What are seed oils?
Oils are lipids (fats) which are liquid at room temperature. Fats are lipids (fats???) which are solid at room temperature - that's right the language here is difficult as lipids are colloquially known as fats and thus fats are fats is a silly sentence that also somehow makes sense. For fats think butter, lard, etc. For oils think olive oil, sunflower oil etc. Fats (lipids that are solid at room temperature) contain a higher proportion of saturated fatty acids than oils (liquid at room temperature) which makes fats more stable, this being the reason that they are not liquid at room temperature. The reverse is therefore necessarily true about oils - they have a higher proportion of unsaturated fatty acids thus are less stable and contain less single bonds (are less saturated with Hydrogen) and they are as such liquid at room temperature... Did anyone manage to read that paragraph without their head exploding? Maybe I could have written it better, oh well...
Seed oils are oils made from seeds (no shit) - for example rapeseed (canola if you're American and feel the need to call things different names, rocket=arugula?!?!), sunflower seeds, corn seeds, safflower seeds, sesame seeds, flaxseeds, soya/soybean. Vegetable oil - an incredibly misleading name for a product - which you may also see and hopefully never buy - is just a generic name for any one of these seed oils, contrary to it's helpfully descriptive name it is not made from vegetables. Check the label of a bottle of 'Vegetable Oil' and it is likely either rapeseed or sunflower oil. Is it called vegetable oil to plant the seed (nice) that it is healthy and thus make it more saleable? - probably. In fact it is anything but, as we shall find out.
Seed oils are extracted from the seed of the plant. There is another team of oils that we see when shopping and use when cooking (hopefully) and I would call these fruit oils (I would never call them this out loud). For example avocado oil, olive oil, palm fruit oil, coconut oil (which interestingly is solid at room temperature due to high sat fat content - is this even an oil? arghhh). These differ from seed oils in that the oil is extracted not from the seed of the plant but from the flesh of the plant's fruit - maybe we should call them flesh oils... maybe not. As would be expected extracting the oil from a plant's fruit is much easier than extracting it from its hard seeds (more on this soon).
It is possible (honestly) to use fats instead of oils for cooking (not that anyone does these days - man that is so early 1900s) - butter, tallow, lard, goose fat, duck fat, ghee. These you could simply call animal fats. They would be classified as fats as they are solid at room temperature and wouldn't be classified as oils because they are not liquid at room temperature - and thus you can't buy a bottle of butter (sadly). Fats contain higher levels of saturated fat than oils and thus as mentioned are solid and more stable (physically, but also emotionally).
Oxidation
Before I go into detail about the extraction of oils I need to define a term which I will be using a lot in this post. That is - oxidation.
Oxidation - "any reaction where a loss of electrons occurs by an atom or any other chemical species". Wow, maybe I will try and make it more relevant to oils...
"Oil oxidation is a complex series of chemical reactions involving oxygen that degrade the quality of an oil." Oxidation will eventually lead to rancidity of an oil giving an off smell and flavour as well as a reduced quality and nutritional profile.
So, oxidation is a chemical reaction involving oxygen which can render oils rancid. Oxidation destroys fat-soluble vitamins (which fats and oils contain a lot of), polyphenols and antioxidants which would otherwise be present within the oil, all of these would have major health benefits when consumed. It also reduces essential fatty acids, replacing them with dangerous trans fats. As if all of this wasn't bad enough, some of the products of oxidation can present toxicity - these being volatile products such as peroxides and aldehydes. Oxidation of linoleic acid (a polyunsaturated fatty acid) forms trans-2,4-decadienal which is a volatile aldehyde and has been shown to present levels of toxicity when consumed.
Oxidation is not good, and needless to say oils that have been highly oxidised are detrimental to your health on consumption. As oxidation occurs free radicals are produced, though these may sound like they are a 1960's hippy pop group, free radicals are actually highly reactive and unstable atoms that can damage important molecules in the body such as DNA, proteins and lipids. They attack critical areas throughout the body which leads to widespread cell damage. An excess of free radicals increases oxidative stress within the body, this has been strongly linked to various degenerative diseases including cancer, Alzheimer's disease, heart disease, kidney disease and type 2 diabetes. Not exactly spreading peace, love and happiness.
