Electron Microscopy of the Gel-forming Ability of Polysaccharide Food Additives

Abstract

Water-insoluble polysaccharides such as curdlan, agar, carrageenan κ and ι, and konjac glucomannan form gels when solutions obtained by heating them are cooled. Curdlan alone and konjac glucomannan with calcium hydroxide still remain as gels at a higher temperature. These polymers were found to form gels by neutralization of their alkaline solutions in the stationary state. Soluble and slimy polysaccharides in water such as carrageenan λ, screloglucan, succinoglycan, xanthan gum, pullulan and dextran are unable to form gels. Electron micrography showed that the gels were composed of long microtibrils of 50Å to 250Å width with interconnections, although the microtibrils of carrageenan ι were shorter, whereas the viscous solutions were composed of short microfibrils of about 10Å to 20Å width. The structures of the gels formed by heating were similar to those formed by neutralization, except in the case of curdlan gel. Dialyzed carrageenan κ and its H-, K- and Na-forms had low or undetectable gel strength and were seen by electron micrography to be composed of short microfibrils. However, the presence of potassium ions in aqueous suspensions of the Na-form enhanced the gel strength and produced longer microfibrils. These results indicate that longer and or wider microfibrils are required for the formation of a gel.