Carbohydrate Polymers in Amorphous States:  An Integrated Thermodynamic and Nanostructural Investigation

Abstract

The effect of water on the structure and physical properties of amorphous polysaccharide matrices is investigated by combining a thermodynamic approach including pressure- and temperature-dependent dilatometry with a nanoscale analysis of the size of intermolecular voids using positron annihilation lifetime spectroscopy. Amorphous polysaccharides are of interest because of a number of unusual properties which are likely to be related to the extensive hydrogen bonding between the carbohydrate chains. Uptake of water by the carbohydrate matrices leads to a strong increase in the size of the holes between the polymer chains in both the glassy and rubbery states while at the same time leading to an increase in matrix free volume. Thermodynamic clustering theory indicates that, in low-moisture carbohydrate matrices, water molecules are closely associated with the carbohydrate chains. Based on these observations, we propose a novel model of plasticization of carbohydrate polymers by water in which the water dynamically disrupts chains the hydrogen bonding between the carbohydrates, leading to an expansion of the matrix originating at the nanolevel and increasing the number of degrees of freedom of the carbohydrate chains. Consequently, even in the glassy state, the uptake of water leads to increased rates of matrix relaxation and mobility of small permeants. In contrast, low-molecular weight sugars plasticize the carbohydrate matrix without appreciably changing the structure and density of the rubbery state, and their role as plasticizer is most likely related to a reduction of the number of molecular entanglements. The improved molecular packing in glassy matrices containing low molecular weight sugars leads to a higher matrix density, explaining, despite the lower glass transition temperature, the reduced mobility of small permeants in such matrices.

Structural and thermodynamic aspects of plasticization and antiplasticization in glassy encapsulation and biostabilization matrices

“…Important properties, which were investigated, are the specific volume and the molecular hole volume as probed by positron annihilation lifetime spectroscopy (PALS). The studies were mainly focussed on starch [46] and starch-derived polymers and oligomers [47][48][49][50], but also included ethyl cellulose [51], gelatin [52,53], poly(vinyl) alcohol (PVA) [54], hydroxypropyl methylcellulose [55] and poly(ethylene) oxide (PEO) [56] as well as several disaccharides, including trehalose [57] and maltose [49,58]. Several of these studies were specifically aimed at investigating the impact of low molecular weight diluents, such as water [46][47][48][49]51,58], glycerol [52,53,59], sorbitol [53] and maltose [49,58] on the structure and thermodynamic properties of the glassy matrix.…”

Understanding polymeric amylose retrogradation in presence of additives

“…For example, the short-chain maltodextrin, in the presence of water, have greater mobility compared with amylose chains 15 and because of the inherent molecular similarity between the amylose and maltodextrin, the extent of cocrystallization of maltodextrin and amylose chains is increased, resulting in water loss. Earlier studies showed that by increasing the water activity of gelatinized starch samples, the net matrix crystallinity was increased, 18 which is similar to antiplasticization behavior. This is due to the increased free volume by smaller plasticizers that lead to greater chain association to coalesce and ''squeeze-out'' water.…”

Food Structure Engineering for Nutrition, Health and Wellness

“…Such glassy matrices show aging during storage and will eventually crystallize (Descamps et al 2009). Amorphous matrices can be very effective for stabilizing oxygen-sensitive bioactives because their permeability can be reduced to a minimum by combining mono-, di-, oligo-, and polymers to a blend with a very low free specific volume (Kilburn et al 2005). Adding an increasing amount of substances with low molecular weight to oligo-or polymers reduces the permeability of the molecular matrix but also increases its moisture sensitivity.…”