Microencapsulation by spray drying of gallic acid with nopal mucilage(Opuntia cus indica)

L. Medina-Torres a,*, E.E. Garca-Cruz b, F. Calderas a, R.F. Gonzlez Laredo c, G. Snchez-Olivares d,J.A. Gallegos-Infante c, N.E. Rocha-Guzmn c, J. Rodrguez-Ramrez b

a Facultad de Qumica, Departamento de Ingeniera Qumica, Conjunto E, Universidad Nacional Autnoma de Mxico (UNAM), Mxico, D.F. 04510, Mexicob Instituto Politcnico Nacional, CIIDIR-IPN-Oaxaca, Hornos No.1003, Santa Cruz Xoxocotln, Oaxaca 71230, MexicocDepartamento de Ing. Qumica y Bioqumica, Instituto Tecnolgico de Durango., Blvd. Felipe Pescador 1830 Ote., 34080 Durango, Dgo., MexicodCIATEC, A.C. Omega 201, Fracc. Industrial Delta, CP 37545, Len, Gto, Mexico

a r t i c l e i n f o

Article history:

Received 7 March 2012

Received in revised form

17 July 2012

Accepted 24 July 2012

Keywords:

Nopal mucilage

Rheological behavior

Bioactive compounds

Gallic acid

Spray drying

a b s t r a c t

The spray-drying process has been previously used to encapsulate food ingredients such as antioxidants.

Thus the objective of this work was to produce microcapsules of gallic acid, a phenolic compound that

acts as antioxidant, by spray drying with an aqueous extract of nopal mucilage (O), which acted as an

encapsulating agent. The rheological response and the particle size distribution of the nal solutions

containing gallic acid at concentrations of 6 g/100 mL were characterized along with the control sample,

no gallic acid added, to elucidate the degree of encapsulation. The drying parameters to prepare the

microcapsules with extract of nopal mucilage were: inlet air temperature (130 and 170 C) and speed

atomization (14,000 and 20,000 rpm). The rehydrated biopolymer showed a non-Newtonian pseudo-

plastic behavior. The Cross Model was used to model the rheological data. Values for m varied between

0.55 and 0.85, and for time characteristic, l, the range was between 0.0071 and 0.021 s. The mechanical

spectra showed that the sample with gallic acid was stable long term (>2 days) and presented a bimodal

particle size distribution. This study demonstrated the effectiveness of nopal mucilage when utilized as

wall biomaterial in microencapsulation of gallic acid by the spray-drying process.

2012 Elsevier Ltd. All rights reserved.

1. Introduction

Polyphenols are chemical compounds or phytochemicals with

diverse biological activities due to their antioxidant capacity.

Ingestion of polyphenol-rich foods should be benecial to human

health as factors associated with cardiac mortality in developed

countries with particular reference to the consumption of wine (St.

Leger, Cochrane, & Moore, 1979). Wine has antimicrobial and

antifungal activity and may play a role in the etiology of migraine.

Red wine may even protect against the common cold. Wine

contains polyphenols from the avonoid type, mostly as grape

tannins (about 35 g/100 g) and anthocyanin pigments (about 20 g/

100 g), not only present mostly in red rather than in white grapes

(Takkouche et al., 2002), but also non-avonoid phenolics such as

stilbenes and gallic acid. Gallic acid (acid 3,4,5-tri-hydroxy-ben-

zoic) and its derivatives are considered natural antioxidants and

their effects and uses have been widely reported (Cho, Kim, Ahn, &

Je, 2011; Pasanphan & Chirachanchai, 2008; Negi et al., 2005).

Stabilization and application of polyphenols in foods and nutra-

ceutical formulations can be improved by microencapsulation

technologies (Senz, Tapia, Chvez, & Robert, 2009). Microencap-

sulation allows protection of bioactive compounds; i.e., an active

material (nucleus) is embedded in a polymer matrix (encapsulating

agent or wall material) to act as a protective barrier against external

or environmental factors (Ahmed, Akter, Lee, & Eun, 2010;

Borgogna, Bellich, Zorzin, Lapasin, & Cesro, 2010; Senz et al.,

2009).

Spray drying is a common technique for producing encapsulated

food materials (Senz et al., 2009). Good microencapsulation ef-

ciency during spray drying is achievedwhen themaximum amount

of core material is encapsulated inside the powder particles, suc-

ceeding in microcapsule stability, volatile losses prevention, and

product shelf-life extension (Seid, Elham, Bhesh, & Yinghe, 2008).

In spray drying, the operating conditions and the dryer design used

depend on the characteristics of the material to be dried and

the desired powder specications (Len Martnez, Mndez, &

Rodrguez, 2010). Studying the effect of operating parameters

* Corresponding author. Tel.: (52) 55 56225360/59703815; fax: 52 55

56225329.

