Thermal transitions in aqueous carrageenan solution 
Carrageenans are the name given to a family of linear sulphated food grade polysaccharides obtained from red seaweeds. They have the unique ability to form an almost infinite variety of gels at room temperature, rigid or compliant, tough or tender with high or low melting point. Carrageenan solutions will thicken, suspend and stabilize particulates as well as colloidal dispersions and water/oil emulsions. They are used extensively in the food and other industries today, for example as a secondary stabilizer in ice cream, in the preparation of evaporated milk, dairy desserts, chocolate milk and in meat coating. The above figure illustrates the multi-frequency ultrasonic analysis of the melting of a carrageenan gel using HR-US 102 spectrometer. Below 30ºC and above 48ºC, ultrasonic velocity exhibits linear temperature dependence, which is a result of the normal (for most materials) decrease in storage modulus with temperature. Between these two temperatures, a transition is shown clearly. The increase in ultrasonic velocity was caused by an increase in the hydration level of the atomic groups of the carrageenan, as a result of the melting of its helical structure and breaking the intermolecular connections between the carrageenan molecules. The drop in ultrasonic attenuation in the transition temperature interval results from the loss of friction between the frozen (unmovable at ultrasonic frequencies) polymer network and the moving solvent in a course of compressions and decompressions in the ultrasonic wave. The frequency dependence of velocity and attenuation (storage and loss moduli subsequently) provide additional information on the dynamic behaviour of the gel network. In this system, high resolution ultrasonic spectroscopy allows detection of the gelation point and interval as well as the analysis of the transformations in the polymer's helical structure (ultrasonic velocity) and characterises the structure of the gel network (ultrasonic attenuation). return to top of the page Progression of milk fermentation by a Lactobacillus strain 
The above figure illustrates the monitoring of enzymatic activity of Lactobacilli culture in milk. These bacteria are used as starter cultures in the manufacture of fermented food products such as cheeses and yoghurts. The bacteria first produce the enzyme lactate dehydrogenase, which turns lactose from the milk into lactic acid. As the pH drops, the negatively charged amino groups on the casein are neutralised, solubilising colloidal calcium phosphate and hence destabilising the inherent structure of the casein micelles within the milk. The destabilised micelles flocculate into clusters, and then link to form a particulate gel. High-resolution ultrasonic spectroscopy can be used to monitor the pre-gelation and gelation processes as they occur. An ultrasonic cell of HR-US 102 spectrometer was filled with milk, and the culture added. As shown in the picture above, the entire fermentation process is complete within three hours. In the pregelation phase during the first hour, the constant level of the ultrasonic attenuation indicates that no structural changes take place in the milk. The rise in ultrasonic velocity represents the production of lactic acid by the culture. However, after approximately one hour, the casein micelles begin to aggregate followed by the formation of the gel network, which leads to an increase in ultrasonic attenuation. After 1.7 hours, the bacterial activity slows down, and the ultrasonic velocity levels off as a result. However, as can be seen from the ultrasonic attenuation measurements, the gel's structure continues to change. At approximately 2 hours, the velocity curves are frequency dependent, which is characteristic of non-homogeneity in the sample. After 3 hours, the gel becomes homogeneous, as indicated by the velocity curves at different frequencies overlapping again. return to top of the page Aggregation and gel formation of cold-setting gel in acidified whey protein solutions 
The kinetics of acid induced cold-gelation consists of pre-gelation, gelation, and the post gelation stages. Pre-gelation occurs over the first two hours after acidification of the whey protein solution, and is shown by an increase in the ultrasonic velocity, (see above) which results from the hydration changes in the system due to unfolding and protonation of the whey protein. The strong frequency dependence of the velocity and attenuation indicates microstructural non-homogeneity of the samples.  An increase in particle size is indicated by a sudden increase in ultrasonic attenuation, as shown in the above figure, beginning at the 80 min mark. This is shown by extrapolation of the dotted lines back to the 80 minute intercept on the time axis. The formation of the gel network is indicated by inflections in the ultrasonic attenuation and ultrasonic velocity profiles beginning at about 110 minutes. After five hours, both ultrasonic parameters reach a plateau as the gel network formation is completed. After being formed, the overall gel network remains stable, as shown by the constant value of the ultrasonic attenuation for 12 hours. The decrease in the ultrasonic velocity with time from five to twelve hours occurs as a result of ongoing conformational and chemical changes within the gel network. A detailed analysis of the magnitude of the changes in ultrasonic velocity and attenuation upon the sol-gel transition allows quantitative characterization of microscopic structure of proteins and their aggregates at various stages of the process. [reference] return to top of the page | 
