*3.2.1. Morphological examinations*

**3.1. Encapsulation efficiency**

270 Cyclodextrin - A Versatile Ingredient

not so important.

As quoted above, we present the encapsulation efficiency of three essential oils (guava oil,

In the case of yarrow oil and carvacrol (yarrow oil major component), there efficiency were 45.05 and 86.59%, respectively [125] see **Table 2**. Black pepper [100] exhibit similar behavior with efficiency of 50.55 and 85.30, respectively, for essential oil and its main component (β-caryophyllene). Finally, guava leaf oil encapsulation efficiency was 52.5%, while it reached

This difference in encapsulation efficiency of the pure compound and the essential oil results from the presence of other minority components. In the case of yarrow oil and carvacrol [125], the other components like 1,8 cineole, thymol, camphor and linalool have also high affinities for CD [6, 121, 128–132] that compete for inclusion complex formations. Kamimura et al. [110] reported that the encapsulation efficiency values of pure carvacrol in HPβCD prepared by

Similar explanation would justify the diferences in encapsulation efficiency of the pure compound and the black pepper oil [100] because the presence of other components in the black pepper oil such as limonene, δ-3-carene and pinene [68] that also have high affinities for HPβCD. In the case of guava leaf oil [112], the large values found are due to minority components, such as β-caryophyllene, 1,8-cineole and α-pinene, exhibit low affinity for the β-CD that are not easily encapsulated and the competition between the other host for the guest in

Similar observation has been reported for other authors in the literature [99] showing that encapsulation efficiencies of cinnamon oil and clove oil were 41.72 and 77.74%, respectively. The encapsulation efficiencies of major components including trans-cinnamaldehyde in cinnamol oil and eugenol in clove oil were also examined and showed higher encapsulation efficiency of 84.70 and 90.15%, respectively. In addition, comparable values of encapsulation efficiency were found in other carriers such as alginate-chitosan system. In this case, the yarrow oil components exhibited 82.4% efficiency of polyphenol encapsulation [133, 134].

**Compound Encapsulation efficiency (%) Compound Encapsulation efficiency (%)**

Black pepper oil 50.55 β-caryophyllene 85.30 Yarrow oil 45.05 Carvacrol 86.59 Guava leaf oil 52.50 Limonene 91.80 Cinnamon oil 41.72 Cinnamaldehyde 84.70 Clove oil 77.74 Eugenol 90.15

**Table 2.** Encapsulation efficiency value in HPβCD.

yarrow oil and black pepper oil) in hydroxypropyl-β-CD (HPβCD).

91.8% for limonene, the major pure compound of guava leaf oil [112].

kneading and freeze-drying methods were around 78 and 84%, respectively.

It is well known that the inclusion complex formation would change the morphology of CDs [135]. **Figure 6** presents the morphology of the encapsulated oils studied by SEM.

The particle shape and morphology of the encapsulated oil were similar to those of free HPβCD in the cases evaluated – guava, yarrow and black pepper – see **Figure 7**. It indicates the hydrogen bonding of the free HPβCD molecules interact with each other in solution producing the cluster of HPβCD [136, 137]. This case not occurs in inclusion complex because inclusion complex formation also induces the conformation change of CD and obstructs the agglomeration among them. Similar observations have been previously reported that the distribution of inclusion complex of carvacrol and β-CD, and the gathering of free β-CD were also found [135].

By contrast, the free HPβCD particle sizes are much larger than those of the encapsulated products. These results are in agreement with Guimaraes et al. [135], who analyze carvacrol encapsulation with β-CD. Considering that HPβCD form clusters in solution through intermolecular hydrogen bonds [136, 137], it seems that the incorporation of different essential oils interferes in these interactions and reduces particle size.

**Figure 6.** SEM micrographs of free HPβCD at 500 times magnification.

Finally, FT-IR spectrum of guava leaf oil showed prominent absorption bands at 2921 cm−1 for C─H stretching vibration of methylene group, 1642 cm−1 for H─O─H bending, 1447 cm−1 for C─H scissoring vibration, 1376 cm−1 for symmetrical deformation vibration of CH3, 886 cm−1 for C─H deformation vibration and 789 cm−1 for S─C absorption. FT-IR spectrum of encapsulated guava leaf oil shows no feature similar to the free guava oil. The bands of guava leaf oil spectrum were almost completely concealed by very intense and broad bands of HPβCD. However, the absorption band at 608 cm−1 of HPβCD disappeared in encapsulated guava leaf oil. This change may be related to the interaction between guava leaf oil and

Encapsulation of Essential Oils by Cyclodextrins: Characterization and Evaluation

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The inclusion complex formation of β-CD was also investigated by Liu et al. [140] using FT-IR analysis. The absorption bands of β-caryophyllene were not detected in the spectrum of inclusion complex. The changes were related to the inclusion complex formation of β-CD and the guests which whole of guest could be contained in the CD cavity. Wang et al. [139] have reported similar results. In their study, the inclusion complex formation of soybean lecithin and β-CD was determined by FT-IR. All the absorption bands of soybean lecithin encapsulated in β-CD were obscured by β-CD spectrum showing that inclusion complex of β-CD and soybean lecithin was formed. However, Gomes et al. [141] reported that the absorption band at 1738 cm−1 of the red bell pepper pigments was observed after encapsulation in β-CD indicating that some region of the encapsulated molecules was not contained in the cavity of β-CD.

Essential oils contain various bioactive chemicals, which adsorb ultraviolet (UV) or visible light (Vis) at different wavelengths. CD host-guest complex formation would alter UV-Vis absorption spectra [142]. Otherwise, the spectra of the guests appear in line of CD [140]. Therefore, UV-Vis spectrophotometry, evaluated the inclusion complex formation of HPβCD and the three essential oils. The UV absorption spectra of guava leaf oil, limonene and their inclusion complexes were compared. Indeed, maximum absorption value of guava leaf oil was at 214.5 nm, which was mainly attributed to limonene. The absorption peak at 205 nm corresponds to β-caryophyllene and/or pinene. The peak at 275 nm of guava leaf oil was

The spectra of the physical mixture of HPβCD with guava leaf oil and with limonene before complexation were consistent with that of guava leaf oil or pure compound. The absorption spectra of the physical mixture of HPβCD with guava leaf oil and with limonene were in accordance to with the spectra of guava leaf oil and pure limonene, respectively. When the active compounds in essential oil or the pure compound were encapsulated into the cavity of HPβCD, the absorption peaks of each compound disappeared in the spectra of the inclusion complexes. To recover active compounds encapsulated in the HPβCD cavity, the active compounds were extracted from HPβCD by dissolving the inclusion complexes in 95% acetonitrile. The encapsulated compounds were released from the cavity of HPβCD and HPβCD was separated from guava leaf essential oil or limonene in solution by centrifugation. The solution was diluted 100 times with acetonitrile and the absorbance was measured by UV

HPβCD in the inclusion complex.

*3.2.3. Ultraviolet-visible spectrophotometry (UV-Vis)*

ascribed to 1,8-cineole.

spectrophotometer.

**Figure 7.** SEM micrographs of encapsulated essential oils at 500 times magnification.
