At Rutgers University in New Jersey, US, Kathryn Uhrich and colleagues have their sights on just such a ‘next generation shrink wrap’, made from a variety of natural, edible, polymers. ‘We haven’t tried eating them yet,’ Uhrich admits, adding that the compounds will need to be approved by regulators before they can be deemed safe. But judging from the curry-like smells coming from the laboratory, the new polymers – derived from the natural oils in cinnamon and cloves – the researchers are hopeful that they should be relatively innocuous.
With the right partner in place to take the films forward to commercialisation, Uhrich says they could be less than a year away from reaching the marketplace.
Rutgers’ researchers are not the only ones interested in developing natural alternatives to conventional petrochemical derived polymers. Biodegradable packaging still accounts for only a tiny fraction of the total food packaging market, but applications are burgeoning as consumers and retailers warm to the idea of environmentally friendly and sustainable packaging options.
Biopolymers that are edible as well as biodegradable are even more attractive. Not only can they reduce waste but potentially they could also be used to fortify foods with ingredients that can sometimes be lacking in the diet. And many of them also have natural antimicrobial properties that should help to increase shelf life as well as killing some of the undesirable pathogens responsible for food poisoning.
At Oregon State University in the US, Yanyun Zhao is focusing her efforts on coatings based on chitosan, a natural polymer made from the shells of crabs and shrimps. As well as its excellent film forming properties, Zhao says, chitosan also readily lends itself to fortification with other ingredients, such as vitamin E and calcium. Strawberries and raspberries coated with the polymer last several days longer than untreated fruits and also boosted levels of vitamin E and calcium.
Chitosan is also well known for its antimicrobial properties, particularly against so-called Gram negative bacteria, differentiated from other Gram positive strains by the nature of their outer membranes. In recent work with OSU colleague Mark Daeschel, Zhao has discovered that this antibacterial activity is further boosted by combining chitosan with the enzyme protein lysozyme, found naturally in human tears as well as in egg whites. In contrast to chitosan, lysozyme is active against Gram positive bacteria, including certain bacteria in wine responsible for the formation of biogenic amines that lead to unpleasant side effects for some drinkers. Hard-boiled eggs coated with this chitosanlysozyme coating have a much wider spectrum of antibacterial activity, including against Listeria and Salmonella, Zhao and Daeschel have found. This finding could keep the eggs safe and fresher for longer, Zhao says.
‘The surface of a boiled egg has lots of pores at high pH and high protein levels that are very nice conditions for microbial growth, even after boiling for half an hour,’ Zhao explains. ‘Coating the egg seals the pores, preventing oxygen transmission and water loss and extends its shelf life.’
Edible films and coatings are probably more common than most people realise. A layer of film in some frozen pizzas, for example, helps to keep moisture from the sauce from being absorbed by the crust; some apples and fruits are waxed to help them fresher for longer, while yet other coatings line ice cream cones and coat battered frozen food. A company in California is also reported to be developing an edible yoghurt pot lid. However, edible coatings with antimicrobial activities would be a new departure and should open up more applications, researchers say.
In the case of the Rutgers’ polymers, their antimicrobial activity stems from the natural oils thymol and carvacrol, from thyme and cinnamon, respectively, from which they are derived. In tests in the laboratory, the resulting polymers have also been found to be active against E. coli and Salmonella responsible for food poisoning. Rather than actually killing the bacteria, in this case the polymers stop them from congregating as biofilms, Uhrich says, which is when these bacterial communities become dangerous. ‘My gut feeling is that this is a good thing as we won’t get antimicrobial resistance. If we killed all the bacteria we would get more problems.’
The team is still exploring the exact mechanism but believes it involves the quorum sensing pathways by which bacteria are known to communicate. The antimicrobial effects are activated by polymer degradation triggered by moisture. Also available in powder form, Uhrich says the polymers could potentially be sprinkled on chicken or other meats to protect against Salmonella and other harmful microorganisms.
