ARTICLE pubs.acs.org/Biomac
Mechanical Properties and Network Structure of Wheat Gluten Foams Thomas O. J. Blomfeldt,† Ramune Kuktaite,† Eva Johansson,‡ and Mikael S. Hedenqvist*,† † ‡
Department of Fiber and Polymer Technology, Royal Institute of Technology, SE-10044, Stockholm, Sweden Department of Agriculture Farming Systems, Technology and Product Quality, P.O. Box 104, The Swedish University of Agricultural Sciences, SE-23053, Alnarp, Sweden ABSTRACT: This Article reports the influence of the protein network structure on the mechanical properties of foams produced from commercial wheat gluten using freezedrying. Foams were produced from alkaline aqueous solutions at various gluten concentrations with or without glycerol, modified with bacterial cellulose nanosized fibers, or both. The results showed that 20 wt % glycerol was sufficient for plasticization, yielding foams with low modulus and high strain recovery. It was found that when fibers were mixed into the foams, a small but insignificant increase in elastic modulus was achieved, and the foam structure became more homogeneous. SEM indicated that the compatibility between the fibers and the matrix was good, with fibers acting as bridges in the cell walls. IR spectroscopy and SE-HPLC revealed a relatively low degree of aggregation, which was highest in the presence of glycerol. Confocal laser scanning microscopy revealed distinct differences in HMW-glutenin subunits and gliadin distributions for all of the different samples.
’ INTRODUCTION There are a number of reasons why materials from renewable resources should be used, including the reduction of waste and green-house gases and the limited amount of oil.1 Wheat gluten is an interesting alternative, especially for making polymer foams, one of the most common applications of polymers.2 In Sweden, wheat gluten is a byproduct from, for example, the manufacture of ethanol (biofuel).3 Because it is a byproduct, wheat gluten is available at a low price.4 It is well known that gluten has great foaming properties (refer to bread)59 as well as interesting elastic properties, which makes it a promising material as a biobased foam. Wheat gluten consists mainly of two protein types: gliadin, which is a low-molar-mass ethanol-soluble part, and glutenin which is ethanol-insoluble and has a high molar mass.10 It is considered that glutenin is the main contributor to the foam elasticity, whereas the gliadins contribute with a viscous behavior.10,11 Foams have a large number of applications, and the mechanical properties required vary significantly; for example, a flexible foam is needed in packaging, whereas a foam for thermal insulation can be rigid.2 Because of the variation in properties of different gluten proteins and the possibility of further polymerizing the structure,12 it may be possible to tailor biobased (protein) foams with different mechanical properties. Recent studies have shown that it is possible to produce foams from wheat gluten by the lyophilization (freeze-drying) process.13 The foaming by freeze-drying relies on the crystallization of water (with or without glycerol), which then precipitates and forms the pores. Very different foam structures can be obtained depending on, for example, material composition and freezing temperature/conditions. The foams obtained in the previous study13 had mainly open pores and were stiff and relatively brittle but could be easily softened with a plasticizer like glycerol. It has also been shown that the gravity-induced r 2011 American Chemical Society
gradient in structure can be minimized by the addition of bacterial cellulose fibers.13 It must be noted that before claiming a totally “green foam solution” the energy cost associated with large scale freeze-drying has to be evaluated. The aim of the present study was to improve the understanding of what parameters are important to obtain “optimal” mechanical properties of wheat gluten foams, plasticized with glycerol, “structure-modified” with bacterial cellulose fibers, or both. In the previous study,13 the main focus was to reveal the cell structure and how it was affected by the material composition. In this study, the focus is on the protein network structure and how it affects the mechanical (compression) properties.
’ EXPERIMENTAL SECTION Materials. The commercial wheat gluten powder was kindly supplied by Lantm€annen Reppe AB, Sweden. According to the supplier, the gluten protein content was 77.7% of the dry weight (modified NMKL Nr 6, Kjeltec, Nx5.7, www.NMKL.org), the moisture content was 6.9% of the total weight (NMKL 23, 1991), and the starch content was 5.8% of the dry weight (Ewers, polarimetric method). The concentration of fats was 1.2% of the dry weight (Soxtec, Lidfett.OA.19, tecator AN 301), and the ash content was 0.9% of the dry weight (NMKL 173 2nd ed). The wheat gluten particle size distribution was: 10% in the interval 160250 μm, 20% 100160 μm, 55% 50100 μm, and 15%
