Sorption Behavior of Mixtures of Glycerol and Starch

Abstract

Glycerol is often added to starches to plasticize the product, but the presence of glycerol may also affect the water content of the samples. To evaluate the effect of glycerol on the sorption properties of starches, waxy maize, rice, and wheat starch were thermomechanically extruded in the presence of glycerol. Sorption isotherms of these extruded samples were ascertained using dynamic vapor sorption (DVS). BET and GAB modeling showed a monolayer (mo) significantly higher for waxy maize than for rice and wheat. Glycerol inclusion changed the model values, indicating reduction in sorption energy at the monolayer and restructuring of the multilayer. An interaction factor (xi) based on weight fraction models was calculated. Differences in xi were obtained when glycerol was added, varying from approximately 0.9 for 5% glycerol to approximately 0.8 for 20% glycerol, supporting the hypothesis of interactions between starch and this polyol.

How does water diffuse in starch/montmorillonite nano-biocomposite materials?

“…A maximum D eff value was observed at a w = 0.7, where the equilibrium moisture content was 0.18 g g −1 d.b. This typical bell-like curve for D eff values as a function of a w is in agreement with other studies (Enrione et al, 2007;Hanson, Cramer, Abraham, & Lancaster, 1971;Karathanos, Villalobos, & Saravacos, 1990;Yu, Schmidt, Bello-Perez, & Schmidt, 2008). Yu et al (2008), using the same method of water sorption kinetic than that used in this study, reported a maximum value of D eff in dent corn starch granules (30% of crystallinity) at 0.5 a w and significantly lower values for a w < 0.5 and >0.5.…”

“…for high a w values. These values were in accordance with those obtained by Enrione, Hill, and Mitchell (2007) on extruded wheat starch containing 20% of glycerol (EMC 0.43 g g −1 against 0.53 g g −1 d.b. in this study at 0.9 a w ).…”

“…They were in the same order than values found for films prepared from pea starch (Zhang & Han, 2008) or potato starch (Rindlav-Westling et al, 1998). A precise comparison is difficult since water vapour transfers are strongly affected by various parameters such as the botanic origin of starch (Enrione et al, 2007), the respective content of amylose and amylopectin (Phan, Debeaufort, Luu, & Voilley, 2005;Rindlav-Westling et al, 1998), the crystallinity (Arvanitoyannis et al, 1994;Mali et al, 2006), the type and content of plasticizer (Enrione et al, 2007;Tang, Alavi, & Herald, 2008b;Zhang & Han, 2008) and the conditions of storage such as relative humidity (Phan et al, 2005). The addition of nanoclays has a significant effect on WVP only for filler content of 6 wt% and for native MMT (MMT-Na) with a decrease of WVP from 2.6 to 2.2 × 10 −11 mol m −1 s −1 Pa −1 .…”

“…Hanson et al (1971), using the sorption kinetic technique, found that the diffusivity of water in individual corn starch granules reached a maximum near a moisture content of 0.15 g g −1 d.b. And finally, Enrione et al (2007) using the same DVS system than that used in this study, found that D eff in thermo-mecanically extruded wheat starch containing 20% of glycerol went by a maximum for an average a w of 0.65. The bell-like curve for D eff vs. a w was also observed for other hydrophilic, porous products such as cerealbased products (Guillard et al, 2003;Roca et al, 2008).…”

“…Indeed, after a threshold value of a w , plasticization of the polymer would not occur anymore and D eff would stabilize to a plateau. It should be noted that in such hydrophilic material, the high amounts of water sorbed in the material for a w > 0.7 could provoke additional phenomena such as polymer relaxation, which may induce a change in the diffusional mechanism as suggested previously by Enrione et al (2007) to explain the unexpected decrease in diffusivity value for a w above 0.6 in their extruded wheat starch. The Fickian model used to obtain D eff would be not entirely appropriate to represent this different diffusional mechanism.…”