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

Abstract

In order to understand the mechanisms behind the undesired aging of films based on vital wheat gluten plasticized with glycerol, films cast from water/ethanol solutions were investigated. The effect of pH was studied by casting from solutions at pH 4 and pH 11. The films were aged for 120 days at 50% relative humidity and 23 degrees C, and the tensile properties and oxygen and water vapor permeabilities were measured as a function of aging time. The changes in the protein structure were determined by infrared spectroscopy and size-exclusion and reverse-phase high-performance liquid chromatography, and the film structure was revealed by optical and scanning electron microscopy. The pH 11 film was mechanically more stable with time than the pH 4 film, the latter being initially very ductile but turning brittle toward the end of the aging period. The protein solubility and infrared spectroscopy measurements 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 did not reach the degree of aggregation of the pH 11 film. Reverse-phase chromatography indicated that the pH 11 films were to some extent deamidated and that this increased with aging. At the same time a large fraction of the aged pH 11 film was unaffected by reducing agents, suggesting that a time-induced isopeptide cross-linking had occurred. This isopeptide formation did not, however, change the overall degree of aggregation and consequently the mechanical properties of the film. During aging, the pH 4 films lost more mass than the pH 11 films mainly due to migration of glycerol but also due to some loss of volatile mass. Scanning electron and optical microscopy showed that the pH 11 film was more uniform in thickness and that the film structure was more homogeneous than that of the pH 4 film. The oxygen permeability was also lower for the pH 11 film. The fact that the pH 4 film experienced a larger and more rapid change in its mechanical properties with time than the pH 11 film, as a 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 volatiles and glycerol within the matrix.

Biobased Materials Production from Biodiesel Residuals of Rapeseed

“…Protein polymerization properties have, in several studies, been shown to be the main determinant of tensile properties of gluten materials [5,13]. The present study gave very little evidence of any higher degree of polymerization of the rapeseed proteins in the rapeseed residuals used.…”

“…The most well known of those, being evaluated in a number of studies, is the wheat gluten, a residue from the starch/bioethanol industry and the protein-rich part from wheat after the starch and other carbohydrates have been washed away [11,12]. Gluten residue from the bioethanol industry has a high protein content (around 78%) and is possible to both compression mold and injection mold into materials of relatively high quality in terms of strength and barrier properties [5,13,14]. Examples of other plant proteins that have been reported to have potentials for materials production are soy and corn proteins [8,15,16].…”

Antimicrobial compression-moulded wheat gluten films containing potassium sorbate

“…monitored through the decrease in the glycerol upper surface concentration (G-USC), was determined. It was estimated from the ratio of the 850 cm − 1 glycerol peak (Olabarrieta, Gällstedt, Sarasua, Johansson, & Hedenqvist, 2006) to the WG 1525 cm − 1 peak. As shown in Fig.…”

Structural Changes of Gluten/Glycerol Plastics under Dry and Moist Conditions and during Tensile Tests

“…25,27,28 Besides the plasticizers, additives, such as acids, bases and urea, have been used to improve the processability of WG. 26,[29][30][31] Interestingly, new tetragonal-packed and HCP nanostructures were discovered in the WG with NaOH and ammonium hydroxide (AH) 28 . A similar HCP structure was also reported in the presence of urea, which act as a combined protein denaturant and plasticizer.…”