E-mail address: luismt@unam.mx (L. Medina-Torres).

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http://dx.doi.org/10.1016/j.lwt.2012.07.038

LWT - Food Science and Technology 50 (2013) 642e650

on the physical properties of powder helps to identify the optimum

operating conditions of spray dryers and their effect on powder

characteristics (Chegini & Ghobadian, 2007; Wang, Lu, Lv, & Bie,

2009). The main factors in spray drying that must be optimized

are feed temperature, air inlet temperature, and air outlet

temperature (Liu, Zhou, Zeng, & Ouyang, 2004; Wang et al., 2009).

Feed temperature modies the viscosity of the emulsion and thus,

its capacity to be homogenously sprayed. When the feed temper-

ature is increased, viscosity and droplets size should be decreased

but high temperatures can cause volatilization or degradation of

some heat-sensitive ingredients. The rate of feed delivered to the

atomizer is adjusted to ensure that each sprayed droplet reaches

the desired drying level before it comes in contact with the surface

of the drying chamber (Zbicinski, Delag, Strumillo, & Adamiec,

2002). Inlet air temperature is determined by the temperature

that can be used safely without damaging the product or creating

operational risks, and comparative costs of heat. Air inlet temper-

ature is directly proportional to the microcapsule drying rate and

the nal water content. An air inlet temperature low causes a low

evaporation rate, the formation of microcapsules with high density

membranes, high water content, poor uidity, and easiness of

agglomeration. However, a high air inlet temperature causes an

excessive evaporation and results in cracks in the membrane

inducing subsequent premature release and a degradation of

encapsulated ingredient or loss of volatiles (Zakarian & King, 1982).

The temperature at the end of the drying zone or air outlet

temperature can be considered as the control parameter of the

dryer. The outlet temperature depends on inlet temperature, and it

has been reported to vary from 50 to 80 C for the microencapsu-

lation of food ingredients with phenolic compounds such as green

tea (Fang & Bhandari, 2010; Gharsallaoui, Roudaut, Chambin,

Voilley, & Saurel, 2007).

For encapsulation purposes, modied starch, maltodextrin, gum

or other substances are hydrated to be used as the wall materials.

Maltodextrins has been used to encapsulate extracts of black

carrots, which contain anthocyanins (Ersus & Yurdagel, 2007);

maltodextrin-gum arabic has been used for procyanidins from

extract grape seeds (Zhang, Mou, & Du, 2007); chitosan has been

used as a wall material in spray drying for olive leaf extract

(Kosaraju, Dath, & Lawrence, 2006); Chiou and Langrish (2007)

used citrus fruit ber as an encapsulating agent for anthocyanin

complexes extracted from Hibiscus sabdariffa L.; colloidal silicon

dioxideemaltodextrinestarch for soybean extract (Georgetti,

Casagrande, Souza, Oliveira, & Fonseca, 2008); another wall

material used for encapsulation of polyphenol was sodium

caseinateesoy lecithin emulsion, which has been used in spray

drying for grape seed extract, apple polyphenol extract and olive

leaf extract (Kosaraju, Labbett, Emin, Konczak, & Lundin, 2008). The

mucilage from Opuntia cus indica is an interesting and promising

alternative due to its emulsifying properties (Medina Torres, Brito

De La Fuente, Torrestiana Snchez, & Katthain, 2000). It is used as

an additive in the food industry, specically as an edible coating to

extend the shelf life of food products (Del Valle, Hernndez Muoz,

& Galotto, 2005). Previous studies have shown that chemical

composition of O. cus-indica mucilage is a complex mixture of

polysaccharides such as L-arabinose, D-galactose, D-xylose and L-

rhamnose, and D-galacturonic acid, which represent up to 10 g/

100 g of total sugars (Medina Torres et al., 2000; Senz, Seplveda,

& Matsuhiro, 2004). Multiple applications have been developed for

this material ranging from a thickener of foods to a turbidity

remover in contaminated water. The usefulness of this hetero-

polysaccharide of high molecular weight (2.3 104 g/mol) relies on

its physicochemical properties, which have been described by

many research groups; emphasizing its electrolyte thickener

capacity and its ow characteristics (Crdenas, Higuera Ciapara, &

Goycoolea, 1997; Medina Torres et al., 2000). High moisture

content in the mucilage limits its applications, generating the need

for previous treatments such as spray drying (SD) to increase its

potential uses. The rheological properties of food products sub-

jected to SD are important and can be used in quality control,

storage and processing, stability measurements, and the nal

texture prediction of the dehydrated product. Rheological studies

are useful, especially when related to the mechanical response and

to the micro-structure of the materials (Abu-Jdayil, Banat, Jumah,

Al-Asheh, & Hammad, 2004).

