Tannins are an essential part of the mouthfeel and ageability of some wines. But scientists believe they could also be used to create plastics that could keep food fresh longer.
A wine’s tannins come mostly from the grape’s seeds and skins. They belong to a class of chemicals known as polyphenols, which are the same antioxidants believed to give red wine its reported health benefits.
Although some are added to wine during fermentation, most tannins remain in the grape marc, or the stalks, seeds and other materials typically thrown out after the pressing process.
Now, these waste tannins could be given a new lease on life.
Paul Kilmartin, a professor of wine chemistry at New Zealand’s University of Auckland, uses the discarded tannins to create plastics that could extend the shelf life of packaged food. He first became interested in tannins due to their antimicrobial properties. Kilmartin wanted to use them to develop antimicrobial plastics to be used in medical settings that could reduce the spread of infection. So, he tried adding tannins to the plastics as they are cast to spread them throughout the material.
However, during the manufacturing process, Kilmartin found that tannins lost their antimicrobial properties.
While disappointed, Kilmartin realized that the plastics retained the antioxidant effect of the tannins. He now uses these plastics to keep foods fresher, as antioxidants can react with chemicals that cause oxidation and slow spoilage.
Kilmartin created different plastic films that contain tannins and tested their effect as packaging for oils. The tannins need to be in contact with the food, so he expects that liquids like oils will benefit the most.
“We can slow down the oxidation of oils—cooking oils, fish oils and vegetable oils—in contact with these films,” says Kilmartin. “We believe the antioxidant effect of those tannins at the surface of those films is slowing down that rate of oxidation.”
Kilmartin found that the tannins could extend an oil’s shelf life up to 30% before they turn rancid.
Not only could this help prevent food waste, but it could also reduce additives often used to preserve oil. The tannins are impregnated into the plastic, so they should avoid leaching into the oil.
“A grape tannin wouldn’t do that,” says Kilmartin. “It stays on the surface of the film. So, it’s things that are in contact with that film which will benefit.”
“There’s a lot of very interesting molecules in nature with a variety of properties, antioxidants, etc.,” says Nicolas Brosse, a professor at the University of Lorraine in France. His research focuses on the use of chemicals extracted from natural sources, including tannins, to develop better materials.
“[Kilmartin’s work is] feasible, but there are a lot of difficulties to overcome,” Brosse says. “For example, in our work with tannins, we demonstrated that it’s possible to include tannins in thermoplastics [plastics which can be reshaped when heated], but the main difficulty is the compatibility between the plastic and the tannin. It means the final properties [of the plastic] are not good enough, and the material is too brittle.”
Kilmartin now works with plastics experts in New Zealand to further develop the packaging to have commercial use.
Extracting the tannins also has an additional benefit, as it makes the waste marc more useful for compost and reduces the risk of contaminating the environment, says Kilmartin.
“Marlborough, a relatively new winegrowing region in New Zealand, has expanded so much over the last 25 years, and they have a major problem with how to deal with this waste stream,” he says. “Those same tannins which we’re trying to extract can be very bad if you get huge amounts washing into rivers and waterways.”
Kilmartin says that extracting the tannins could have long-term environmental benefits.
“If we take the tannins out through an extraction step, we believe that makes that remaining material more suitable to go into a soil compost,” Kilmartin adds. “We’ve been doing some trials to see how seedlings will grow, and it seems to help if you take some of those tannins out to make it more usable as a compost mix.”