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Accepted Manuscript
Evaluation of the oxidative stability of chia oil-loaded microparticles by thermal,
spectroscopic and chemometric methods
Alexandre Guimarães-Inácio, Cristhian Rafael Lopes Francisco, Valquíria Maeda
Rojas, Roberta Souza Leone, Patrícia Valderrama, Evandro Bona, Fernanda Vitória
Leimann, Ailey Aparecia Coelho Tanamati, Odinei Hess Gonçalves
PII:
S0023-6438(17)30706-5
DOI:
10.1016/j.lwt.2017.09.031
Reference:
YFSTL 6546
To appear in:
LWT - Food Science and Technology
Received Date: 11 April 2017
Revised Date:
15 September 2017
Accepted Date: 18 September 2017
Please cite this article as: Guimarães-Inácio, A., Francisco, C.R.L., Rojas, Valquí.Maeda., Leone, R.S.,
Valderrama, Patrí., Bona, E., Leimann, Fernanda.Vitó., Tanamati, A.A.C., Gonçalves, O.H., Evaluation
of the oxidative stability of chia oil-loaded microparticles by thermal, spectroscopic and chemometric
methods, LWT - Food Science and Technology (2017), doi: 10.1016/j.lwt.2017.09.031.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
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EVALUATION OF THE OXIDATIVE STABILITY OF CHIA OIL-LOADED
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MICROPARTICLES BY THERMAL, SPECTROSCOPIC AND CHEMOMETRIC METHODS
GUIMARÃES-INÁCIO, ALEXANDREa; FRANCISCO, CRISTHIAN RAFAEL LOPESb; ROJAS,
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VALQUÍRIA MAEDAa; LEONE, ROBERTA SOUZAb; VALDERRAMA, PATRÍCIAa; BONA,
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EVANDROa; LEIMANN, FERNANDA VITÓRIAa; TANAMATI, AILEY APARECIA COELHOb;
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GONÇALVES, ODINEI HESSa*
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a
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Via Rosalina Maria dos Santos, 1233, CEP 87301-899, POBox 271, Campo Mourão - PR –
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Brasil. Phone/Fax: +55 (44) 3518-1400
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Federal University of Technology - Paraná, Post-graduation Program of Food Technology
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Federal University of Technology - Paraná, Food Department
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Via Rosalina Maria dos Santos, 1233, CEP 87301-899, POBox 271, Campo Mourão - PR –
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Brasil. Phone/Fax: +55 (44) 3518-1400
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*corresponding author: odinei@utfpr.edu.br
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ABSTRACT
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Chia oil has been widely studied due to its benefits to human health, being source of
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important omega-3 and omega-6. Encapsulation has been studied in order to protect the oil
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from thermal degradation. In this work, oil from chia seeds was microencapsulated in
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carnauba wax and thermal stability was evaluated not only by thermal methods but also
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Spectroscopy and chemometric based analyses. High encapsulation efficiency of both
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omega-3 and -6 (up to 97%) was verified. Particles presented micrometric sizes, spherical
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shape and no fissures. Differential Scanning Calorimetry was applied in isothermal and non-
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isothermal modes and kinetic parameters were determined using the Ozawa-Flynn-Wall
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(OFW) method. An effective protection of the chia oil was verified by an increase in the
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oxidative stability in both isothermal (activation energy increased from 73.5 ± 1.5 to 91.8 ±
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1.6 kJ.mol-1) and non-isothermal (from 87.4 ± 2.1 to 97.3 ± 2.9 kJ.mol-1) tests. Increase in the
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onset oxidation temperature and in the oxidation induction time were also detected. Chia oil
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and oil-loaded microparticles were subjected to accelerated stability test. Degradation was
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evaluated by the UV-Vis spectroscopy coupled to chemometry and also by the extinction
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coefficients, corroborating the increase in the oxidative stability.
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Keywords: DSC; kinetic parameters; OFW methodology; MCR-ALS; chemometry.
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1. Introduction
Chia seed oil is a rich source of polyunsaturated fatty acids (PUFAs), containing a
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high proportion of α-linolenic acid (approximately 60%) when compared to other known
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vegetable sources (Sandoval-Oliveros and Paredes-López, 2013). This fatty acid is part of
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the omega 3 group and its consumption is fundamental to growth and development of human
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body (Simopoulos, 2002).