Oxidation will progress at different rates depending on a number of factors - these include temperature, light, availability of oxygen and presence of moisture/metals (such as iron). Higher temperatures, more light, more oxygen and more moisture result in more rapid oxidation ergo more rapid rancidity. Temperature is the number one contributing factor in the oxidation of oil, high temperatures speed up the oxidation process. High temperatures for example those used in extraction and cooking.
Something else that influences the rate of oxidation of an oil is its constituent fatty acid content. As discussed in my post on macronutrients, all fats and oils are made up of fatty acids. Each different oil and fat has a different proportional composition of the various fatty acids (main ones being - SFAs, MUFAs, PUFAs). An oil containing a higher proportion of polyunsaturated fatty acids (PUFAs) will oxidise more rapidly than one with a lower proportion. PUFAs have double bonds between their carbon atoms, these unsaturated double bonds are more reactive than the single bonds of saturated fatty acids (SFAs). The more reactive double bonds of PUFAs are more prone to oxidation. Monounsaturated fatty acids (MUFAs) contain only one double bond compared to PUFAs multiple double bonds and thus MUFAs are less reactive, and less prone to oxidation than PUFAs. In a league table of reactivity and oxidation rate polyunsaturated fatty acids would be the clear leaders (this ain't good, nothing to brag about), monounsaturated fatty acids would take second place and saturated fatty acids would be 3rd and bottom of the league (which in this case is the best place to be!).
So it is clear that fatty acids that are more unsaturated are oxidised more rapidly during heating in the presence of oxygen. Linoleic acid - an Omega-6 polyunsaturated fatty acid - for example oxidises up to 25 times faster than oleic acid, a monounsaturated fatty acid. The general rule is the higher the levels of unsaturation in an oil the more rapid the oxidation when heated. This means that seed oils - which all contain high levels of polyunsaturated fatty acids (highly unsaturated), mostly linoleic acid, oxidise at a much faster rate than oils containing less unsaturation. So for example olive oil which is mostly comprised of the monounsaturated oleic acid will have an oxidation rate lower than that of sunflower oil - which as you can see below is mostly PUFAs. The chart below in fact underlines the message that seed oils (rapeseed, sunflower, corn, flaxseed etc) all have higher PUFA levels than fruit oils and fats (olive oil, palm oil, lard, butter, coconut).
In the graph below the grey and orange bars make up total PUFAs, blue bar MUFAs and yellow is SFAs. The graph shows for example butter has a total PUFA content of 4%, olive oil 10%, rapeseed (canola) oil 32%, sunflower oil 72%. These four are possibly the most commonly used fats and oils in the UK. As you can see from the graph seed oils are, without exception higher in PUFAs than fruit oils and fats. This information is worth bearing in mind as we explore seed oils further.
Extracting oils
Anyway, this post is about seed oils. I have told you what they are, but how are these oils extracted from the seeds in the first place? Well, as I said earlier it is more difficult to extract oil from a hard seed (e.g. rapeseed or sunflower seed) than from a soft fleshy fruit (e.g. avocado or olive). Avocado and olive oil can therefore be easily extracted through purely mechanical means...
Fruit oil extraction
Olive oil has been made for millennia, the Greeks first began making it 5,000 years ago. The process they used is still in widespread use today. This process for extracting olive oil can be summarised as follows:
1. Olives are washed, 2. Olives are ground in to a pulp, 3. the pulp is placed into something similar to a woven basket and a stone placed on top to release the oil from the pulp, 4. the extracted liquid containing both oil and vegetable water mixed together is collected in a pit, 5. after it settles the hydrophobic (water fearing) oil rises to the surface and is collected by decantation, 6. the oil is then filtered and ready for consumption
Iterations of this process are still used today, with more modern equipment such as millstones and hydraulic machine presses but the process itself is fairly similar. Another more modern method of producing the oil is by a process called decanter centrifugation (sounds complicated) - in essence it still just involves crushing the olives in to a paste and then separating the oil from the other more water based liquids of the fruit, this is also a mechanical method of extraction, that is to say no chemicals or direct heat is required to extract the oil. Water which is used in the separation phase of the extraction is heated though and this is done in order to produce a higher yield of oil at the cost of quality
You may have seen the term cold pressed printed on bottles of various oils - it is possibly most ubiquitous with olive oil. Cold pressed means no/minimal heat is added during extraction and the olives were pressed, or crushed, in a mill to extract the oil.
