Ordered conformation of xanthan in solutions and “weak gels”: Single helix, double helix – or both?

Abstract During the progress of xanthan from a recent discovery to its present status as a well-established food hydrocolloid, some investigations have indicated that the ordered structure, which underlies most of the practical applications of xanthan, is a single helix stabilised by ordered packing of sidechains along the polymer backbone, giving first-order kinetics for the disorder–order transition. Others favoured a coaxial double helix, whose formation causes a doubling of molecular weight and mass per unit length. It is proposed here that both interpretations are correct, and that ordering is a 2-stage process: formation of single helices followed, under favourable conditions, by enthalpically-driven conversion to coaxial double helices. Comparison of recent evidence from atomic force microscopy with models from analysis of X-ray fibre diffraction data suggests that the most likely coaxial arrangement is an antiparallel 5 1 double helix. Graphical abstract Image Highlights • Rapid progress of xanthan from recent discovery to major food hydrocolloid. • Accompanying evidence of both single-helix and double-helix structures. • Proposed interpretation in terms of 2-stage process of conformational ordering. • Stage 1: coil–single helix transition; stage 2: conversion to 5 1 double helices. [ABSTRACT FROM AUTHOR]