Submitted to: Polymer Degradation and Stability
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 27, 2006
Publication Date: October 18, 2006
Citation: Imam, S.H., Gordon, S.H., Mohamed, A., Harry O Kuru, R.E., Chiou, B., Glenn, G.M., Orts, W.J. 2006. Enzyme catalysis of insoluble cornstarch granules: impact on surface morphology, property and biogradability. Polymer Degradation and Stability. 91(12): 2894-2900. Interpretive Summary: In general, biopolymers have poor physical properties. Because renewable polymers are being used as raw material in the design and development of single-use biodegradable consumer products, it is critical that we use chemical as well as enzymatic methods not only to improve polymer property but to create novel functionalities. We have utilized commercial glucoamylase to modify regular cornstarch granules. Enzyme specifically degraded the amorphous region of the granules. Additionally, modified starches exhibited higher crystallinity, modified gelatinization temperature, and were resistant to degradation in compost. Such enzyme-modified starches could be useful in the development of consumer products with controlled biodegradation property.
Technical Abstract: Granular cornstarch (Buffalo; 70% amylopectin, 30% amylose) was treated with microbial glucoamylase (50mM Sodium Acetate buffer at pH 5.5 at 30 0C, 150 rpm) for up to eight hours. Treated starch was recovered and evaluated for changes in granular morphology, chemical property, thermal property, crystallinity, and impact on its biodegradability. As the enzyme treatment progressed, reducing sugars begun to accumulate in the liquid culture media (total of 6% in 8 hours) and the granule suffered roughly 6% weight-loss within eight hours of incubation. While the granules appeared intact morphologically, numerous small pits developed throughout the surface of the granules as a result of the enzyme treatment. Even after 8 hours of enzyme treatment, pitted granules were not disrupted and remained intact. X-ray diffraction indicated no loss of crystallinity in the enzyme treated granules but rather an increase in relative crystallinity, suggesting that the enzyme preferentially catalyzed the anhydroglucose units in amorphous regions of the granule. These findings were further supported by the FTIR data suggesting that starch becomes more resistant to enzymatic attack as hydrolysis of noncrystalline domains proceeds at a faster rate than hydrolysis of crystalline domains. These results also suggest that variations in the crystallinity of different types of starches have the potential to affect their rates of biodegradation. Interestingly, enzyme treated starch granules exhibited resistance to biodegradation, and the degree of resistance was proportional to the length of enzyme treatment. Granules treated with enzyme for a total of 7 hours and subjected to biodegradation in soil produced 40-50% less CO2 in a closed circuit respirometer compared to the untreated samples. Differential Scanning Calorimetry (DSC) thermograms showed an endothermic reaction with a slight shift in the gelatinization temperature from 73oC to 78oC in enzyme-modified granules.