponding author at: Departamento de Graduados e Investigación en Alimentos, Escuela Nacional de Ciencias Biológicas, Instituto Politécnico Nacional, Carpio y Plan de Ayala s/n, CP 11340 México DF, Mexico. E-mail addresses: [email protected], [email protected] (L. Alamilla-Beltrán). http://dx.doi.org/10.1016/j.jfoodeng.2014.11.014 0260-8774/Ó 2014 Elsevier Ltd. All rights reserved.

objective of this technique is to protect the nucleus material or active agent against environmental factors, this also improves flavour, aroma, stability, nutritional value, and appearance of the microencapsulated agent. The encapsulated products can be obtained as powders or agglomerated form, at low moisture content and water activity values; these aspects ensure the physiochemical and microbiological stability (Cuq et al., 2011). For an efficient microencapsulation process, it is necessary to control aspects such as operation conditions, and the selection of wall materials which depends on the core material to be encapsulated (Georgetti et al., 2008; Kim et al., 2009). By other hand, a blend of wall materials may to increase the microencapsulation process efficiency, due to synergetic effect of functional properties of each material used (Madene et al., 2006; Gharsallaoui et al., 2007); carbohydrates and proteins are usually used, some of them are skimmed milk powder, whey protein, soy protein isolate, gum Arabic, maltodextrin, modified starch, gum mesquite, gelatin, starch (Baranauskiené et al., 2006; Krishnan et al., 2005; RodríguezHuezo et al., 2007; Kurozawa et al., 2009; Wang et al., 2009;

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Guadarrama-Lezama et al., 2012). As composition of microparticle surface plays an important role in the end use of powders, understanding the developing mechanism of the surface of microparticles in terms of compositional aspects, and the ability to control its formation, can be useful to improve microencapsulation process by spray drying. Gaiani et al. (2006) and Jafari et al. (2008) reported that it is possible to determine the distribution of elements in particles of dairy powders through the technique of X-ray induced photoelectron spectroscopy (XPS). Murrieta-Pazos et al. (2013) reported the use of X-ray dispersion spectroscopy (EDX) to analyse the atomic composition of sample; this technique is coupled with scanning electron microscopy (SEM). The SEM technique has been used to visualise morphology and microstructure of particles, and confocal laser scanning microscopy (CLSM) has been used as evaluation and characterisation tool, as a non-destructive technique by its ability to obtain images of the internal structure of the microparticles using the autofluorescence own of the material (Hambrecht et al., 2000; Paramita et al., 2010). Given the importance of using wall materials to obtain microcapsules with appropriate structural and functional properties, the objective of this study was to examine the effect of wall materials such as maltodextrin, gum Arabic and soy proteins isolate on the development of external and internal structure of microparticles, as well as to determine the distribution of elements within them. 2. Materials and methods 2.1. Materials Soy protein isolate (SPI) (Cenit, Mexico); maltodextrin 20 DE (MD) (Globe Corn Product International, Illinois, U.S.A.) and gum Arabic (AG) (E number E414) (Morevo Quick Gum) were used as wall materials. 2.2. Preparation of samples Although in industrial spray drying processes, dispersions with high solid concentration are recommended (>30%) to reduce costs and increasing drying efficiency (Filoková et al., 2006), in this study dispersions with low concentration of total solids were selected in order to avoid obstruction of the pneumatic nozzle when using blends with the highest concentration of gum. Suspensions of the wall materials SPI, MD, and AG were prepared (Table 1), keeping constant a concentration of 20% of total solids. Samples were hydrated during 12 h at room temperature (22 °C) using distilled water and applying mechanical agitation.

cylinder and weighing the sample and measuring the volume it occupied within the cylinder. The mass of the sample was divided by the volume it occupied (Jumah et al., 2000; Niro Analytical Methods, 2009). Hygroscopicity (Hp) was calculated using the method described by Tonon et al. (2008). 1 g of sample was placed into a glass cell, after that the cell was introduced in desiccator containing a saturated NaCl solution for maintaining equilibrium relative humidity at level of 75.29%, at 25 °C. After one week, samples were weighed, reporting the hygroscopicity value as g of absorbed humidity per 100 g of dry solids (g/100 g). The wetting time (WT) is the time required for all of the particles to submerge in solvent; it was evaluated placing 1 g of sample into 10 mL of water, according to the static method of wetting time described by Gaiani et al. (2010). The protein content was assessed by Kjeldahl method (AOAC, 2005) was applied using a conversion factor of nitrogen to protein of 6.25. This measurement was done to confirm concentration of protein and N2 in wall materials. All measurements were carried out in triplicate, and all results were reported as the mean and standard deviations. 2.5. Microstructural analysis of particles 2.5.1. Scanning electron microscopy (SEM) To evaluate the morphology of microparticles, powders were immobilized on carbon tape collocated on a specimen slide and removed the excess. After that, samples were observed by means of a double-beamed scanning electron microscope (Dual Beam Nova 200 Nanolab, FEI) operated at 1.00 kV, at amplifications of 3000 (Nijdam and Langrish, 2005). To study the influence of the wall materials on the microstructural properties of powders and determine the thickness of the shell of the microparticles, an induced-fracture on microparticles was applied. The powder was placed on a copper tape fixed to a specimen slide. In order to fracture the particles, the tape was stuck and unstuck, making use of a second piece of copper tape. After that, the slide was covered with gold using an evaporator (Danton Vacuum, Desk II) and the powder sample was observed through the double-beamed scanning electron microscope (Dual Beam Nova 200 Nanolab, FEI, USA), operated at 3 kV. Once the fractured particles were localised, the thickness of the shell was measured (Paramita et al., 2010). 2.5.2. Focused ion beam (FIB/SEM) The focused ion beam of the double-beamed scanning electron microscope (Dual Beam Nova 200 Nanolab, FEI) was used to rough dressing the shell of microparticles, controlling the current of the ion beam from 1 pA to 20 nA. SEM was utilised to acquire images of the particle wall at different times of cutting (Wang et al., 2013).