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Despite showing a favorable nutritional profile for heath, the high percentage of
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PUFAs in chia oil results in low oxidative stability (Ixtaina, Nolasco, & Tomás, 2012; Santos,
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Santos, Souza, Prasad, & Santos, 2002). Therefore, protection against lipid oxidation is a
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crucial factor when aiming for chia oil quality. Oil microencapsulation is an alternative that
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has been studied to protect vegetable oils presenting high amounts of unsaturated fatty
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acids. Encapsulation of PUFAs-rich oils is reported in the literatures (González, Martínez,
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Paredes, León, & Ribotta, 2016; Ixtaina, Julio, Wagner, Nolasco, & Tomás, 2015; Timilsena,
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Adhikari, Barrow, & Adhikari, 2016) demonstrating that the encapsulation efficiency is highly
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dependent on the encapsulant as well as the encapsulation technique used. The production
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of solid lipid particles has been reviewed in the literature demonstrating the advantages of
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such kind of system mainly due to the compatibility of the lipid encapsulant and the oil
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(Müller, Radtke, & Wissing, 2002; Tamjidi, Shahedi, Varshosaz, & Nasirpour, 2013). Oil-
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loaded microparticles may be readily obtained by the hot homogenization technique in which
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the oily phase is melted and dispersed in an aqueous medium. Solid lipid powders are
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obtained after total water evaporation (Milanovic, Levic, Manojlovic, Nedovic, & Bugarski,
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2011). In this context, carnauba wax is a promising encapsulant material since it is accepted
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in food formulations and is classified as Generally Accepted as Safe (GRAS). Moreover, it
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presents high melting temperature meaning that it may not melt during most food processing
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conditions.
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Since oxidation is an exothermic reaction, thermal analyses have been used to follow
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the reaction course through continuous monitoring the thermal effects of lipid oxidation.
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Vecchio Ciprioti and coworkers (2017) reported that Differential Scanning Calorimetry (DSC)
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has been used to evaluated thermal oxidation of oils for over 60 years and DSC experiments
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may be carried out in dynamic (linear increase in temperature) or in isothermal (constant
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temperature) conditions (Ostrowska-Ligeza et al., 2010). Moreover, the correlation between
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classical testes like Rancimat and DSC was demonstrated showing that DSC is reliable, fast
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and less expensive technique to evaluate oxidative stability of vegetable oils (Noello,
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Carvalho, Silva, & Hubinger, 2016; Pardauil et al., 2011; Tan, Man, Selamat, & Yusoff, 2002;
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Vecchio Ciprioti et al., 2017). A wide array of edible oils were evaluated by DSC such as
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olive oil (Qi et al., 2016), blackberry and raspberry oil (Micić et al., 2015), toogga (Balanites
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aegyptiaca) oil (Gardette & Baba, 2013), buriti, rubber tree and passion fruit oil (Pardauil et
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al., 2011).
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Integral isoconversional methods are well stablished methodologies to perform kinetic
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analysis of thermal data which may be obtained isothermally or non-isothermally (Vyazovkin
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et al., 2011). It is worth noting that the kinetic parameters found must be associated to
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processes and not to compounds. As described by Ostrowska-Ligeza and coworkers (2010),
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the correct use of the OFW method in DSC experiments relies on the comparison of systems
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at the same degree of conversion (α), defined by Equation 1, where ∆Hτ is the heat evolved
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at an specific time τ and ∆Htotal is the heat evolved during the process. It may be assumed
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that the degree of conversion at the beginning of the oxidation process is low but constant.
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=
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ΔH
ΔH
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The approach proposed by Ozawa, Flynn and Wall (OFW) (Flynn & Wall, 1966;
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Ozawa, 1970; Vyazovkin et al., 2011) is one of the most used when evaluating the stability of
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edible oils (Grompone, Irigaray, Bruno. Rodríguez, & Sammán, 2013; Litwinienko &
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Kasprzycka-Guttman, 1998; Micić et al., 2015; Musialik & Litwinienko, 2007; Ostrowska-
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Ligeza et al., 2010; Qi et al., 2016). Integral isoconversional methods are powerful tools to
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determine the relationship between the activation energy and the degree of conversion
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throughout the reaction. However, in the case of the stability of edible oils, researchers agree
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that the evaluation of a single activation energy at the start of the oil degradation is sufficient
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to compare different samples that were subjected to the same experimental conditions (same
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heating rates, oxidizing atmosphere and so on). This may be also valid when comparing pure
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and encapsulated oils in order to determine the actual gains in stability caused by
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encapsulation.
