(265c) Analyzing the Effects of Time and Crosslinker Ratio on the Mechanical Properties of Biodegradable Zein Super Gels

Analyzing
the Effects of Time and Crosslinker Ratio on the Mechanical Properties of Biodegradable
Zein Super Gels

Hazal
Turasan, Jozef Kokini, Department of Food Science, Purdue
University, West Lafayette, IN

Zein is the main
storage protein found in corn and comprises almost 80% of all the corn
proteins. Due to the lack of essential amino acids and insolubility in water, zein
is not a good choice as a nutrition source for humans. However, its unique
properties, such as film forming capability, make zein one of the most commonly
chosen plant-based materials for many applications including textile production,
adhesive industry, creating various analytical platforms or tissue engineering.
Zein also has the ability to undergo gelation in different solutions. Many
researchers have explored the solubility of zein in different solutions^[1]–[4]. Based on the need
for another solvent, the solvents for zein are classified in three. The
solvents that does not need a secondary solvent to dissolve zein are classified
as the primary solvents, and those that require the addition of either water or
another solvent were defined as secondary solvents. Acetic acid is under the
category of primary solvents. Ethanol, on the other hand, is classified in the
secondary solvent for zein since it requires the addition of water to dissolve
zein completely ^[4]. It has been
reported that gelling properties of zein in solution depends on the type of the
solution zein dissolved in, such that zein has less tendency to form a gel in its
primary solvents ^[4]. It has also been
reported that the addition of crosslinker to zein solutions increase the gelation
process ^[5]. In this study,
the gelation process of zein protein in a primary solvent, acetic acid solutions,
have been explored to produce a stiff biomacromolecule gel from a plant-based
polymer. The zein solutions have been crosslinked with glutaraldehyde solution and
the results have been compared to control solutions with no glutaraldehyde. Rheological
measurements were conducted on zein solutions to observe the changes in the
chemistry and to determine the time of gelation. In addition, compression tests
have been conducted on the fully gelled zein solutions to understand the
highest compression the gels can endure without breaking.

The zein solutions
were prepared with 70% aqueous acetic acid solution. The concentration of zein
was kept constant at 1:5 (g/ml) ratio. Oleic acid and monoglyceride was added as
plasticizer and the emulsifier at a ratio of 1:1 (g/g zein) and 1:0.15 (g/g
zein), respectively. Different crosslinking ratios (0%, 4%, 8%, and 12% (w/w of
zein)) have been tested to understand the minimum effective concentration of
glutaraldehyde for stiff gel formation. The solutions were sonicated to remove
the gas bubbles for 30 seconds for every 10 ml solution and left in room
temperature without stirring for 30 days. Starting from Day 0 amplitude sweep
tests and frequency sweep tests were conducted in replicates. The results
showed that all the solutions showed liquid like behavior in Day 0, such that
the loss modulus (G”) values were greater than the storage modulus (G’) values
for all the crosslinker ratios. Uncrosslinked zein solutions did not show any
sign of crosslinking throughout the experiment, such that neither the loss
modulus values nor the storage modulus values showed an increasing trend in 30
Days. In addition, the loss modulus values were always greater than the storage
moduli values for 0% solutions the entire experiment. This result suggests that
zein in acetic acid solution does not gel without crosslinker. For 4% crosslinked
solutions the loss moduli were greater than the storage moduli for 28 Days,
indicating no gelation. However, at Day 30, the G’ values crossed the G”
values, showing gelation onset for 4% crosslinked solution at Day 30. For 8%
crosslinked zein solutions, the transition from sol to gel phase occurred at
Day 12, shown by the crossover of G’ and G” values. For 12% crosslinked zein
solutions, the gelation started the earliest, at Day 9. The highest G’ value
reached in 12% crosslinked zein gels was 2000 Pa where it was 1100 Pa for 8%
crosslinked zein gels. The mechanical compression tests showed that the gels
formed from 4% were not stiff enough to resist compression; however, gels with
8% and 12% crosslinker ratio were stiff enough to take 70% compression to fully
recover back to their original structure after the compression. At 80% strain,
the gels collapsed irreversibly (Figure 1).

Figure 1. Compression
test on zein gels. A) Before 70% strain, B) at 70% strain, C) Recovery after
70% strain, D) Before 80% strain, E) at 80% strain moment before failure and F)
at 80% strain gel failure.

In
summary, this study focuses on producing bio-based polymer super gels from zein-acetic
acid solutions and understanding the rheological behavior of zein solutions
during gelation. The results showed that with enough concentration of
crosslinker, a super gel could be formed from corn protein. The uncrosslinked
zein-acetic acid solutions did not show any gelation. 4% crosslinking was
enough to start gelation, however, the formed gel was not stiff enough for
compression test. Both 8% and 12% crosslinking caused gelation at day 12 and
day 9 respectively resulting in stiff gels. The mechanical compression tests
showed that both gels recovered from strains up to 70%. However, 80%
compression caused gel failure.

[1]  Y. Li, Q.
Xia, K. Shi, and Q. Huang, “Scaling Behaviors of alpha-Zein in Acetic Acid
Solutions,” J. Phys. Chem. B, vol. 115, no. 32, pp. 9695–9702, Aug.
2011.

[2]  R.
H. Manley and C. D. Evans, “Binary Solvents for Zein,” Ind. Eng. Chem.,
vol. 35, no. 6, pp. 661–665, Jun. 1943.

[3]  D.
Fu, “Zein properties and alternative recovery methods,” ETD Collect. Univ.
Neb. - Linc., pp. 1–188, Jan. 2000.

[4]  J.
W. Lawton, “Zein: A History of Processing and Use,” Cereal Chem. J.,
vol. 79, no. 1, pp. 1–18, Jan. 2002.

[5]  D.
J. Sessa, A. Mohamed, and J. A. Byars, “Chemistry and Physical Properties of
Melt-Processed and Solution-Cross-Linked Corn Zein,” J. Agric. Food Chem.,
vol. 56, no. 16, pp. 7067–7075, Aug. 2008.