Ever stood in front of the shelf spaces full of oil bottles and cans and wondered why there are so many different ways to pack oils? Large non-transparent bottles, slim and tall transparent or greenish plastic bottles, others in non-see through cans and again others have been packed in white or green glass. Olive oil especially can be found in a lot of very ‘authentic’ looking bottles and cans and then there’s the virgin, extra virgin, mild, basic, with an extra bit of flavour, etc.
An overload of choice makes it hard to choose, so time to understand what those differences are, why they’re there and how to choose. What better way to do so than by studying the chemistry of olive oil.
How olive oil is made
Olive oil is made from olives that aren’t completely ripe yet, they should still be green. This is the point they contain most oil. After cleaning, crushing and removing the pits the olives are ground into a paste which is pressed to expel the oil. Extra pressing and a slight heat treatment can be used to expel even more olive oil. However, the more ‘work’ is done to expel the oil, the lesser the quality of the olive oil tends to be.
This is partly where all those different olive oils you can find in a shop come from. The ‘purer’ less treated ones tend to have a different colour and flavour than those which have been pressed somewhat more vigorously and which way have been refined. In the European Union rules (Annex 16) apply to the naming of these different types of olive oil. These rules state for instance whether the oil may have been heated and which other process steps are allowed to name it extra virgin or virgin olive oil.
What is olive oil made of?
The olive oil that is obtained is rich in oil of course, but it will contain a lot of other chemical components as well. Below you can find more about the composition of this oil and the triglycerides and fatty acids in olive oil. However, it’s not just oil that olive oil is made of. Olive oil contains a lot of other chemical components which give olive oil its characteristic taste and smell. For instance, olive oil contains chlorophyll and carotenoids. These give olive oil their greenish colour.
Apart from these colour molecules there are a lot of flavour molecules as well such as terpenes and esters or loose fragments of fatty acids. All these components are released during processing.
Chemical structure of olive oil – fatty acids
Since the main component of olive oil is the actually oil molecules themselves, let’s focus on them first. Olive oil is mostly made up of triglycerides. This is a group of molecules with a very similar build: a glycerol molecule with three long fatty acid chains. Some triglycerides are liquid at room temperature (these are oils) others are solid (the fats). Despite the different names they are quite similar, as we noted when comparing olive oil, butter and lard.
An proper introduction on the topic of triglycerides can be found in our food chemistry basics post.
Olive oil does not have one chemical structure. Instead, olive oil is made up of a mixture of triglycerides. These trilgycerides have a variety of fatty acids attached to them. Below you can find the most common fatty acids in olive oil. The most prevalent fatty acid is oleic aicd, but remember that this again sits in a triglycerides
Free fatty acids in olive oil
Even though there are a lot of fatty acids in olive oil, most of these are part of the triglycerides. However, processing can cause the release of some of these fatty acids, allowing them to become ‘free’. These free fatty acids are actually a component we want to avoid in olive oil. A larger quantity of these free fatty acids shows that the olives have been treated more intensely, resulting in these free fatty acids. In European legislation therefore a limit has been set to the maximum allowable content of free fatty acids for unrefined olive oil.
Refining olive oil can remove these fatty acids. However, refining will also remove other components from the oil, overall resulting in a lower quality oil. This is why this refining step has to be mentioned within Europe.
Why is olive oil packed in green bottles?
The oleic acid in olive oil is quite a stable molecule, as are most of the other fatty acids when part of the triglycerides. However, chlorophyll isn’t as stable. Chlorophyll is the molecule in plants that helps them to catch sunlight and it will continue doing so in the olive oil. As a result the chlorophyll can be oxidized. This can unbalance the rich flavour profile of olive oil and make it stale or harsh.
Therefore, unrefined olive oil has to be protected from light. Packing the olive oil in green bottles does the job. The green colour of the bottle will not let the correct wavelength of light come in for the chlorophyll, preventing the oxidation. Aluminium cans have a similar effect, they don’t let in any light at all.
On food and Cooking, written by Harold McGee was an important source for this article.