In the present work, an antioxidant compound (gallic acid) was

encapsulated using aqueous extracts from O. cus-indica (O)

mucilage as wall material by spray drying; the thermal (differential

scanning calorimetry, DSC) and scanning electron microscopy

(SEM) analysis were used to evaluate the effectiveness of wall

material as an encapsulating agent. The response through DSC

coupled and the release of the microcapsules was evaluated as to

assess the feasibility of this encapsulating process. The biome-

chanical response in simple shear and oscillatory ow and the

particle size distribution (PSD) were evaluated as quality measure

of microcapsulation. The microcapsules obtained represent an

interesting option for incorporation of food antioxidants and

additives into functional foods applications.

2. Materials and methods

2.1. Materials

Cladodes of nopal (O. cus-indica) (O) of approximately 6-

month-old plants, with 92.21 g/100 g dry solid, moisture content,

were collected randomly from the same plantation (August 2010),

at Milpa Alta, Mexico City. The moisture content of fresh cladodes

was determined with an infrared balance (AND, model AD-4714A).

Cladodes were washed thoroughly with water at 25 C, using

a plastic bristle brush. Cladodes were macerated (500mL deionized

water per kg of material) to facilitate the extraction of mucilage.

The material was let to stand 24 h, and the solid material separated

by decantation. Extract was ltered to 149 mm pore size, and the

remaining ne particles were separated with nylon canvas and

using a centrifuge Dinacclay at 11,000g for 15 min (Medina Torres

et al., 2000). This will be called mucilage extract from here on.

Mucilage extract was stored in refrigeration containers at 4 C and

its Brix measured using a manual refractometer with temperature

compensation (WestoverRHB-32ATC model).

2.2. Aqueous dispersion previous to the spray-drying process

The mucilage extract was diluted in deionized water at 1 Brix,

due to its initial high viscosity and solids content it was not possible

to spray drying. The encapsulated samples were prepared with

0.3 g of gallic acid/L mucilage extract (E1eE5) to analyze the effect

of spray drying. All samples were homogenized at constant stirring

for 30 min using a mechanical shaker.

2.3. Spray drying

A pilot scale spray dryer with co-current ow Niro atomizer

(Production Minor Spray Dryer, Niro Inc., Denmark) (Niro, Copen-

hagen, Denmark), equipped with rotary atomizer (TS-Minor, M02/

A) was used to spray drying. Distilled water at room temperature

(25 C) was used to stabilize the equipment. The mucilage extract

was fed into the drying chamber using a peristaltic pump

(WatsoneMarlow 505S/RL). A 22 factorial design was used to

evaluate the effect of the independent variables: inlet air temper-

ature (130 and 170 C), and atomizer speed (20,000 and

L. Medina-Torres et al. / LWT - Food Science and Technology 50 (2013) 642e650 643

14,000 rpm) on the encapsulated properties of samples E1eE4

(Table 1). The drying conditions for control samples (with no

gallic acid added) B1 were 130 C and 14,000 rpm, while for B2,

170 C and 20,000 rpm. Samples E5 and B3 (control) were double

processed (dried, reconstituted and dried once again) at 130 C and

14,000 rpm. All the experiments were carried out in duplicate.

2.4. Reconstituted solutions of control and encapsulated samples

Reconstituted solutions at a concentration of 6 g/100 mL were

used for all rheological measurements, at this concentration could

make out clearly the effect of the variables; the powders were

dispersed in deionized water (pH w5.6), using a magnetic stirrer

(Lighting mark) at 500 rpm for 2 h at 25 C. The pH of the solution

was taken with a Thermo Orion 420 Apotentiometer Plus.

2.5. Rheological measurements

Rheological characterization was performed in simple and

oscillatory shear ow, using a controlled stress rheometer (Model

AR-G2 TA Instruments) with the concentric cylinders geometry

(21.96 mm outer cylinder diameter, 20.38 mm inner cylinder

diameter, 59.50 mm height, and 500 mm gap), maintaining

a constant temperature (25 C) with a circulatory water bath (Cole

Parmer Polystat and a Peltier AR-G2).

2.5.1. Steady-shear viscosity measurements

Steady-shear viscosity measurements were monitored as

a function of increasing shear rate h _g over the range 0.1e300 s1.

Experimental data were adjusted properly to the Cross model,

expressed in the Eq. (1)

h hN

h0 hN

1

1 l _gm (1)

where h is the shear viscosity (Pa s), g is the shear rate (s1), l is

a relaxation time (s),m is the dimensionless index ow [m 1 n],

and hN and h0 are the limit viscosities at high and low shear rates,

respectively (Kirkwood & Ward, 2008).