2.3. Spray drying of wall materials blends

2.4. Physical and chemical properties

2.5.3. Confocal laser scanning microscopy (CLSM) In order to determine the quantity of hollow particles, samples were analysed using a Confocal Laser Scanning Microscope LSM 710 (Carl Zeiss, Germany). The samples were excited with a diode laser (405 nm), argon laser (488) and a diode-pumped solid-state laser (DPSS) (561 nm). The images of the autofluorescence of the particles were captured with an objective of 40/1.3 in both 2D (flat) and 3D (z-stack). The hollow particles in a selected sample of 300 particles were counted, and the percentage of hollow particles was calculated according to the method described by Paramita et al. (2010).

Water activity (aw) of each powder was analysed using an Aqualab (4 TE model, Decagon Devices, USA). The moisture content (MC) was determined gravimetrically following the official AOAC method (AOAC, 2005). Apparent density (Dp) was calculated by collecting the powder, without compacting it, in a 10 mL graduated

2.5.4. X-ray photoelectron spectroscopy (XPS) In order to quantify elements as C, N, O and S in the surface of microparticles, X-ray photoelectron spectroscopy was applied using a spectrometer K-alpha (Thermo Scientific, U.K.). Powder was placed in a sample holder and kept for 24 h in an ultra-high

Suspensions were fed into a spray dryer (Mobile Minor™ 2000, GEA Niro, Denmark) using a peristaltic pump (Watson-Marlow 520S, USA). The equipment was operated with a co-current flow and pneumatic nozzle as atomizer; the inlet temperatures of drying air were 250 °C, 200 °C, 175 °C and 150 °C; the outlet temperature of drying air was kept constant at 70 °C (Table 1). The powders were collected and placed in polyethylene bags, hermetically sealed and stored until further assesses were performed.

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J. Porras-Saavedra et al. / Journal of Food Engineering 152 (2015) 105–112 Table 1 Experimental design to evaluate the effect of drying-air temperature and composition of different blends of wall materials. Experiment

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 a

Wall materiala (%) SPI

MD

AG

10 20 3.33 0 10 10 0 0 0 0 10 0 10 20 10 0 20 0 0 13.33 20 10

0 0 13.33 10 0 0 20 10 0 10 0 20 10 0 10 10 0 20 0 3.33 0 10

10 0 3.33 10 10 10 0 10 20 10 10 0 0 0 0 10 0 0 20 3.33 0 0

Inlet temperature of drying-air (°C)

Sample

250 150 175 250 150 150 150 150 150 200 250 250 250 150 250 200 250 200 250 200 250 150

SPI/AG/250 SPI/150 SPI/13MD/AG/175 MD/AG/250 SPI/AG/150 SPI/AG/150 MD/150 MD/AG/150 AG/150 MD/AG/200 SPI/AG/250 MD/250 SPI/MD/250 SPI/150 SPI/MD/250 MD/AG/200 SPI/250 MD/200 AG/250 13SPI/MD/AG/200 SPI/250 SPI/MD/150

Formulations at 20% of total solids.

vacuum chamber. After that, the sample was analysed using a monochromatic source of X-rays Al K-alpha, and X-ray spot of 400 lm. The spectra obtained was analysed using the Thermo Scientific Avatage software (ver. 5.9). 2.5.5. X-ray dispersive spectrometry (EDX) Using X-ray dispersive spectrometry a qualitative elemental chemical characterisation of the samples can be estimated. The equipment utilised was an EDX analyser coupled to SEM (operated at 3 kV) and equipped with an X-ray detector. EDX is a technique coupled with SEM to characterise the atomic composition of the sample (Murrieta-Pazos et al., 2011). 2.6. Experimental design An experimental design of combined mixes was applied, using the statistical software Design-Expert version 8.5 (State-Ease Inc., Minneapolis, United States). One-way analysis of variance (ANOVA) maintaining an a = 0.05 was used for all results. 3. Results and discussion 3.1. Physical and chemical composition of blends In case of formulations containing SPI/AG, MD/AG and SPI/MD, and dried at 250 °C, differences in moisture content were observed, being the highest moisture content in the blend of SPI/AG. For all cases, the moisture content was between 2.21% and 6.79%, and water activity values between 0.07 and 0.2; similar results were reported by Pereira et al. (2009). According to Bhandari (2008), the powder moisture content should be less than 5%, and water activity values between 0.15 and 0.2. For all formulations, increasing the inlet temperature of drying-air and keeping constant the outlet temperature, powders with high moisture content were obtained. Bhandari (2008) reported similar behaviour, explaining that it was due to the high humidity content of the inlet airflow. When fed product at room temperature (24 °C) and it contacts the drying-air, the shell of particles is developed faster at high temperatures (250 °C) than lower ones (150 °C). At low dryingair temperatures, a substantial quantity of water is evaporated

