Emulsification efficacy of Quillaja saponins at very low concentration: Model development and role of alcohols

doi:10.1016/j.colsurfb.2017.08.041

Colloids and Surfaces B: Biointerfaces

Volume 159, 1 November 2017, Pages 829-837

https://doi.org/10.1016/j.colsurfb.2017.08.041 Get rights and content

Highlights

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A model was developed to quantify surface coverage of Quillaja saponins.
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Quillaja saponins showed a head surface of 1.37 nm² with a lay-on configuration at interface.
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The model was used to understand surface activity of terpenic and medium chain alcohols.
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Alcohols enhanced emulsification efficacy of saponins in a surfactant-oil-matrix system.
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Distribution of alcohols at interface was driven by partitioning.

Abstract

The present study aims at quantifying interfacial coverage of a biosurfactant (Quillaja saponins) and understanding the impact of flavor and fragrance alcohols on emulsification efficacy of the biosurfactant in a surfactant-oil-matrix system. Emulsions were prepared using limonene, alkanes (C₈, C₁₂, and C₁₆) or limonene ̶ alcohol (linalool and C₆ single bond C₁₀ alcohols) mixtures at different ratios as oil phase stabilized by Quillaja saponins at very low concentrations (0.005–0.05% w/w). Droplet size was measured and size distributions were numerized to determine surface and volume average droplet diameters of bimodal emulsions. Using a model developed in the present study, Quillaja saponins showed an interfacial coverage of 5.0 × 10⁶ cm²/g and a head surface of 1.37 nm² with a lay-on configuration at interface. The model proved to discriminate between surface active (alcohols) and non-active (alkanes) compounds. The apparent interfacial coverage of saponins increased linearly with increasing alcohol concentration. The type of alcohol (terpene alcohol vs. medium chain alcohols) and alcohol chain length (C₆ single bond C₁₀) showed little impact on emulsification efficacy of Quillaja saponins. The molar ratio of heptanol to saponin at interface increased from 0 to 8.6 corresponding to 0–30% w/w heptanol in limonene. This study revealed that the distribution of alcohol at interface was mainly driven by partitioning in the surfactant-oil-matrix system. The practical implication of the present study is to enhance emulsification efficacy of Quillaja Saponins at very low concentration by incorporating surface active compounds, i.e. flavor or fragrance alcohols.

Graphical abstract

Introduction

Emulsification is one of the first unit operations in most of the oil encapsulation processes (e.g. spray dry and hot melt extrusion) and is the most important step for successful encapsulation [1], [2]. An optimal encapsulated product is built upon a good emulsion. Stable and smaller droplet size emulsion often results in higher active retention; in contrast, unstable or large droplet size emulsion is often associated with poor active retention, poor yield, high surface oil content, and accelerated deterioration [3], [4], [5], [6]. Therefore, it is important to understand the emulsification process from both molecular and process standpoints. At a molecular level it is necessary to be able to describe and identify the parameters controlling the formation and stability of an emulsion [7]. Models have been developed to quantify the surface activity of various emulsifiers (e.g., surfactants [8], proteins [9], nanoparticles [10], etc.). A well fit model adapted to a practical surfactant-oil-matrix system (including bimodality) could be utilized to determine the efficiency of surfactants in stabilizing an interface [7]. However, even the most efficient surfactant system faces challenges when complex materials, such as flavor or fragrance oils, are at the core of the emulsion. The challenges are attributed to the great variety of chemical substances in the core and their significant impact on the interface.

Terpene alcohols and medium chain alcohols (C₆ single bond C₁₀) are important aroma or odor compounds present in many flavor and fragrance oils. Solubilization of these alcohols has been studied in micelles [11], liquid crystals [12], [13] and microemulsions [14], [15]. Strey and Jonströmer [16] reported that medium chain alcohols (C₄C₁₀) preferentially dissolve in surfactant monolayer of nonionic bicontinuous microemulsions. Tchakalova and Fieber [17] demonstrated that perfume raw materials can be classified by interfacial solubility according to their functional groups following the order of alcohols > aldehydes > terpenes > aromatics > alkanes. All these studies suggest that alcohols have a significant impact on the formation and the stabilization of an interface. In order to develop high quality emulsion-based delivery system, there is a need to better understand surface activity of flavor and fragrance compounds.

