Oil-in-water emulsions are an important vehicles for the delivery of hydrophobic bioactive compounds into a range of food products. The preparation of very fine emulsions is of increasing interest to the beverage industry, as novel ingredients can be added with negligible impact to solution clarity. In the present study, both a batch and focused flow-through ultrasonic cell were utilized for emulsification with ultrasonic power generation at 20–24 kHz. Emulsions with a mean droplet size as low as 135 ± 5 nm were achieved using a mixture of flaxseed oil and water in the presence of Tween 40 surfactant. Results are comparable to those for emulsions prepared with a microfluidizer operated at 100 MPa. The key to efficient ultrasonic emulsification is to determine an optimum ultrasonic energy intensity input for these systems, as excess energy input may lead to an increase in droplet size.Industrial relevanceThe preparation of oil-in-water emulsions is a common feature of food processing operations. The use of ultrasound for this purpose can be competitive or even superior in terms of droplet size and energy efficiency when compared to classical rotor­stator dispersion. It may also be more practicable with respect to production cost, equipment contamination and aseptic processing than a microfluidisation approach. The present paper shows that ultrasound can be effective in producing nanoemulsions for use in a range of food ingredients.