before the particle shell is developed, and the components have enough time to migrate to the surface of the droplet. Whereas, at high temperatures, the particle surface solidifies quickly, being this a barrier to the water diffusion from core to the surface and to the environment within the dryer (Kim et al., 2003, 2009; Nijdam y Langrish, 2005). Bulk density is related with powder flowability properties as a technological functional properties of powders (Thalberg et al., 2004), in this context, mechanical and chemical interactions are related and depend on environmental conditions, material processing, moisture content, particle size and composition (Ganesan et al., 2008). In this study, the bulk density values vary between 0.33 g/cm3 and 0.60 g/cm3, similar results were reported by Carneiro et al. (2013). The water absorption is a critical factor to keeping stable structure of microparticles; when the powders get moisture, the microparticles change in their physical structure, such as collapse, stickiness and caking (Bhandari and Howes, 1999), losing their individual structure and function. In this work, gum Arabic was the most hygroscopic wall material, after this maltodextrin and soy proteins isolate were less hygroscopic. The values vary between 7.28 and 17.87 g of water/100 g of total solids. However, the structural changes due to hygroscopicity were quite evident in all formulations containing maltodextrin, being the most marked change in ternary mixture SPI/13MD/AG. Hartmann and Palzer (2011) reported that the chemical composition is decisive in powder caking, in this phenomenon the microparticles are binding together due to adhesion forces (Palzer, 2011). Similar results (12 and 8 g of humidity/100 g of solids) were reported in powders of dairy cream and whole milk when they were exposed at relative humidity of 75% (Murrieta-Pazos et al., 2011). For all samples, the wetting time was less than 4235 s. However, important differences among analysed formulations were observed. As expected, in formulations with high maltodextrin content the wetting time was lower, but this wetting time was increased when MD was mixed with SPI and AG. This point is consistent with results reported on recent studies of Gaiani et al. (2010) and Murrieta-Pazos et al. (2011), whom found that dairy powders with higher protein content in their particle surface take more time to be moisturised.

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Fig. 1. Images acquired by SEM of the external structure of microparticles of different powdered wall materials spray dried at 150 °C/70° as inlet/outlet drying air temperatures. (a) MD, (b) GA, (c) SPI, (d) MD/GA, (e) MD/SPI, (f) GA/SPI, (g) SPI/13MD/GA and (h) 13SPI/MD/GA.

In this work, bulk density, hygroscopicity and wetting time were affected by concentration of wall material; drying air temperature did not suggest effect on these characteristics. Taking in count these results as selecting criteria, the samples SPI, AG, MD, SPI/AG, SPI/MD, AG/MD, SPI/13MD/AG and 13SPI/MD/AG (spray dried at 150 °C/70° as inlet/outlet drying air temperatures)

were chosen for the microstructural analysis by scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM); and for the compositional analysis by X-ray photoelectron spectroscopy (XPS) and X-ray dispersive spectrometry (EDX). All these analysis were done with particles of powdered samples.

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Fig. 2. Images acquired by CLSM and SEM of hollow microparticles of different powdered wall materials spray dried at 150 °C/70° as inlet/outlet drying air temperatures. (a) MD, (b) GA, (c) SPI, (d) MD/GA, (e) MD/SPI, (f) GA/SPI, (g) SPI/13MD/GA and (h) 13SPI/MD/GA.

Fig. 3. Images acquired by FIB/SEM of compact and hollow microparticles of SPI/GA/13MD blend spray dried at 150 °C/70° as inlet/outlet drying air temperatures. (a) centre of a compact particle observed at 10,000, (b) centre of a compact particle observed at 40,000, (c) hollow particles and shell observed at 10,000 and (d) proposed structure of shell.

3.2. Microstructural analysis of the particles

a-tocopherol using AG and MD (Quintanilla-Carvajal et al., 2010).