Quillaja saponins have gained attention in recent years because of their properties as foaming agents and natural surfactant, as well as their applications in cholesterol-reduction and flavor enhancement [18], [19]. The basic structure of Quillaja saponins consists of glycosides linked to triterpene aglycones. The hydrophilic groups of the molecule consist of sugars such as rhamnose, xylose, arabinose, galactose, fucose, etc., while the hydrophobic portions of Quillaja saponins are triterpenoid rings [20]. Purified Quillaja extract has been commercialized as a natural alternative to gum Arabic and modified starch in stabilizing flavor emulsions and nanoemulsions [21], [22], [23], [24]. The main surface properties of saponin molecules including surface activity [25], surface rheology [26] and adsorption kinetics [27] have been characterized by a few authors. All these studies suggest Quillaja saponin extract is an efficient natural surfactant capable of stabilizing various interfaces which lead us to use it in the present study.

Here, a model is developed to describe a typical bimodal emulsion and characterize the interfacial coverage of Quillaja saponins at very low concentrations in limonene-in-water emulsions. The low concentration of surfactant is particularly interesting because of cost reduction and concerns of surfactant usage (i.e. off-taste and regulation). Furthermore, this model was applied in emulsions containing limonene ̶ alcohol (i.e. linalool and C₆ single bond C₁₀ alcohols) mixture and the impact of alcohol on emulsification efficacy of Quillaja saponins was investigated. The mechanism of alcohol distribution at interface is discussed.

Section snippets

Model development

The surface coverage of a surfactant is defined as the stable surface generated from a given quantity of surfactant with respect to the oil volume to be stabilized. The total surface (S) of a number (N) of spherical oil drops with a known surface average diameter ( $\bar{d_{s}}$ ) is expressed as Eq. (1). $S = N (π {\bar{d_{s}}}^{2})$

The number of oil drops (N) is also attainable through volume (V) relationship using the volume average diameter ( $\bar{d_{v}}$ ) and Eq. (2). $V_{o i l} = N (\frac{π}{6} {\bar{d_{v}}}^{3})$

The total surface created (S) for a given volume

Materials

Quillaja saponin extract (Q-Naturale^® 100) was obtained from Ingredion (Bridgewater, NJ, USA) and the material was used as received without further purification. This liquid extract contains about 20% dry matter with 14% active saponins and the remainder being mainly sugar, tannins and phenols [28], [29]. It has to be noted that in present study surfactant concentration is expressed on saponin basis rather than total mass basis. Maltodextrin (10 DE) was purchased from Tate & Lyle (Decaur, IL,

Determination of d_s and d_v

The model presented was applied to evaluate the performance of Quillaja saponins in a model flavor emulsion serving as spray dry feed material. The model emulsions contain 45% limonene, saponin from 0.0014% to 0.161%, and a matrix solution (15% sucrose, 15% 10 DE maltodextrin, and 70% deionized water). It should be noted that all percentages are expressed by weight in the present study. The emulsions showed bimodal size distribution at lower surfactant concentrations (Fig. 1a) and the size

Conclusion

In the present study, a model was developed to characterize the emulsification efficacy of a biosurfactant, Quillaja saponins, at very low concentrations (<0.05% w/w). Using the developed model, a head surface of 1.37 nm² for Quillaja saponins with a lay-on configuration at interface was determined which is comparable to reported values varying from 0.98 to 1.19 nm² [25], [26]. The model proved to be able to discriminate between surface active (linalool and medium chain alcohols) and non-active

Acknowledgements

Prof. T. Zemb and Dr. D. Benczedi are acknowledged for their support and fruitful discussions. This manuscript has been submitted in the memory of our colleague A. Subramaniam who tragically passed away after initiating this study.

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ProtocolsEmulsification efficacy of Quillaja saponins at very low concentration: Model development and role of alcohols

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Model development

Materials

Determination of ds and dv

Conclusion

Acknowledgements

Trends Food Sci. Technol.

Carbohydr. Polym.

J. Colloid Interf. Sci.

Colloids Surf. A Physicochem. Eng. Asp.

Colloids Surf. A Physicochem. Eng. Asp.

Food Hydrocolloid

Colloids Surf. B Biointerfaces

Food Hydrocolloid

LWT-Food Sci Technol.

Food Chem.

J. Food Eng.

Colloids Surf. B Biointerfaces

Food Hydrocolloid

Food Chem.

Food Chem.

Biomaterials

J. Colloid Interf. Sci.

Food Chem.

Phytochem. Lett.

Adv. Colloid Interface Sci.

Biophys. J.

J. Colloid Interf. Sci.

Protocols
Emulsification efficacy of Quillaja saponins at very low concentration: Model development and role of alcohols

Determination of d_s and d_v