2.5.2. Activation energy at shear rate ow

Viscosity-temperature dependence was observed from 25 to

45 C at a constant shear rate of 10 s1, and data were adjusted to

the Arrhenius equation (Eq. (2)) (Medina Torres et al., 2000; Sengl,

Ertugay, & Sengl, 2005)

h Aexp

Ea

R

1

T

(2)

2.5.3. Steady oscillatory ow measurements

The viscoelastic properties, storage modulus (G0) and loss

modulus (G00) were determined through small amplitude oscil-

latory shear ow experiments at frequencies ranging from 1 to

100 rad s1. Prior to any dynamic experiments, a strain sweep

test at a constant frequency of 10 Hz was performed, xing the

upper limit of the linear viscoelastic zone at a strain value of 30%

(which was used in all dynamic tests). All rheological measure-

ments were carried out in duplicate. The experimental rheo-

logical data were obtained and analyzed directly from the TA

Rheology Advantage Data Analysis software V.5.7.0 (TA Instru-

ment Ltd., Crawley, UK).

2.6. Particle size distribution of resuspended solutions (PSD)

Particle size distributions (PSD) of samples (6 g/100 mL) were

quantied with a Master-sizer 2000 laser diffraction particle

analyzer (Malvern Instrument Ltd, UK). The dispersant was

deionized water (particle R.I. 1.336, and dispersant R.I. 1.33).

2.7. Differential scanning calorimetry (DSC)

DSC analysis was performed in a DSC-7 calorimeter (Perkin

Elmer, Norwalk, CT, E.U.A.), previously calibrated with Indium

(melting temperature 156.6 C, melting heat 28.45 J/g) and

equipped with Perkin Elmer DSC pan cells No. 02190062. An empty

pan was used as reference to develop the baseline from 20 to

140 C. The sample (18 0.6 mg) previously weighted in aluminum

pans, was initially heated to 80 C for 30 min in the corresponding

thermocell of the DSC. In all stages the heating rate used was 5 C/

min. Temperatures for the different transitions (i.e., the onset

temperature, T0; peak temperature, Tp; ending temperature, Te)

were determined using the rst derivative of the heat capacity

calculated from the DSC program library and by comparison to the

baseline.

2.8. Scanning electron microscopy (SEM)

Detailed sample preparation for SEM measurements has been

described elsewhere (Medina Torres, Brito De-La Fuente, Gmez

Aldapa, Aragn Pia, & Toro Vzquez, 2006). Essentially, the sample

was placed onto an aluminum slide using electrically conductive

tape (Bal-Tec, Frstentum Liechtenstein), and coated with gold at

10 mbar for 90 s (Polaron SC-7610, Fisson Instruments, CA, USA).

The images were obtained with an electron microscope Leica

Stereoscan S420i (Cambridge, England).

2.9. Controlled release of gallic acid from microcapsule

The release of the microcapsules of sample E5 was carried out in

Franz cells with a membrane of 0.22 mm HV in a water bath (37 C)

with constant stirring (300 rpm). The sample was resuspended to

6 g/100 mL in buffer at pH 5 in order to try to simulate the condi-

tions of the intestinal tract. The concentration of gallic acid liber-

ated was used in a spectrophotometer at an intensity range of

1.0e0.8 counts (Sez, Hernez, & Lpez, 2003). The calibration

curve of gallic acid ranged from 5.5 105 to 5.0 106. The

sampling was carried out with 2 mL of the receiving cell (Franz

cells) which were recovered with the solvent (buffer pH 5). Results

were analyzed at a wavelength of 273.71 nm (Ferk et al., 2011).

Dilutions were performed 0.05 g/mL. Measurements were per-

formed at least two times for accuracy.

3. Results and discussion

3.1. Effects of spray-drying conditions on steady-shear rate ow

The effect of each SD factor on the viscous behavior was studied

graphically, comparing the samples with one degree of freedom

Table 1

Samples drying conditions.

Treatment Ti (C) Sa (rpm)

E1 130 14000

E2 130 20000

E3 170 14000

E4 170 20000

E5 130 14000

B1 130 14000

B2 170 20000

B3 130 14000

L. Medina-Torres et al. / LWT - Food Science and Technology 50 (2013) 642e650644

(L 1), i.e., temperature, pressure or rotor speed. Drying temper-

ature and speed atomizer were found as the factors that inuence

the most on the sample viscous behavior.