To investigate the effect of the wall materials on the microstructural development of particles, their morphology were observed with CLSM and SEM. Micrographs of samples acquired by SEM are illustrated in Fig. 1. All samples revealed individual particles (primary) with spherical shape and shape factor of 0.90–0.98; when this factor approaches 1.00 it indicates a perfect circle. Similar values have been reported in particles of encapsulated

The size particle evaluated as Feret’s diameter was in an interval between 2 and 7 lm; this variability is a typical characteristic due to the process of spray drying (Vehring et al., 2007). In addition, microparticles without apparent breakage or fissures of microstructure were observed (Fig. 1), this fact is important because any damage in particle surface could reduce the protection and retention of the core material. The particle surface of GA are smooth, while samples of SPI and MD are rough with dents

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Table 2 Effect of the composition on the wall thickness of the microparticles of blends spray dried at 150 °C/70° as inlet/outlet drying air temperatures. Sample

Wall thickness Min (nm)**

MD SPI SPI/GA MD/GA SPI/MD SPI/13MD/GA 13SPI/MD/GA GA

Mean (nm)* a

44 92 121 107 101 136 100 158

132 ± 81 250 ± 113b 259 ± 118b 264 ± 104b 290 ± 115b 336 ± 139c 361 ± 138c 432 ± 166d

Max (nm)*** 392 753 553 558 631 698 691 911

*

Mean value of three measurements (±standard deviation). The lowest value measured. *** The highest value measured. Equal letters in the same row indicate no significant differences with p < 0.05. **

Table 3 Relative elemental composition of different wall material powders measured by XPS. Sample

SPI AG MD SPI/AG SPI/MD AG/MD SPI/13MD/AG 13SPI/MD/AG

Relative atomic concentration (wt%) C

O

N

S

68.17 ± 0.75 65.34 ± 0.50 67.21 ± 1.21 65.08 ± 1.27 64.62 ± 0.10 65.23 ± 2.06 66.37 ± 1.31 67.77 ± 1.78

15.97 ± 2.78 31.67 ± 0.56 32.79 ± 1.21 19.77 ± 0.10 24.56 ± 0.19 32.03 ± 1.99 23.89 ± 0.80 19.77 ± 0.49

15.48 ± 2.21 2.98 ± 0.08 nd 14.04 ± 0.37 10.37 ± 0.30 2.73 ± 0.27 9.16 ± 0.66 11.81 ± 1.27

0.36 ± 0.015 nd nd 0.42 ± 0.14 0.43 ± 0.00 nd 0.37 ± 0.08 0.33 ± 0.06

nd is not detected.

Table 4 Protein content evaluated applying a chemical method and a microscopic technique. Sample

% Protein (Kjendhal method)a

% Protein (XPS method)

APS AG MD APS/AG APS/MD AG/MD APS/13MD/AG 13APS/MD/AG

84.17 ± 2.19 2.60 ± 0.04 nd 42.60 ± 0.30 42.73 ± 0.67 1.77 ± 0.07 14.00 ± 0.00 55.13 ± 1.17

96.06 ± 0.69 20.03 ± 0.57 nd 94.28 ± 2.46 69.64 ± 1.95 18.33 ± 1.86 61.51 ± 4.37 84.26 ± 1.62

nd is no detected. a Calculated using N  6.26.

(Fig. 1). This irregularity is due to shrinkage phenomena observed during the drying and cooling of the particles, and could be related to the collapse (Frascareli et al., 2012). In case of SPI/13MD/AG blend, the surface is modified, and the microparticles tend towards spherical form with a smooth surface. All these morphological characteristics of particles influence the powder flowability properties (Kurozawa et al., 2009). Analysing the internal structure of microparticles by the conventional process (applying mechanic force to break them), just some particles were fragmented and internal zone (vacuole) was identified. Fig. 2 illustrates images taken with CLSM and SEM of the internal microstructure of the microparticles, which can be identified as compact or hollow with a continuous matrix wall, also the internal zone of the hollow microparticles and the thickness of the wall. Vacuoles are originated during solvent evaporation of the solvent; it is due to the great partial pressure generated in the centre of the microparticle during the spray drying process (Alamilla-Beltrán et al., 2005; Mezhericher et al., 2010; Handscomb and Kraft, 2010). Paramita et al. (2010) indicate that the vacuole is formed only in microparticles with one or more gas bubbles undissolved before solidifying.

