Volume Fraction Dependence of Linear Viscoelasticity of Starch Suspensions
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When starch granules are gelatinized, many complex structural changes occur as a result of large quantity of water being absorbed. The enlargement of granule sizes and the leaching out water-soluble macromolecules contribute to the viscoelasticity. Starch pasting behavior greatly influences the texture of a variety of food products such as canned soup, sauces, baby foods, batter mixes etc. It is important to characterize the relationship between the structure, composition and architecture of the starch granules with its pasting behavior in order to arrive at a rational methodology to design modified starch of desirable digestion rate and texture. Five types of starch used in this study were waxy maize starch (WMS), normal maize starch (NMS), waxy rice starch (WRS), normal rice starch (NRS) and STMP cross linked normal maize starch. Evolution of volume fraction φ and pasting of 8% w/w starch suspension when heated at 60, 65, 70, 75, 80, 85 and 90 °C were characterized by particle size distribution and G’, G” in the frequency range of 0.01 to 10 Hz respectively. As expected, granule swelling was more pronounced at higher temperatures. At a fixed temperature, most of the swelling occurred within the first 5 min of heating. The pastes exhibited elastic behavior with G’ being much greater than G”. G’ increased with time for waxy maize and rice starch at all times. G’ and G’’ were found to correlated only to the temperature of pasting and not change much with the rate of heating. For WMS, WRS and STMP crosslinked NMS, G’ approached a limiting value for long heating times (30 min and above) especially at heating temperatures of 85°C and above. This behavior is believed to be due to the predominant effect of swelling at small times. For normal maize and rice starch, however, G’ reached a maximum and decreased at longer times for temperatures above 80 °C due to softening of granules as evidenced by peak force measurements. For each starch sample, the experimental data of G’ at different heating temperatures and times could be collapsed into a single curve. The limiting value of G’ at high volume fraction was related to granule size and granule interfacial energy using a foam rheology model. The interfacial free energy of granules were obtained from contact angle measurements and was employed to evaluate the limiting G’. The experimental data of G’ for all starches when subjected to different heating temperatures and times were normalized with respect to the limiting value at high volume fractions. The master curve for normalized G’ was employed to predict the evolution of G’ with time for different starches which was found to agree well with experimental data of storage modulus. A mechanistic model for starch swelling that is based on Flory Huggins polymer swelling theory was employed to predict the evolution of volume fraction of swollen granules. The model accounts for the structure and composition of different types of starches through starch-solvent interaction as quantified by static light scattering, gelatinization temperature and enthalpy of gelatinization, porosity and its variation with swelling and crosslinking of starch molecules within the granule from equilibrium swelling. Consequently, one could predict the evolution of texture of these starch suspension from the knowledge of their swelling behavior. Expressing the limiting storage modulus of complete swelling (volume fraction approaching unity) of starch suspension in terms of foam rheology, we were able to normalize the storage modulus of different types of starches with respect to its limiting value which is found to fall into 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.