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Besides thermal analyses, spectroscopy techniques have been used to evaluate the
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degradation of edible oils such as the use of UV-Vis (Gonçalves, Março, & Valderrama,
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2014; Poyato, Ansorena, Navarro-Blasco, & Astiasarán, 2014; Valderrama, Março, Locquet,
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Ammari, & Rutledge, 2011; W. Zhang et al., 2015) and FTIR (Mahboubifar, Hemmateenejad,
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Javidnia, & Yousefinejad, 2017) spectroscopy coupled to chemometric methods. The
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degradation process leads to the formation of oxidized compounds with intense absorption at
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232 and 270 nm (conjugated dienes and trienes) which enables efficient monitoring (AOCS,
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2009).
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DSC technique is well established when it comes to vegetable oils and oxidative
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stability evaluation. However, few studies use this technique to determine the actual gains in
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stability caused by encapsulation. The microparticles stability during storage also needs to
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be evaluated by conventional or chemometric methods. The objective of this work was to
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evaluate the oxidative stability of chia oil encapsulated in carnauba wax microparticles. For
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the stability tests, Differential Scanning Calorimetry analyses in isothermal and non-
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isothermal modes were carried out, comparing encapsulated and in natura (before
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encapsulation) oil. The accelerated stability test of the microparticles were also carried out
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and the samples were subjected to classical extinction coefficient determination and also to
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chemometric procedures.
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2. Materials e methods
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2.1 Materials
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Chia oil (Veris do Brasil Ltda) was stored at -10°C and protected from light until use.
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Distilled water was use as continuous medium in the microparticles production. Carnauba
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wax (Sigma-Aldrich, analytical grade) and sodium caseinate (Sigma-Aldrich, analytical
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grade) were used as wall material and stabilizer, respectively. Neutral ether-alcohol solution
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(2:1 v/v); phenolphthalein (Dinâmica, analytical grade) solution (1%); sodium hydroxide
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(Dinâmica, analytical grade) solution 0.01 mol.L-1; methyl tricosanoate (23:0 Me, Sigma-
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Aldrich,
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esterifying reagent, methanolic sodium hydroxide solution 0.5 mol.L-1 and saturated sodium
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chlorate solution were used in the characterization analyzes.
standard);
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chromatographic
grade);
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2.2 Methods
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(Dinâmica,
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isooctane
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chromatographic
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2.2.1 Microparticles production
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The aqueous phase was prepared dissolving sodium caseinate (0.0550 g) in water
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(50 g) and heating up to 368.15 K under gentle stirring. Separately, a jacketed borosilicate
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flask was connected to a thermostatic bath at 368.15 K and, then, carnauba wax (3.350 g)
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was added to be melted. After, chia oil (1.650 g) was added to the flask under gentle
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agitation for 1 minute. Then, the aqueous phase was added under stirring (16,000 rpm, 5
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minutes) using a high efficiency disperser (Ultraturrax IKA, T25). At the end of this step, the
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obtained dispersion was poured in a recipient in ice bath for quick quenching and
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microparticles solidification. Lastly, the dispersed microparticles were freeze-dried without
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separation and stored at 263.15 K protected from light. The same proceeding also was
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carried out to microparticles without chia oil (blank microparticles).
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2.2.2 Chia oil and microparticles characterization
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Physicochemical parameters (acidity, peroxide and humidity index) of the chia oil
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were determined according to the methodology described by the Adolfo Lutz Institute in
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triplicate (Instituto Adolfo Lutz, 2008).
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Fatty acids quantification was performed by Gas Chromatography (GC) using methyl
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tricosanoate (23:0, Sigma-Aldrich) as internal standard according to Hartman and Lago
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methodology (Milinsk, Matsushita, Visentainer, Oliveira, & Souza, 2008). Fatty acid methyl esters
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(FAMEs) were separated, identified and quantified using chromatograph standards (Sigma-
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Aldrich, F.A.M.E. Mix C14-C22) (Joseph & Ackman, 1992). Transesterifications were performed
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in triplicate.
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Encapsulation efficiency was measured for omega 3 and omega 6 by GC. An aliquot
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of the dispersion containing the microparticles were washed with absolute ethanol and
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filtered (3 µm average porosity) to remove the oil on the particles surface (non-encapsulated
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fraction). Then, the washed microparticles were dissolved in cyclohexane at 323.15 K then
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quenched (283.15 K) to precipitate the carnauba wax. Sample was then centrifugated
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(14,500 rpm, 15 min) and the supernatant was carefully separated, esterified and analyzed
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by GC. This procedure was also applied to the microparticles without being washed to
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determine the total amount of fatty acids present in the sample. Encapsulation efficiency
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(EE%) was determined according to Equation 2 considering the total concentration of fatty
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acids present in the microparticles ([FA]total) and the concentration of each fatty acid in the
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washed microparticles ([FA]encapsulated).