3.1.1. Effect of inlet air temperature

Drying inlet air temperature (Ti) was shown to affect the

viscosity of reconstituted samples, by increasing Ti, the viscous (h0)

response at low shear rates _g < 10 s1 was found to decrease as

shown in Fig. 1(A), which shows ow curves from samples B1, E1

and E3 (microcapsules of mucilage with 0.3 g/L gallic acid). The

viscous response of B1 is the highest of all; h in E1 is greater than in

E3. The effect of Ti may be attributed to material thermal degra-

dation when exposed to high temperature, as Kha, Nguyen, and

Roach (2010) reported previously, stating that an increase in inlet

drying temperature results in thermal degradation and oxidation.

The evidence is the fact that the shape of the ow curves does not

essentially change, at high shear rates _g > 10 s1 all acquire the

same shear thickening slope and most of the curves overlap except

for E2. Spray drying (SD) may be causing the decrease in viscosity

due to thermal effects and shear stress experienced by the uid

since typical shear rates for spray drying range from 103 to 104 s1

(Barnes, 2000). McGarvie and Parolis (1981) and Medina Torres

et al. (2000) reported molecular effects due to partial hydrolysis

caused by thermal effects and the pH in the O mucilage

components, generating a higher concentration of galacturonic

acid, causing a structural reconguration. Abu-Jdayil et al. (2004)

observed that thermal effect alters structure of pectic substances

mainly by hydrolysis. Pectic and other carbohydrate polymers can

be largely hydrolyzed by heat resulting in smaller molecules. High

temperature (>170 C) has been reported to cause thermal degra-

dation of the mucilage molecular structure and a low viscosity

(Len Martnez, Rodrguez Ramrez, Medina Torres, Mndez

Lagunas, & Bernad Bernad, 2011). The effect of thermal degrada-

tion on viscosity was not observed for samples prepared at high

spray speed (20,000 rpm): E2 (Ti 130C) and E4 (Ti 170

C). In

fact, E2 showed the lowest viscosity of all samples and does not

overlap at high shear rates. In this case, the PSD had a dominant

effect on the sample viscosity, samples E2 and E4 showed the most

dispersed PSD (broadest distribution) of all samples, with PSD of E2

broader than for E4 which directly affected viscosity, this is also an

evidence of poor encapsulating effect for this sample, so

20,000 rpmwas considered as a non favorable process condition for

encapsulation purposes.

3.1.2. Effect of air pressure (rotor speed)

Viscous response was also found to be affected by spray speed

(Sa), where a high speed (20,000 rpm) caused the uid fed into the

dryer to exhibit a decrease in viscosity at low shear rates (h0) as

shown by the ow curves of B1, E1 and E2 samples on Fig. 1(A).

Again, the viscosity of the control sample is the highest (B) of all

and the viscosity of sample E1 (14,000 rpm) is higher than that of

E2 (20,000 rpm), this effect was not observed for samples at high

inlet temperature (E3 and E4, 170 C) which is, attributed to

differences in PSD. Theoretically, rotor speed is proportional to the

particle size distribution of mucilage powders obtained. This means

that at a higher fragmentation rate, the greater the contact surface

between the drop and hot air, the thinner and more porous parti-

cles are obtained by the incorporation of air with lower moisture

content. A similar effect was studied on O mucilage (Len

Martnez et al., 2011) and spray drying of milk (Walton &

Mumford, 1999), where higher spray pressure decreases particle

size. Hill and Carrington (2006) suggest that viscosity increases due

to the presence of very small particles, causing more

particleeparticle interactions and increasing the ow resistance,

especially at higher shear rates, of course this is also affected by

particle concentration and has to be considered carefully.

3.1.3. Activation energy at shear rate ow

Inuence of temperature from 25 to 45 C on viscous response

of encapsulated samples and control samples at concentration 6 g/

100 mL are shown on Table 2, where Ea represent the activation

energy, A is a factor of Arrhenius equation and R2 is the square of

the correlation coefcient. The viscosity of liquids generally

decreases as temperature increases (Sengl et al., 2005). This

relationship can be represented by the Arrhenius equation, where

high activation energy (Ea), indicates a more rapid change in

A

B

Fig. 1. Effect of spray-drying conditions on: A) Viscosity curves and B) Storage (G0) and

loss (G00) modulus. (C B1, ; B2, - E1, A E2, :E3, E4) // Cross Model. Filled

symbols are G0 , blank symbols represent G00 .

Table 2

Arrhenius equation parameters for encapsulated and control samples.

Treatment Ea (kcal/mol) A (Pa s) 105 R2

E1 1.068 3.2 0.978

E2 1.124 1.8 0.982

E3 1.156 3.1 0.978

E4 1.130 2.1 0.980

E5 1.095 2.0 0.978

B1 1.056 3.8 0.975

B2 1.091 2.9 0.994

B3 1.083 1.1 0.997

L. Medina-Torres et al. / LWT - Food Science and Technology 50 (2013) 642e650 645

viscosity with temperature. Hassan and Hobani (1998) have re-

ported that the intermolecular forces and wateresolute (inter-

phase) interactions restrict the molecular motion and inuence the

viscosity of a solution. Therefore, as temperature increases, the

thermal energy of the molecules increases and the intermolecular

distances raise as a result of thermal expansion (Koocheki,

Mortazavi, Shahidi, Razavi, & Taherian, 2009).

