Investigation of oil distribution in spray-dried chia seed oil microcapsules using synchrotron-FTIR microspectroscopy
Introduction
Microencapsulation is one of the techniques commonly used to protect highly sensitive, yet nutritionally beneficial, polyunsaturated fatty acids (PUFAs) from oxidative damage by environmental stressors (such as oxygen, heat, light and moisture) during processing and storage. Shielding the PUFA oil inside an inert matrix also facilitates their incorporation in food, pharmaceutical and cosmetic products. Nevertheless, shell materials and process-based parameters reportedly play an important role in the stability and bioavailability of the encapsulated PUFAs (Augustin & Hemar, 2009).
Distribution of the PUFA-rich oil on the surface and within the microcapsules is the most important parameter that affects the stability of the microcapsules, and thereby determines the quality of the encapsulation. PUFAs exposed to the surface of the microcapsule undergo an oxidative degradation more readily than those shielded inside the wall matrix (Timilsena et al., 2016a, Wang et al., 2014). Although various aspects of microencapsulation of PUFA-rich oils have been previously reported (Wang et al., 2014, Eratte et al., 2014), there is a paucity of information on the spatial distribution of the PUFAs in the microcapsules and its relationship with the oxidative stability, release and digestion in the gastro-intestinal environment.
Fourier transform infrared (FTIR) spectroscopy is a non-destructive technique widely used to obtain molecular information of materials with only minimal sample preparation required. Advanced FTIR technique in microscopic configuration further enables in-depth characterization of chemical compositions, particularly lipid and protein conformation, and their distribution in different layers of the microcapsules (Leroy, Lafleur, Auger, Laroche, & Pouliot, 2013). This is because the absorption peaks specific to α-helix (1648–1660 cm−1), β-sheet (1625–1640 cm−1), turns (1660–1685 cm−1) and random coil (1652–1660 cm−1) structures reside in the frequency range of the amide I band, and these peak positions can be used to identify particular protein conformations presented in the analyzed samples (Tamm and Tatulian, 1997, Barth, 2007, Araki et al., 2015). Similarly, FTIR is very effective in providing detailed information on the content and nature of lipid present in a material because the absorption peak of the carbonyl bond found between 1800 and 1700 cm−1 is characteristic of total lipids, whereas the absorption peak of olefinic double bonds (–HC = CH–) in unsaturated fatty acids (UFAs) can be observed in the range of 3050–3000 cm−1. In addition, FTIR spectra can be used to obtain detailed information on chemical bonding and molecular structure of materials, which cannot be obtained using common microscopic methods (Araki et al., 2015). Furthermore, FTIR technique can be used to carry out multi-component analyses. By combining microscopy with FTIR spectroscopy, FTIR microspectroscopy has since become a widely used technique for imaging changes in secondary protein structures across a sample by observing the variation of intensity and peak position of the amide I band. Thus, FTIR microspectroscopy combines the molecular specificity of FTIR spectroscopy with the spatial resolution of a microscope to provide detailed information on the distribution of targeted molecules and their interactions with other molecules of interest (Mendelsohn, Chen, Rerek, & Moore, 2003).
Nevertheless, the spatial resolutions of most laboratory-based FTIR microspectroscopic instruments are not sufficient to allow studies of oil (lipid) distribution in micron or submicron-sized solid microcapsules, such as spray dried powders used in this study. To overcome such a limitation, the highly collimated synchrotron IR beam, which essentially offers 100–1000 times higher brightness than those of laboratory-based internal Globar™ IR sources, was used in this study. Such unique properties of the synchrotron IR light source result in enhanced spectral quality, in terms of signal to noise (S/N) ratio, which enables acquisition of high-quality FTIR spectra at diffraction-limited spatial resolution (Carr, 2001, Carr et al., 1995). Therefore, synchrotron-FTIR (S-FTIR) technique is crucial for examining very fine structures in small samples, thus allowing for semi-quantitative examination of lipid and protein distribution in the microcapsules.
In this work, we studied the distribution of chia seed oil (CSO) in spray dried solid microcapsules produced using three different shell materials, including chia seed protein isolate (CPI), chia seed gum (CSG) and CPI-CSG complex coacervates. S-FTIR microspectroscopic experiments were performed in macroscopic attenuated total reflection (macro ATR) and transmission modes, to obtain information on the distribution of protein, oil and gum specifically on the surface and in the interior of the microcapsules, respectively.
Section snippets
Materials and methods
CSO was extracted from Australian chia seeds (Salvia hispanica L.) which were purchased from The Chia Company (Melbourne, Australia). The oil extraction procedure used in this study was based on our recently published protocol (Timilsena, Vongsvivut, Adhikari, & Adhikari, 2017a). Briefly, the seeds were ground into a fine powder using a portable grinder (InCafe MEC-559, Shanghai, China) and mixed with petroleum ether at flour-to-solvent ratio of 1:10. The mixture was agitated for 4 h at ambient
Comparison of ATR-FTIR spectra of oil, wall materials and microcapsules
First, reference ATR-FTIR spectra of the freshly extracted liquid CSO and the compounds used as shell materials in the production of CSO microcapsules, were obtained and are presented in Fig. 1A. Our detailed band assignments were performed according to the previous literatures (Vongsvivut et al., 2014, Leroy et al., 2013) and are presented in Table 2. The spectrum of the pure CSO showed strong characteristic bands of lipids in the high wavenumber region (3100–2800 cm−1), including a single
Conclusion
This study presents the spatial distribution of lipid and protein on the surface and interior of CSO microcapsules encapsulated in three types of shell materials (CPI, CSG and CPI-CSG complex coacervates) using spray drying method. S-FTIR microspectroscopy was used to observe the chemical distribution of the major components particularly lipids and proteins in the finished CSO microcapsules. The S-FTIR results suggested that the composition of shell materials affected the distribution of the
Acknowledgements
We wish to thank Mr. Ian Boundy (Senior Histologist, Hawthorn Histology, Melbourne) for his technical assistance in sectioning of the microcapsules. This research was undertaken on the Infrared Microspectroscopy (IRM) beamline at the Australian Synchrotron, part of ANSTO, through the merit-based access program (Project ID. M10488). This work was also partially supported by Australian Research Council’s Linkage Project (LP140100722).
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