A scientific review published on April 21, 2016 in the peer-reviewed journal Trends in Food Science and Technology summarizes the main crosslinking agents used for protein and polysaccharide films and coatings with potential applications as food contact materials (FCMs). Henriette Azeredo from the Embrapa Tropical Agroindustry, Fortaleza, Brazil, and Keith Waldron from the Institute of Food Research, Norwich, UK, discuss the chemical and physical methods that can be used for crosslinking, their advantages and limitations, and potential applications in FCMs.
Crosslinking is defined as “the process of forming tridimensional networks”, where polymer chains may be linked by covalent or noncovalent bonds. Crosslinking of polysaccharide- and protein-based polymers is advantageous because it allows improving their mechanical and barrier properties, most notably enhancing their water resistance. However, crosslinkers intended for use in FCMs must present low toxicity.
Disulfide crosslinking can be promoted by denaturation in sulfhydryl-rich proteins, such as whey proteins or gluten. This type of crosslinking improves water resistance but also increases film opacity, which could be seen as a negative feature in FCM applications where transparency is required.
Aldehydes such as formaldehyde (CAS 50-00-0) and glutaraldehyde (CAS 111-30-8) are very effective as crosslinkers of both polysaccharides and proteins. However, their use in FCMs is usually avoided due to their considerable toxicity. Less toxic aldehydes, such as cinnamaldehyde (CAS 104-55-2), have been tested by some researchers. Oxidation of some polysaccharides by periodates under acidic conditions leads to the formation of 2,3-dialdehyde polysaccharides. These compounds can form crosslinks with protein side chains or with amino groups present in chitosan (CAS 9012-76-4).
Several plant-derived phenolic compounds, including tannic acid (CAS 1401-55-4), ferulic acid(CAS 1135-24-6), and gallic acid (CAS 149-91-7), can be used as crosslinkers for protein and polysaccharide polymers. The antioxidant properties of natural phenolic compounds could bring additional benefits in FCM applications, while the changes in film color may present a disadvantage.
Alginic acid (CAS 9005-32-7), a polysaccharide from brown seaweeds, has been used to crosslink collagen (CAS 9007-34-5) films, while di- and polycarboxylic acids, such as citric acid (CA, CAS 77-92-9), may be used to as crosslinking agents for polysaccharide films. The advantage of CA as a crosslinker is its very low toxicity. Another naturally occurring low-toxicity crosslinker of both protein and polysaccharide polymers is genipin (CAS 6902-77-8), present in fruit of Gardenia jasminoides. Genipin’s disadvantage is the formation of a dark blue color in crosslinked films.
Biopolymers can also be subjected to enzymatic crosslinking, which is advantageous due to the specificity of enzymes and mild reaction conditions. One prominent example of such enzymes is microbial transglutaminase that can catalyze intra- and intermolecular crosslinking between glutamine and lysine residues in proteins.
Physical crosslinking mechanisms discussed by the authors include ionic crosslinking between polyuronates (e.g. pectin, CAS 9000-69-5) and divalent cations (e.g. Ca2+), ionic interactions between polycarboxilic acids (e.g. oxalic acid, CAS 14-62-7) and amino groups of chitosan or proteins, ionic interactions between tripolyphosphate and chitosan, crosslinking by hydrophobic interactions and hydrogen bonds between polyphenols and polysaccharides, formation of polyelectrolyte complex between cationic (e.g. chitosan) and anionic (e.g. alginate, CAS 9005-32-7) polyelectrolytes.
As the authors note, many of the alternative crosslinkers have not yet been assessed in FCM applications, and most knowledge on these applications has come from the biomedical field so far. Many of the reviewed chemicals and mechanisms look promising for FCM applications, but their safety will need to be ensured by FCM- relevant migration and toxicity testing.
Azeredo, H., and Waldron, K. (2016). “Crosslinking in polysaccharide and protein films and coatings for food contact – A review.” Trends in Food Science & Technology 52: 109-122.