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% = 100 [
]
[
(2)
]
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For the determination of peak melting temperature of the microparticles, Differential
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Scanning Calorimetry (DSC, Perkin Elmer 4000) was performed at 20 K.min-1 (Hayati,
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Aminah, & Mamot, 2000) from 273.15 K to 713.15 K under nitrogen atmosphere (50 mL.min-
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1
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(Perkin Elmer).
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) in closed aluminum sample holders. DSC data were evaluated using software Pyris 11
Size and morphology of the microparticles were analyzed by Scanning Electron
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Microscopy (Carl Zeiss - EVO MA15) operating at 15 kV with a secondary electron detector.
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Particles diameter were measured with an image analyzer (SizeMeter software). Medium
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infrared spectra of chia oil and the microparticles were obtained in a spectrophotometer
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equipped with the Attenuated Total Reflectance accessory (FTIR-ATR, PerkinElmer Frontier
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Spectrum 100). Oil samples were directly analyzed while microparticles were dissolved in
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cyclohexane moments before reading to expose the encapsulated oil. X-ray diffraction trial
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(DRX, Bruker model D8 Advance) were performed with Cu-Kα radiation, generated at 40 KV
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to 35 mA, varying from 3° to 60° (2θ) at 5.9°.min-1.
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Thermogravimetric analyses were performed with chia oil and with the oil-loaded
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microparticles. Samples (approximately 4 mg) were conditioned in aluminum sample holders
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in a thermobalance (Shimadzu, TGA-50) and submitted to heating from 303.15 K to 923.15 K
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at 10 K.min-1 using synthetic air atmosphere (mixture 79% N2, 21% O2, 150 mL.min-1).
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2.2.3 Thermal stability evaluation
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Samples (5 - 10 mg) of chia oil or microparticles were submitted to Differential
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Scanning Calorimetry (DSC, Perkin Elmer model 4000) in isothermal and non-isothermal
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modes. Heating rates of non-isothermal experiments were 1, 5, 10 and 20 K.min-1 in a
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temperature range of 273.15 K to 713.15 K. Isothermal tests were performed at 383.15 K,
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393.15 K, 403.15 K and 413.15 K during 220 minutes. In both tests, open aluminum sample
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holders and synthetic air (mixture 79% N2, 21% O2) at 100 mL.min-1 were used. Calorimeter
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calibration was performed for each analysis condition with zinc and indium standards (429.75
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K and 692.68 K, respectively).
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For the non-isothermal tests, calculation of the activation energy (Ea) involved in the
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lipid oxidation reaction was carried out with the data obtained during the non-isothermal tests
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by the method proposed by Ozawa, Flynn and Wall (OFW) (Flynn & Wall, 1966; Ozawa,
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1970; Qi et al., 2016). The onset oxidation temperature was found by extrapolating the
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baseline and the tangent line (leading edge) of each thermogram (Ostrowska-Ligeza et al.,
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2010). The onset temperature was used because the onset of the reaction occurs at about
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the same extent of conversion which is in turn close to zero (Micić et al., 2015). Activation
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energy was calculated using a set of onset oxidation temperature obtained at different
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heating rates (β) using Equation 3 and 4.
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!"
(3)
(4)
# $%
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where β is the heating rate (K.min-1), T is the onset oxidation temperature (Ton in K),
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“a” and “b” are the slope and intercept of Equation 3, respectively and R is the universal gas
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constant (8.314 J.mol-1K-1).
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For the isothermal DSC condition, the isoconversional method was used to determine
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the activation energy using Equation 5 (Grompone et al., 2013; Micić et al., 2015). The
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oxidation induction time (Ot) was found in the DSC curves as the intersection of the
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extrapolated baseline and the tangent line (leading edge) of the exothermic peak. The
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activation energy was then calculated as the slope of the curve found by the least squares
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regression of the ln(1/Ot) and T-1.
1
= ln
/
−
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(5)
For both isothermal and non-isothermal modes, oxidation induction time and onset
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oxidation temperature, respectively, were measured in duplicate and the averages for each
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condition were used in the calculations. In all experiments, the maximum deviation found for
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the parameters were less than 1.1% in the non-isothermal mode and 2.3% in the isothermal
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mode, which is in accordance with the literature (Ostrowska-Ligeza et al., 2010). Linear
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regression analyses for fitting of kinetics data (p-values and lack of fit) were carried out using
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STATISTICA 7.0 (StatSoft Inc., Tulsa, OK, USA).
