Non-contact ultrasonic measurements in food materials

A novel air-coupled transducer and signal generator is used to measure the speed of sound and thickness (5–15 mm) of samples of various food products (cheese, reduced fat cheese, luncheon meat, reduced fat luncheon meat, and cranberry sauce). Measurements were made with two transducers (1 MHz) placed one either side of the sample at a distance of approximately 30 mm from the food surface. Thicknesses measured by this method were similar to measurements made using calipers for all samples. At a single point the ultrasonic measurements were very precise (s.d. 0.003 mm, n=12) but when several points on the same slice were averaged the precision reflected unevenness in the slicing operation. Ultrasonic velocity measurements were made using non-contact and conventional contact-mode ultrasound. The precision of both methods was similar but the apparent speed of sound measurements made in contact mode were significantly higher, suggesting the soft solids studied were compressed during analysis.