The health benefits of eating fresh fruit are undeniable, but wouldn't it be great if it could be used for something else other than fuelling your body. Like, say, making electricity?

As it turns out, with some lateral thinking, fruit can be used for just that purpose! 

This is not only interesting as a concept but with ever-increasing amounts of waste food, like fruit, being generated every day, it could also be a way of making "money for old rope." In fact, in the US alone, somewhere in the region of 150,000 tons of waste fruit and vegetables are thrown away every single day.

This is not only incredibly wasteful, but it is potentially harmful to the environment. If only this wasted food could be used for something else? 

Like sustainable energy, for example. 

Interestingly enough, you can, well, sort of. 

One way to generate electricity from fruits is to make basic batteries using electrodes and whole pieces of fruit. The acids within the fruit, namely citric acid, can be used, with the application of some science, to turn the fruit into a kind of rudimentary battery.

Just like in conventional batteries, the acids interact with electrodes to generate small amounts of current that can be tapped and used to power small electronic devices. 

A fun piece of home science, you can experiment with different pieces of fruit (or vegetables, for that matter) to find the best fruit-battery subjects. 

Using some basic materials, like two different metals (zinc and copper work best) to make electrodes, this experiment is a relatively simple and effective activity to share with your kids. Either suspend the metal electrodes in some fruit juice or insert them into an intact piece of fruit to generate some small amounts of voltage. 

Amazing, but how does it work? 

As in regular batteries, the electrodes will act as the cathode and anode, allowing a small electrical current to be generated. The citric acid in the fruits acts as a kind of electrolyte that reacts with the metal electrodes to generate ions. 

This is because organic material can act as an ionic conductor. These substances allow ions to freely move through them. 

It works because the zinc electrode (cathode) will also react with the citric acid to produce cations and free electrons. These electrons then slowly build up on the zinc cathode and, once a wire is attached, will flow through the wire. 

With the wires connected to the copper cathode to complete a circuit, the electrons begin to react with hydrogen cations in the fruit, eventually leading to the formation of hydrogen gas. This process will continue as long as the electrolytes are present and the electrodes are intact -- i.e. until the fruit dries out or an electrode dissolves.

The juice of the fruit will also conduct electricity, effectively completing the circuit. This works in a similar way that salt solution can also conduct electricity. 

Unless you have ready-made examples lying around the house, you can use a copper-plated penny and a galvanized nail. Galvanized nails are often made of iron with a coating of zinc.

With this kind of setup, you may be able to generate a small amount of current (about 0.5 to 3/4 of a volt) from a single piece of fruit. Vegetables, like potatoes, can produce a little more, often just over 1 volt. 

Lemons, as it turns out, can produce up to 0.96 volts. 

Not much, but you can connect several fruit pieces in parallel to up the voltage. You may even be able to produce enough current to charge something like a mobile phone. The more fruits you need, it might become easier and more efficient to just use a regular charger. 

The  highest voltage achieved from a  fruit battery was 1,521 volts by Alssundgymnasiet Sønderborg high school in Sønderborg, Denmark, on 29 January 2020. However, it took 1,964 lemons to achieve.

Depending on the type and number of fruits, the voltage might be enough to power a small LED or even a small motor. As it turns out, the best fruits and vegetables to use are those that high in conductive ions like potassium or sodium.

It also helps if they have the "right" internal structure to create working currents. More homogenous fruits, or vegetables like potatoes or pickles, are excellent choices. 

Fruits like oranges are actually often a poor choice as their internal structure is partitioned into compartments, separated by membranes, which act as barriers. This will block the free flow of current. 

In a different approach from the basic fruit-battery described above,  researchers are looking into creating electricity from fruit more sophisticatedly. One example is called a microbial fuel cell (MBCs) that can be used to convert fruits and vegetables into useable electricity. 

MBCs are effectively bio-electrochemical devices that can generate electricity using a wide range of microorganisms. A team of researchers from Ecuador experimented with this technology using membrane-less single chamber MCBs over 60 days. 