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2.2.4
Accelerated stability test
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Microparticles samples and the chia oil were conditioned in an open recipient and
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evaluated by the Schaal oven test (González et al., 2016; Poyato et al., 2013). Ultraviolet-
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visible spectra were acquired from 200 to 550 nm at 353.15 K and analyzed by Multivariate
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Curve Resolution Alternating Least-Squares (MCR-ALS) as described by Gonçalves et al.
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(2014), using software Matlab R2007b. Spectra were assigned to their respective
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compounds according to Valderrama et al. (2011) in respect to degradation products and
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also to tocopherols. Similarly, UV-Vis spectra were acquired at 353.15 K and the specific
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extinction coefficients were calculated at 232 and 270 nm (American Oil Chemists Society,
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2009; Instituto Adolfo Lutz, 2008). In both experiments, aliquots were removed at time
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intervals defined in preliminary experiments, and dissolved in cyclohexane moments before
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analysis in triplicate.
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3. Results and Discussion
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3.1 Chia oil and microparticles characterization
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Table 1 presents the physicochemical parameters determined for the chia oil before
encapsulation (in natura oil).
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(Table 1)
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Ixtaina et al. (2011) found acidity index from 0.07 to 0.20 goleic
acid.100goil
-1
when
studying the properties of chia oil. In similar work, Segura-Campos et al. (2014) also found
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small variations when comparing their results with other studies and suggested that this
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difference may be influenced by cultivation or the oil extraction method. However, in both
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works and also in the present study values found are within the established standards by
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Codex Alimentarius (Codex Alimentarius Commission, 1999).
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In Table 2, composition of chia oil and chia oil loaded-microparticles is presented.
Figure 1 shows the obtained chromatograms.
(Table 2)
(Figure 1)
Omega 3 concentration found was 63.8% which is consistent with the data available in
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the literature (González et al., 2016; Julio et al., 2015). The encapsulation efficiency found for
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omega-3 and omega-6 were 96.4 ± 0.9 and 96.6 ± 1.0 %, respectively. Both results were
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considered efficient and demonstrate the affinity between the chia seeds oil and the
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encapsulating matrix chosen.
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Proportions between polyunsaturated and saturated fatty acid (PUFA:SFA, Table 2) higher
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than 0.45 and between omega 3 and 6 (n-6: n-3) from 1:1 to 2:1 are recommended for daily
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human ingestion (Wood et al., 2004) meaning the oil have high nutritional quality. This is an
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important data, since an imbalance in n-6: n-3 proportion, as in the modern western food
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ingestion, are related with chronic diseases and metabolic disorders (Simopoulos, 2008). The
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encapsulated oil presented characteristics similar to chia oil before encapsulation, meaning that
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the microparticles may be used to protect the oil and to formulate products with the same high
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nutritional values of chia oil since the wall material may be considered a food formulation
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component.
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Figure 2 (a) presents the image obtained by Scanning Electron Microscopy of the oil-loaded
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microparticles, figure 2 (b) presents Infrared spectra and Figure 2 (c) the X-Ray diffractograms
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also for the oil-loaded-microparticles and blank microparticles.
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(Figure 2)
Microparticles presented spherical morphology, continuous surface, without fractures
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and monomodal size distribution varying from 1.5 to 15 µm with average diameter of 3.71 ±
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1.85 µm. This is important since porous and irregular structure may lead to oxygen diffusion
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and oil migration which could accelerate the oxidation process.
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DSC thermograms carried out under nitrogen flow (Figure 1S, Supplementary
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Material) showed a decrease in the melting temperature of 4.2 K (from 357.37 K to 361.57 K)
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and in the melting enthalpy of 47.68 J.g-1 (from 162.70 to 115.02 J.g-1) when comparing blank
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and oil-loaded microparticles was observed. This may be related to the decrease in the
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carnauba wax crystallinity caused by the partial solubilization of the wax in the oil which
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suggests an effective encapsulation of the chia oil. Ribeiro et al. (2012) observed similar
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behavior when producing lipid microparticles using different proportions of oleic acid and
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stearic acid. Since oleic acid melts at 6 ºC, its presence decreased the melting temperature
290
and enthalpy of the stearic acid.
