three different bottles with oil

The Chemistry of Olive Oil (And Why It’s Stored in Green Bottles!)

Ever stood in front of the shelf spaces full of olive oil bottles and cans and wondered why there are so many different types of packaging? Non-transparent plastic bottles, slim and tall transparent plastic bottles, oils in greenish plastic, some in dark green or just plain transparent glass, and others in cans. Olive oil probably has the most variety in its packaging type in our supermarkets. Add to that the different types of olive oil, virgin, extra virgin, mild, basic, flavored, etc., and your olive oil confusion may well be complete.

Luckily, olive oil is ‘just’ an oil, so once you know its chemistry, it will hopefully, become a little easier to understand the differences. And, yes, all those different types of packaging do impact the olive oil and its quality!

How olive oil is made

As the name says, olive oil is made from olives grown specifically for being made into oil. After taking care of their olives for the whole season, harvesting at the right time, is especially important for farmers.

Oil content increases during ripening

When the olives start to ripen on the tree their oil content increases. At the same time, the connection between the olive and tree starts to weaken, to allow the olive to fall off the tree. The optimal harvest time for farmers is when olives contain as much oil as possible (generally 15-25%), without yet having fallen off the tree (since those can’t be used for the higher quality olive oil products).

Each variety ripens slightly differently. Also, the weather, location, etc. all impact this process and as a result the types and amount of oils produced within the olive. The chemical reactions that cause the formation of the oils depend on factors such as temperature.

Cleaning & Grinding

Once harvested from the tree, it is important olives are processed quickly. As is the case for other fruits, they can spoil quickly after harvest, especially in warm, sunny climates. The olives are first cleaned, branches and dirt is removed.

Next up, the olives are ground into a paste. The pits can be left in, or taken out after grinding, depending on how a manufacturer operates their equipment. During grinding the structure and thus cells of the olives are broken down. As a result, the oils and water can leaven the cells and can be separated in next steps.

Malaxation

Before separation takes place though, the ground paste is left to ‘rest’ for a while. During this phase, called malaxation, the olives are still moved around. Even though it might be tempting to skip this step, there are two benefits from taking some extra time here.

First of all, more cells of the olives are broken down, making it easier to extract even more oil from the olives. Also, the small oil droplets will find each other and start to coalesce. This makes it easier in a later step to separate the oil from the rest of the olive.

But it’s not just for efficiency’s sake, a lot of the flavor of olive oil is formed during this step. A wide range of chemical reactions takes place during this step, producing a wide range of desirable aromatic compounds. Enzymes, naturally present in the olives, play a big role in facilitating these reactions.

This step is a balancing act. At higher temperatures the oil separation happens more quickly and so do the chemical reactions. Whereas you don’t want too few chemical reactions taking place, the opposite is undesirable as well. This can lead to a loss of flavors or the formation of not so desirable ones!

Pressing / Centrifugation

Even though people might still refer to olives being ‘pressed’, most nowadays aren’t actually pressed. Instead, to separate the oil from the water and solids of the olive, manufacturers use centrifugation. Centrifugation is a technique that uses the difference in density of oil vs the other components to separate them. It is a sped-up version of just leaving the mixture sit in a tank. Over time, the oil will float to the top and the water to the bottom (as would happen when you try to mix oil and water in a glass). This is actually a common technique in the food industry, it also plays an important role in processing milk for instance!

At this point, you have a good olive oil which is slightly cloudy. However, the shelf life can still be quite short due to some remaining pieces of olives and some additional remaining water. Therefore, there might be an additional centrifugation or filtration step to ‘finish’ the olives.

Storing & Packaging

Once the olive oil is finished it is stored under a “nitrogen blanket” if it is not packaged in its final packaging immediately. This means that the oil is stored in a container where all the air has been replaced by nitrogen. This is essential to ensure the olive oil stays of a high quality over time. Oxygen causes oxidation of the oils, making it rancid. It’s one of the most important processes that greatly reduce the shelf life (and quality) of olive oil (and many other oils and fats).

As we noticed at the start, the final olive oil can be packaged in a wide range of different packaging materials. Which is used depends on consumer preferences, as well as on the product itself.

The shelf life of olive oil is limited by the occurrence of off-flavors in the oil over time. These flavors can form due to various chemical reactions, most notably oxidation. Bot the pigments (e.g. chlorophyll) as well as the unsaturated fatty acids as well as various other minor components can oxidize under influence of light and/or oxygen. As such, it’s best to store olive oil away from light (to prevent auto-oxidation) and oxygen (so keep the packaging container closed).

Why is olive oil packed in green bottles?

The stability of the olive oil is also why you see so many different packaging types of olive oil. Non-transparent metal containers protect the olive oil well against light and air. Dark-colored, especially green (to prevent chlorophyll oxidation), glass bottles protect it well also. Transparent plastic bottles on the other hand provide less protection. If an oil has been refined that is less of a problem, but for non-refined oils, this can accelerate the loss of quality of the oil.

three different bottles with oil
The right bottle doesn’t provide much protection to the oil.

