The design of novel food micro-structures aimed at the quality, health and pleasure markets will probably require unit operations where the scale of the forming device is closer to the size of the structural elements (i.e., 1–100 μm). One emerging possibility is microfluidics or devices that employ small amounts of fluids (10−6 to 10−9 l) flowing in channels where at least one dimension is less than 1 mm. However, under these conditions, the predominant effects are not necessarily those present in conventional macroscopic unit operations. Dominant physical effects at the microfluidic scale are introduced through the use of dimensionless numbers. Different types of geometries to generate multi-phase flows in micro-channels, techniques and materials to construct the micro-devices, principally soft lithography and laser ablation, as well as methods used to modify surface properties of channels, are reviewed. The operation of micro-devices, the role of flow regimes, rheological behaviour of fluids in micro-channels and of transient time is discussed. Finally, systems developed to generate emulsions and foams, fluid mixing and dispersion, and future applications of these devices in food processing and food analysis are presented.

Applications of Microfluidic Devices in Food Engineering

Bubble formation and breakup dynamics in microfluidic devices: A review

Aerated gels contain both bubbles and entrapped water, thus offering ample versatility in product development. Dispersed air (or other gases) provides an additional phase within the gel that may accommodate new textural and functional demands. Many food polymers form gels and their target properties may be enhanced by combining materials (mixed polymer gels) or introducing a finely dispersed fat phase (emulsion gels). Traditional methods to generate bubbles in foods as well as non-conventional technologies (membrane processes, microfluidics, etc.) are revised and their potential applications in producing aerated gels are discussed. Aerated gels may find applications in reducing the caloric density of foods and inducing satiety, as carriers of flavors and nutrients, and in novel gastronomic structures.

Aerated food gels: fabrication and potential applications

Mechanical properties of calcium alginate fibers produced with a microfluidic device.

The present study aims at scaling the formation of slug bubbles in flow-focusing microfluidic devices using a high-speed digital camera and a micro particle image velocimetry (μ-PIV) system. Experiments were conducted in two different polymethyl methacrylate square microchannels of respectively 600 × 600 and 400 × 400 μm. N2 bubbles were generated in glycerol–water mixtures with several concentrations of surfactant sodium dodecyl sulfate. The influence of gas and liquid flow rates, the viscosity of the liquid phase and the width of the microchannel on the bubble size were explored. The bubble size was correlated as a function of the width of the microchannel W
 c, the ratio of the gas/liquid flow rates Q
 g/Q
 l and the liquid Reynolds number. During the pinch-off stage, the variation of the minimum width of the gaseous thread W
 m with the remaining time could be scaled as $$ W_{\text{m}} \propto ({\frac{{Q_{\text{g}} }}{{Q_{\text{l}} }}})^{ - 0.15} (T - t)^{1/3} . $$
 The velocity fields in the liquid phase around the thread, determined by μ-PIV measurements, were obtained around a forming bubble to reveal the role of the liquid phase.

P. Bouchon

Papers

Formation of bubbles and foams in gelatine solutions within a vertical glass tube