Orange juice is a delicious and healthy drink that millions of people around the globe enjoy every day. New research suggests that orange juice could be used for more than just breakfast. Orange peel chemicals could be used to create new building blocks for products such as paracetamol, plastics and other pharmaceuticals. This would help us to reduce our dependence on crude oil.
The world’s population is totally dependent on fossil fuels for the chemical and material products they need. In order to produce biofuels, there has been a growing global focus on developing renewable chemicals using a variety of sustainable sources. These include sugarcane as well as fatty acid. The chemically rich oils found in waste citrus peels are another source being explored with zest.
It is encouraging since the orange juice industry has a highly inefficient and wasteful process of juicing, where almost half the fruit is thrown out. The peels contain a variety of molecules that can be used to create chemicals.
Limonene – a versatile building block
According to recent figures, around 20 million tonnes are wasted every year. The oil from the waste rinds contains limonene, which is a colorless liquid known as C 10H 16. This waste can yield 125,000 tonnes of this liquid hydrocarbon each year.
The current extraction method relies on steaming through waste solids and then collecting the oil that results. Researchers at the University of York have been investigating microwave extraction methods in an effort to find a more environmentally friendly alternative. The team placed orange peel, organic solvent, and microwave for 30 minutes. The water molecules in the peel begin to boil and rupture the cells, allowing limonene to leach out. The process produces higher-quality limonene, is less energy-intensive, and yields twice as much as the conventional method.
Limonene structural formula. Ju/Wikimedia, CC BY-SA
What is it about limonene that makes it so useful? It is a hydrocarbon that shares many similarities to the chemicals we get from fossil fuels. For example, its liquid state and boiling point. The technology that we use to convert petrochemical feedstocks into useful products can also be used for limonene. This can be done by utilizing the two double-carbon bonds in limonene for various chemical transformations.
Renewable plastics
The polymer industry may be the best use for limonene. Chemists constantly seek new ways to combine molecules into long chains that can be used for a variety of products. For example, polypropylene fibers (a chain consisting of propylene molecules) are used for carpets, and polyethylene, a chain consisting of ethylene molecules, is used for plastic bottles.
Researchers at Cornell University demonstrated the potential of this strategy in 2006. In 2006, scientists at Cornell University demonstrated how this strategy could be used to create polyesters. In recent years, this groundbreaking work was followed up by a variety of other polymer-forming reactions. Other forms of polyesters were made by substituting carbon dioxide with alternative construction blocks.
There may be a new player on the market. Kumar’s Edit/Flickr, CC BY-SA
Polyurethanes are a group of synthetic polymers found in many products, from shoe soles to foam cushions. The raw materials used in the production of polyurethanes are currently derived from fossil fuels. However, limonene-based alternatives have recently been identified. Researchers in Germany, for example, have developed a group of new polyurethane-like plastics that contain limonene as the central component. These materials’ properties and hardness could be tuned easily to create a variety of household products.
The development of materials based on limonene does not end with everyday consumer goods. Recently, advanced optical materials were prepared using limonene to create thin luminescent film.
One day, the production of medicine from limonene could also be a possibility. My team at the University of Bath investigates the production of paracetamol by using the carbon skeleton of limonene as its base. We plan to transform waste orange peels, using a combination of state-of-the-art engineering and chemical transformations, into this pain-relieving drug.
Hurdles that you must overcome
Limonene has the potential to be an important component in the bio-based chemicals industry. This field is in its infancy, with no commercially available limonene-based plastics. There are still many issues to address, including finding a reliable source and greener processing. Many of the processes used for making polymers from limonene require petrochemicals.
Flower wrapping made from PLA. Achim Raschka/wikimedia, CC BY-SA
Limonene and its family of terpenes (also known as terpenoids) are just one part of the puzzle when it comes to renewable feedstocks. Polylactic acid is a sustainable polymer made from corn starch that has been available commercially for over ten years, primarily in disposable packaging. Coca-Cola’s plant bottle technology is one example of a plastic building block derived from plants. These renewable alternatives can contain large quantities of oxygen atoms, giving the polymers different and complementary properties compared to limonene-based plastics.
Also, the availability of limonene depends on the weather conditions in Florida and Brazil, as well as our desire to consume orange juice. The amount of limonene currently available from citrus waste would not make much of a difference to global plastic production.
If we were to make better use of the citrus fruit that is wasted, then the amount of limonene available would rise. Biotechnology and more efficient extraction methods are also expected to increase availability. My team is investigating ways to modify bacteria so that they can only produce limonene from municipal waste. Although these kinds of advancements may still be some time off, they are crucial first steps in breaking our dependence on oil. Molecules like limonene can be an important part of the future.
