Research Note Droplet size determination in food emulsions: comparison of ultrasonic and light scattering methods John N. Coupland * , D. Julian McClements Department of Food Science, Food Colloids and Biopolymers Laboratory, University of Massachusetts, Amherst, MA 01003, USA Received 26 November 1996; received in revised form 18 September 2000; accepted 4 November 2000 Abstract Ultrasonic velocity and attenuation spectra 1±5 MHz) were measured for a series of corn-oil-in-water emulsions 20 wt% oil in 2 wt% polyoxyethylene sorbitan monolaurate) with a range of dierent mean droplet diameters 0.5±1.5 lm). Multiple scattering theory was used to calculate mean diameters for each emulsion from both spectra. Droplet size was also determined using two commercial light scattering instruments. All techniques showed the same general trend of decreased size with increased degree of homogenization but there was only limited quantitative agreement between the various measurement techniques. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Emulsion; Ultrasound; Particle-sizing; Light-scattering 1. Introduction The colloidal properties of a food are important in determining bulk qualities such as mouthfeel, appear- ance and rheology Dickinson & McClements, 1995); this is particularly true for food emulsions. A food emulsion consists of a suspension of small oil droplets in an aqueous continuum e.g., milk, salad dressings, ice cream mix) or vice versa e.g., butter, margarine, choc- olate). It is therefore important to develop methods to accurately characterize the colloidal properties of food emulsions. The properties of a food emulsion that most con- tribute to its perceived characteristics are particle size distribution and oil concentration. Direct determina- tion of these parameters is dicult because droplets are often too small to be resolved by conventional light microscopy. Electron microscopy provides ade- quate resolution, but sample preparation is more dis- ruptive and may generate artifacts. The most commonly used technique for the characterization of food emulsions is laser light scattering, but this is only suitable for very dilute systems / < 0:05 wt% and dilution of more concentrated emulsions may disrupt delicate aggregates. Nuclear magnetic resonance has been used to characterize concentrated food emulsions but the apparatus is expensive and dicult to operate Dickinson & McClements, 1995). Measurements of the electrical properties of an emulsion can also be related to its particle size distribution e.g., Coulter Counter, electroacoustic techniques), but these tech- niques are often limited to charged particles in a conducting medium. Techniques to extend the ease-of-use and versatility of laser scattering particle sizing instruments to the more concentrated emulsions commonly found in foods would therefore be valuable. An ideal technique would be reliable, rapid, and applicable to real food emulsions under a range of process conditions. It would also be advantageous to make on-line, non-invasive measure- ments for process control. Ultrasonic sensors have the potential to meet many of these needs. It has been shown that the scattering of ultrasound by an emulsion is related to the size and concentration of the droplets as well as the physicochemical properties of the component phases McClements, 1996). Various theories exist to predict the ultrasonic properties of an emulsion Allegra & Hawley, 1971; Harker & Temple, 1988) as a function of droplet size and concentration. By calculating the particle size distribution that generates the best theo- retical ®t to experimental measurements it is possible to characterize an unknown emulsion. Journal of Food Engineering 50 2001) 117±120 www.elsevier.com/locate/jfoodeng * Corresponding author. Present address: Department of Food Science, 103 Borland Laboratory, The Pennsylvania State University, University Park, PA 16802, USA. Tel.: +1-814-865-2636; fax: +1-814- 863-6132. 0260-8774/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII:S0260-877400)00201-6