This process is divided into two phases according the classic drying theory. In the first one, the droplet heats up when contacting the hot air, and its surface remains saturated due to water diffusion from the interior to the exterior; at the end of this phase, the solutes are concentrated, the moisture content is removed almost completely and the development of the solid surface begins. In the second phase, the surface is totally formed and particle temperature is increased until reaches the end of the process. The moisture content diminishes to a minimal value, and the product is considered as dry particle (Masters, 1985; Handscomb and Kraft, 2010; Mezhericher et al., 2010; Paramita et al., 2010; Nandiyanto and Okuyama, 2011). Fig. 3 illustrates the effect of the composition on the percentage of hollow microparticles, which was found in an interval between 10% and 26%. The formulations MD/SPI and MD/AG recorded the highest percentage of hollow microparticles (26%) and the lowest percentages of compact microparticles (74%); the results of the sample SPI/AG suggested the lowest content of hollow microparticles (10%), it means that particles are not forming shell, so they are principally compacts (90%). A similar study was reported by Paramita et al. (2010), with 27% of hollow microparticles, mixtures of AG/MD (at 20% of total solids, in ration 1:2). Carneiro et al. (2013) reported different results with MD, SPI, AG, modified corn starch and tapioca starch spray dried at 180 °C; in all cases the microparticles obtained were hollow capsules with active material embedded inside the wall. In case of shells mechanically divided, the thickness of the wall of the microparticles was found between 44 and 911 nm (Table 2). The lowest wall thickness was identified in MD blend (132 nm) and the highest wideness was observed in GA blend; when MD was mixed with GA or SPI the thickness of the wall was expanded. The use of focused ion beam (FIB), coupled SEM was a useful tool to examine internal structure of microparticles. In this case, the ion beam was applied to the surface of the material to fragment it; in Fig. 3a and b, the polished microparticles (hollow and compact) are illustrated. When observing the interior of the compact microparticles at 10,000 and 40,000, the presence of pores or vacuoles was not distinguished (Fig. 3a and b); in case of hollow particles, three zones of the shell were identified: an external surface, a middle section and an internal surface (Fig. 3c and d), confirming that microparticles could be compact and hollow. A structure of shell is proposed in Fig. 3d. 3.3. Composition of microparticles The elemental atomic composition of surface microparticles was performed using XPS technique, analysing the composition of C, N, O, S; this technique allows to assess an area of 1 mm2, and a depth of 10 nm. The relative elemental composition of different wall material powders measured by XPS is are exposed in Table 3, in case of the atomic composition at the surface of the particles prepared with SPI, in addition to C, N and O atoms identified, traces of S (less than 1%) were observed. S atom is due to sulphureted amino acids (methionine and cysteine) forming the SPI. This model was developed considering aspects reported by MurrietaPazos et al. (2012). Similar results were reported by Zhao et al. (2011), in lyophilized SPI, the sulphur is only present those samples that incorporated SPI as wall material. In Table 3, N is detected in matrices in which APS and AG are incorporated, but in samples containing 100% of maltodextrin, the N was undetected. MurrietaPazos et al. (2011) reported that the N atom is indicative of the presence of proteins. To calculate protein distribution based on the nitrogen concentration on the surface of microparticles of samples prepared with APS and AG, a conversion factor of 6.25 was considered (the most of food proteins contain 16% of N). The higher protein concentration on particle surface was found in all samples containing SPI;

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J. Porras-Saavedra et al. / Journal of Food Engineering 152 (2015) 105–112 Table 5 Distribution of chemical elements (C, N and O) in three sections of the shell of hollow microparticles measured by EDX. Sample

Atomic element

Relative atomic concentration (%) External surface

Middle section

Internal surface

MD

C N O

89.93 ± 4.37a – 10.40 ± 1.73a

89.93 ± 1.44a – 10.37 ± 1.77a

90.47 ± 0.71a – 9.50 ± 0.71a

GA

C N O

63.97 ± 6.87a 21.93 ± 1.53a 14.10 ± 3.18a

59.53 ± 5.56a 28.02 ± 2.63b 12.45 ± 2.94a

66.10 ± 4.58a 23.30 ± 1.21ab 10.60 ± 3.85a

SPI

C N O

66.00 ± 1.57a 20.34 ± 0.31a 13.66 ± 1.30a

72.59 ± 6.22ab 15.95 ± 3.84a 11.46 ± 2.39ab

76.52 ± 3.08b 15.76 ± 1.75a 7.71 ± 1.58b

MD/GA

C N O

72.75 ± 2.60a 11.75 ± 0.98a 15.50 ± 2.61a

84.53 ± 8.26a 4.04 ± 1.38b 11.42 ± 5.32a

82.65 ± 1.53a 8.07 ± 2.16ab 9.29 ± 0.68a

MD/SPI

C N O

63.97 ± 0.59a 26.25 ± 0.50ab 9.57 ± 0.30a

48.91 ± 4.11b 32.35 ± 1.35b 18.75 ± 2.77b

67.43 ± 9.55a 23.74 ± 5.11a 8.82 ± 4.73a

GA/SPI

C N O

67.60 ± 1.91a 17.16 ± 2.00a 17.81 ± 0.91a

65.18 ± 1.86a 17.01 ± 2.08a 15.23 ± 0.11a

80.79 ± 4.94b 11.06 ± 2.63b 8.14 ± 3.87b

SPI/13MD/GA

C N O

66.28 ± 4.49a 16.28 ± 2.44a 17.44 ± 2.054a

81.77 ± 5.04b 8.66 ± 0.75b 10.57 ± 4.10ab

89.21 ± 4.73b 7.17 ± 2.13b 7.39 ± 2.27b

13SPI/MD/GA

C N O

67.17 ± 4.62a 19.61 ± 0.93a 13.22 ± 3.71a

75.23 ± 5.43a 11.22 ± 1.74b 13.56 ± 5.49a

75.81 ± 4.04a 12.77 ± 2.80b 10.09 ± 2.19a

Mean value of three measurements (±standard deviation). Equal letters in the same row indicate no significant differences with p < 0.05. (–) Undetected. Blends spray dried at 150 °C/70° as inlet/outlet drying air temperatures.

