Enhanced stability and controlled release of menthol using a β-cyclodextrin metal-organic framework
Introduction
Menthol is a naturally occurring volatile, cyclic terpene alcohol with a characteristic minty smell, which is generally found in the Mentha species of plants(Yildiz, Celebioglu, Kilic, Durgun, & Uyar, 2018). It can be produced synthetically or isolated from other mint oils (Nazıroğlu et al., 2018). Various biological properties have been ascribed to menthol, including anti-itching, anti-tumor, anti-microbial, analgesic, antiphlogosis, local anesthesia and cooling effects. Menthol has been currently applied in a wide range of consumer products, including oral-care products, candies, pharmacy, cosmetics and chewing gum, to name but a few (Kamatou, Vermaak, Viljoen, & Lawrence, 2013). In most food formulations, menthol is generally among one of the most valuable ingredients because it plays an essential role in consumer satisfaction and influences the consumption of food.
However, menthol exhibits high volatility and low solubility in water, which causes its limitation in some potential applications. Moreover, production and storage processes, packaging materials, and other ingredients in foods usually give rise to modifications in the integral aroma via lowering the intensity of the flavor compound, even producing off-flavor constituents (Ciobanu, Landy, & Fourmentin, 2013). To prevent flavor loss during manufacturing and storage, it is beneficial to encapsulate the menthol molecules that produce these effects prior to use. Consequently, many attempts for menthol encapsulation have been widely investigated. Various materials, including cyclodextrin (CD), starches and maltodextrin have been reported. In one of these studies, water-soluble β-cyclodextrin grafted with chitosan was prepared by reacting chitosan and O-p-toluenesulfonyl-β-cyclodextrin under acidic conditions and used to capture menthol with an encapsulation efficiency of up to 5% (Phunpee et al., 2016). In other studies, starches with different amylose content were used for menthol entrapment with the highest menthol concentration of 5.5% (Ades, Kesselman, Ungar, & Shimoni, 2012). Interestingly, a metal–organic framework based on β-CD (β-CD-MOF) was successfully determined to be a better encapsulation material with a menthol content (MC) and encapsulation efficiency (EE) of 21.76 (w/w) and 22.54% (w/w) respectively in our previous study, which was 3–4 times higher than those previously reported (Hu, Li, Wang, Zhang, & Huang, 2021).
Metal-organic frameworks (MOFs) composed of polydentate bridging ligands and metal ions or metal clusters have emerged as a new type of highly versatile materials with large specific surface area and easily adjustable porous structures (Mao, Lu, Zhang, & Huang, 2018). For food acceptability and non-toxicity, a novel class of β-CD-based bio-friendly relevant MOFs have been discovered, β-CD-MOF, prepared from β-CD and potassium ions using a vapor diffusion method (Forgan et al., 2012; S. Li, Zhang, Li, Fu, & Huang, 2020). To date, β-CD-MOF has been critical for various potential applications in drug delivery (Yang, Liu, Li, Li, Li, & Liu, 2019), sensitive detection (Mao et al., 2018), herbicide adsorbents (Liu, Wang, Liu, Yi, Zhou, & Liu, 2019) and nanocomposite hydrogels (Heydari & Sheibani, 2015). In our previous study, the extended β-CD-MOF was monoclinic with a P 21 space group. Its repeating unit consisted of one β-CD molecule, one K+ ion and one water molecule. Adjacent β-CD molecules were connected via the coordination of the potassium cations to the –OCCO- units located on the d-glucopyranosyl residues of β-CD, forming double channel subunits, shaped like “Figure-8”. β-CD-MOF has a large specific surface area and high porosity when compared with traditional reported materials, which results in its great affinity for the inclusion of menthol and use as a menthol carrier. However, these experimental results presented some uncertainties including the major factors affecting its menthol encapsulation under environmental conditions and the physico-chemical properties of its inclusion complex (IC).
Developing an efficient and stable method to achieve the encapsulation of a large amount of menthol is of great interest. To further facilitate the interfacial interaction between menthol molecules and β-CD-MOF, the effect of different factors on the encapsulation menthol were investigated to verify the experimental parameters. The common optimization approach changes the level of one parameter at a time, while keeping other variables constant (Shahbaz Mohammadi, Mostafavi, Soleimani, Bozorgian, Pooraskari, & Kianmehr, 2015). In this study, we investigated four parameters: Temperature, mass ratio of menthol to β-CD-MOF, time and menthol-loading method during our encapsulation optimization study. The menthol content (MC), stability, thermal properties and release properties of menthol from β-CD-MOF-IC upon oral digestion were investigated. This is the first report to study the influencing factors in the interfacial interaction between an aroma compound and MOF. This study has allowed us to identify the optimal inclusion condition for menthol encapsulation and determine its release kinetics from the β-CD-MOF-IC, in particular, for its controlled release under production and storage conditions.
Section snippets
Chemicals and materials
l-Menthol (purity ≥ 99%), l-Menthol (purity ≥ 99.5%, GC grade) was used as a standard, β-cyclodextrin (β-CD, purity ≥ 96%) was obtained from Aladdin Biochemical Technology Co., Ltd (Shanghai, China). Potassium hydroxide (KOH, purity > 85%), ethyl acetate, absolute ethanol and methanol were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). All other chemicals were of analytical grade and used as received without further purification.
Preparation of menthol encapsulated β-CD-MOF
The β-CD-MOF was obtained using a modified
Encapsulation under different temperature conditions
Temperature had a significant influence on the EE and MC of the IC samples (Fig. 1, a1 and a2). The MC of menthol/β-CD-MOF-IC was significantly changed upon increasing the encapsulation temperature from 60 to 75 ℃ and then to 90 ℃, increasing from 8.0 to 21.7% and then to 26.5%. A similar trend was observed in the EE results, as its value increased from 6.3 to 20.3% and then to 30.6%. Both the MC and EE of the IC samples reached a maximum value when the temperature was 90 ℃ and it then began to
Conclusions
The results of the present work support the possibility of preparing a high menthol-loading IC and the development of a controlled release delivery system for menthol based on an β-CD-MOF. We successfully determined a set of parameters that influence the MC and EE; namely, a temperature of 90 ℃, a menthol-β-CD-MOF mass ratio of 1:1, a reaction time of 60 min, and our grinding method was found to be the best combination of process conditions to induce the highest MC and EE of 27.1 and 30.6%,
CRediT authorship contribution statement
Ziman Hu: Methodology, Software, Investigation, Formal analysis, Writing – original draft. Miao Shao: Writing – review & editing. Bin Zhang: Writing – review & editing. Xiong Fu: Writing – review & editing. Qiang Huang: Conceptualization, Supervision, Writing – review & editing, Funding acquisition, Validation.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors acknowledge the financial support received from the National Natural Science Foundation of China (31771929), Natural Science Foundation of Guangdong Province (2019A1515012174), 111 Project (B17018) and Science and Technology Project of Zhuhai City (ZH22036207200022PWC), Guangdong, China.
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