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Chemistry


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Olives (Olea europaea)

Figure 1: A watch-glass with some olives.

The european olive ‘Olea europaea’, grows on a small tree commonly found in the Mediterranean. These trees have been producing olives for quite some time: the oldest tree in the world is said to be found on the island of Crete and is estimated to be around 4,000 years old! Olives are delicious when eaten whole, but their oil is much more valuable and finds use in cooking, cosmetics and as a lubricant. This makes demand for olive oil incredibly high, with Spain being the largest producer of olive oil despite Greece consuming the most. Read on to find out more about the interesting properties of olive oil, and to uncover its biggest secret!

1. Olive oil extraction begins with removing the stems, leaves, and twigs from the olives. They are then cleaned to remove any dirt or debris left over after picking. Next, the olives are ground into a paste to release the oil from the vacuoles. The paste is mixed so that smaller oil droplets combine to become larger ones. This process usually takes between 20 and 45 minutes. If the oil is mixed for too long it can oxidise in the air, which causes a bitter taste and decreases the shelf life of the oil. The oil must then be separated from the fruit. This is achieved using a centrifuge, which spins the oil-vegetable mix at high speeds to separate the components. This leaves you with a viscous golden oil (for more information see http://www.madehow.com/Volume-3/Olive-Oil.html). Given that it takes 7 litres of olives to make just one litre of olive oil, it’s unlikely you’ll be making your own oil any time soon!

The main use of olive oil, and the one you will likely be most familiar with, is in cooking. It is one of the main components of the ‘Mediterranean diet’, and is probably one of the healthiest fats around given that it has been associated with reducing the risk of cardiovascular disease (see this article from the British Heart Foundation). In addition, olive oil can replace palm oil in the soap-making process (known as saponification) and finds other uses as a lubricant and as a fuel in traditional oil lamps. Many moisturizers and other cosmetics use olive oil as a base ingredient, and it is even regarded by some as being beauty’s best kept secret! Given its myriad uses, it’s no surprise that high quality extra-virgin olive oil can set you back as much as £10 per litre at your local supermarket.

Figure 2. The triglyceride ester of oleic acid, the main component of olive oil.

2. Olive oil contains a mixture of saturated and unsaturated fatty acids, present as triglyceride esters which are formed from the condensation of glycerol and three fatty acids (figure 2). Although the precise composition of olive oil can vary as a result of the crop, time of harvest and extraction process used, it predominately contains up to 83% oleic acid, and approximately 20% of linoleic acid and palmitic acid (figure 3). In addition olive oil contains small amounts of phosphatides, pigments, flavour compounds and sterols.

Fatty acids such as oleic acid, linoleic acid and palmitic acid are usually non-polar molecules containing around 14-24 carbon atoms, with the general formula CH3(CH2)nCOOH. Depending on the presence and number of double bonds in the alkyl chain, fatty acids can be classed as saturated (no double bonds) or unsaturated (containing at least one double bond). Olive oil usually contains more monounsaturated fatty acids because they are more resistant to oxidation. The higher the number of double bonds the more unstable the molecule is to factors such as heat and light.

Figure 3. Structures of the free fatty acids that make up olive oil.

The double bonds present in fatty acids lead to some interesting properties. There is restricted rotation around a double bond and this gives rise to cis isomers (where bulky groups adopt positions on the same side of the double bond) and trans isomers (where bulky groups adopt positions on opposite sides of the double bond). For more information on isomerism see this helpful guide. Cis isomers have a ‘bend’ in the molecule which is more pronounced than trans isomers. For this reason, trans fatty acids are able to pack together and solidify at room temperature whereas cis fatty acids struggle to pack together and remain as a viscous liquid. Olive oil only contains cis fatty acids and so remains an oil at room temperature. Chain length is also is a factor in the viscosity of the oil, with longer chains causing more viscous oil. Usually olive oil will contain fatty acids with no more than 20 carbons.

3. You may be aware that extra-virgin olive oil commands a premium price compared to other edible oils. This makes olive oil adulteration a serious problem in the food industry. Olive oil can be mixed with cheaper oils such as soybean oil, rapeseed oil, sunflower seed oil or corn oil and it has been estimated that approximately 30 percent of retail olive oil is fraudulent.

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Comparing rapeseed oil (left) and extra virgin olive oil (right), on top of an IR sprectrometer

One method chemists have used to detect olive oil purity is by using Raman spectroscopy. This technique measures the intensity of laser light that is scattered from a sample. Molecules containing different functional groups interact with laser light in different ways. This means that the spectrum of scattered light from a sample produces a unique molecular ‘fingerprint’ of what’s in that sample. Olive oil contains a lower proportion of unsaturated fatty acids compared with other oils (see section 2) and thus produces a Raman spectrum that is distinct from other edible oils. Chemists typically quantify these differences in terms of the intensity ratio of the cis-(=C-H) and cis-(C=C) vibrational Raman bands, allowing pure olive oil to be distinguished from adultered oils (J. Agric. Food Chem., 2009, 57, 6001-6006).

Whilst you probably won’t be able to characterise the composition of your own olive oil (unless you have your own Raman spectrometer), you can be safe in the knowledge that adultered olive oils don’t pose any serious risks to your health. In fact, olive oil is reported to have numerous health benefits, so you can carry on enjoying your favourite olive oil as normal.

Contributors: Emily Boys (Writing), Angus Voice (Editing), Chris Adams (Editing), Natalie Fey (Pictures).