Fig. 4. Atomic elements distribution model showing gradients C, N and O in all sections of the shell of hollow microparticles assessed by EDX. (Case: SPI/13MD/GA blend spray dried at 150 °C/70° as inlet/outlet drying air temperatures).

these results agree with total protein content determined by Kjeldahl method (Table 4), the results of % protein contained in pure samples of SPI and AG, were 84.17 and 2.6, respectively. Similar values have been reported by Cassini et al. (2010) for APS samples and by Alftrén et al. (2012) for AG samples. Other important results are the minimal differences in the N content of the surface of the microparticles of AG and AG/MD; these results suggest the diffusion of the proteins to the surface so rapidly that they saturate it, and any increases in the protein content does not modify the surface concentration (Shrestha et al., 2007). Adhikari et al. (2009) studied the effect of whey proteins (WPI) on the surface of sucrose microparticles obtained by spray drying. The authors reported that incorporating 1% of WPI would cover up to 54% of this sugar, and using different relations of components, just minimal differences were found; this implies that, when wall materials contains protein, the surface of the droplet reaches saturation and the rest of the protein molecules cannot occupy the surface, even if the concentration in the alimentation is increased (Shrestha et al., 2007). In fact, the 0.125% nominal protein concentration in the droplet can reach equilibrium in terms of protection of the surface (Adhikari et al., 2009). Other important result was the minimal differences in the N content in particle surface of AG and AG/MD blends; the results suggest fast protein diffusion to the particle surface, so it gets saturate the external surface, after that, any increase

of protein does not affect the nitrogen content on the particle surface. A greater protein content in particle surface increases the wetting time; this could be related to the hydrophobic interactions which promote the protein–protein association and the reduction of solubility (Murrieta-Pazos et al., 2011); and also the presence of proteins in the outer shell favours the elasticity. These properties may be favourable in the case of requiring longer times for release of the microencapsulated bioactive agent. The results of EDX analysis (Table 5) proved the distribution of chemical elements (C, N and O) in three sections of the wall (or shell) of hollow microparticles, this allowed to propose an atomic elements distribution model (as example, the distribution of chemical elements of microparticles of SPI/13MD/GA blend is illustrated in Fig. 4). The greater composition of N in the external surface of microparticles of samples SPI, MD/AG, AG/SPI, SPI/13MD/AG, SPI/MD/AG was found. This facts is due to the fast diffusion of proteins to the external surface, as previously explained. Other theory was reported by Jayasundera et al. (2009) explaining that the protein adsorption process in the air–water interphase is realised in three steps: (a) rapid adsorption in the interphase, (b) unfold and reorient in the interphase and (c) capacity to interact with neighbouring molecules and to form a viscoelastic film. This fact implies that proteins form an external layer in the particle surface and depending of the interaction with other components, as well as the drying air temperature, the microparticles may suffer structural changes as expansion, collapse either keep the same size.

4. Conclusions Technological functional properties (flowability and rehydration) of blends of wall materials evaluated as function of bulk density, hygroscopicity and wetting time are acutely affected when the components relation is changed; the same properties are not affected with changes in inlet temperatures of drying air, however,

