Proteins are promising precursors to be used in production
of sustainable
materials with properties resembling plastics, although protein modification
or functionalization is often required to obtain suitable product
characteristics. Here, effects of protein modification were evaluated
by crosslinking behavior using high-performance liquid chromatography
(HPLC), secondary structure using infrared spectroscopy (IR), liquid
imbibition and uptake, and tensile properties of six crambe protein
isolates modified in solution before thermal pressing. The results
showed that a basic pH (10), especially when combined with the commonly
used, although moderately toxic, crosslinking agent glutaraldehyde
(GA), resulted in a decrease in crosslinking in unpressed samples,
as compared to acidic pH (4) samples. After pressing, a more crosslinked
protein matrix with an increase in β-sheets was obtained in
basic samples compared to acidic samples, mainly due to the formation
of disulfide bonds, which led to an increase in tensile strength,
and liquid uptake with less material resolved. A treatment of pH 10
+ GA, combined either with a heat or citric acid treatment, did not
increase crosslinking or improve the properties in pressed samples,
as compared to pH 4 samples. Fenton treatment at pH 7.5 resulted in
a similar amount of crosslinking as the pH 10 + GA treatment, although
with a higher degree of peptide/irreversible bonds. The strong bond
formation resulted in lack of opportunities to disintegrate the protein
network by all extraction solutions tested (even for 6 M urea + 1%
sodium dodecyl sulfate + 1% dithiothreitol). Thus, the highest crosslinking
and best properties of the material produced from crambe protein isolates
were obtained by pH 10 + GA and pH 7.5 + Fenton, where Fenton is a
greener and more sustainable solution than GA. Therefore, chemical
modification of crambe protein isolates is effecting both sustainability
and crosslinking behavior, which might have an effect on product suitability.