The structural behavior of a well-characterized gelatin sample has been revisited to investigate the morphology of its network in the presence of sugar. This was then contrasted with the corresponding properties of the gelling polysaccharides agarose, kappa-carrageenan, and deacylated gellan. Small deformation dynamic oscillation, differential scanning calorimetry in plain and modulated mode, visual observations, and transmission electron microscopy were used to identify the structural characteristics of the biopolymers from the rubbery plateau through the transition region to the glassy state. In contrast to the collapse of the polysaccharide gels at intermediate levels of co-solute, gelatin forms reinforced networks. The drop in polysaccharide network strength is accompanied by a decline in the enthalpy of the coil-to-helix transition, whereas the transition enthalpy is more pronounced in gelatin gels in accordance with their strengthening. Tangible evidence of the molecular transformations was obtained using microscopy, with polysaccharides disaggregating and dissolving in the saturated sugar environment. Gelatin, on the other hand, is visualized in an aggregated form thus producing a phase-separated topology with sugar.

No abstract

Over the past few years, a considerable amount of work has been done in several laboratories on the measurement of structural properties of low-solid biopolymer mixtures or high-solid materials of a single biopolymer in the presence of co-solute. The main objective of this work has been to establish a correlation between the two types of systems and extend it to a binary mixture in a high-solid environment. In doing so, it employed well-characterized kappa-carrageenan and gelatin samples in an aqueous preparation or in the presence of glucose syrup and sucrose. The phase behavior of the composite gel was ascertained using small-deformation dynamic oscillation, differential scanning calorimetry, and light microscopy. Experimental observations were built into polymer blending laws that argued for an explicit phase topology and distribution of solvent between the two networks. A working hypothesis was formulated and applied to high-solid mixtures thus identifying phase or state transitions in the time/temperature function. This led to the development of a mechanical glass transition temperature as the threshold of two distinct molecular processes governing the "rubber-to-glass" transformation. A stage was reached at which the predictions of the hypothesis were found to be in good agreement with the experimental development of viscoelasticity in the high-solid kappa-carrageenan/gelatin mixture ranging from the rubbery plateau and the transition region to the glassy state.

The structural properties of four gelatin fractions in mixture with sucrose and glucose syrup have been investigated extensively using small deformation dynamic oscillation. The total level of solids was 80%, the number average molecular weight of the protein ranged from 29.2 to 68 kD, and the temperatures were between 60 and -60 degrees C. Remarkably, the nature of the time and temperature dependence on the viscoelastic functions of all samples could be reduced to master curves using horizontal shift factors. The construction of master curves indicates a common mechanism of structure formation, which, in accordance with the synthetic polymer literature, comprises the rubbery zone, glass transition region, and glassy state. Application of Ferry's free-volume formalism and Rouse theory suggests that there is no change in the thermodynamic state of materials during vitrification, with changes in molecular weight simply introducing shifts in the time scale and temperature range of contributions to viscoelasticity. The thermorheological simplicity allowed development of the concept of "rheological" Tg. This was defined as the point between free-volume phenomena of the polymeric backbone occurring in the glass transition region and an energetic barrier to rotation required for local chain rearrangements in the glassy state. Mechanical relaxation and retardation distribution functions were calculated, thus obtaining values for the effective friction coefficient per monomer unit of the protein. It appears that the local friction coefficient is governed by a linear relationship between fractional free volume and the decreasing molecular weight of the protein, which introduces additional voids due to molecular ends.

The understanding of synthetic polymer viscoelasticity was applied to the small deformation properties of high-sugar gelatin and kappa-carrageenan mixtures. The glass transition zone in sugar/gelatin mixtures exhibited a dominant liquid-like response, which was followed by the method of reduced variables. The glass transition temperature predicted by the WLF/free volume approach coincided with the crossover of storage and loss modulus at the onset of the glassy state. The viscoelastic spectrum was resolved into a basic function of temperature and a basic function of time, thus rationalizing their effect on the vitrification of the mixture. Vitrification was only one in a plethora of viscoelastic properties. Manipulation of the composition of the mixture generated a continuous gelatin matrix or a viscous sugar phase suspending a dispersion of ice crystals. Besides the small-molecule crystallinity, vitrification can be halted by ordered macromolecular helices observed in the kappa-carrageenan/potassium system. Thus, the solid-like component of the sugar/kappa-carrageenan network remained prevalent over the predicted frequency window. The WLF equation was unable to follow the progress of viscoelastic functions, which were better described by an energetic barrier of rotation from one state to another.