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Infrared analyses of the oil showed bands corresponding to –CH, –CH2 and –CH3
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groups from the fatty acids chains and other components present in the sample (Moros,
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Roth, Garrigues, & Guardia, 2009). In Figure 2 (b) the region from 2800 to 3000 cm-1
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presents few absorption bands that correspond to the C-H bonds such as asymmetric
295
stretching (CH2 at 2926 cm-1) and to symmetric stretching (CH2 at 2853 cm-1). The region
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below 1600 cm-1 corresponds to a fingerprint region presenting a more complex profile of
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absorption of various bands in 1450, 1256, 1040, 902 and 860 cm-1 presenting the
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absorption bands of functional groups –C-H2, -C-H3, -C-O (ester), -HC=CH (cis) and =CH2
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(Zhang et al., 2012). The characteristic bands of the chia oil were attenuated in the oil-loaded
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microparticles indicating that the oil was located inside the microparticles. It is worth pointing
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out that all spectra were normalized in order to allow comparison between samples.
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In Figure 2 (c) low intense peaks were found at 21.6° and 23.8° which correspond to
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0.41 nm and 0.37 nm, respectively (Villalobos-Hernández & Müller-Goymann, 2006). They
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are characteristic of β’, orthorhombic crystalline sub-cell arrangement of the carnauba wax
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(Blake, Co, & Marangoni, 2014; Révérend, Fryer, Coles, & Bakalis, 2010). It was possible to
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observe the appearance of an amorphous region from 15 to 20° in the oil-loaded
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microparticles as well as a decrease in the peaks intensity. This may indicate that the
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presence of the oil led to the disorganization of the crystalline structure of carnauba wax.
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3.2 Thermal analysis
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3.2.1
Non-isothermal Differential Scanning Calorimetry
In Figure 3 the thermograms obtained in the non-isothermal DSC tests are presented
314
(the adjusted curves are presented in the Supplementary Material, Figure 2S). Table 3
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shows the activation energy and the adjusted coefficient of determination of each adjusted
316
curves found in the non-isothermal and isothermal Differential Scanning Calorimetry
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experiments.
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(Figure 3)
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(Table 3)
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For both samples (chia oil and oil-loaded microparticles) linear regression presented
324
satisfactory adjusted coefficient of determination (Table 3). Also, all linear models were
325
statistically significant (p<0.05) while lack of fit was statistically not significant (p>0.05)
326
meaning that the linear models may satisfactorily describe the experimental data.
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It is worth pointing out that chia oil (non encapsulated) is liquid under the conditions
328
used in all experiments meaning that the variations detected in the DSC runs could not be
329
caused by other thermal events like glass transition. Moreover, PUFAs rich oils like chia oil is
330
prone to degrade under heating under oxidizing atmospheres. The activation energy values
331
for the onset of the oxidation reaction of chia oil and for oil-loaded microparticles were of 87.4
332
± 2.1 kJ.mol-1 and 97.3 ± 2.9 kJ.mol-1, respectively. Grompone et al (2013) found similar
333
value (97 kJ.mol-1) when analyzing the oxidation of chia oil. As stated by Santos at al. (2002),
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the first step of thermal decomposition of edible oils is the most important one when
335
determining stability, as decomposition of unsaturated fatty acid starts in this step. Thermal
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decomposition of fatty acids is considered a multistep process which is highly dependent on
337
fatty acids composition.
When comparing the chia oil and the oil-loaded microparticles it was possible to
339
observe an increase in both oxidation temperatures and in the activation energy (increase of
340
9.89 kJ.mol-1, corresponding to 11.3% increase). This denotes a greater resistance of the oil
341
to oxidize, probably because the encapsulant acted as a barrier to oxygen while it was still
342
solid delaying the onset of the oxidation reaction. The encapsulant could still protect the oil
343
even after melting due to a dilution effect resulting in an efficient protection of the chia oil by
344
the carnauba wax. Grompone et al. (2013) carried out non-isothermal DSC experiments and
345
found different induction temperatures for the chia oil of those described in Table 3. This
346
variation may be associated with different analysis conditions, such as the flow gas in the
347
DSC or differences in oil composition and harvest conditions. Litwinienko and Kasprzycka-
348
Guttman (1998) also mentioned the influence of the atmosphere applied during DSC
349
analyses.
350
353
354
355
Isothermal Differential Scanning Calorimetry
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3.2.2
In Figure 4 the thermograms obtained in the isothermal DSC tests are presented (the
adjusted curves are presented in the Supplementary Material, Figure 3S).
