**3.1. Encapsulation efficiency**

As quoted above, we present the encapsulation efficiency of three essential oils (guava oil, yarrow oil and black pepper oil) in hydroxypropyl-β-CD (HPβCD).

**3.2. Characterization of host-guest complex**

interferes in these interactions and reduces particle size.

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

It is well known that the inclusion complex formation would change the morphology of CDs

Encapsulation of Essential Oils by Cyclodextrins: Characterization and Evaluation

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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

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

[135]. **Figure 6** presents the morphology of the encapsulated oils studied by SEM.

*3.2.1. Morphological examinations*

also found [135].

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

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

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 not so important.

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].


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