The Ea of encapsulated samples E1 and E3 (1400 rpm) is lower

than their corresponding samples E2 and E4 (2000 rpm) this

indicates a higher stability with temperature for samples prepared

at low rotor speeds and is an evidence of the impact of high shear

rates on the sample, this also indicates that at the conditions

studied rotor speed has more inuence on sample integrity than

the temperature. This also holds for control samples with Ea for

sample B1 being lower than that for sample B2, while the effect of

temperature cannot be distinguished here. Ea from control samples

showed a similar trend to the 1.16 kcal/mol reported previously for

O mucilage at 5 g/100 mL (Medina Torres et al., 2000).

3.2. Oscillatory shear curves on spray-drying conditions

The effect of drying conditions in storage (G0) and loss (G00)

dynamic modulus, as a function of oscillating frequency of samples

prepared with mucilage extract (B1, E1eE4), is presented in

Fig. 1(B). This data has been reported to be characteristic of the

random coil conguration of polymeric networks (Medina Torres

et al., 2000). As shown on Fig. 1(B), at low inlet temperature (Ti,

at 130 C), magnitudes G0 and G00 increase, while a decrease is

observed with the addition of Gallic acid. However, for sample E2

there is a slight solid-like response (G0 tending to be frequency

independent while being lower than G00), implying a higher phys-

ical interaction of components (mucilage extract-gallic acid) and in

principle, a more stable matrix. The solid-like response has been

reported elsewhere to conrm strong polymer matrix-disperse

phase interactions (Medina Torres, Calderas, Gallegos Infante,

Gonzlez Laredo, & Rocha Guzmn, 2009). Len Martnez et al.

(2011) suggest a similar effect of spray-dried O mucilage due to

partial hydrolysis of mucilage pectin chains.

Fig. 1(B) shows the evolution of G0 and G00 modulus showing the

effect of spray speeds (Sa) maintaining a constant temperature

(130 C). Its evident that the structure response modies (G0

decreases) as Sa increases. The encapsulated and dried sample at

20,000 rpm shows a similar trend, more stable at longer times, and

magnitude G0 is higher at lower frequency rates ( G0). Viscoelastic behavior

of a biopolymer mixture (sodium alginate and hydroxypropyl

methyl cellulose, HPMC) used as excipient was reported with G00

values larger than G0 (Borgogna et al., 2010) as observed in this

study.

3.3. Analysis of simple shear and oscillatory curves at the optimal

drying conditions

3.3.1. Analysis of simple shear curves

Simple shear ow curves [h vs _g] of double processedmucilage

samples: E5 and control (B3) have shown a non-Newtonian shear-

thinning type (n < 1) behavior (Fig. 2(A)). Shear-thinning behavior

is the resulting orientation effect of large polymer chains aligned to

the ow direction caused by the shearing rate, showing less

interaction between adjacent chains and thus viscosity decreases.

This behavior is typical of these macromolecules and has been

previously reported (Medina Torres et al., 2000; Orozco, Daz, &

Garca, 2007). At concentrations of 6 g/100 mL and lower shear

rates (

represent macromolecular polysaccharide solutions with a random

coil conguration similar to galactomannan and some other gelling

polysaccharides such as dextran, l-carrageenan, and cellulose

derivatives (Morris, Cutler, Ross-Murphy, & Rees, 1981), for O

mucilage (Medina Torres et al., 2000), and Alyssum homolocarpum

mucilage (Koocheki et al., 2009). Table 3 shows that viscosity (h0) at

lower shear rates ( G0. In

this case (E5), the interaction between mucilage and gallic acid

increases, and such change inuences the elastic modulus (G0), this

effect is more evident at low frequencies (i.e., tendency to solid-like

behavior). This was similar to a report for a gel system of sodium

alginate and HPMC (Borgogna et al., 2010).