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these functional properties could be changed if the outlet temperature or the flow arrangements are modified. The experimental descriptive model proposed try to explain the possible arrangement of wall materials during microparticle development, combining the knowledge of particle microstructure and the elemental analysis of components. Currently, the combined use of FIB/SEM, CLSM, XPS and EDX represent an useful tool, which can be applied to understand the particle microstructural development process during microencapsulation by spray drying processes, impacting the design of drugs, food and biotechnological products of high quality. Although, in this work the distribution of components was proposed, the relation of wall materials and active agent has to be studied by applying microscopy techniques. Acknowledgments The authors wish to thank the Instituto Politécnico Nacional – México (National Polytechnic Institute) for the financial support provided through the SIP Project: 20140253 and 20140554, BEIFI-IPN, COFAA-IPN, CONACyT – México, Project 216044 and the scholarships for the PhD studies of Josefina Porras-Saavedra. The authors are grateful for experimental support of Instituto Mexicano del Petróleo and CNMN-IPN. References Adhikari, B., Howes, T., Wood, B.J., Bhandari, B.R., 2009. The effect of low molecular weight surfactants and proteins on surface stickiness of sucrose during powder formation through spray drying. J. Food Eng. 94, 135–143. Alamilla-Beltrán, L., Chanona-Pérez, J.J., Jiménez-Aparicio, A.R., Gutiérrez-López, G.F., 2005. Description of morphological changes of particles along spray drying. J. Food Eng. 67, 179–184. Alftrén, J., Peñarrieta, M.J., Bergenstahl, B., Nilson, L., 2012. Comparison of molecular and emulsifying properties of gum arabic and mesquite gum using asymmetrical flow field-flow fraction. Food Hydrocolloids 26, 54–62. AOAC, 2005. Official methods of analysis of AOAC Internacional, 16th ed. Gaythersburg, USA. Baranauskiené, R., Rimantas, V.P., Dewettink, K., Verdhé, R., 2006. Properties of oregano (Origanum vulgare L.), citronella (Cymbopogon nardhus G.) and marjoram (Majorama hortensis L:) flavors encapsulated into milk proteinbased matrices. Food Res. Int. 39, 413–425. Bhandari, B., 2008. Spray drying and food powder properties. In: Hui, Y.H., Clairy, C.C., Farid, M.M., Fosina, O.O., Noomhorm, A., Welti-Chanes, J. (Eds.), Food Drying Science and Technology Microbiology, Chemistry, Applications. Destech Publications Inc., USA. Bhandari, B.R., Howes, T., 1999. Implications of glass transition for the drying and stability of dried foods. J. Food Eng. 40, 71–79. Carneiro, H.C., Tonon, R.V., Grosso, C.R., Hubinger, M.D., 2013. Encapsulation efficiency and oxidative stability of flaxseed oil microencapsulated by spray drying using different combinations of wall materials. J. Food Eng. 115, 443– 451. Cassini, A.S., Tessaro, I.C., Marrczack, L.D., Pertile, C., 2010. Ultrafiltration of wastewater from isolated soy protein production: a comparison of three UF membranes. J. Clean. Prod. 18, 260–265. Cuq, B., Rondet, E., Abecassis, J., 2011. Food powders engineering, between knowhow and science: constraints, stakes and opportunities. Powder Technol. 208, 244–251. Filoková, I., Xin Huang, L., Mujumdar, A.S., 2006. Industrial spray drying systems. In: Mujumdar, A.S. (Ed.), Handbook of Industrial Drying. CRC Press, USA, pp. 215– 256. Frascareli, E.C., Silva, V.M., Tonon, R.V., Hubinger, M.D., 2012. Effect of process conditions on the microencapsulation of coffee oil by spry drying. Food Bioprod. Process. 90, 413–424. Gaiani, C., Ehrhardt, J.J., Scher, J., Hardy, J., Desobry, S., Banon, S., 2006. Surface composition of dairy powders observed by X-ray photoelectron spectroscopy and effects on their rehydration properties. Colloids Surf., B 49, 71–78. Gaiani, C., Morand, M., Sanchez, C., Arab, T.E., Jacquot, M., Schuck, P., Jeantet, R., Scher, J., 2010. How surface composition of high milk proteins powders is influenced by spray-drying temperature. Colloids Surf., B 75, 377–384. Ganesan, V., Rosentrater, K.A., Muthukumarappan, K., 2008. Flowability and handling characteristics of bulk solids and powders-a review with implications for DDGS. Biosyst. Eng. 101, 425–435. Georgetti, S.R., Casagrande, R., Fernandes, S.C., Pereira, O.W., Vieira, F.M., 2008. Spray drying of the soy bean extract: effects on chemical properties and antioxidant activity. LWT-Food Sci. Technol. 41, 1521–1527. Gharsallaoui, A., Saurel, R., Roudaut, G., Chambin, O., Volley, A., 2007. Applications of spray-drying in microencapsulation of food ingredients: an overview. Food Res. Int. 40, 1107–1121.

