Aging Properties of Films of Plasticized Vital Wheat Gluten Cast from Acidic and Basic Solutions

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• 作者：Idoia Olabarrieta ; Sung-Woo Cho ; Mikael G&auml ; llstedt ; Jose-Ramon Sarasua ; Eva Johansson ; and Mikael S. Hedenqvist
• 刊名：Biomacromolecules
• 出版年：2006
• 出版时间：May 2006
• 年：2006
• 卷：7
• 期：5
• 页码：1657 - 1664
• 全文大小：446K
• 年卷期：v.7,no.5(May 2006)
• ISSN：1526-4602

文摘

In order to understand the mechanisms behind the undesired aging of films based on vital wheat gluten plasticizedwith glycerol, films cast from water/ethanol solutions were investigated. The effect of pH was studied by castingfrom solutions at pH 4 and pH 11. The films were aged for 120 days at 50% relative humidity and 23 ges/entities/deg.gif">C, andthe tensile properties and oxygen and water vapor permeabilities were measured as a function of aging time. Thechanges in the protein structure were determined by infrared spectroscopy and size-exclusion and reverse-phasehigh-performance liquid chromatography, and the film structure was revealed by optical and scanning electronmicroscopy. The pH 11 film was mechanically more stable with time than the pH 4 film, the latter being initiallyvery ductile but turning brittle toward the end of the aging period. The protein solubility and infrared spectroscopymeasurements indicated that the protein structure of the pH 4 film was initially significantly less polymerized/aggregated than that of the pH 11 film. The polymerization of the pH 4 film increased during storage but it didnot reach the degree of aggregation of the pH 11 film. Reverse-phase chromatography indicated that the pH 11films were to some extent deamidated and that this increased with aging. At the same time a large fraction of theaged pH 11 film was unaffected by reducing agents, suggesting that a time-induced isopeptide cross-linking hadoccurred. This isopeptide formation did not, however, change the overall degree of aggregation and consequentlythe mechanical properties of the film. During aging, the pH 4 films lost more mass than the pH 11 films mainlydue to migration of glycerol but also due to some loss of volatile mass. Scanning electron and optical microscopyshowed that the pH 11 film was more uniform in thickness and that the film structure was more homogeneousthan that of the pH 4 film. The oxygen permeability was also lower for the pH 11 film. The fact that the pH 4film experienced a larger and more rapid change in its mechanical properties with time than the pH 11 film, asa consequence of a greater loss of plasticizer, was presumably due to its initial lower degree of protein aggregation/polymerization. Consequently, the cross-link density achieved at pH 4 was too low to effectively retain volatilesand glycerol within the matrix.