Novel extensional device to efficiently form fine oil‐in‐water food emulsions

Abstract

Two immiscible liquids are commonly mixed by mechanically dispersing one into the other to form emulsions. Surfactants or emulsifiers confer stability. Mechanical mixing, in practice, is an energy‐intensive shear flow that is ineffective when the ratio of the dispersed‐phase viscosity to the continuous‐phase viscosity exceeds about four. Extensional flows are not subject to this viscosity ratio limit. This superiority of extensional flow was exploited to fabricate a novel, continuous‐flow, cone‐shaped device with an extensional strain of eight to make fine soybean oil‐in‐water emulsions. A spherical insert having a wall clearance of 25 μm was an effective design factor. Starting with ‘coarse’ 50 wt.% oil emulsions, two stretching episodes were needed for size reduction. The temperature rise was negligible, and the results were independent of the emulsifier type employed. Increasing flow rate and stretching episodes, reducing wall clearance, enhancing emulsifier concentration, and multiple passes through the device gave progressively smaller drops; the volume‐averaged diameter became less than 2 μm, and the number‐averaged diameter reached 0.5 μm, narrowing the size distribution. The emulsions that formed had a high viscosity and were stable. The performance of a scaled‐up device was compared with other mixers. At equivalent energy density and 50 wt.% oil, drop sizes were similar for a valve homogenizer but larger for a rotor‐stator mixer. At 80 wt.% oil, the rotor stator‐mixer again required more energy for the same drop size, but emulsions prepared with the valve homogenizer broke. The findings of this study can help to design industrial‐scale energy‐efficient extensional‐flow dominant devices for the formation of food emulsions.