The experiment was very fruitful, excuse the pun, and was able to produce an output voltage of 330 mV. 

Using fresh fruit as a kind of ad hoc battery is one way to generate electricity from fruit, but there is another way. As the fruit decomposes, methane gas is produced, which can, in turn, be combusted to generate power on a much larger scale.

This is exactly what authorities in Seville, Spain, have proposed as a way to kill two birds with one stone. The city's streets are filled with Seville orange trees. Farms in the region export almost all of the Seville oranges they grow to Britain, where they are turned into marmalade. But the residents of Seville don't eat the bitter fruits, so the city ends up with streets littered with rotting oranges. 

These fallen oranges not only present a hazard for pedestrians but are also a headache to clean up for city authorities. The oranges are often squashed underfoot or by the wheels of cars, leaving streets sticky with juice and swarms of flies. 

Currently, 200 people are employed to collect the fruit every year. 

In February 2021, a new pilot scheme was announced to put this waste fruit to use, rather than simply chucking them all into a landfill. 

The fruit is gathered up and taken to a municipal water company, Emasesa, which generates clean energy from it to run one of its water purification plants. The juice is extracted, combined with other organic matter, and used to produce biogas, while the peels are being composted and transformed into fertilizer for fields.  

“The juice is fructose made up of very short carbon chains, and the energetic performance of these carbon chains during the fermentation process is very high,” said Benigno López, the head of Emasesa’s environmental department in an interview with the Guardian. “It’s not just about saving money. The oranges are a problem for the city, and we’re producing added value from waste.”

It is hoped that the scheme could eventually be extended to supply surplus electricity to the national grid too. According to the team behind the initiative, with some more investment and upscaling, otherwise wasted oranges from the city could be used to power as many as 73,000 homes. 

That is based on trials using 1,000 kg (2,205 lbs) of oranges that could be used to generate 50 kWh of electricity -- enough to power five homes for a single day. 

Rotting oranges are not the only way to produce methane gas, obviously. In Corvallis, Oregon, a pair of farmers spent $10 million and 14 months building their very own biogas plant.

This plant turns discarded or unwanted fruit and vegetables into thermal and electrical energy. Like in Seville, Spain, this waste would otherwise be dumped into a landfill.

Their biogas plant has been so successful that it was even recently connected to the national grid. It produces so much power, in fact, that it could help the farm save around $500,000 in electricity and natural gas costs. 

The plant works much the same as the stomachs (rumens) of a cow. Biomass is fed in, and methane gas comes out. 

Corn spillage, pumpkin waste, and other organic material are ground up, placed inside an anaerobic digester, and left to turn into a soupy liquid. Within the digester, bacteria's sets convert the waste into volatile fatty acids and then into methane gas. 

This gas then bubbles up through the soup and is collated at the top. From here, it is fed into an internal combustion engine that can burn it to generate electricity. 

Because the digester runs on a completely natural process, it can work continuously so long as new biomass is added. According to its owners, once the plant is fully operational, it could be able to produce twice the power needs of the farm -- enough to power 1,000 homes. 

A similar process is also used by a company called Gills Onions in Oxnard, California. A fresh-onion puree producer, around 136,078 kg (300,000 pounds) of waste onion solids are used to produce renewable energy.

By feeding this onion waste into an anaerobic digester, methane gas is produced, which can then be burned to power 600-kilowatt fuel cells, which provide enough electricity to power 460 homes. 

Their system required around $9.5 million in investment and utilized existing technology with the University of California's assistance. 

Whether you simply wire up an existing piece of fruit or build a more sophisticated piece of apparatus, fruit has some genuine applications beyond just eating it, Or, indeed, throwing it away. 

The use of technologies like anaerobic digesters might just be the ticket for reducing filling up landfills and releasing the very potent greenhouse gas methane into the atmosphere. Why waste it?