Rheological Study of the Gelation Process of Agarose-Based Solutions

The gelation of agarose is investigated by rheological methods and electron microscopy, as well as the thickening properties of xanthan. The gelling and thickening agents have been investigated in pure water to compare the results with theoretical models. The gelation of agarose was shown to follow two steps upon cooling, which could be addressed to the formation of helices and their aggregation. In addition to the rheology, transmission electron micrographs of freeze-dried samples have been taken to underline the date by corresponding structures at different stages of the gelling process. The xanthan molecules, which have been approximated by rigid highly charged rodlike molecules, undergo a jamming transition at a critical concentration. This concentration shows a strong dependence on the length of the molecules, which supports the high thickening effect of xanthan. When both, agarose and xanthan are mixed, the gel structure becomes very different. The gelling process is now determined by one step only. It is proposed that the jamming xanthan molecules prevent the formation of the aggregates of the agarose gel. The gels themselves appear then less elastic, and should yield a better mouth feeling.