Influence of sugar surfactant structure on the encapsulation of oil droplets in an amorphous sugar matrix during freeze-drying
Introduction
O/W emulsion is one of the forms of oil that is frequently used as in foodstuffs. A surface active substance is usually used as an emulsifier, and the characteristics of the surfactant are determinants of the stability of such O/W emulsions. A wide variety of surfactants have been examined for their contributions to emulsion stability, and considerable knowledge and know-how in terms of tuning an O/W emulsion stability are currently available (Benita and Levy, 1993, Capek, 2004, Tadros et al., 2004).
On the other hand, in the case of oily flavoring substances, hydrophobic therapeutic agents, insoluble taste components, and related food additives, it is often necessary to convert them into a solid (powder) form from the viewpoint of the handling and usage. In the case of solidification (powderization), the oily materials are emulsified into O/W emulsions with the aid of a surfactant and then dehydrated in the presence of bulk forming agents. Consequently, the emulsified oil droplets, or oil droplets containing hydrophobic substances, are embedded in the resulting dried bulk matrix. The encapsulation of emulsified oil droplets has been extensively investigated (Corveleyn and Remon, 1999, Desai and Park, 2005, Gharsallaoui et al., 2007, Gibbs et al., 1999, Gouin, 2004, Porter et al., 1996, Shimada et al., 1991), in which the spray drying is frequently employed (Anwar and Kunz, 2011, Desobry et al., 1997, Gharsallaoui et al., 2007, Hogan et al., 2003). As a result, a close relationship has been found between encapsulation efficiency, emulsion size distribution (Minemoto et al., 2002, Soottitantawat et al., 2003, Soottitantawat et al., 2005) and types of bulk-forming agent (Matsuno and Adachi, 1993, Moreau and Rosenberg, 1996, Paramita et al., 2010) as well as drying conditions (Anwar and Kunz, 2011, Desobry et al., 1997, Hansen et al., 2004, Jonsdottir et al., 2005).
Oil droplets can be encapsulated via freeze drying. In freeze-drying, the low temperature below the freezing point and the absence of air thoroughly minimize the chemical degradation of labile components (Anwar and Kunz, 2011, Longmore, 1971) although it is reported to cost 30–50 times more than spray-drying (Desobry et al., 1997). Another merit of freeze-drying is that the amorphous state of encapsulation matrix is easily obtained, even when easy-to-crystallize materials, such as sucrose and trehalose, are used as matrix-formers (Bayram et al., 2005, Drusch et al., 2006). The deposition of dried preparations on the dryer wall and consequent yield loss is less likely to occur in the freeze-drying compared with spray drying (Anwar & Kunz, 2011).
It would naturally be expected that the combination of oil phase, surfactant, and matrix materials is essential for the efficiency of oil encapsulation in a dried solid (powder). In our previous study, the characteristics of various types of sugars in the encapsulation of O/W emulsion droplets during freeze-drying were investigated (Imamura et al., 2013). A model O/W emulsion, comprised of linoleic acid methyl ester and sucrose monolaurate as the oil phase and surfactant, respectively, was freeze-dried in the presence of various types of sugar and mixtures, and the amount of oil droplet encapsulated in the dried matrix was determined. The findings indicated that the loss of oil droplets mainly occurred at the primary stage of the freeze-drying process, i.e., during the sublimation of frozen water, and the oil droplets that failed to be encapsulated were segregated on the wall surface of the sample container. The final encapsulation efficiencies for different matrix-formers (sugars) after freeze-drying could be correlated with the glass transition temperatures of sugars in the fully hydrated state (Tg⁎). Based on these findings, it was possible to optimize oil droplet encapsulation efficiency by combining different types of sugars so as to give a Tg⁎ of around − 50 °C (Imamura et al., 2013).
In the next step, the influence of the molecular structure of the surfactant on oil droplet encapsulation in a freeze-dried matrix of amorphous sugar was investigated in this study. Sugar-derived surfactants were employed in this study, since they would be expected to have a high compatibility with the bulk-forming sugar matrix. Linoleic acid methyl ester (LME) was used for an oil phase, and trehalose was mainly used as a matrix-former, respectively. As emulsifiers, various types of sugar surfactants, with different alkyl chain lengths, sugar head groups, type of bond linking the sugar head group to the alkyl chain, and alkyl chain length, were used as well as other categories of surfactants. The O/W emulsions, emulsified with various surfactants, were freeze-dried and the efficiencies of encapsulation of oil droplets in freeze-dried amorphous sugar matrix were compared. The molecular structure of the sugar surfactant that is the most suitable for the oil droplet encapsulation in freeze-drying was discussed. Furthermore, combinations of different types of sugar surfactants were also investigated for their effect on oil droplet encapsulation in a freeze-dried sugar matrix.
Section snippets
Materials
d-(+)-Xylose, xylitol, fructose, α-glucose, trehalose, α-maltose, and sucrose were products of Wako Pure Chemical Industries, Ltd. (Osaka, Japan). Maltotriose, -tetraose, and -pentaose were obtained from Hayashibara Biochemical Laboratories, Inc. (Okayama, Japan). Dextrans with mean molecular masses of 1500 and 40,000 were from Sigma-Aldrich Co. (St. Louis, MO). Twenty four types of surface active substances, including eighteen types of sugar surfactants and seven types of commercial sucrose
Results and discussion
Sugar surfactants were compared with other categories of surfactants in terms of the contributions to the LME encapsulation efficiency during freeze-drying in the presence of trehalose (Fig. 1). The data shown in Fig. 1 suggests that ionic surfactants are inferior to non-ionic ones for LME encapsulation, including a commercial sugar ester mixture (L-1695), arabic gum, and lecithin. This can occur when the removal of solvent water enhances the repulsion among neighboring ionic head groups of
Conclusions
Different types of sugar surfactants were compared for their contributions to oil droplet encapsulation during freeze-drying an O/W emulsion in the presence of a sugar. An optimum alkyl chain length of sugar surfactant for the oil droplet encapsulation in a freeze-dried sugar matrix exists. The ester bond of a sugar surfactant was a major factor in oil droplet encapsulation during freeze-drying, relative to an ether bond. Concerning sugar esters, a larger sugar head group appeared to be better,
Acknowledgments
This work was supported by Grant-in-Aids for Science Research (C) (No. 23560908) from the Ministry of Education, Science, Sport and Culture of Japan, the Japan Food Chemical Research Foundation, the Information Center of Particle Technology, Japan, and Iijima Foundation for Food Science.
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