The development of bio-based polymers from proteins has gained attention for their large availability and renewable and biodegradable nature. However, protein-based plastics have limited commercial applications because of several drawbacks, such as poor processability, brittleness, moisture sensitivity, and inferior mechanical and thermal properties. Extensive studies have been conducted to solve or ameliorate these issues by pretreatment or modification of proteins or protein-derived biopolymers before or during wet processing or dry processing at elevated temperatures. This review provides an overview of research efforts conducted in the area of physical and chemical treatment of proteins to achieve better processability, mechanical properties, and other physical performance based on a literature review in this subject.
INTRODUCTIONBiopolymers derived from agricultural resources such as wool, cotton, leather, and silk have been extensively used by humans for a long time. Industrial use of agricultural commodities for fuels and consumer products began in the 1920s. They were soon replaced by petroleum-based chemicals after World War II because these synthetic polymers were cheaper, more durable, and more versatile. Advances in petroleum-based fuels and polymers have benefited mankind in numerous ways. However, sustainability of their production and use has become a societal concern. Besides, petroleum resources are finite, and their prices are likely to continue to rise in the future. The disposal of petroleum-based plastic has also been causing environmental concerns. 1 Since the early 1990s, the use of renewable biopolymers as a substitute for nonbiodegradable petrochemical-based polymers has again attracted considerable attention because of their environmental benefits such as renewability and biodegradability. Nowadays, natural polymers can even be tailored to meet specific needs.Protein is one of three main categories of renewable biopolymers aside from polysaccharides and polynucleotides that are derived from plant or animal resources. Plant proteins that can be used for bio-based plastics include soy protein, corn zein, wheat protein, cottonseed protein, and sunflower protein. Animal proteins such as blood meal, gelatin and collagen, keratin and feather quills, egg protein, whey protein, and meat and bone meal can also be used as a feedstock for such bio-based polymers. 2 Despite their environmental friendliness, proteinbased biopolymers have many shortcomings like lower mechanical properties when compared to petroleum-based polymers. 3 Due to the hydrogen bonds between C@O and NH groups, protein-based polymers have poor processability and exhibit high sensitivity to moisture. As a result, the protein-based polymers are very difficult to process without plasticizers or pretreatment. To date, overcoming these shortcomings of proteinbased bioplastics has proven difficult. Numerous methods have been investigated, including physical treatment, denaturation of the original protein in crystalline form, cleava...