Abstract
Increased environmental and health concerns over the use of plastic packaging or fluorine-containing coatings, in combination with increased market demand for products with a longer shelf life, make bio-based materials one of the most important research candidates for alternative paper packaging materials for oil resistance. These bio-based materials have excellent oxygen and oil barriers, which are critical for food packaging. Moreover, they are biodegradable, naturally renewable, and safe. In this artical, two main groups of bio-based oil repellents for paper food packaging, including polysaccharide-based biopolymers and protein-based biopolymers, are enumerated, and the advantages and weaknesses of bio-based oil repellents are discussed, and effective solutions are proposed. Finally, research status and prospects on the development of bio-based oil-resistant coatings for the food packaging industry are presented.
The food-packaging revolution began when humans learned to cook food. Food packaging occupies a substantial part of the modern packaging industry, accounting for approximately 60%–70% of the industry. Among all types of food packaging materials, plastic occupies a large share of the food packaging market because of its
light weight, low cost, flexibility, and good barrier properties. However, because of their non-degradability and potential toxicity, plastics in landfills and marine debris have a considerable impact on the environment and human health
However, the rapid development of the fast-food industry and the increased popularity of take-out food in recent years have promoted the continuous expansion of the paper food packaging market. There is a great, unsatisfied demand for paper packaging including wrapping paper, paper boxes and bags in the fast-food and take-out industries. Paper packaging is more environmentally friendly, biodegradable, and recyclable
The most common method of improving the oil resistance of paper packaging is to use oil repellents in the pulp as additives or coatings on the paper surface. Perfluoroalkyl and polyfluoroalkyl substrates (PFAS) are the most widely used and effective oil repellents because of their low surface energies. However, PFAS pose a huge threat to human health because of their toxicity and degradation difficulty resulting from perfluorooctanesulfonyl compounds (PFOA/PFOS), which are produced in the production and utilization process
Bio-based packaging materials are derived from natural renewable sources. Paper coatings, polysaccharides, proteins, and lipids, or combinations of these components, can form a dense film on the surface of paper or paperboard to prevent oil from permeating the paper and paperboard
As a result, many studies successfully used bio-based materials as coatings on paper or paperboard to provide an effective oil barrier for food packaging applications. This review aims to provide a synopsis on the current status of bio-based oil repellents for paper food packaging and a view of prospective developments in sustainable next-generation paper coatings.
Bio-based oil repellents, which are different from fluorine-containing oil repellents, can form a barrier layer on the paper surface to resist the permeation of oil because the higher critical surface tension makes it difficult to prevent paper from being wetted by oil drops. Therefore, materials with good film formation may perform better as oil barriers. Currently, research on bio-based oil repellents mainly focuses on two categories: polysaccharide-based biopolymers (chitosan, starch, and nanocellulose) and protein-based biopolymers (zein, whey protein, and soy protein). The status and prospects of these two kinds of biopolymers used for grease barrier coatings on food paper packaging were reviewed in this artical.
Polysaccharide-based biopolymers primarily include chitosan, starch, and nanocellulose. They have been used as adhesives in papermaking and paper coatings, and have received considerable attention for application in oil-barrier coatings for food packaging in recent years. As environmentally friendly materials, polysaccharide-based biopolymers can form a smooth and dense film on the surface to improve paper barrier performance owing to strong hydrogen bonding interactions. However, their natural hydrophilic characteristics limit their applications in the food packaging industry
Chitosan is an edible and biodegradable material that exhibits excellent oxygen and oil barrier properties owing to its semi-crystalline nature, hydrogen bonds between molecular chains, and the positive charge on the amino group
Ham-Pichavant et al
Despite the good barrier properties that could be achieved with chitosan-coated paper or paperboard, the combination of paper and chitosan is not yet fully suitable for all food packaging applications; even setting aside the cost, water sensitivity is inherent to a majority of polysaccharides, which contain a large number of hydrophilic groups
Chitosan has the potential to replace plastics in food packaging, although many problems need to be resolved before it can be commercially used; these concerns include the difficult process of chitosan coating during coating process, high viscosity and high cost. Optimizing both the cost and performance to realize the multiple functions (waterproofing, antibacterial, and water vapor barrier) of chitosan by chemical modification or compounding with other materials may be a viable alternative.
Starch and its derivatives are naturally available, inexpensive, and biodegradable polymers commonly used in paper coating. In its native form, it can be utilized as a sizing agent, whereas after modification, it can be used as a coating agent to enhance paper properties owing to its excellent film-forming ability. Starch and its derivatives were early biomaterials used in oil-resistant food packaging. Recently, some modified products have entered the market, such as Filmkote 370, a modified oil-proof starch produced in the USA; however, these products are not as effective as expected.
