Abstract
In the present study, a better knowledge of the influence of plasticizer content, storage relative humidity and film thickness on the mechanical properties of high amylose corn starch based films gelatinized at low temperature, is presented. The mechanical properties, tensile strength and percentage of elongation at break of high amylose corn starch films plasticized with glycerol were evaluated using tension tests. The films exhibited an increase in elongation and a decrease in tensile strength with increasing plasticizer concentration. When the glycerol level was high, some fissures were detected in the dry films, possible due to phase separation (starch-glycerol) phenomena. Film crystallinity is related to the reorganization capacity of the polymer chain, and thus the relative film crystallinity should increase with plasticizer content (glycerol and water). The mechanical properties were found to be strongly dependent on the water content due to the hydrophilic nature of starch films. The influence of moisture sorption on tensile strength was similar to that of plasticization with glycerol. The relationship between polymer chain mobility and water content explained this behavior. Elongation suffered a different effect and maximum values were reached at 45% relative humidity. The final drop in elongation was due to a softening of the structure at high relative humidity. The thicker the film the longer the drying time required, leading to greater relative crystallinity due to the corresponding increase in the possibility for chain reorganization. As a consequence, linear increases in tensile strength and elongation were observed with film thickness over the whole range studied (30 to 100 µm).
Highlights
The thickness of the film specimens was measured with an electronic micrometer (0.001 mm accuracy, Fowler, Cole-Parmer Instrument Co.), the thickness reported being the mean value of five measurements, which was used as the specimen thickness in the calculations
Myllärinen et al (2002) and Zhang and Han (2006) obtained high amylose corn starch films using the traditional method under high pressure and temperature conditions
The plasticizer seems to facilitate crystal growth and recrystallization due to higher mobility of the polymeric chains and greater possibilities for polymer chain alignments during the film drying process and storage. This is in agreement with the results reported by Rindlav-Westing et al (1998) and by Van Soest and Vliegenthart (1997)
Summary
A better knowledge of the influence of plasticizer content, storage relative humidity and film thickness on the mechanical properties of high amylose corn starch based films gelatinized at low temperature, is presented. The mechanical properties, tensile strength and percentage of elongation at break of high amylose corn starch films plasticized with glycerol were evaluated using tension tests. Mechanical properties of a high amylose content corn starch based film, gelatinized at low temperature BERTUZZI, M. The purpose of the present contribution was to gain a better understanding of the relationship between the mechanical properties of high amylose corn starch films obtained by a low temperature method with an alkaline pretreatment of the starch, and the plasticizer concentration, water activity and film thickness
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