Guadarrama-Lezama, A.Y., Dorantes-Alvarez, L., Jaramillo-Flores, M.E., PérezAlonso, C., Niranjan, K., Gutiérrez-López, G.F., Alamilla-Beltrán, L., 2012. Preparation and characterization of non-aqueous extracts from chilli (Capsicum annuum L.) and their microencapsulates obtained by spray-drying. J. Food Eng. 112, 29–37. Hambrecht, A., Schäfer, U., Lehr, C.-M., 2000. Structural analysis of microparticles by confocal laser scanning microscopy. AAPS Pharm. Sci. Technol. 1 (3), 17. Handscomb, C.S., Kraft, M., 2010. Simulating the structural evolution of droplets following shell formation. Chem. Eng. Sci. 65, 713–725. Hartmann, M., Palzer, S., 2011. Caking of amorphous – material aspects, modelling and applications. Powder Technol. 206, 112–121. Jafari, M.S., Assadpoor, E., Bhandari, B., He, Y., 2008. Nano-particle encapsulation of fish oil by spray drying. Food Res. Int. 41, 172–183. Jayasundera, M., Adhikari, B., Aldred, P., Ghandi, A., 2009. Surface modification of spray dried food and emulsion powders with surface-active proteins: a review. J. Food Eng. 93, 266–277. Jumah, R.H., Tashtoust, B., Shaker, R.R., Zray, A.F., 2000. Manufacturing parameters and quality characteristics of spray dried jameed. Drying Technol. 18, 967–984. Kim, E.H., Chen, X.D., Pearce, D., 2003. On the mechanisms of surface formation and the surface compositions of industrial milk powders. Drying Technol. 21 (2), 265–278. Kim, E.H.-J., Chen, D.X., Pearce, D., 2009. Surface composition of industrial spraydried milk powders. 2. Effects of spray drying conditions on the surface composition. J. Food Eng. 94, 169–181. Krishnan, S., Bhosale, R., Singhal, R.S., 2005. Microencapsulation of Cardomom oleoresin: evaluation of blends of gum arabic, maltodextrin and a modified starch as wall materials. Carbohydr. Polym. 61, 95–102. Kurozawa, L.E., Park, K.J., Hubinger, D.M., 2009. Effect of carrier agents on the physicochemical properties of a spray dried chicken meat protein hydrolysate. J. Food Eng. 94, 326–333. Madene, A., Jacquot, M., Scher, J., Stéphane, D., 2006. Flavour encapsulation and controlled release – a review. Int. J. Food Sci. Technol. 41, 1–21. Masters, K., 1985. An Introduction to Principles, Operational Practice and Applications. In Spray Drying. Leonard Hill, London. Mezhericher, M., Levy, A., Borde, I., 2010. Spray drying modelling based on advanced droplet drying kinetics. Chem. Eng. Process. 49, 1205–1213. Murrieta-Pazos, I., Gaiani, C., Cuq, B., Desobry, S., Scher, J., 2011. Comparative study of particle structure evolution during water sorption: skim and whole milk powders. Colloids Surf., B 87, 1–10. Murrieta-Pazos, I., Gaiani, C., Galet, L., Scher, J., 2012. Composition gradient from surface to core in dairy powders: agglomerations effect. Food Hydrocolloids 26, 149–158. Murrieta-Pazos, I., Galet, L., Rolland, C., Scher, J., Gainai, C., 2013. Interest of energy dispersive X-ray microanalysis to characterize the surface composition of milk powder particles. Colloids Surf., B 111, 242–251. Nandiyanto, A.B.D., Okuyama, K., 2011. Progress in developing spray-drying methods for the production of controlled morphology particles: from the nanometer to submicrometer size ranges. Adv. Powder Technol. 22, 1–19. Nijdam, J.J., Langrish, T.G., 2005. An investigation of milk powders produced by a laboratory-scale spray dryer. Drying Technol. 23, 1043–1056. Niro Analytical Methods, 2009 . Palzer, S., 2011. Agglomeration of pharmaceutical, detergent, chemical and food powders – similarities and differences of materials and processes. Powder Technol. 206 (1–2), 2–17. Paramita, V., Iida, K., Yoshii, H., Furuta, T., 2010. Effect of additives on the morphology of spray-dried powder. Drying Technol. 28, 323–329. Pereira, R.H., Peixoto, K.S., Jaeger, C.L., Rodrigues, A.L., Pedrosa, C., Rocha, P.A., 2009. Legumes seeds protein isolates in the production of ascorbic acid microparticles. Food Res. Int. 42, 115–121. Quintanilla-Carvajal, M.X., Camacho-Díaz, B.H., Meráz-Torres, L.S., Chanona-Pérez, J.J., Alamilla-Beltrán, L., Jiménez-Aparicio, A., Gutiérrez-López, G., 2010. Nanoencapsulation: a new trend in food engineering processing. Food Eng. Rev. 2, 39–50. Rodríguez-Huezo, M.E., Durán-Lago, R., Prado-Barragán, L.A., Cruz-Sosa, F., LobatoCalleros, C., Alvarez-Ramírez, J., Vernon-Carter, E.J., 2007. Pre-selection of protective colloids for enhanced viability of Bifidobacterium bifidum following spray-drying and storage, and evaluation of aguamiel as thermoprotective prebiotic. Food Res. Int. 40, 1299–1306. Shrestha, A.K., Howes, T., Adhikari, B.P., Wood, B.J., Bhandari, B.R., 2007. Effect of protein concentration on the surface composition, water sorption and glass transition temperature of spray-dried skim milk powders. Food Chem. 104, 1436–1444. Thalberg, K., Lindholm, D., Axelsson, A., 2004. Comparison of different flowability tests for powders for inhalation. Powder Technol. 146, 206–213. Tonon, R.V., Bravet, C., Hubinger, M.D., 2008. Influence of process conditions on the physicochemical properties of acai (Euterpe oleraceae Mart.) powder produced by spray drying. J. Food Eng. 88, 411–418. Vehring, R., Foss, W.R., Lechuga-Ballesteros, D., 2007. Particle formation in spray drying. J. Aerosol Sci. 38, 728–746. Wang, Y., Lu, Z., Lv, F., 2009. Study on microencapsulation of curcumin pigments by spray drying. Eur. Food Res. Technol. J. 229, 391–396. Wang, Y., Schulmerich, M., Padua, G.W., 2013. Characterization of core-shell structures formed by zein. Food Hydrocolloids 30, 487–494. Zhao, X., Chen, J., Zhu, Q., Du, F., Ao, Q., Liu, J., 2011. Surface characterization of 7S and 11S globulin powders from soy protein examined by X-ray photoelectron spectroscopy and scanning electron microscopy. Colloids Surf., B 86, 260–266.