Tannins are an essential part of the taste and aging of certain wines. But scientists think they can also be used to make plastics that keep food fresher for longer.
The tannin in the wine comes mainly from the seeds and skins of the grapes. They belong to a class of chemicals known as polyphenols. These are the same antioxidants that are thought to give red wine its health benefits.
Although some are added to the wine during fermentation, most tannins remain in the grape lees or in the stems, kernels and other materials normally discarded after pressing.
These worn out tannins can now be brought back to life.
Paul Kilmartin and Charlotte Vandermeer at the University of Auckland, New Zealand / Photo courtesy of Paul Kilmartin
Paul Kilmartin, professor of wine chemistry at the University of New Zealand in Auckland, uses wasteannins to make plastics that can extend the shelf life of packaged foods. He first became interested in tannins because of their antimicrobial properties. Kilmartin wanted to use it to develop antimicrobial plastics for use in healthcare that could limit the spread of the infection. He therefore tried to add tannins to the plastics as they were cast to distribute them over the material.
During the production process, however, Kilmartin discovered that the tannins had lost their antimicrobial properties.
Kilmartin was disappointed that the plastics retained the antioxidant properties of the tannins. Nowadays these plastics are used to keep food fresh, because antioxidants can react with the chemicals that cause oxidation and slow decay.
Kilmartin has produced several plastic films with tannins and tested their effectiveness as packaging for oils. Tannins have to come into contact with food, so he expects liquids like oils to be most beneficial.
We can slow down the oxidation of cooking oil, fish oil and vegetable oil if they come into contact with these films, Kilmartin says. We believe that the antioxidant effect of these tannins on the surface of these films slows down the rate of oxidation.
Kilmartin has discovered that tannins can extend the shelf life of oil by up to 30% before it becomes rancid.
Grape waste in New Zealand / Photo courtesy of Paul Kilmartin.
This not only prevented food waste, but also reduced the amount of additives often used to conserve oil. The tannins are impregnated with plastic, so leaching in the oil must be avoided.
The tannins in the grapes don’t work, Mr. Kilmartin says. It stays on the surface of the film. So it’s the things that come into contact with this movie that do the trick.
In nature, there are many very interesting molecules with different properties, antioxidants, etc., explains Nicolas Brosset, professor at the University of Lorraine in France. His research focuses on the use of chemicals from natural sources, including tannins, to develop better materials.
Kilmartin’s work is possible, but there are many challenges, says Brosset. In our work on tannins, for example, we have shown that tannins can be processed in thermoplastics [plastics that can be formed hot], but the main difficulty is the compatibility between the plastic and the tannin. This means that the final properties [of the plastic] are not good enough and that the material is too brittle.
Kilmartin is currently working with plastics specialists in New Zealand to further develop commercial packaging.
Eliminating tannins also has the advantage of making pumpkin waste more suitable for composting and reducing the risk of environmental pollution, Kilmartin explains.
Marlborough, a relatively new wine region in New Zealand, has developed rapidly over the past 25 years and there is a big problem with how to deal with this waste stream, he says. The tannins we try to extract can be very harmful in large quantities and can end up in rivers and streams.
According to Kilmartin, tanning can have long-term benefits for the environment.
If we extract the tannins at the extraction stage, we think that the remaining material is better suited for composting in the soil, Kilmartin adds. We did some tests to see how the seedlings would grow, and it seems useful to take some of these tannins to make them more suitable for use as a compost mix.
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