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(Figure 4)
356
One may observe in Table 4 an increase of 77 minutes in the oxidation induction time
357
for the analysis at 393.15 K (120ºC). The experimental run of the microparticles at 383.15 K
358
did not present signs of oxidation during the whole analysis time (220 min) confirming the
359
protection given by the encapsulant. The activation energy values found was 73.5 ± 1.5 and
360
91.8 ± 1.6 kJ.mol-1 for the onset of the oxidation reaction of chia oil and for the oil-loaded
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microparticles, respectively. As in the case of the non-isothermal DSC mode this
362
demonstrates that encapsulation led to an increased stability of the chia oil. Differences
363
between results from isothermal and non-isothermal setups were expected because the
364
mechanisms involved in the oil oxidation are influenced by the heating time and temperature
365
(Adhvaryu, Erhan, Liu, & Perez, 2000; Grompone et al., 2013; Litwinienko & Kasprzycka-
366
Guttman, 1998).
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3.2.3
Thermogravimetric analysis (TGA)
Figure 5 presents the TGA thermogram and its derivative for chia oil and oil-loaded
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microparticles.
(Figure 5)
371
The onset oxidation temperature for the oxidation of the chia oil and the oil-loaded
373
microparticles were 426.15 K and 497.55 K, respectively. Oil oxidation is a complex chain
374
reaction involving a multi-step mechanism (Ostrowska-Ligeza et al., 2010). As pointed out by
375
Qi and coworkers (Qi et al., 2016), oxidation follows a first order reaction when excess of
376
oxygen is used. The first stage of thermal decomposition of vegetable oils is the most
377
important in the thermal stability study because is in this step that the unsaturated fatty acid
378
decomposition begins (Santos et al., 2002). Mass loss was detected in all samples while the
379
temperature in which degradation took place was lower for the oil before encapsulation than
380
for the microparticles, demonstrating the gain in stability due to the encapsulation. It is worth
381
reminding that the application of the isoconversional method is not immediately invalidated
382
due to the occurrence of multi-step processes (Vyazovkin et al., 2011).
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3.3 Accelerated oxidative stability tests
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Samples of oil and microparticles containing chia oil were evaluated in oven
386
accelerated stability tests (Schaal oven test). Results of Multivariate Curve Resolution with
387
Alternating Least Squares (MCR-ALS) are presented in Figures 6 (UV-Vis spectra recovered
388
by the MCR-ALS for the chia oil and for the oil-loaded microparticles are presented in the
389
Supplementary Material, Figure 4S (a) and Figure 4S (b), respectively).
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(Figure 6)
391
It is worth noting that the test temperature (353.25 K) was lower than the melting
392
temperatures of the blank and oil-loaded microparticles (361.55 and 357.35 K) meaning that
393
they remained solid during all the duration of the experiments.
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The oxidation of polyunsaturated fatty acids is followed by an increase in absorption
395
in ultraviolet region (Vieira and Regitano-d’Arce, 1999) with conjugated dienes and trienes
396
presenting intense absorption at the 234 nm and 270 nm, respectively. For chia oil, this
397
absorption profile is consistent with the spectral profile recovered by MCR-ALS. The relative
398
concentration profiles showed that oxidation started at the beginning of the oven test for the
399
oil but started after 54 hours for the encapsulated oil demonstrating the protection against
400
degradation due to the encapsulation. The dashed lines in the spectral profiles may be
401
attributed to tocopherol (Gonçalves et al., 2014) which relative concentration decreased from
402
the beginning of the exposure at 353.15 K for the oil but started after 48 hours of for the
403
encapsulated oil. The encapsulant played in important role protecting the oil from the oxidant
404
atmosphere thus preventing degradative oxidation. Migration of the oil from the interior of the
405
capsules to the surface after long exposure times may be responsible for the degradation at
406
the later stages of the experiment.
407
408
409
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Oil stability was also evaluated by the specific extinction coefficients (K) as showed in
Figure 6.
(Figure 7)
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It is worth noting that monitoring the oil sample after 48 hours was not possible due to
411
intense deterioration of the oil which became insoluble in the cyclohexane. Blank
412
microparticles were also evaluated, presenting an extinction coefficient of 15.5 at 232 nm
413
and 13.2 at 270 nm, which was used in the calculation of the coefficient value of the
414
oil-loaded microparticles. This value was constant over the storage time which may be
415
attributed to the encapsulating material as already demonstrated by Freitas and colaborators
416
(2016) and Villalobos-Hernández and Müller-Goymann (2006). Variations in the extinction
417
coefficients were observed after 36 storage hours for oil while the oil-loaded microparticles
418
started to degrade after 54 hours.