Many attempts have been made to improve the gas, oil, and water vapor barrier properties of starch, including plasticization, blending with other materials, modification (chemical, physical, or enzymatic method), and using different combinations of such interventions. Song et al
Starch-based oil repellents are a hot topic in the field of oil repellent research for food packaging. It not only has excellent performance, safety, and environmental protection property, but can also be combined with a variety of other materials to optimize its waterproofing and oil repellent effects, mechanical properties, antibacterial properties, thermal stability, and food preservation properties
The nanocellulose mentioned in this paper includes cellulose nanofibers (CNF), microfibrillated cellulose (MFC), and cellulose nanocrystals (CNC). Owing to its good barrier properties, the application of nanocellulose in food packaging has emerged as a promising alternative to plastic, attracting an increasing number of researchers in recent years. Their nanometer scale, high surface energy, and ability to form a nanoporous network make MFC an ideal material in paper coatings
To evaluate the potential application in packaging materials, Aulin et al
Nanocellulose-based coatings exhibit excellent gas and oil barriers. However, the difficulty in the application progress is due to the very high viscosity at low solids and susceptibility to moisture. These two limitations limit their commercialized prospects in application such as barrier coatings for food packaging. The solution, in this respect, is to chemically modify nanocellulose or develop nanocellulose-based barrier composite coatings with other materials such as nanofillers and lipids. Tyagi et al
Another approach to protect nanocellulose-based coatings from moisture is to have a multilayered structure consisting of alternating layers of nanocellulose and hydrophobic polymers such as guar gum, polyvinyl alcohol, polyhydroxyalkanoates (PHAs), shellac, and poly lactic acid ( PLA)
Although there are still significant gaps in the performance and cost between nanocellulose and potential replacements for petroleum-based materials, nanocellulose-based materials with competitive performance for food packaging seems attainable.
The excellent barrier properties and satisfactory mechanical performance of protein-based biopolymers make them promising materials for food packaging application. Compared with polysaccharide-based biopolymers, the viscosity of the protein solution is lower at the same concentration, which is beneficial for the coating process
In particular, zein is hydrophobic because of its high content of nonpolar amino acids
Research on the application of zein-based polymers in food packaging began early. Trezza et al
These results demonstrate the potential of zein in paper-based food packaging. Although zein-based coatings have relatively good water and water vapor resistances, they are insufficient for food packaging. Therefore, lipids or crosslinking agents have been used to improve the water and water vapor barrier characteristics of zein-based coatings
The potential barrier ability of zein has spurred considerable research on its application in food packaging. However, to commercialize these applications, intensive researches on improving their performances of mechanical properties and flexibility are necessary. Current methods for increasing the tensile strength of zein-based coatings often result in reduced flexibility
Whey protein has excellent film-forming properties along with barrier properties against oxygen, aroma, and oils, which can be further enhanced by crosslinking the polymeric chains, making the whey protein-based coating material water-insoluble and more suitable for paper-based food packaging
Han et al
The incorporation of plasticizing agents is necessary to overcome the intrinsic brittleness of whey protein coatings
As mentioned above, whey protein coatings containing plasticizers are flexible, safe, biodegradable, and have excellent barrier characteristics for oxygen, oil, and aroma
Soy protein isolate (SPI) also has a remarkable film-forming capacity, resulting in better oxygen barrier performance than zein and wheat gluten
Park et al
As reported, soy protein-coated paper was found to impart gas and oil barriers as well as adequate mechanical properties
Given the above, we may reasonably arrive at the conclusion that adding nanoparticles, fatty acids, and lipids to overcome the defects that have limited the application of SPI films or coatings in food packaging seems to be a promising approach.
Casein, especially sodium caseinate (NaCAS), is usually used as caseinate because of its poor solubility in water. NaCAS appears to provide better gas barrier properties than nonionic polysaccharides
Aloui et al
Although caseinates have the aforementioned advantages, some drawbacks still need to be addressed to adapt them to practical application, such as the lack of water resistance and flexibility. Currently, the use of functional additives, such as wax, fatty acids, plasticizers, and crosslinkers, is an effective solution
As described above, problems that are similar with most biopolymers, such as hydrophilicity, crystallization behavior, lack of flexibility, and high cost, prevent their full commercial exploitations. We also discuss some corresponding solutions proposed in many studies as representative examples. Although bio-based materials, compared with fluorinated oil repellents, still have a great shortage in effect and cost, extensive researches are needed in the paper food packaging industry towards developing methods for new coating formations, conveying costs and improving the properties of paper coatings
In the future, numerous combinations can be used cooperatively on an industrial scale, depending on consumers demands, products characteristics, and operating conditions. In our view, the research directions of bio-based grease barrier will focus on the following three aspects in the years ahead.
The barrier properties of bio-based materials may be improved by chemical and physical crosslinking or by the incorporation of additives and other bio-based materials. Chemical modifications have been specifically used to provide good water or water vapor barrier properties, including grafting, acetylation, and alkylation. The moisture sensitivity of bio-based coatings can be decreased by crosslinking, leading to improved barrier properties against water vapor and oxygen transmission at high humidity
Paper coating technology has matured abroad and at home, including bar coating, knife coating, dip coating, and curtain coating. However, innovative processing methods need to be applied to contribute to the bio-based coatings properties as auxiliary means. Drawing lessons from successful practices in other related industrial areas is an accessible approach, such as in the fields of extrusion, enrolling, fluidization, spraying, and UV polymerization. Currently, extrusion and compression molding, which are well-known commercial methods applied to develop films and plastic granules, are considered the most likely to be successful in the process of preparing bio-based barrier coatings. However, many problems such as plasticization and process monitoring still need to be solved.
Given that most bio-based materials do not present a sufficient water vapor barrier for food packaging, the combination of individual layer with a multilayer coating structure is often necessary to improve their competitive edge against synthetic films
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