In the decanter centrifugation method mentioned, during one of the extraction steps hot water is used to separate the water from the oil. In order for an oil to be labelled cold pressed in the EU the water used needs to have been below 27 degrees Celsius. There is no requirement for it to be above this temperature to extract the oil - as discussed earlier olive oil can be extracted by purely mechanical means. The good quality oils are those that are cold pressed, low quality olive oils use higher temperature water to extract the oils - the higher the temperature of water used during the processing, the higher the yield of olive oil obtained from the paste and this is where those lower quality manufacturers opt for quantity over quality (just like with factory farming). The problem with using higher temperatures (as we shall see with seed oils) is that higher temperatures cause a reduction in quality of the oil, an increase in oxidation (bad), and a reduction in antioxidants, vitamins and polyphenols in the oil (also bad) thus it is poor quality oils that use higher temperatures. Olive oil does not contain as high PUFA content as the seed oils as shown earlier so to that point oxidation would occur at a slower rate, though it is still an issue when heated.
So in summary fruit oils can be and still are extracted by purely mechanical means, methods which cause minimal loss of quality and minimal oxidation. More modern methods also used e.g. decanter centrifugation use heated water to separate oil from vegetable water, the higher the temperature of this water the lower the quality of the oil. So the advice would be to only buy cold-pressed where you can, as this uses lower temperature water and thus lower oxidation and higher quality oil.
Seed oil extraction
Now on to seed oils. Seeds are different to fruits in that they do also contain oils made up of fatty acids, however these oils are harder to extract as they are locked up within the solid seed. The three main methods of extracting oil from seeds are expeller pressing, solvent expelling and the lesser used cold pressing.
Expeller pressing - a mechanical method for extracting oil from seeds. Seeds are squeezed under high pressure, as the seeds are pressed (at much higher pressures needed than pressing fruits) friction causes heat to be produced, this can be temperatures between 50-100 degrees Celsius. Not only this but the raw materials (the seeds) are often heated up to around 120 degrees Celsius before pressing to make the pressing more efficient. As discussed earlier heating these oils causes a reduction in quality, but not only this, also a degradation of any possible healthy elements including vitamins, antioxidants and polyphenols. Worst of all heat causes increased oxidation. And heating up to this degree causes a lot of oxidation within these oils thus creating rancidity and making these oils something which you're not going to want to consume.
Solvent expelling - the seeds are ground to a paste and then this paste is washed in a solvent to release the fat from the seed paste. The solvent most commonly used is Hexane - a chemical which is commonly extracted from petroleum and crude oil, as a solvent it is also used commonly in industrial cleaner, degreaser, paints, lacquers and glues (yes you read all of that correctly). Hexane is added to the seed paste and this concoction is heated to high temperatures (again heat is bad) and distilled to remove the Hexane. This method gives high yields of oil and thus is a widely used seed oil extraction method - it is in fact considered "standard practice in today's modern oilseed-processing facilities". This solvent use along with high temperatures while providing a high yield leaves the oils in a rancid and unpleasant tasting state, the oil therefore has to be bleached and deodorised to make it edible (barely still). The whole process destroys antioxidants and vitamins within the oil.
Cold pressing - it is possible to cold press seeds, just as with fruit. However as you can imagine they require a lot of pressing to give up those oils, which are locked away within the seed. Seeds are cold pressed using a screw press which grinds the seeds and presses squeezing out the oil, the oil is then filtered. This process delivers a lower yield than the other methods mentioned and is therefore not at all common. Though I am starting to see more cold-pressed rapeseed oil and the like appearing in the more high end shops. The general bog standard rapeseed, sunflower or vegetable oil you will buy in a shop, or that will be used in every restaurant and fast food place, or that you will find in basically every non-whole food item in a shop is of the non cold-pressed variety.
