Sesame Seed Protein Bioplastic Films Modified with Maleic Acid: Structural, Morphological and Functional-Group Characterizations

Abstract

The development of protein-based bioplastics from renewable agricultural resources provides a promising route towards alternative polymeric materials derived from non-petroleum feedstocks. This study investigated the effect of maleic acid concentration on the structural, morphological, and spectroscopic characteristics of sesame (Sesamum indicum L.) protein bioplastic films. Sesame protein isolate was prepared through alkaline extraction followed by acid precipitation and subsequently modified with 4% and 5% maleic acid to obtain 4MX and 5MX formulations, respectively. The modified protein materials were plasticised with glycerol and processed into films before characterisation by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier-transform infrared (FTIR) spectroscopy. XRD patterns exhibited broad diffraction features characteristic of predominantly disordered protein-based structures. The principal reflection shifted from 2θ = 16.800° for 4MX to 17.800° for 5MX, corresponding to calculated interplanar spacings of 5.2778 and 4.9835 Å, respectively. Thus, increasing maleic acid concentration produced a 1.000° shift in the principal reflection and an approximately 5.57% reduction in calculated d-spacing. SEM examination revealed concentration-dependent differences in surface organisation, including variations in surface texture, interconnected regions and voids. FTIR spectra retained characteristic protein amide absorptions, while the broad O–H/N–H stretching band shifted from 3261.46 cm−1 for 4MX to 3248.38 cm−1 for 5MX, representing a 13.08 cm−1 shift. The combined XRD, SEM and FTIR findings demonstrate that increasing maleic acid concentration from 4% to 5% alters the structural organisation, surface morphology and molecular environment of the sesame-protein matrix. However, the available data do not independently establish covalent esterification, increased crosslink density, or improved functional performance. Further mechanical, barrier, thermal and controlled chemical analyses are required to establish the structure–property relationships of the resulting films.