Multifunctional carbohydrate-based soft materials from cereal by-products
Sammanfattning: Cereal production generates large quantities of by-products every year, which still remain underutilized. The hemicellulose fractions from cereal by-products are anticipated to play an important role in tomorrow's sustainable and bio-based circular economy. This thesis addresses the valorization of hemicelluloses from cereal bran into films and hydrogels, starting from their isolation and expanding to the evaluation of material properties and potential use in future applications.Initial isolation of arabinoxylans (AX) from wheat bran was achieved by a lab-scale subcritical water extraction (SWE) maintaining their functional groups (i.e. ferulic acid) and the effect of prior protein isolation on AX extraction was studied. The protein isolation resulted in a looser structure of wheat bran, which increased the polysaccharide yields in subsequent SWE. The polymeric structure and ferulic acid groups were preserved to a large extent after both protein isolation and SWE. The extracted AX fractions had considerable antioxidant activity, rendering them potential sources for further material development.Further isolation of larger quantities of wheat bran AX was achieved by pilot scale SWE and alkaline extraction, resulting in feruloylated and non-feruloylated AX fractions, respectively. The film formation and properties of these AXs were investigated in comparison with a wheat endosperm AX. The three AX were also chemically modified by acetylation and applied in films. Higher purity, molecular weight, and degree of substitution of the AX extracts led to better thermal and mechanical properties of their films. The thermal stability of the films was significantly improved after chemical acetylation however, the mechanical performance and permeability properties did not change. Bound ferulic acid in feruloylated AX films was found to have considerably higher antioxidant activity than external incorporation of free ferulic acid.Hydrogels were produced by enzymatic crosslinking of feruloylated AX from both wheat and corn bran, which show distinct molecular structure and ferulic acid content. For wheat bran AX, hydrogels were obtained by laccase crosslinking and the following regeneration process, and their biochemical and biophysical properties were studied. The rheological properties of feruloylated AX were enhanced by enzymatic crosslinking and further improved by the regeneration, proving that their mechanical strength can be modulated by chemical and physical adjustments. For corn bran AX, crosslinking was applied by laccase and peroxidase to compare the properties of the resulting hydrogels. Laccase formed a more elastic hydrogel network whereas peroxidase crosslinking resulted in hydrogels with larger covalent polymer networks. As a result of these differences between the two enzymes, the hydrogel obtained by peroxidase crosslinking contained larger aggregates with lower clustering strength. The crosslinking was followed by a cell application of the AX hydrogels, where their protective effect against chemically induced oxidative stress was demonstrated. Both corn bran AX hydrogels provided adequate scavenging against reactive oxygen species produced by human colon cells. It was shown that the gelation of wheat bran AX is governed by physical interactions between the xylan backbones of adjacent chains and interactions between larger scale aggregates in addition to the covalent crosslinks. The gelation mechanism of highly substituted corn bran AX was instead hypothesized to proceed by interactions between side chains together with covalent crosslinks. This thesis demonstrated that AX-based hydrogels could find potential use in food and biomedical applications. The outputs of this thesis will contribute to the bioeconomy and sustainable development by valorizing food side fractions into new high-value materials.
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