Fixation and stability enhancement of beta-carotene by organo-modified mesoporous silica

https://doi.org/10.1016/j.micromeso.2015.08.019Get rights and content

Highlights

  • The inner pore of mesoporous silica was modified by alkyl chains using alcohols.

  • β-carotene was incorporated in the pore of the organo-modified mesoporous silica.

  • Incorporated β-carotene showed improved stability against visible irradiation.

  • Durability against elution was enhanced by the incorporation into the mesopore.

  • Stabilization of the dye depended on the hydrophobic nature of the inner pore space.

Abstract

β-carotene is one of naturally occurring dyes, and has been used as a colorant because of its nontoxicity and biological functions. However, the color stability of the β-carotene is not sufficient. The aim of this study is to enhance the stability of the β-carotene by incorporation into the pore of mesoporous silica. Although the highly lipophilic β-carotene does not generally meet rather hydrophilic inner pore space of mesoporous silica, the modification of the inner pore by alkyl chains enables the incorporation of the β-carotene into the mesopore. The incorporated β-carotene is successfully stabilized against visible light irradiation. Also improved is the durability of the dye in the composite sample against elution to the solvent. The effect of the complexation on the stability enhancement depends on the affinity of the lipophilic β-carotene to the organo-modified inner pore space.

Introduction

Carotenoids are one of the groups of naturally occurring dyes. They are found in many vegetables and fruits, and very common to the human life. Among them, the carotenoids that are made only of carbon and hydrogen are called carotenes. β-carotene is a typical compound belonging to carotenes, and is known as a precursor of vitamin A [1].

The most characteristic structure of the β-carotene is its highly conjugated double bonds with a center of symmetry (Scheme 1). Due to this hydrocarbonaceous structure, the β-carotene has a very high hydrophobic nature and an affinity to the hydrocarbon tails of surfactants [2]. The highly conjugated polyene structure is also responsible for the yellow to orange color of the β-carotene deriving from the light absorption of visible region. In addition, the β-carotene is known to show some important biological functions such as antioxidant properties and inhibition of cancer [3], [4], [5]. Because of its nontoxicity, as well as its color and biological functions, the β-carotene has been widely used as a food colorant [1].

However, the conjugated π-bonding is generally reactive and not so resistant to the oxidation. Like other compounds with the highly conjugated double bonds, the β-carotene easily loses its color by oxidative degradation in the air, especially under illumination [6], [7]. Therefore, the improvement of the color stability is a requisite step for the extensive use of the β-carotene as a general colorant [8].

The composite of various dye molecules and inorganic host materials is one of the promising technique to acquire the improved stability, and various inorganic materials, such as clay, zeolite and mesoporous silica, have been found to be effective for this purpose [9], [10], [11], [12], [13], [14], [15]. Among those inorganic host materials, silica-based one possesses non-acidic or non-basic nature and can be adopted for more general usage. In addition, the mesoporous structure provides the large surface area including inner pore space, which may result in a more dye adsorption and a vivid color. We have reported that the natural anthocyanin [16], [17], [18] or its model compounds, flavylium [19], [20], can be stabilized by the incorporation into the pore of mesoporous silica. Although the dyes with a hydrophobic nature is not suitable for the adsorption to the surface of the mesoporous silica or clays [21], it has been reported that the modification of the clay interlayer spaces with the alkyl chains of surfactants (organo-modified clays) enables the adsorption and stabilization of the hydrophobic natural dyes [10], [22], [23], [24]. The hydrophobic environment can also be achieved in the pore of the mesoporous silica by the modification of the sidewall inside the mesopore with the alkyl chains [17]. Since several research groups reported the stability enhancement of the β-carotene using hydrophobic host materials such as carbon nanotubes [25] or organo-modified clays [22], the stabilization of the β-carotene can be expected through the incorporation into the organo-modified mesoporous silica.

Here, we report the possibility to improve the stability of the β-carotene using the mesoporous silica as an inorganic host material. As the mesoporous silica samples, we used the MCM-41 and HMS type ones, which possessed the straight and wormhole-like mesopore, respectively [26], [27]. In order to give the hydrophobic nature to the mesopore space, the inner surface of the mesoporous silica was modified with the alkyl chains using alcohols (organo-modification). The shape of the mesopore and the length of the alkyl chains may have some effects on the light fastness of the dye and its resistance to the elution. Since the β-carotene is highly lipophilic, nonaqueous solvents are usually required to dissolve and disperse the β-carotene molecules before adsorption onto the inorganic hosts. However, from the environmental point of view, the use of organic solvents is not favorable. Therefore, in this study we employed a simple method to obtain the composite materials: mixing both the β-carotene and the mesoporous silica in a solid state. This physically mixing technique is superior to the adsorption from the dye solution, because the mixing of the powdery crude materials is an easy operation as a dry process. The light fastness and the resistance to the elution of the obtained composite samples were investigated.

Section snippets

Sample preparation

The HMS type mesoporous silica was prepared in the manner described in the literature [20]. Dodecylamine (Wako Chemical Co., 0.91 g) was dissolved in 5 cm3 of ethanol and mixed with 45 cm3 of water. Tetraethoxysilane (TEOS, Wako Chemical Co., 4.60 g) was dropped to the solution slowly, and the mixture was kept at 333 K for 22 h. The resulting sediment was collected by filtration and calcined at 903 K for 6 h under dry air stream. After grinding in mortar, the powdered HMS sample was obtained.

Characterization of the organo-modified mesoporous silica samples

The XRD patterns of the HMS and MCM-41 samples before and after organo-modification with 1-butanol and 1-octanol are represented in Fig. 1. As shown in Fig. 1a and d, both the HMS and MCM-41 samples exhibited a diffraction peak at around 2°, suggesting the mesoporous structure in those samples. The modification of HMS and MCM-41 with the alcohols caused almost no change in the peak position and intensity of the XRD patterns. This means that the procedure of the organo-modification induced no

Concluding remarks

The β-carotene was successfully included in the pore of the two types of mesoporous silica, HMS and MCM-41, when the inner pore was modified with alkyl chains. The stability of the dye against visible light irradiation and the resistance against washout with ethanol were both improved by the incorporation into the mesopore. The degree of the improvement was increased along with the hydrophobic environment. The stabilization effect on the β-carotene was more obvious in the MCM-41 composite

Acknowledgment

This work was financially supported by JSPS KAKENHI Grant Numbers 24700787 and 25289279.

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