3.3.3. Analysis of shear rate and oscillatory tests curves using the

CoxeMerz relationship

Applicability of the CoxeMerz relationship was investigated for

encapsulated E5 and control B3 samples. Fig. 2(C) shows the

CoxeMerz rule for double processed samples: control (B3) and

encapsulate (E5), where the relationship h*h h holds for low shear

rates (

phase transition. Glass transition temperature (Tg) was taken at the

midpoint of the glass transition zone. The samples used as control

had a Tg of 48C, this value increased upon the addition of gallic

acid to 60 C. Len Martnez et al. (2010) reported a Tg for Opuntia

mucilage of 45 C, which closely resembles the value estimated in

this study. Gonzlez Campos, Prokhorov, Luna Brcenas, Fonseca

Garca, and Snchez (2009) reported a transition temperature of

51 C for chitin and 59 C for chitosan. This value is associated with

the extensive characteristic hydrogen bonding for polysaccharides

and polypeptides, signicant thermal disruption of H-bonding, and

the onset of main chain molecular motions, which are probably

closely related (Gonzlez Campos et al., 2009). The results of this

study also suggest that the mucilage encapsulation increases the

transition temperature. The latter effect is associated to the pres-

ence of encapsulated gallic acid, which somehow restricts molec-

ular chain mobility, possibly by the creation of encapsulated

structures where gallic acid is surrounded bymucilage components

and thus reducing mobility while enhancing the rheological

properties and PSD. This is in agreement with Senz et al. (2009),

who observed that mucilage gum showed afnity to encapsulate

different bioactive composites. This behavior is associated with

stickiness, which reduces performance because material adheres to

the drier chamber. However, the outlet temperatures from spray

drier were observed between 77 and 86 C for the encapsulated and

control samples, respectively, therefore the encapsulated product

cannot have a rubbery behavior output under these conditions.

Chiou and Langrish (2007) explained that spray drying produces

mainly amorphous products which, when heated above Tg, become

a gummy and sticky material. This transformation usually occurs at

20 C above Tg. These results suggest that encapsulated products

should be stored at room temperature and in dry conditions to

maintain the moisture content (less than 10 g/100 g dry solid) due

to the hydrophilic characteristic of mucilage. Water sorption in

polysaccharides is usually a non ideal process leading to plastici-

zation, the presence of water increases the amount of hydrogen

bonds producing an increase in cooperative motion. The mucilage

may be readily hydrated, forming macromolecules with rather

disordered structures (Gonzlez Campos et al., 2009). DSC analysis

suggests that the thermal degradation starts above 140 C.

Gonzlez Campos et al. (2009) reported that the thermal degra-

dation process of chitosan (w170 C) can occur by a pyrolysis of

polysaccharides, which starts by a random split of the glucosidic

bonds, followed by a further decomposition.

3.6. Scanning electron microscopy

Scanning electron microphotographs for the O (B3) and gallic

acid (E5) systems were evaluated at two water activities

(0.2 < aw < 0.4) shown in Fig. 4. The microphotographs 4a and 4b

clearly show themucilagemicrocapsules alone, andwith the added

gallic acid, respectively. The morphology of microcapsules with

encapsulating agents was irregularly spherical in shape with an

extensively dented surface. The formation of these dented surfaces

on spray-dried particles was attributed to the shrinkage of the

Temperature (C)20 40 60 80 100 120

Hea

t fl

ow

, (J

/g)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

Fig. 3. Heat ow vs temperature of double processed samples, C B3, 7 E5.

Fig. 4. Micrographs of control single processed sample B1 and double processed sample B5 at 1000, aw 0.2 (A and C), hydrated aw 0.4 (B and D). Samples of treatment B1 are A

and B, to E5, C and D.

L. Medina-Torres et al. / LWT - Food Science and Technology 50 (2013) 642e650648

particles during the drying process. Similar morphology was

observed in microcapsules of cactus pear cultivars (Opuntia lasia-

cantha) pigments with maltodextrin (Daz, Santos, Kerstupp,

Villagmez, & Scheivar, 2006), Amaranthus using maltodextrin of

different dextrose equivalents (Cai & Corke, 2000), and b-carotene,

using modied tapioca starch and maltodextrin as encapsulating

agents (Loksuwan, 2007). Nevertheless, smooth spheres have

primarily been observed in microcapsules of black carrot pigments

(Daucuscarota L.) with maltodextrin (Ersus & Yurdagel, 2007).

Gharsallaoui et al. (2007) mentioned that changes in morphology

are related to inlet temperature during the drying process. The

microphotographs of the mucilage presented a macromolecular

dispersion that became less agglomerated by the addition of gallic

acid. The intermolecular mucilageegallic acid interactions become

favorable and thusly, considerably reduce the size of the aggregates.

The results conrm the encapsulation by mucilage. It is interesting

to note here that the chemical compositions of Opuntia mucilage

have been described by several research groups (Senz et al., 2009).

On the other hand, in the early 2000s, some authors presented

evidence on the neutral character of the mucilage, but more recent

reports have shown that it has acidic residues and, therefore,

polyelectrolyte behavior (Medina Torres et al., 2000).

