Edible / biodegradable packaging for food

A major group of alternative and novel materials which possess future commercial potential are those derived from utilised and under-utilised food ingredients, or food grade ingredients. Consequently, they provide packaging materials which are not just biodegradable in nature, but which are edible also, thereby presenting greater opportunities for commercial application in a more sustainable manner. Therefore, the potential advantages that such packaging materials have over more current conventional packaging forms used by the food industry are obvious.

Consumers demand eco-friendly packaging solutions

To date, the majority of food packaging materials and formats consist primarily of laminates which can be comprised of plastics, metals and/or paper and glass. These materials are, and have traditionally been, manufactured and engineered for specific food packaging applications. However, consumer demands are changing with respect to food product purchasing and consumers are becoming increasingly aware of the presence, role and implications of the food packaging that surrounds their retail food purchases.

Issues pertaining to sustainability, environment, ethics, food safety, food quality and product cost are all becoming increasingly important factors for modern-day consumers when purchasing food products and a number of these issues are also enforced by food packaging legislative regulations. Changes in consumer packaging demands are being informed by continual drip feeding of negative information pertaining to conventionally-used packaging. For example, reports¹ claim that plastics formed 38 per cent of all food packaging materials used in the US last year and that most of this would end up in landfill or cause an environmental risk when processed in incinerators. The author also went on to say that only small amounts of the 38 per cent of plastic waste would go on to be recycled.

Additionally, most of the packaging used today is synthetic in nature and has been derived from fossil fuels (from naphtha or natural gas) for over half a century. Commercial usage of such forms of packaging for a whole host of goods from food and pharmaceutical products through to electronic and fragile items have relied on a plentiful supply of cheap packaging materials. However, there are now global concerns about the depletion of non-renewable raw materials for manufacturing plastics. The global economy is quickly facing the scenario that greater oil demands are being faced with falling supplies.

The need for sustainability in food packaging

It is estimated that at some point between today and 2030, oil extraction will have peaked and oil fuel for transportation will have become increasingly uneconomic². The world’s annual consumption of plastic materials has increased from approximately five million tonnes in the 1950s to nearly 230 million tonnes today. The total production of plastics in Europe was approximately 57.5 million tonnes in 2005, representing 25 per cent of the total worldwide production of 230 million tonnes, at similar levels to that of North America, at 24 per cent. The total demand of plastics in Europe was 47.5 million tonnes in 2005, of which 17.6 million tonnes (37 per cent) were used for manufacturing packaging materials. Since 37 per cent of plastics are used for packaging, it is not surprising that this category has attracted most attention from policy-makers and environmentalists³. Food packaging is a significant part of that total and thus even a small reduction in the amount of materials used for each package would result in a significant cost reduction and may improve solid waste disposal⁴. The current trend in new food packaging development is that, wherever possible, it should not only be natural and ‘environmentally friendly’, but also functional and cost-effective.

Thus, development of edible/biodegradable films/coatings for effective food packaging has generated considerable interest in recent years due to their potential to reduce and/or replace conventional, non-biodegradable plastics. As food manufacturers require packaging materials to be food grade, maintain/enhance product shelf-life stability and safety and utilise nominal values of packaging, the reduction or replacement with alternative biodegradable forms would clearly allow improvement in overall operating costs while reducing waste streams. EU regulatory pressures, coupled with indirect demands via consumer groups on EU food processors and packaging manufacturers, to develop/utilise ‘environmentally friendly’ packaging systems are increasing. Research development in the area of edible/biodegradable films/coatings is a key and unique field of exploration within food packaging which possesses enormous commercial and environmental potential.

Edible/biodegradable packaging research

Scientific research on the production, quality and potential applications of edible/biodegradable films in food manufacturing has been carried out by several research groups worldwide and has been reported in research publications^5-9. The enormous commercial and environmental potential in the area of edible/biodegradable films/coatings has often been stressed^5,10,11 and numerous publications have primarily addressed issues relating to mechanical properties, gas migration, and the effects of other factors on these properties, such as type and content of plasticisers, pH, relative humidity and temperature etc.^6,8,10-15.

However, research into edible/biodegradable films is still in its infancy and research on industrial application of edible/biodegradable films has received more attention in recent years, however, coverage is still quite limited.

Researchers in the Food Packaging Group, Department of Food and Nutritional Sciences, University College Cork, Ireland, have developed several functional, biopolymer-based, edible/biodegradable films over the last few years.

Testing food-grade polymers

In our most recent study, more than 20 food ingredients were investigated for their film-forming abilities and a number of optimised food-grade polymers were produced over a range of concentrations and processing parameters in an attempt to evaluate their mechanical and permeability properties (tensile strength, puncture resistance, tear strength and oxygen/water vapour permeability)^16-19. The properties of biopolymer films were found to be significantly affected by pH adjustment and corn oil addition and laminated films showed the most desirable properties among all the film formats evaluated. Laminate films consisting of sodium alginate emulsion, gelatine emulsion and whey protein isolate emulsion had the highest tensile strength, puncture strength and tear strength values (55.77 MPa, 41.36 N, 27.32 N, respectively); films formed from gelatine emulsion (pH = 10.54, CO = 27.25 per cent) had the highest elongation values (351.12 per cent); composite films formed from WPI, G and SA (WPI: G: SA = 10.0:16.0:14.0) had the lowest oxygen permeability values 8.00 (cm3·mm/m2·d·kPa); Bilayer films formed from gelatine emulsion and sodium alginate emulsion had the lowest water vapour permeability values (22.63 g·mm/kPa·d·m2).

