Production of water-in-oil nanoemulsions using high pressure homogenisation: A study on droplet break-up

Highlights

• Oil continuous nanoemulsion produced with 50 nm droplet sizes.

• Both high pressure devices produced similar droplet sizes and distributions.

• Addition of salt into the dispersed phase reduced droplet size.

• The droplet size was independent of viscosity ratio indicating elongational flow.

Abstract

In this paper we compare the efficiency of a Microfluidizer to a high pressure valve homogeniser (HPH) for the production of oil continuous emulsions by investigating the effect of pressure, number of passes, phase viscosities and salt addition on droplet size. The results obtained show that the Microfluidizer and HPH have similar emulsification efficiency, giving droplets of ∼60 nm diameter at a pressure of 50 MPa. By increasing the pressure to 100 MPa the droplet diameter was reduced by 10 nm in both devices. Increasing the number of passes in the HPH caused an increase in droplet size probably as a consequence of the temperature increase from multiple passes causing an increase in coalescence. Changing the viscosity ratio from 0.001 to 1 resulted in a minimal change in droplet size indicating that the flow is elongational when break-up occurs rather than turbulent as expected.

Introduction

There is an increasing level of interest in the production of nanoemulsions and submicron emulsions in the food industry. These emulsions have several advantages including a much longer stability, enhanced mouthfeel, faster flavour release and if the droplets are small enough (below 50 nm droplet diameter) will deliver oil soluble micronutrients and bioactives in an imperceptible way (McClements, 2011).

Nanoemulsions can be produced using low energy techniques that exploit chemistry to cause phase inversion (Solans et al., 2005) or high energy techniques including high pressure valve homogenisation (HPH) and high pressure impinging jet devices (e.g. Microfluidizer).

Droplet break-up in high-pressure systems has been reported to have areas of both turbulent and elongational flow (Håkansson et al., 2011, Floury et al., 2004, Lee and Norton, 2012). When the flow regime is turbulent, droplet break-up is either from cavitation or shear. Droplet break-up from shear is described by the Kolmogorov–Hinze theory (Kolmogorov, 1949, Hinze, 1955). Two types of droplet break-up regimes are identified: turbulent inertial and turbulent viscous (Walstra and Smulders, 1998). Turbulent inertial break-up occurs when the droplet size is of a similar size to the smallest scale eddies in the system, this scale is known as the Kolmogorov length scale. In the turbulent viscous break-up regime, droplet sizes are reduced to below the size of the smallest eddies in the system by the shearing forces created within these eddies. Droplet deformation and break-up in turbulent viscous flow is considered to be mechanistically similar to simple shear (Walstra, 2003), and typically occurs between viscosity ratios of 0.1–5 (dispersed phase viscosity/continuous phase viscosity) (Walstra, 2005).

A HPH consists of a piston pump and a narrow gap, where the operating pressure is up to 150 MPa. Droplet break-up occurs within the region of the valve gap and in the jet after the gap, where the flow is elongational and then turbulent, respectively (Floury et al., 2004). The advantage of a HPH is that it is scalable for industrial production. A Microfluidizer operates to a similar maximum pressure generated via an air-driven piston pump, and droplet break-up occurs from high turbulence and shear created by the collision of two impinging jets oriented at 180° to each other (Cook and Lagace, 1985, Siddiqui et al., 2009). The Microfluidizer is currently used within pharmaceuticals, however, the production rate is low and as such is thus not yet suitable for food production.

In order to determine the droplet break-up mechanism it is important to understand the geometry of the systems and the factors that affect energy dissipation including the volume over which the energy dissipates. It has been reported that in a HPH a jet is formed at the exit of the gap where the majority of the energy dissipates: producing a stable and large eddy that causes the jet to become unstable and attach to a wall (Innings and Trägårdh, 2007). It has been proposed that the majority of the droplet break-up occurs at the outer regions of the jet, this is because the difference in velocity of the jet and the surrounding fluid produces the highest shearing forces.

The Microfluidizer consists of a small chamber, where an impinging plane is formed by the collision of two inlet jet streams at 180° (each with approximately 75 μm diameter). The region of impingement is characterised by its fast dissipation of turbulent kinetic energy (Gavi et al., 2007, Siddiqui et al., 2009). The droplet break-up occurs during energy dissipation of the jets impinging creating high shear forces for droplet deformation and break-up.

A recent study (Lee and Norton, 2012) investigated the droplet break-up in a Microfluidizer and HPH for the production of oil in water (O/W) nanoemulsions. In this study it was shown that the Microfluidizer produced similar droplet sizes to the HPH, however, the HPH required several passes to achieve the final droplet size. It was suggested that the impinging jet in the Microfluidizer creates large shearing stresses within the highly turbulent impingement region thus creating droplet deformation and breakup in the first pass, the presence of elongational flow subsequent to impingement increased the time for emulsifier adsorption and therefore the droplet size distributions produced showed minimal evidence of coalescence whereas the geometry of the HPH caused some droplet coalescence.

This study aims to compare the same homogenising techniques as (Lee and Norton, 2012) for the purpose of water in oil (W/O) nanoemulsion production. There has been limited work on W/O nanoemulsions produced from high pressure devices and as such the parameters affecting emulsion formation explored; homogenisation pressure, number of passes, emulsifier concentration and viscosity ratio. In addition to this the effect of salt on the droplet size of the emulsion was studied. This follows other W/O studies on the micron scale that suggest the addition of salt into a polyglycerol polyricinoleate (PGPR) stabilised oil continuous emulsion increased stability and reduced the size of the water droplets (Márquez et al., 2010, Pawlik et al., 2010).