Understanding structure/rheology relationships of amidated low-methoxyl citrus and apple pectin gels: Implications of sucrose, pectin types and characteristics

In this study, the relationships between structure and rheology (at small and large deformations) of amidated low-methoxyl (LMA) citrus and apple pectin gels were investigated considering how does these relationships can be modulated by soluble solids (sucrose), pectin types and characteristics. For that purpose, various LMA pectins with varying degrees of methyl esterification (DE) and amidation (DA), processed from citrus peels and apple pomace, were used. Scaling models of biopolymer gels have been critically discussed in respect to pectin critical gelling concentration (C0) and their structural variabilities. It was demonstrated that, whatever the types of LMA pectins and their structural variabilities, scalar percolation and quadratic models can be efficiently used for the prediction of the storage modulus G′ and C0 as other biopolymer gels. Fractal approach reveals that the origin of the LMA pectin gel elasticity is more in harmony with an enthalpic network of rod-like filaments with bending rigidity ruling the network's elasticity. The addition of sucrose induced an anticipation of the gelation temperature and an important improvement of the viscoelastic properties of the resulting gels. It was also shown that the most tough gels are obtained for citrus pectins due to the higher level of neutral sugar side chains in apple pectins that can hinder preponderant ca-bridging sites unlike citrus pectins. Finally, it was shown that unlike the firmness and hardness of the gels which appear to be more driven by the amide groups, the deformability of the gels is rather governed by the methyl ester groups.