3.7. Controlled release of gallic acid

The release of the microcapsules of mucilage with gallic acid

was performed in Franz cells with a 0.22 mm membrane. The

sample E5 was double processed at 6 g/100 mL in buffer pH 5 in

order to try to simulate the conditions of the intestinal tract (Ferk

et al., 2011; Sez et al., 2003). The results were analyzed at

a wavelength of 273.71 nm. The controlled release is designed with

the conditions of the small intestine as it is the place where gallic

acid is absorbed (Sez et al., 2003). Sample follow-up was per-

formed until no signal in the spectrum was shown. However, after

3.3 days there was still an increase in the signal spectrum respect to

mucilage without gallic acid. Fig. 5 shows the controlled release of

gallic acid, which indicates that 65% is released in 2.47 days, the

microcapsules showed high efciency (>60%) using mucilage gum

this is attributed to microencapsulation conditions, which showed

quasi-modal particle size and are, in principle, more stable (Sez

et al., 2003).

New ndings suggest that mucilage may have both neutral and

acidic fractions depending on the extraction method used. The

previously stated hypothesis, that mucilage might encapsulate

gallic acid and that the interaction is only controlled by the elec-

trostatic charge of the mucilage, is then supported by these results.

Subsequently, the co-existence of two types of micro-structure was

conrmed by SEM and controlled release of gallic acid, this is

attributed to microencapsulation conditions (Walton & Mumford,

1999).

4. Conclusions

This study has shown that using spray drying to process O. cus-

indica mucilage extract produces a stable powder with small

particle size and, consequently higher viscosity, while also exhib-

iting higher resistance to ow, mainly due to encapsulated struc-

tures. Moreover, the viscous modulus G00 predominates over the

elastic modulus G0 for the spray-dried samples at concentrations

6 g/100 mL, and showed in some cases solid-like qualities,

indicating a strong biopolymeregallic acid interaction. Thus, the

viscosity and viscoelastic properties (G0 and G00) were signicantly

affected by high inlet air temperatures and behavior, under steady

ow for all systems, was non-Newtonian shear thinning (n < 1).

This study showed that samples can achieve stability during

storage and subsequent usewith extract aqueous of mucilage, dried

at 130 C and 14,000 rpm, as well as samples dried at 130 C and

20,000 rpm. The rheological properties were affected inversely by

the increase in inlet temperature and the atomizer speed, and

directly by the increase of feed ow rate.

The nopal mucilage microcapsules described in this study

represent a promising food additive for incorporation into func-

tional foods (gallic acid). The DSC analysis conrmed this with

activation energy and glass transition temperature results. Based

on the calorimetric and SEM data obtained, it is proposed that

nopal mucilage serves as an effective encapsulating agent on

bioactive functional foods, providing additional structure.

Finally, the liberation proles of encapsulating principles for

these systems are still under evaluation and it is observed that the

cactus mucilage has a good ability to encapsulate, however more

study and research is recommended over a longer and continuous

time period in order to obtain more reliable and conclusive results.

This study conditions may serve to understand properties of

encapsulated active ingredients (i.e., gallic acid) and their stability,

as well as serve as a precedent for future investigations on drying

yields and encapsulation efciency.

Acknowledgments

The authors would like to acknowledge the support received by

Ivan Puente-Lee (Laboratorio de Microscopa, USAI, Facultad de

Qumica, UNAM., Mexico., D.F.).

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Microencapsulation by spray drying of gallic acid with nopal mucilage (Opuntia ficus indica)1. Introduction2. Materials and methods2.1. Materials2.2. Aqueous dispersion previous to the spray-drying process2.3. Spray drying2.4. Reconstituted solutions of control and encapsulated samples2.5. Rheological measurements2.5.1. Steady-shear viscosity measurements2.5.2. Activation energy at shear rate flow2.5.3. Steady oscillatory flow measurements

2.6. Particle size distribution of resuspended solutions (PSD)2.7. Differential scanning calorimetry (DSC)2.8. Scanning electron microscopy (SEM)2.9. Controlled release of gallic acid from microcapsule

3. Results and discussion3.1. Effects of spray-drying conditions on steady-shear rate flow3.1.1. Effect of inlet air temperature3.1.2. Effect of air pressure (rotor speed)3.1.3. Activation energy at shear rate flow

3.2. Oscillatory shear curves on spray-drying conditions3.3. Analysis of simple shear and oscillatory curves at the optimal drying conditions3.3.1. Analysis of simple shear curves3.3.2. Analysis of the linear viscoelastic data3.3.3. Analysis of shear rate and oscillatory tests curves using the CoxMerz relationship

3.4. Particle size distribution (PSD)3.5. Differential scanning calorimetry (DSC)3.6. Scanning electron microscopy3.7. Controlled release of gallic acid

4. ConclusionsAcknowledgmentsReferences