Edible films formed from protein-polysaccharide powders (whey protein concentrate (WPC)-45, alginate, pectin, carrageenan, or konjac flour) were also investigated by our group at UCC. Results showed that films formed from co-dried powders had lower water vapour permeability, higher tensile strength, elastic modulus and elongation than equivalent films formed from the dry blended powders and there was potential to alter the physical properties of hydrophilic films by combining whey protein and polysaccharide components²⁰. Previous work by our research group also involved the extrusion of pectin, pea starch and gelatine/sodium alginate blends for the preparation of sausage casings^21,22.

The limitations of edible packaging

Generally, edible films have limited application primarily because of their inferior physical characteristics. For example, single, lipid-based films have good moisture barrier properties but contain no mechanical strength²³. Consequently, laminated films were formed by adhering two or more biopolymer films together. However, laminated films are advantageous to single, emulsion-based biopolymer films due to their possession of enhanced barrier properties. The creation of laminated structures has the potential to overcome these shortcomings by engineering edible/biodegradable films with multiple functional layers.

Edible films and coatings based on water-soluble proteins are often water-soluble themselves but possess excellent oxygen, lipid and flavour barrier properties. Proteins act as a cohesive, structural matrix in multicomponent systems, yielding films and coatings with good mechanical properties. Lipids, on the other hand, act as good moisture barriers, but have poor gas, lipid and flavour barriers. By combining proteins and lipids in emulsion or bilayer (a membrane consisting of two molecular layers), the positive attributes of both can be combined and the negatives minimised.

From the research conducted by the Food Packaging Group at UCC, the general characteristics of developed edible/biodegradable films are as follows:

• Thickness of manufactured edible/biodegradable films range from 25μm to 140μm
• Films can be clear, transparent, and translucent or opaque depending on the ingredients used and the processing technique employed (Figure 1: a, b)
• Aging specific film types under controlled environmental conditions (Figure 2) improved mechanical properties and gas barrier properties
• Storing films at ambient condition (18-23°C, 40- 65 per cent RH) for five years did not significantly alter structural characteristics
• Films formed from various ingredients can be relatively easily laminated together (Figure 3)
• Manufactured films can be labeled, printed on or heat sealed
• Small variations in film microstructure (e.g. biopolymer phase separation) affects film properties (Figure 4)

Edible/biodegradable films versus traditional synthetic film (Polyethylene terephthalate (PET)/Polystyrene (PS))

In comparison to synthetic film (Figure 5), edible/biodegradable films (4G) had the best elongation (351.12 per cent), followed by PET (136.94 per cent). PS showed the highest tensile strengths (77.17 MPa), followed by edible/biodegradable films SAOGOWPIO (55.77 MPa). However, puncture strength of edible/biodegradable films SAOGOWPIO was nearly seven times greater than either synthetic film used (6.34 N, 6.36 N for PET and PS, respectively). Although edible/biodegradable films showed an overall lower mechanical strength than synthetic films, they were still within a sufficiently acceptable range to be capable of holding most food products. One great advantage that edible/biodegradable films possess is that they generally have significant resistance to oxygen permeability (e.g. GWPISA-4, 8 cm3·mm/m2·d·kPa). Conversely, water vapour permeability of edible/biodegradable films is in general, higher than that of synthetic films, which is a major disadvantage but one which is being addressed.

Application example of developed edible/biodegradable packaging on meat

Manufactured films were tested in food packaging/processing applications to assess their performance during food preparation and storage. Figure 6 shows one example of meat products prepared, wrapped and stored using edible film wrappings. From this one example, observed advantages in using such materials included:

• Increasing cooking yield by 12 per cent
• Decreasing moisture loss by seven per cent during storage (-18°C, 60 days).
• Improved product flavour and juiciness

Future packaging

The production of edible/biodegradable films is mostly at research level and the commercial use of films is limited at present. Research is currently ongoing, however, it is expected that the use of edible/biodegradable packaging materials in foods has a bright future as edible/biodegradable packaging can offer a natural protection to foods according to specific packaging requirements. Despite the advantages of edible/biodegradable materials which have been presented by scientists, a number of obstacles in their development potential have to be overcome, such as cost effectiveness, improved water vapour permeability barrier and technological application methods. Full commercial production and application is still some way off, but the potential that edible/biodegradable films possess is now being realised.

Acknowledgement

The authors would like to acknowledge Dr. Mark A. E. Auty, Senior Research Scientist, Manager of the National Food Imaging Centre (NFIC), Teagasc Moorepark, Co. Cork for assistance in preparing and imaging edible/biodegradable film samples (SEM & CLSM) for this article.

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