Escalating concern around food waste and the use of plastic packaging is driving interest and advancements in edible biofilms, both in academic and industry circles. Researchers and startups are exploring the development of “invisible” second skin coatings that inhibit microbial activity, dehydration, and oxidation, thereby extending the life of food products and eliminating the need for plastic packaging.
Israel-based Sufresca and Spanish company Bio2Coat are two of the startups forging ahead with such innovations.
Sufresca has developed coating solutions for various fruits and vegetables from water, glazing agents, emulsifiers, stabilisers, and acidity regulators.
Bio2Coat has also been focusing on fresh produce and was awarded a prize in the 2022 Fi Europe Startup Innovation Challenge for its edible coating innovation.
Other startups in the field include Liquidseal, based in the Netherlands, and California-based company Apeel.
Natural polymer potential
Polysaccharide-based coatings, such as chitosan, alginate, and cellulose, have received particular attention owing to their excellent film-forming ability, natural credentials, and non-allergenic properties. The downside is that they are lacking in certain physical and chemical properties, which has limited their commercial adoption.
Chitosan is a polysaccharide derived from shrimp shells. It is widely available at an affordable cost and exhibits several desirable characteristics, including non-toxicity, biodegradability, and biocompatibility.
However, its application in edible coatings has been hindered by its low solubility in water and organic solvents, weak barrier properties, and inadequate antibacterial activity.
Creating compounds to address limitations
Researchers have sought to address these shortcomings by combining chitosan with polyphenolic compounds that have antioxidant and antimicrobial properties that help prevent food oxidation and spoilage.
Among these compounds, gallic acid – which is abundant in grapes and green tea – has been investigated. The trouble is that conventional methods of applying gallic acid to improve the solubility of chitosan generally involve acidic solvents, which may affect the aroma of the fruits.
Researchers from Chungnam National University in South Korea set out to overcome this issue by conjugating gallic acid with chitosan using EDC/NHS coupling chemistry. According to the researchers, this is a sustainable way of enhancing the film’s properties.
“When gallic acid, as a naturally derived compound, is introduced into chitosan, chemical crosslinking between polymer chains can take place without the use of an initiator or crosslinking agent, thereby improving the coating efficiency and stability of the film,” Professor Won Ho Park, lead researcher, told this publication.
The researchers compared the new film with chitosan films to assess gallic acid’s enhancement effects and tested its efficacy on stored mini bananas and cherry tomatoes.
“Optimal conditions for coating were established for the application of biofilms as coating materials, and the antibacterial and antioxidant properties of coating materials, weight change of fruits according to coating, and firmness were measured to evaluate their applicability as food packaging materials,” Park said.
Key findings
The researchers observed that the film exhibited enhanced mechanical strength, offering protection against food damage during transportation and improved antioxidant properties – leading to extended shelf-life.
“After eight days, the condition of the coated fruit did not show much difference to the normal fruit after one or two days of storage,” Park said.
Explaining the mechanism responsible for the shelf-life extension, he added: “The deterioration of fruits is caused by the oxidation of the surface or by microorganisms. When we apply the biofilm to the surface of the fruit, the shelf-life of the fruit is extended by inhibiting the oxidation reaction on the surface and suppressing the growth of microorganisms by the antioxidant activity of gallic acid.”
The film also showed improved antibacterial activity against two types of bacteria. This confirmed its effectiveness against multiple microbial species and superior UV-blocking capabilities to prevent photo-discolouration and damage, said Park.
“In our study, Staphylococcus aureus, a gram-positive bacterium, and Escherichia coli, a gram-positive bacterium that can cause food poisoning, were used to evaluate antimicrobial properties,” he added.
Consumer concerns
Whilst some of the barriers to the commercialisation of natural polymer-based biofilms are technical, others relate to consumer acceptance.
Park acknowledged that consumers are concerned about remaining coating residues on the fruit. However, he said that the washability of this conjugate alleviates this concern.
A second potential deterrent is the animal origin of chitosan, given that a growing number of consumers are shunning animal-derived foods in favour of plant-based alternatives.
However, Park does not believe that this will be an issue, saying: “Although the definition of animal food is not clear, the use of chitosan as a coating agent is not expected to be a major problem, except for the fruitarian, as it is a polysaccharide present in the crustacean exoskeleton and structurally resembles vegetable cellulose rather than animal protein.”
Early days
While the researchers appear to have hit on a coating that is made from sustainable, natural materials and may offer a viable and biodegradable alternative to conventional packaging materials, it is still in the early stages of development.
“Further research into food packaging and large-scale production methods beyond fruit-based products is necessary for commercial viability,” Park added.
The full study is detailed in Food Chemistry.