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4. Conclusions
Microencapsulation of chia oil using carnauba wax as wall material showed to be
422
effective since encapsulation efficiencies around 97% were found for important fatty acids
423
such as omega 3 and omega 6.
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Encapsulation led to gains in thermal stability which was verified by Differential
425
Scanning Calorimetry, both in isothermal and non-isothermal modes, by the increase in the
426
activation energy of the degradation reaction due to the encapsulation. During the
427
accelerated oven stability test the microencapsulated oil presented greater stability as
428
detected by the extinction coefficient, disfavoring the appearance of degradation products.
429
Multivariate analyses also showed the effectiveness of encapsulation in protecting
430
tocopherols present in the oil.
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The effective protection of the oil by the encapsulation process was demonstrated by
432
thermal, spectroscopic and advanced mathematical methods since all analyses agreed that
433
the encapsulated oil presented improved oxidative stability.
434
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5. Acknowledgments
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Authors thank CNPq, CAPES and Fundação Araucária for the support.
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FIGURE CAPTIONS
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Figure 1. Chromatograms (a) of the chia oil and (b) oil-loaded microparticles.
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Figure 2 – (a) Scanning Electron Microscopy of the oil-loaded microparticles, (b) Infrared
616
spectra (FTIR-ATR) of the (red line) oil-loaded microparticles, (black line) blank
617
microparticles and in natura chia oil (before encapsulation, blue line) and (c) X-Ray
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diffractograms of the (black line) oil-loaded microparticles and (red line) blank microparticles.
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Figure 3 – Non-isothermal DSC for the (a) chia oil and (b) oil-loaded microparticles (onset
621
oxidation temperatures are indicated).
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Figure 4 – Isothermal DSC for the (a) chia oil and (b) oil-loaded microparticles (oxidation
624
induction times are indicated).
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Figure 5 - Thermogravimetric analyses (TGA) and its derivative for the chia oil and oil-loaded
627
microparticles.
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Figure 6. Relative concentration of (- - -) tocopherol and (—) degradation products for (a)
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chia oil and (b) oil-loaded microparticles.
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Figure 7. Specific extinction coefficients at 232 nm (K232, ●: chia oil; ▲: oil-loaded
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microparticles) and 270 nm (K270, ○: chia oil; ∆: oil-loaded microparticles) obtained during the
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accelerated stability test
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Table 1. Physicochemical parameters of chia oil.
Maximum values
Parameter
Result
(Codex alimentarius
0.050 ± 0.002
Peroxide index (meq.kgoil-1)
0.784± 0.195
Humidity (%)
0.381± 0.125
0.4
15
*
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Acidity (goleic acid.100goil-1)
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Table 2. Quantification of major fatty acids present in the chia oil and in the oil-loaded
microparticles.
Concentration (mgFA.goil-1)
Fatty acid
Oil-loaded microparticles
C16:0
72.8 ± 1.6
72.5 ± 2.1
C18:0
30.1 ± 0.5
C18:1 n-9
59.1 ± 1.3
C18:2 n-6c
200.8 ± 5.4
C18:3 n-3
643.6 ± 18.9
Total*
1006.7 ± 27.6
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33.9 ± 0.6
57.1 ± 2.2
179.0 ± 6.6
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SFA (%)
MUFA (%)
PUFA (%)
PUFA: SFA
955.1 ± 33.6
11.8
18.2
5.1
6.0
83.1
75.8
7.1
4.1
0.3
0.3
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n6:n3
545.3 ± 20.6
*Sum of all fatty acids identified in the samples (some not shown in the table).
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Table 3. Activation energy and adjusted coefficient of determination of the adjusted
curves for the non-isothermal and isothermal Differential Scanning Calorimetry
experiments.
Non-isothermal mode
Oil-loaded microparticles
Ea (kJ.mol-1)
87.4 ± 2.1
97.3 ± 2.9
R2
0.993
Chia oil
73.5 ± 1.5
R2
0.991
Oil-loaded microparticles
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Ea (kJ.mol-1)
0.991
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Isothermal mode
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91.8 ± 1.6
0.999
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Oxidative stability gains from microencapsulation of edible oils must be correctly assessed.
Chia oil encapsulated in carnauba wax showed improved oxidative stability.
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Accelerated stability tests corroborated the improved stability verified in the thermal tests.
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MCR-ALS showed the effectiveness of encapsulation in protecting tocopherols present in the
oil.
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