Ultrasonic studies of foods containing air
Aerated foods are an important sector of the processed food market, but they are poorly characterised by the available techniques. Ultrasound has a complex interaction with bubbly liquids, which has the potential to be used as a means of non- destructively characterising bubbly food liquids. The ultrasonic velocity, attenuation coefficient and reflection coefficient can be related to the volume fraction of gas and bubble size distribution using theories which are known to work at low volume fraction. Ultrasonic equipment has been described which can measure the ultrasonic velocity and attenuation coefficient very accurately, and which can make reflection measurements on systems which defy transmission measurements. A dilute model bubbly system, a concentrated model bubbly system, yogurt, cream, bread dough and beer foam have been characterised ultrasonically, and by densitometry, light microscopy, electron microscopy and light scattering. The model system has been shown to be stabilised by an unusual and novel structured surfactant layer at the interface. Conventional measurements on the dilute model system did not agree with ultrasonic transmission measurements. The ultrasonic technique was able to measure volume fractions as low as 10'7, but it could not measure the bubble size. Reflection measurements on the concentrated bubbly liquid demonstrated that the theory fails at high volume fractions of air. The ultrasonic technique has been shown to be able to measure the bubble size in whipped yogurt. The presence of an agglomerated fat globule network has been shown to have a dramatic effect on the ultrasonic properties of whipped cream. The ultrasonic technique was able to discriminate between conventional whipped cream and spray cream, and the technique has been shown to be sensitive to the time-dependent properties of spray cream. The small volume fraction of air present in unfermented dough was detected by the ultrasonic reflection technique. Fermented dough could not be characterised ultrasonically. Marked inter sample variability was demonstrated in measurements on beer. The ultrasonic technique was of limited use for characterising beer foam. Realisation of the full potential of ultrasound for characterising real aerated foods will not be possible without an improved theory for concentrated systems.