Strawberries dipped in the seaweed coating solution. Image credits: Clare Kiernan

Strawberries are notoriously difficult to keep fresh without refrigeration. But what if, instead of keeping fruit cold throughout the supply chain, we could give it a protective second skin? Even better, what if that protection was non-plastic and natural?

Researchers at the University of British Columbia (UBC) have developed a thin, edible coating made from seaweed-derived agar that helps strawberries, grapes and apple slices stay fresh for longer.

In tests, coated strawberries remained mould-free for at least four days at room temperature. They also lost far less moisture and stayed considerably firmer than untreated berries.

“Our hope is to help keep produce fresh for longer throughout the food system. If we can reduce spoilage during storage and transportation, we can reduce food waste while using less energy to preserve food,” Tianxi Yang, one of the researchers and an assistant professor at UBC’s dept. of Food Science, said.

Turning seaweed into a protective shield Graphic illustration of what a coated strawberry might look like. Illustration: ZME Science.

Fresh produce keeps respiring after harvest and gradually loses water. As fruit dries out, it softens, shrivels and becomes more vulnerable to bacteria and mould.

Refrigeration slows these processes dramatically. The problem is that maintaining a cold chain from farms through warehouses, trucks and supermarkets requires substantial energy and infrastructure.

The UBC team approached the problem from the opposite direction. Instead of changing the environment around the fruit, they put a protective barrier directly on its surface.

The coating starts with agar, a substance derived from red seaweed that is already widely used in food as a thickener and plant-based alternative to gelatin. The researchers combined it with tannic acid, a naturally occurring plant compound found in foods such as grapes and tea, and zinc, an essential micronutrient.

Zinc ions coordinate with tannic acid, causing the ingredients to self-assemble into tiny particles within the agar. Dip a strawberry into the mixture and, once it dries, the coating forms a thin, transparent layer across the fruit.

“It was exciting to watch the particles self-assemble in real time, from a cloudy, milky liquid into this incredibly uniform, protective layer. To our knowledge, no one had made an agar-based microparticle coating like this before,” Ivy Chiu, lead researcher and a doctoral student at UBC, said.

The coating does two jobs at once. It slows the loss of water from the fruit while also showing antibacterial activity, helping protect the produce against microorganisms associated with spoilage.

Strawberries show the biggest difference

Strawberries produced some of the clearest results.

After four days at room temperature, untreated strawberries had lost 30.59% of their weight, compared with just 13.36% for coated berries.

The coated fruit was also much firmer. It registered a firmness of 41.77 newtons, compared with 18.53 newtons for untreated strawberries — meaning it resisted deformation more than twice as strongly during the firmness test.

Vitamin C and antioxidant compounds were better preserved as well. UBC reports that untreated strawberries at room temperature began developing mould within about two days, while the coated berries remained mould-free for at least four days without refrigeration.

“We wanted to find a simple alternative to cold storage. Once we added the coating, the fruit became far less sensitive to changes in temperature and moisture and stayed fresh longer,” Yang added.

Moreover, the coating wasn’t limited to strawberries. It produced preservation benefits lasting up to 14 days for grapes and about 24 hours for apple slices.

A coating that can be washed away

The coating is incredibly thin and tasteless. But it still has to pass a practical consumer test: what if someone doesn’t want it on their fruit?

The researchers found that most of the coating could be removed by rinsing it with tap water for two minutes. They also substituted laboratory-grade agar with commercially available food-grade agar and found that the coating still reduced spoilage and moisture loss in strawberries.

The environmental comparison was also notable. A life-cycle assessment estimated that the coating could have a 14% lower carbon footprint than conventional refrigeration and reduce freshwater ecotoxicity by about 85%. “Freshwater ecotoxicity” is a metric that estimates how chemical emissions can harm aquatic species and ecosystem functioning. The researchers attributed much of the difference to reduced electricity and refrigerant requirements.

Researchers see this coating as useful especially in the transportation and storage phases. It’s not so much about keeping fruits fresh on a kitchen counter, but rather by reducing the amount of cooling needed while produce is stored and transported, and keeping products safer for long.

However, the current results are not enough to eliminate refrigeration. The experiments were conducted under controlled conditions, and produce that travels long distances or requires extended storage may still need cold temperatures. 

The technology will also have to prove that it can be manufactured cheaply and consistently, applied efficiently at commercial scale—and at the same time—meet food-safety and regulatory requirements. 

The researchers are now testing the coating on more fruits and vegetables and investigating how to scale the process and integrate it into commercial food systems.

The study is published in the Journal of Agricultural and Food Chemistry.