Refining olive oil

The oil made using the steps above is a virgin (or, depending on the quality, extra virgin) olive oil. An important characteristic of this olive oil is that is contains a lot of aromas and flavors that aren’t necessarily oils (as we’ll discuss below). However, if the olives were not of a high enough quality or if processing did not happen as intended, the quality of this oil might not be suitable for consumption. For example, the olive oil might contain too many free fatty acids.

In such a case, the oil is refined further. During this refining process a lot of additional molecules are stripped from the oil. This makes it more neutral in flavor and increases shelf life, but decreases the overall quality (and value) of the oil.

What makes up olive oil?

The main component of olive oil is oil, making up more than 97% of the oil. Apart from the oil, there are some minor chemical components such as polyphenolics and pigments. Even though they make up only a tiny amount of the olive oil, they are very important for the overall quality of the oil!

Triglycerides

Fats and oil, whether it’s lard, olive oil, butter, sunflower oil, or another common fat or oil in your kitchen, are all a mixture of triglycerides. Triglycerides are a group of molecules, with a very similar structure. The backbone of the molecule is made up of a glycerol molecule (hence triglyceride). Attached to this backbone are three (hence tri) fatty acid chains. Fatty acids are linear chains of carbon atoms. They might be attached to one another with just one bond, or a double one.

Note: A proper introduction on the topic of triglycerides can be found in our food chemistry basics post.

Triglyceride with glycerol and fatty acid
A = varying chain of carbon atoms

Triglycerides can contain three of the same fatty acids, or three different ones. Within a fat or oil you might have a wide range of different types of triglycerides. The types of fatty acids present in a fat or oil impact the behavior of that fat or oil. A longer chain of carbon atoms for instance tends to decrease the melting point of a fat/oil, thus increasing the chances it is liquid at room temperature.

You might have also heard that certain fats and oils are ‘healthier’ than others. An important factor here is the presence (or absence) of one or two double bonds between carbon atoms in a molecule. The ones with double bonds, called unsaturated fatty acids, tend to be perceived as healthier.

Most common triglycerides in olive oil

The major fatty acid in olive oil is oleic acid, it makes up over half of all the fatty acids in the olive oil (see table below). Apart from oleic acid, there are 5 other common fatty acids in olive oi. Aside from these, there are a few minor other fatty acids, each generally making up less than 1% of the oil

Fatty acidNo. of carbon atomsSaturationApprox. w% in oil*
Palmitic acid16Saturated7.5 – 20
Stearic acid18Saturated0.5 – 5
Palmitoleic acid16Unsaturated0.3 – 3.5
Linoleic acid18Unsaturated3.5 – 21
Linolenic acid18Unsaturated0 – 1.5
Oleic acid18Unsaturated55 – 83
Most common fatty acids in olive oi. (note: w-% from Tsimidou, 2003)
most-prevalent-fatty-acids-in-olive-oil

Polyphenolics

Polyphenols are a group of molecules with a similar molecular structure. They are relatively common in a variety of foods. The bitterness of tea is mostly caused by polyphenols and they undergo various reactions during the processing of tea. They also play an important role in enzymatic browning reactions (which you might see on cut apples and bananas for instance).

A common characteristic is that they tend to be bitter and as such, they contribute to the flavor of olive oil. Olives naturally contain various polyphenols, a common one that can be found in olive oil is oleuropein aglycone. How much of these polyphenols actually end up in the final olive oil and which types, depends strongly on just how the olive oil was processed. Temperature, time, presence of oxygen, they all impact just how these molecules react and behave. You’re essentially extracting these molecules into the oily phase, so the rules of extraction apply here.

Pigments

The color of olive oil comes from pigments within said oil. The pigments that make it into the oil are those that dissolve in oil and not in water. The water-soluble pigments have been washed away during processing. Common pigments in olive oil are carotene, chlorophyll (which has reacted into pheophytin), and lutein (also present in yellow tomatoes).

two different bottles of olive oil
Just two of the many different types of olive oil!

References

Blatchly, Richard., O’Hara, Patricia., Nircan, Zeynep Delen. The Chemical Story of Olive Oil: From Grove to Table. United Kingdom: Royal Society of Chemistry, 2017. link

M. Tsimidou, G. Blekas, D. Boskou, OLIVE OIL, Encyclopedia of Food Sciences and Nutrition (Second Edition), Academic Press, 2003, Pages 4252-4260, ISBN 9780122270550,
https://doi.org/10.1016/B0-12-227055-X/01347-X.

Regulation (EU) No 1308/2013 of the European Parliament and of the Council of 17 December 2013 establishing a common organization of the markets in agricultural products and repealing Council Regulations (EEC) No 922/72, (EEC) No 234/79, (EC) No 1037/2001 and (EC) No 1234/2007, Part VIII, link; European legislation of olive oil type terminology

Olive Oil: Chemistry and Technology. United States: Elsevier Science, 2015. Chapter 6, link

The Extra-Virgin Olive Oil Handbook. Germany: Wiley, 2014. Chapter 16, link

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