Prolong Freshness: Fruits & Vegetables

It’s a problem that man has faced since the first moment when we had more food than we could eat at one time. When food is plentiful, how do you store it to make it last? The question has almost as many answers as there are foods. The ancient Greeks washed figs in seawater and dried them in the sun, while in medieval China lemons and oranges were coated in wax. In 15th century Japan, vegetables were coated in soy milk to prevent moisture loss and extend their shelf life. In 16th century England, they were coated with lard.

The problem of rotten apples and moldy grain may have been a matter of season-to-season survival for our ancestors. Today, the prevention of food waste is no less of a challenge, even if the stakes have changed somewhat. Global greenhouse gas emissions from food waste are around 10 times higher than in the UK. Wasted meat contributes the most to these numbers, as the energy needed to produce it is typically several times that of plant-based foods. Throw away 100g of steak and you may have just wasted the equivalent of 10kg of CO2.

By mass, however, fruits and vegetables account for the largest volume of food wasted: around half a billion tons a year. In the UK, oranges and tangerines lead the way in wasted produce, followed by apples and tomatoes.

So how can we best preserve our fruits and vegetables so that they arrive in greater numbers on our plates?

Many of the tools available to producers today to reduce food waste involve the use of plastics and chemicals.

A Swiss study published in 2022 showed that the climate benefits of packaging cucumbers in plastic are almost five times greater than the negative climate impacts of the packaging itself.

As for chemicals such as chlorine, hydrogen peroxide and trisodium phosphate, they have long been used to kill various microorganisms on fresh produce to prevent spoilage and extend shelf life.

Yet customers are turning away from both chemical treatments and plastics. Chlorination can lead to the formation of compounds suspected of being carcinogenic, which can either end up in drinking water (as a result of industrial processing of fruits and vegetables) or remain on produce.

When it comes to plastics, many of us feel guilty about the amounts we use. According to David McClements, a food science researcher at the University of Massachusetts, there is “strong pressure today to replace plastics” and find other ways to preserve fruits and vegetables without resorting to chemical treatments. .

While many of these new technologies are still confined to research labs, others are beginning to appear on supermarket shelves, or will soon.

Build a Barrier

One of the potentially promising technologies is the edible coating: this involves covering fruits and vegetables with a film of protective material that can be eaten with the food. Modern commercial coatings have come a long way since the first experiments with soybeans and lard in Japan, England and elsewhere.

Coatings made from beeswax or paraffin took off in the 1930s, when waxing fruits such as apples became popular. Apples have a natural wax coating when picked from the tree, but this is often lost in the washing process. Today, an artificial coating is often reapplied to apples, oranges, lemons and other fruits to help retain moisture and extend shelf life.

Although these coatings are quite effective in limiting the dehydration of fruits and vegetables, there is still a lot to be done. To create the perfect edible coatings, scientists are now experimenting with many different substances, from silk fibroin (a protein secreted by the silkworm) to chitosan (a sugar from the outer skeleton of crustaceans) to gum cashew nuts, fish gelatin, fenugreek protein, soy protein, cellulose and seaweed derivatives – the list goes on.

These coatings, applied by dipping, brushing or spraying, form a thin membrane on the surface of strawberries or tomatoes, for example, reducing gas and water vapor transfer, limiting browning and flavor loss, and ultimately extending the shelf life.

Ideally, these coatings should keep the fruit or vegetable well sealed, but not too much – otherwise you risk causing anaerobic fermentation (this is when your apple turns into cider, for example).

According to McClements, chitosan, which can be obtained as a byproduct of shrimp fishing, figures prominently in current efforts to find the perfect edible coating.

In a recent study, coating strawberries with chitosan and whey protein isolate (a by-product of cheese making) extended their shelf life by 60% if stored at temperatures near freezing. those of the refrigerator. The tomatoes coated with chitosan and green algae, on the other hand, remained nearly perfect even 30 days after harvest (the untreated tomatoes were in very unattractive shape after the same period).

A number of companies around the world are now working to commercialize the results of research into edible coatings. A California start-up, Apeel Sciences, makes edible coatings from vegetable oils that can double the shelf life.

In the United States, their coatings can be found on apples, avocados, and limes; in the UK, they have teamed up with Tesco to sell coated oranges and lemons – products that are usually peeled before eating, as the UK and EU have strict food regulations. edible coatings. (The treatments applied to the rind of citrus fruits are one of the reasons many recipes call for uncoated fruit).

Another company, Liquidseal, sells polyvinyl alcohol-based coatings for mangoes and avocados in the UK – again, these are hard-skinned fruits only. However, Liquidseal has already developed an edible coating for cucumbers to replace those infamous plastic wrappers, and hopes to sell it in Europe soon.

Nanoprocessing

Another emerging way to improve edible coatings is to use nanomaterials – materials with particle sizes less than 100 nanometers (nm) in at least one dimension (1,000 times smaller than a human hair).

“If you reduce the particle size, you can improve the functional performance of edible films and coatings, for example by increasing their strength and barrier properties,” says McClements.

You can produce these tiny particles using lasers, vibrations, plant extracts, or even certain microorganisms. In one study, after a week of storage at room temperature, most regular strawberries were covered in fungus.

On the other hand, only 10% of the strawberries coated with chitosan and nanosilver were damaged. Freshly cut carrots coated with silver nanoparticles remained intact for 70 days. Uncoated carrots lasted only four days.

Nanoparticles aren’t just for edible coatings, though. As some of them are powerful antimicrobials, they can be added to regular plastic packaging to extend the shelf life of fruits, vegetables or leafy greens.

Additionally, they can be used in sensors that could notify retailers or customers that food is no longer safe to eat, which could help prevent premature trips to the trash. Researchers at Canada’s McMaster University, for example, have developed patches that could be applied to product packaging to predict spoilage.

However, these nanoscale interventions raise safety issues. In mice and rats, ingestion of zinc oxide nanoparticles causes liver and kidney damage. Studies on silver nanoparticles, on the other hand, have revealed toxicity to model organisms such as the roundworm Caenorhabditis elegans and to human cells.

“All new technologies carry risks and we have to be careful,” says Gustav Nyström, a scientist at the Swiss Federal Laboratory for Materials Science and Technology.

Nyström points out that silver and zinc nanoparticles can bioaccumulate in tissues. However, if these nanoparticles are well encapsulated in plastic packaging, the risk of them migrating into food is low.

This article is originally published on bbc.com