Effectiveness of encapsulating biopolymers to produce sub-micron emulsions by high energy emulsification techniques

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Abstract

In this study, different emulsifying ingredients were used to produce sub-micron emulsions for encapsulation purposes. Maltodextrin combined with a surface-active biopolymer (modified starch, or whey protein concentrate), or a small molecule surfactant (Tween 20) were used as the continuous phase, while d-limonene was the dispersed phase. Results showed that biopolymers are not efficient ingredients to produce very small emulsion droplets compared with small molecule surfactants because of their slow adsorption kinetics. The main problem with surfactants also is instability of the resulted emulsions due to “depletion and bridging flocculation” caused by free biopolymers and competition between surfactant and surface-active biopolymers. In general, it was not possible to produce a fairly stable microfluidized emulsion with surfactants for encapsulation purposes.

Introduction

Emulsification is one of the important and critical steps in microencapsulation of food oils and flavours through spray drying and emulsion properties such as stability and droplet size1 play a key role in optimizing the encapsulation efficiency during the process (Barbosa et al., 2005, Danviriyakul et al., 2002, Liu et al., 2000, Liu et al., 2001, Risch and Reineccius, 1988). A stable emulsion with minimum droplet size can increase the retention of volatiles and shelf-life of encapsulated oil products through reduction of unencapsulated oil at the surface of powder particles (Ishido et al., 2002, Minemoto et al., 2002, Soottitantawat et al., 2003, Soottitantawat et al., 2005). So, sub-micron emulsions can be of real benefit for encapsulation purposes.

The production and control of submicron emulsions with a narrow size distribution have been attracting considerable attention in recent years. Nano-(submicron) emulsions are kinetically stable systems that can be transparent (EDS <200 nm) or “milky” (EDS  500 nm) (Izquierdo et al., 2002, Tadros et al., 2004), and because of their very small EDS and high kinetic stability, they have been applied in various industrial fields, for example, personal care and cosmetics, health care, pharmaceuticals, and agrochemicals (Forgiarini et al., 2001, Schulz and Daniels, 2000, Solans et al., 2005, Sole et al., 2006, Sonneville-Aubrun et al., 2004). Production of nano-emulsions by “High-energy emulsification” methods like Microfluidization involves an application of very high amounts of energy (e.g., high pressures) on a previously prepared coarse emulsion to produce very small emulsion droplets. Some workers believe Microfluidization is superior because, EDS distributions appeared to be narrower and smaller in Microfluidized emulsions than in the traditional emulsifying devices (Dalgleish et al., 1996, Pinnamaneni et al., 2003, Robin et al., 1992, Strawbridge et al., 1995). It is shown, however, that Microfluidization is unfavourable in specific circumstances such as higher pressures and longer emulsification times, as it leads to “over-processing”, which is re-coalescence of emulsion droplets (Jafari et al., 2006a, Jafari et al., in press, Lobo and Svereika, 2003, Olson et al., 2004).

Final EDS is the result of equilibrium between droplet break-up and re-coalescence. Between new droplet formation and its subsequent encounter with surrounding droplets, emulsifiers adsorb onto the created interface to prevent re-coalescence. If the timescale of emulsifier absorption is longer than the timescale of collision, the fresh interface will not be completely covered and will lead to re-coalescence, i.e., an EDS increase (Desrumaux and Marcand, 2002, Kolb et al., 2001, Marie et al., 2002, Perrier-Cornet et al., 2005). Fast stabilization of new interfaces by sufficient emulsifier molecules is an efficient way to prevent re-coalescence (Brosel and Schubert, 1999, Floury et al., 2003, Karbstein and Schubert, 1995, Schulz and Daniels, 2000, Stang et al., 1994, Stang et al., 2001). An effective emulsifier should adsorb rapidly at the fresh interface created during emulsification, reduce interfacial tension appreciably to facilitate droplet disruption, and prevent new droplets from flocculation by providing a protective layer around them.

There are many different emulsifiers available to incorporate into emulsions; some of them are solely emulsifier such as Spans and Tweens (Floury et al., 2004, Marie et al., 2002) and some have both emulsifying and stabilising properties such as milk proteins and modified starches (Mohan and Narsimhan, 1997, Tesch et al., 2002). Slow emulsifiers, like biopolymers and high molecular weight surfactants can only be used effectively in emulsification systems with high residence times, such as colloid-mills, or multistage high pressure systems because, they get the chance to stabilize newly broken up droplets more than once. Small-molecule emulsifiers such as Tween 20 stabilize new interfaces in milliseconds, so that the droplets are unlikely to re-coalesce. When a mixture of emulsifiers is present, different molecules compete to adsorb at oil-water interface and lower the interfacial tension (Arboleya and Wilde, 2005, Dickinson, 2003, Klinkesorn et al., 2004, McClements, 2004). Since low molecular weight surfactants are much smaller in size than biopolymers, and because they can reduce interfacial tension more efficiently and quickly by adsorbing a large number of molecules within the same surface area, they are likely to dominate at the interface after equilibration, if both are present at high enough bulk concentrations (Kerstens et al., 2006, Mackie et al., 2000, Pugnaloni et al., 2004).

By the advent of modern emulsification systems and their potential application in encapsulation of food ingredients, understanding the mechanisms of emulsification and the behaviour of emulsion components along with the knowledge of factors affecting the emulsion properties during emulsification is essential. Also, there has been limited work to produce sub-micron emulsions with small molecule surfactants for encapsulation purposes. In fact, most of the published work in the emulsion territory is dealing with pure emulsions consisting water, oil and emulsifier. While in emulsification for subsequent encapsulation purposes, there is another constituent involved, so-called wall material or encapsulation matrix, which is mainly a biopolymer and has some direct and indirect influences on the emulsion properties. Therefore, the objectives of this work are to determine the optimum emulsification conditions and evaluate the influence of extreme emulsification conditions of Microfluidization on emulsion stability and droplet size by applying different surface-active biopolymers and surfactant.

Section snippets

Materials

d-Limonene (ρ = 840 kg/m3, η = 8.8 mPa s at 25 °C, RI = 1.487) was supplied by Quest International (NSW, Australia). Modified starch (Hi-Cap 100, waxy corn starch-modified, 5% moisture, solubility > 90%) and Maltodextrin (DE = 16–20, 5% moisture, bulk density = 600 kg/m3) were purchased from National Starch and Chemical (Sydney, Australia), and Penford Limited (NSW, Australia), respectively. Whey protein concentrate (73% protein, 9% fat, 4% moisture, 5% lactose, 4% ash) was purchased from New Zealand Milk

Influence of different surface-active biopolymers

Since our final goal was to prepare sub-micron emulsions for spray drying to produce encapsulated powders, we investigated the behaviour of two different biopolymers having emulsifying properties (Hi-Cap vs. WPC) during Microfluidization. The results (Table 2) showed that the biopolymer type had a very significant effect (P < 0.01) on EDS during Microfluidization. The effect of oil content and pressure was also very significant (P < 0.01). Another interesting result was the significant interaction

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