(181am) Prediction of Swelling and Linear Viscoelasticity of Starch Suspensions

Starch pasting is the process by which starch granules uptake water in a heated aqueous medium and thicken a starch dispersion. This process is common in a variety of food products in order to produce a given texture/feel, but is also important in a wide variety of non-food applications that range from paper coating to the fabrication of paints. Currently, the pasting of starches in industry is controlled in an Edisonian fashion where one judiciously adjusts the processing conditions (temperature history, flow conditions, etc.), or adjusts the starch structure/composition through starch choice, cross-linking, and/or chemical modification. The development of physics-based models of starch pasting will be useful in order to design these materials more cheaply and quickly. This would require understanding the swelling of starch granules, the conditions under which they will rupture, the extent of release of its contents to the aqueous medium upon rupture and the effect of these on the rheology of suspension.

In the first part of this talk, we describe a mathematical model that describes the swelling kinetics of starch granules when subjected to heating. The model is based on a Flory-Rehner theory of gel swelling, and accounts for the structure and composition of different types of starches through (i) starch-solvent interaction as quantified by static light scattering (ii) gelatinization temperature and enthalpy of gelatinization (iii) porosity and its variation with swelling and (iv) crosslinking of starch molecules within the granule from equilibrium swelling. The mechanistic model is able to quantitatively predict the evolution of granule size distribution of a variety of starches such as normal and waxy maize, normal and waxy rice, and normal (Penpure 80) and modified potato (Novation 1600) starches when subjected to heating at different temperatures in the range of 60 to 90 ï‚°C.

In the second half of the talk, we discuss the rheology of starch dispersions during heating and swelling. We observe two distinct regimes. When the granule volume fraction is above the closed packed limit of spheres (65%), the granules have a packing microstructure that is akin to a foam, and hence one can use classical ideas of foam rheology to describe the storage modulus, which is roughly constant in this regime. In the volume fraction below 65%, the storage modulus depends mainly on the volume fraction of swollen granules and not on heating temperature and time. These experiments were done over a wide range of starch types, heating rates, and heating temperatures. Consequently, it appears that the rheology in this low-volume fraction regime can be described reasonably well from only the knowledge of their size distribution, which is obtained from the swelling models described above. We compare our experimental results of linear viscoelasticity to Stokesian dynamics simulations to test the former hypothesis. Lastly, we find through appropriate scaling of the storage modulus, the storage modulus of a wide range of starches onto a master curve. This master curve when employed along with the swelling model resulted in the successful prediction of development of texture for different types of starches. The above methodology can quantify the effects of structure and composition of starch on its pasting behavior and would therefore provide a rational guideline for modification and processing of starch-based material to obtain desirable texture and rheological properties.