The effect of mixing conditions on the material properties of an agar gel - microstructural and macrostructural considerations

The effect of mixing on the properties of agar gels was investigated with consideration being given to both macrostructural and microstructural characteristics of the gel through rheological techniques that include conventional and ultrasound based methods and Differential Scanning Calorimetry. Agar gels of 1 and 3% concentrations were prepared. The gels were subjected to three different mixing conditions: no mix, slow mix (400 rpm), and fast mix (900 rpm). A pulse echo ultrasound technique was utilized to obtain velocity measurements and these velocity measurements were used to obtain ultrasound derived mechanical modulus values. A controlled-stress rheometer was used to obtain rheological properties of the gel as well. Differential Scanning Calorimetry (DSC) was performed to determine thermal transitions of the various systems studied in order to obtain information on the degree of microstructural association of the agarose fractions. Mixing speed affected the degree of porosity induced in the system, by incorporation of the air bubbles, and the rheology of the system. Both ultrasound derived mechanical properties and those obtained in a conventional rheometer showed that mixed gels were stronger than the no mix gels in spite of the no mix gels exhibited negligible porosity, i.e. they did not have bubbles that could weak the macrostructure of the gels. In addition, DSC results indicated that gels mixed at different conditions exhibited thermal transitions at different temperatures. These results showed that material properties of agar gels might be more sensitive to changes in the microstructure than the macrostructure of these systems. This highlights the importance on preparation conditions and utility of an agar gel. (c) 2005 Elsevier Ltd. All rights reserved.