At the University of California at Davis, US, meanwhile, John Krochta and co-workers are making their edible polymers starting from excess whey produced by the dairy industry. ‘Whey protein can be used to make films and coatings that are flexible, transparent, colourless and without any flavour or aroma,’ Krochta says. These coatings could be used to protect fragile foods, such as freeze-dried chicken pieces destined for dry soup mixes, for example, against breakage into small fragments.
Whey protein already has natural antibacterial activity of its own, but researchers can supplement this by adding other natural antimicrobial compounds – often extracted from whey – to boost the level available in the films. Depending on the compound added, Krochta says that these have proven effective against E. coli, Salmonella and Listeria. And he adds that the films and coatings can also carry natural antioxidants, such as vitamin C, to enhance their oxygen barrier properties and protect food from going rancid.
Applications closest to market include a whey coating for peanuts – and potentially other nuts – that slows down oxidation of peanut oil and ‘significantly increases the time before the peanuts go rancid,’ Krochta says. ‘This means that simpler, cheaper packaging and/or more recyclable packaging can be used.’
The group is also exploring alternatives to the high gloss coatings used on confectionary products such as chocolate-covered nuts and jelly beans. ‘Most often this coating is made of shellac, an exudate from a beetle that has to be solubilised and applied from an ethanol solvent,’ Krochta says. ‘Many people would be unhappy to know the source of the shellac – usually referred to as confectioner’s glaze on product labels – and the ethanol solvent is a worker safety and air pollution problem.’
Whey protein by contrast is a wholesome material that is also soluble in water, which does not pose worker safety or air pollution problems, he says.
Coatings used by the food industry have to be registered as safe to eat. In the US, the Food and Drug Administration classifies such compounds as Generally Regarded As Safe (GRAS), and any edible ingredients need to be clearly labelled. Whey protein has the advantage that it already has GRAS status. Although a very small number of individuals are allergic to the compound, the protein is clearly shown on product labels so it can be avoided if needed. However, chitosan coatings may be more problematic, Zhao acknowledges. Although chitosan is consumed in China, people with shellfish allergies may prefer to avoid it and the polymer does not yet have GRAS status in the US. One way forward, Zhao continues, could be to screen the coatings and remove any allergycausing proteins. She adds that Swiss company Fordras, which funded the chitosan-lysozyme film/ coating research, now holds the licence on this technology.
Elsewhere, other commercial partnerships have already borne fruit. Work by US firm Origami Foods and US Department of Agriculture researcher Tara McHugh, for example, has led to a commercial edible film: an alternative sushi wrap made from carrots. The carrot film is one of a number of fruit or vegetable-based films, containing 85% of the natural material, the researchers have made, McHugh says. She and Origami are also currently codeveloping edible drink straws made from similar fruit-based films.
‘Fruit or vegetable straws could never compete [on cost] with plastic ones, which cost fractions of a penny to produce,’ says Matthew de Bord, owner of Origami Foods: ‘But they are environmentally friendly. I never realised how many straws end up in landfill they come with every fast food meal so something made of fruit or vegetables is biodegradable and eliminates a massive amount of waste.’
McHugh’s main interest in the films is as a way of introducing more fruit and vegetables into the diet, she says. The group also has a collaboration with East West Medical Institute in Los Angeles, with the aim of encouraging children to eat more vitamins and minerals, possibly by fortifying the straws or fruit based strips with other nutrients.
Commercialisation of edible films and coatings, however, is still fraught with technical difficulties. To be competitive with conventional packaging polymers, the coatings generally need to be supplemented with plasticisers and other additives to improve their flexibility and durability, which usually leads to some reduction of barrier properties to water and oxygen. Another challenge is that edible polymers are biodegradable and readily dissolve in water, which can be a problem during transport or storage, or when making edible straws, for example. Plus, they are also likely to be more expensive than conventional plastics.
Advocates argue that many of these problems are already being overcome and say the advantages of such edible products may outweigh the cost difference. However, when some of these new films will reach the marketplace is ‘always impossible to know’, Krochta says. ‘I hope that, with the increased attention to increasing food quality and safety and decreasing packaging cost and waste, food manufacturers will explore the advantages of edible films and coatings.’
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