New Biodegradable Films To Reduce Plastic Waste
New biodegradable films made from edible biopolymers from renewable sources could become an important factor in reducing the environmental impact of plastic waste. Proteins, lipids, and polysaccharides are the main biopolymers employed to make edible films and coatings. Which of these components are present in which proportions determines the properties of the material as a barrier to water vapour, oxygen, carbon dioxide, and lipid transfer in food systems. Films composed of lipids 25 are more moisture-resistant, but they are usually opaque, relatively stiff, and more vulnerable to oxidation. For these reasons the current trend in designing biodegradable materials for food packaging is to combine different biopolymers.
Films composed of lipids 25 are more moisture-resistant, but they are usually opaque, relatively stiff, and more vulnerable to oxidation. For these reasons the current trend in designing biodegradable materials for food packaging is to combine different biopolymersFish muscle protein Protein, the most important functional components in muscle, confers many desirable physicochemical and sensory attributes of muscle foods. Muscle proteins comprise 15-22% of the total muscle weight (about 60-88% of mass) and can be divided into three major groups on the basis of their solubility: sarcoplasmic proteins (water-soluble), myofibrillar proteins (salt-soluble) and stroma proteins (insoluble).
Sarcoplasmic proteins
Sarcoplasmic proteins are located inside the sarcoplasm and are soluble in water or low salt concentrations (ionic strength < 0. 15). Generally, the sarcoplasmic proteins comprise about 20-30% of the total amount of proteins in fish muscles. The contents of sarcoplasmic proteins are higher in pelagic fish muscle than in dermersal fish muscle. Sarcoplasmic proteins consist of heme protein such as myoglobin and hemoglobin as well as enzymes involving in glycolysis, citric and electron transfer cycles.
Myofibrillar proteins
Myofibrillar proteins are the major structural proteins in fish muscle, which accounts for 55 to 60% of total protein muscle. These proteins can be extracted with neutral salt solutions of ionic strength above 0. 15. Myofibrillar proteins can be further divided into three subgroups.
Contractile proteins
Contractile proteins, including myosin and actin, are directly responsible for muscle contraction. Myosin makes up 50 to 58% of the myofibrillar fraction. It consists of six polypeptide subunits, two heavy chains and four light chains. The two globular heads with ATPase activity are relatively hydrophobic and are able to bind acti. When myosin is digested by trypsin or chymotrypsin for a short period, it is divided into two heavy meromysin chains with a size of 220, 000 Daltons, depending on species and fiber types (Actin is the second most abundant myofibrillar protein, comprising about 22% of the myofibrillar protein. Each actin molecule contains five sulfhydryl groups and is free of disulfide bond. It also contains a myosin binding site, which allows myosin to form temporary complexes via non-disulfide bonds, which can be split by high-energy compounds such as ATP at high ionic strengthsProtein describes as a random copolymer of aminoacids and contains a side chain that can be modified easily by chemical and physical mean and because of this property it can get a large importance to be used as a packaging material. protein get easily denatured by acid, by an alkali or by any enzymatic treatment and make more resitable structure which is required for the edible films, several kind of protein used for edible fims.
Regulartory proteins
The major regulartory proteins are tropomyosin and troponin, located on the thin filaments. Tropomyosin represents approximately 8-10% of the total myofibrillar proteins. Molecule of tropomyosin consists of acidic and basidic amino acids (isoelectric point = 5. 1). Troponin is a globular protein found in thin filament with a molecular weight of 37, 000 Daltons.
Cytoskeletal proteins
The proteins in this group include titin, connectin, nebulin, desmin and other proteins. Cytoskeletal proteins are functioned to support and stabilize the contractile proteins of the muscle. The contractile proteins vary in susceptibility to postmortem proteolytic degradation, contributing to the varying meat tenderness.
Stroma proteins
Stroma protein is the residue after extraction of the sarcoplasmic and myofibrillar proteins. The stroma protein is insoluble in dilute salt solution. It can be extracted by water, acid or alkaline solution and neutral salt solution. It constitutes about 3% of total muscle proteins. The stroma protein is composed of the main connective tissue proteins such as collagen and elastin. Protein-based filmsProteins cover a broad range of polymeric compounds that provide structure or biological activity in plants or animals. Various proteins can be used as film-forming materials such as soy protein isolate, whey protein isolate, wheat gluten egg white, and fish myofibrillar protein.
Protein-based films generally have the superior mechanical and barrierproperties to polysaccharide-based films. Proteins consisting of about 20 amino acidshave a specific structure which confers a wider variety of functional properties, compared with polysaccharides which are mostly homopolymers. Furthermore, inter-and intra-interaction between protein molecules, such as hydrogen bonds, ionic-ionicinteractions, hydrophobic interactions and covalent bonds, could be formed during drying condition. Properties of protein-based films are most likely dependent on the protein sources which are different in sequential order of the amino acids, protein structure and the degree of structure.
Mechanism of protein film formation
Protein-based films can be formed in three steps.
- Break intra- and inter-molecular bonds (non-covalent and covalent bonds) that stabilize polymers in their native forms by using chemical or physical rupturing agents (by solubilization or thermal treatment). As a result, polymer chains became mobile.
- Arrange and orient mobile polymer chains in the desired shape.
- Allow the formation of new intermolecular bonds and interactions to stabilize the three-dimensional network.
The shape obtained in step 2 is maintained by eliminating agents used in step 1 (e. g. , solvent removal or cooling). Based on these three steps, solvent process is based on dispersing and solubilizing the proteins in various solvents and then casting, spraying, or dipping, followed by drying. This process has been extensively studied and applied to produce films from various proteins, particularly from myofibrillar proteins.
Basic properties of protein-based films
Protein films possess different properties depending upon the sources of protein, protein concentration, extrinsic factors, etc. Barrier properties Protein films provide the advantage of being excellent oxygen and carbon dioxide barriers but their hydrophilic nature makes them rather ineffective moisture barrier. Barrier property of protein films decrease with increasing film thickness. However, barrier property can 8 be varied with the source of protein, which can be associated with amino acid composition.
