Electron Microscopic Studies on the Ultrastructure of Curdlan and Other Polysaccharides in Gels Used in Foods

Curdlan gels form either by neutralization of alkaline solutions or by heating aqueous suspensions. The former gels consist of long rnicrofibrils, 20 to 25 nm wide, made up of subunits, 2-3 nm wide. The microfibrils disintegrate on sonication into fibril units which on heating loosen and release thinner elementary fibrils, 1-3 mm wide. Heated gels are composed of electron-dense structures in pseudocrystalline form. Preparations having their average degree of polymerization (DPn) 200, are unable to form gels. Agar, K- and t-carrageenans, and konjac glucomannan gels formed by cooling of heated sols have structures similar to gels produced by neutralization of their alkaline solutions. Both neutralized and heat-induced gels are composed of long microfibrils, about 5-25 run wide. Viscous solutions of A-carrageenan, scleroglucan, succinoglycan, xanthan gum, pullulan, and dextran are composed of shorter rnicrofibrils, 1-2 nm wide. Sodium salts of K-carrageenan, alginate, gellan gum, and low-methoxyl pectin form gels in the presence of potassium or calcium ions. The microfibrils in these gels are considerably longer and/or wider than in the sols. Locust bean gum does not gel alone but yields gels containing fibrous or globular forms when mixed in solution with xanthan gum or K-carrageenan and potassium ions. Fibrous structures of amylose and amylopectin in starch gels appear to be similar. Short-chain amylose (D!\u27n 57 and 78) consists of branched microfibrils about 10 mm wide. Molecular association resulting in the formation of long and/or wide rnicrofibrils in rod-like or globular forms is supposedly one of the prerequisites for polysaccharides to form gels