Time dependent studies of foam stability using image analysis, electrical resistivity and ultrasound
The production of highly aerated foods remains a challenge that requires skill to obtain desired appearance and texture. Since foams are fragile and inherently unstable, evaluation of structure changes with time requires a delicate approach. Non-invasive but informative evaluation of changes in aerated food properties is a desired goal to be achieved in this thesis. I studied the aging of egg white foams of different void fraction using two noninvasive techniques - ultrasonic spectroscopy and electrical resistivity - with a view to understanding aging mechanisms that would affect the quality of the final product. To help in interpretation of the results, a commercial high void fraction foam, Gillette (Sensitive), was also analyzed. As a support technique for ultrasound results I used image analysis in order quantitatively evaluate the size of bubbles in the foams. Large increases in attenuation were observed with increasing aging time and frequency. Results were modeled using an effective medium theory originated by Foldy (1945) in order to understand the factors governing foam lifetime and texture. From electrical resistance measurements, liquid fraction changes in an egg white foam column were evaluated quantitatively and qualitatively by using a generalized free drainage model. Although egg-white foams were more prone to changes due to drainage, I observed that for all foams the ultrasonic scaling parameter alfa*lambda/freq was proportional to the square of average bubble size (indicative of a diffusively driven aging process due to disproportionation of bubbles). Slopes of alfa*lambda/freq versus aging time were 6*10-8 and 11*10-8 for egg white foams of void fraction 0.65 and 0.78, respectively, indicating that disproportionation progressed approximately twice as fast in the high void fraction foam. The slopes of alfa*lambda/freq versus aging time were similar for both Gillette foam void fractions (0.93 and 0.91) at a value of 1.5*10–8 s-1, attributable to a lower solubility of isobutane compared to air. By combining ultrasound and electrical resistivity, this thesis has provided novel insights into understanding instability processes occurring in foams. Potentially, ultrasound techniques could be used instead of imaging for foam aging studies, since non-invasive and non-destructive measurements of attenuation and phase velocity permit interrogation of opaque foam structures.
ultrasound, food foams, electrical resistivity, foam imaging