Ultrasound Viscosity Measurements Allow Determination of Gas Volume Fraction in Foamed Gels

Abstract

Aerated whey protein gels were obtained by gelation of preheated solutions at room temperature by calcium, magnesium and ferrum (II) ions at different pH and protein concentration. The density of aerated and non-aerated gels was determined by the flotation method. Dynamic viscosity of aerated and non-aerated gels was measured using an ultrasonic viscometer. Air fraction of obtained aerated gels ranged from 24.5 to 76.8%. There was a linear increase in the product of viscosity and density values with increasing concentration of non-aerated gels. For aerated gels, the highest values of the product of viscosity and density were noted at 7.5 and 8.0% (w/w). A formula for calculation of the air fraction in aerated gels by ultrasound viscosity measurements was derived. Air fraction values obtained by density measurements correlated with the air fraction values obtained by ultrasound viscosity measurements (R2 = 0.99). Measurements of the product of viscosity and density values by an ultrasound viscometer allow the determination of the air fraction of aerated gels. Practical Applications Different methods can be applied to obtain aerated whey protein gels. One common method capitalizes on the gel reversibility. Another common method is based on the gelation of preheated solutions induced at room temperature by salt addition. Foamed gels can be applied in food technology and as matrices for active ingredient release. Ultrasonic viscometers are used for continuous measurement of viscosity under conditions where measurement is difficult and does not influence the sample shape. Viscoelastic properties of the gels can be measured using oscillatory rheometry, but ultrasound viscosity measurements can also characterize the viscosity of the gel samples. Ultrasound viscometers are cheaper than oscillatory rheometers and the measurements are simpler.