<p>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.</p>
<p> </p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/7771058 |
| Date | 25 June 2020 |
| Creators | Jinsha Li (6400343) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/thesis/Volume_Fraction_Dependence_of_Linear_Viscoelasticity_of_Starch_Suspensions/7771058 |
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