Now, a new research indicates that it has the potential well beyond the breakfast table.
The chemicals in orange peel can be used in products ranging from plastics to paracetamol – and help break our dependence on crude oil.
Our society is totally dependent on the chemicals and materials extracted from our diminishing stock of fossil fuels.
Almost 50 percent of the citrus fruit goes to waste
There is a global increase in focus on the development of renewable, chemical raw materials from a variety of renewable sources, such as sugar and fatty acids for the production of biological fuel.
The essential oils of citrus fruit peels are filled with many different chemicals, and it is being explored with great enthusiasm.
It looks very promising. When orange juice squeezed industrially, they use inefficient and a waste-producing manufacturing method, where nearly 50 percent of the fruits are thrown away.
It creates a real opportunity to develop a sustainable chemical supply from the many different molecules that lies inside the peel.
Limonene – a versatile building block
New calculations estimate that there are approximately 20 million tons of citrus spilled as waste annually.
About 95 percent of the oil that can be extracted from the peel,consisting of the so-called ‘limonen’ molecules, just go to waste.
Limonene, 4-isopropenyl-1-methyl-cyclohexane, C 10 H 16, is a colorless liquid with a lemon-like odor and a boiling point of 176 ° C (limonene occurs widely in nature in as much as 90 percent of the oils from, for example, citrus fruits , caraway and dill, ed.).
We can produce approximately 125,000 tonnes of limonene extracted from citrus fruit peel annually.
Currently limonene is extracted by distillation – that is, of separation or cleaning the volatiles by boiling and then cooling the steam, so that it condenses to a liquid, or in this case oil.
A new, fast and energy-saving process during development
Recently, scientists at York University in the UK began to investigate the microwave extraction techniques as a greener alternative.
Limonene, 4-isopropenyl-1-methyl-cyclohexane, C10H16, is a colorless liquid with a lemon-like odor and a boiling point of 176 ° C.
The team simply put orange peels and an organic solvent in a microwave and heat it for 30 minutes.
The water molecules in the skin began to boil, causing the cells to rupture, so limonene fluid could flow out.
It gave a very good result. It is a more rapid and energy-efficient process which produces twice as much liquid-limonene, which is of a higher quality than in the conventional method.
Limonene can be converted to useful products
But what makes limonene so useful?
The technology that we currently use for petrochemical raw materials, can be used directly to transform limonene into usable products.
This can be done by utilizing limonenes two double bonds between the carbon atoms. These compounds are sometimes referred to as unsaturated and can be used for a variety of chemical transformations.
Ground-breaking discovery: Scientists made polyester from limonene
Limonene can also be used for the preparation of polymers. The chemists are always looking for new ways to link the molecules into long polymer chains so that they can be used for a variety of products such as polypropylene (propylene molecular chain), fibers used for carpets or polyethylene (ethylene molecule chain) for the manufacturing of plastic bottles.
In 2006, scientists at Cornell University was convinced about how good this strategy could be. By combining limoneneoxid – limonene one molecule and an oxygen atom – with carbon dioxide in an alternating pattern, created the polyester which is used in many woven and knit fabrics.
This groundbreaking discovery in recent years has been accompanied by other polymer-propagating reactions, among other types of polyester, by replacing carbon dioxide with alternative building blocks.
Polyurethanes are synthetic polymers that have found their way into our homes – from foam cushions for shoe soles. At this stage the raw materials are mostly derived from fossil fuels, but a number of limonene-based substitutions have recently been identified and candidate to take place of fossil fuels based materials.
Limonene for the production of paracetamol
In Germany, researchers have produced a new polyurenthan-like plastic that is based on limonene. Product properties and hardness is available to be used in the development of a number of household items.
The development of limonene-based materials does not stop at consumer goods.
Recently, limonene was used as the binder in the luminescent thin strips for the production of advanced optical materials.
Limonene will continue to be used for the production of pharmaceuticals. My research team at the University of Bath is currently producing paracetamol from limonenes .
Obstacles to be overcome
Limonene certainly has a great potential to become an important component of a bio-based chemical industry. It is yet a new field, and there is no limonene-based plastics on the market.
We still need to find a reliable supply of limonene as an environmentally good processing method, since many of these manufacturing methods are still using petrochemical additives.
Limonene, terpenes and terpenoids – chemical name for a group of organic substances released from many plants – is only a small part of the renewable raw materials.
There are basically three types of bioplastics: starch polymers, polymers obtained by chemical synthesis (e.g., PLA, polylactic acid) and polymers produced by biological processes (e.g., PHA, polyhydroxyalkanoate). They all have in common that they are produced from renewable raw materials such as corn.
Other plastic building blocks derived from plants – like Coca Cola’s Plant Bottle technology. Plastic raw material from these sources can contain a large amount of oxygen, which results in the other – but complementary – properties.
The availability of limonene is dependent on the weather in Brazil and Florida and our craving for orange juice.
Municipal waste material can be used for limonene
Even if all the current limonene from waste from citrus fruits were transformed into polymers, it is only a drop in the ocean of all the raw materials needed to meet the global plastic production.
But if you can use all the waste from citrus fruits, the amount of limonene would increase. More efficient extraction techniques and biotechnological advances will also be a big help.
Such advances can help to break our dependence on petroleum, even though they are just around the corner.
Molecules as limonene will no doubt be a colorful part of our common future.