In summary, while it is possible to extract seed oils by mechanical means - cold-pressing is very rare as it is difficult and delivers a much lower yield than solvent expelling or expeller pressing. Solvent expelling is standard practice for extraction of oil from seeds and this process involves the use of a petroleum extract called Hexane, which is also used commonly in industrial cleaners, paints and glues. After being mixed with Hexane the oil is so rancid it has to be bleached and deodorised to make it remotely edible. Both non cold-pressing methods of extracting seed oils involve high heats which cause oxidation and rancidification of the oils, as well as diminishing any vitamins, antioxidants and polyphenols. Heat and oxidation is a bigger deal with seed oils - as shown earlier- as they contain higher levels of PUFAs and are thus more reactive and prone to more rapid oxidation.
Heat when cooking
We have examined the extraction process of the oils and I have broken the news to you that the vast majority of seed oils found in a supermarket are rancid, toxic and have been mixed with a petroleum extract (yes the Hexane is removed, but still) and that they have had to be bleached and deodorised to render them edible. Maybe more crucial than this to your health is the oxidation causing free radicals and toxic compounds to build up within the oils as a product of the high heats they have been exposed to. Now this is the case for the seed oils you buy in bottles to cook with (hopefully not after reading this), the seed oil that is used in (I guarantee) every restaurant you would care to visit, every fast food place certainly, not only that but also the seed oil that is used in almost every non-fresh/whole food item in the shop.
Honestly, don't believe me, just wonder around any shop picking up non-fresh food/non-whole-food items at random. Its an almost 100% certainty that it will contain seed oils. Check the labels... Mayonnaise? - seed oil. Hummus - seed oil. Crisps - seed oil (obviously?). Popcorn - seed oil. Pasta sauces - seed oil. And in the biggest headbanging clusterfuck of all time fresh olives - seed oil. That's right check out the next tub of "fresh" olives you buy - the ones in the fridge- you can almost guarantee they're covered in sunflower or rapeseed oil. Now I could go on and on here but I won't. Just have a look yourself. You can buy olive oil versions of these items but would have to go out of your way to do so.
Back to the restaurant issue, these oils are used for frying everywhere, and I mean everywhere. I worked in quite a nice restaurant when I was younger (bar staff not the chef I hasten to add) and they had barrels of seed oil out the back. They used it for frying anything that needed frying. And speaking of frying, the oil that is used in deep fryers throughout the land is seed oil - deep frying exposes these oils to temperatures of above 180 degrees Celsius. This is after they have already been made rancid, oxidised and barely edible during the extraction phase, they are then re-exposed to high temperatures to finish them off, just in case there wasn't enough oxidation and rancidity. Not only this but the oils used for deep frying are reused all day - and only for one day if you're lucky - it has been know for fast food places to reuse their oil for a week and more. Re-frying the same oil over and over again. It cannot be understated just how bad consumption of this toxic, rancid, disease inducing oil is for your body.
Now you may not eat in restaurants or fast food places often and if you do maybe you don't order fried food and therefore you are avoiding the worst. However even using seed oils in your cooking at home is something you should not be doing. As I have said, but will continue to repeat, these oils are rancid and toxic while sitting there on the shelf waiting for you to buy them by means of their very extraction. Say you purchase a bottle of seed oil, take it home and use it for shallow frying onions for example to start off a Bolognese or curry, you will be heating this oil again in the presence of oxygen and thus further oxidation will occur on top of the oxidation already instigated through extraction.
Other sources of oxidation to be aware of
- Light -
Exposure to light will also increase oxidation rate and thus lead to the deterioration of fats/oils. The process is called photo-oxidation. The same effects apply as detailed earlier regarding heat. Photo-oxidation leads to rancidity, build up of free radicals, build up of volatile toxic harmful products within the oil. Also leading to the oils losing their flavour and quality.
- Air -
This one seems like a no-brainer. Air contains Oxygen and therefore exposure to air increases the oxidation rate of oils. Again increased oxidation leads to the negative and harmful effects discussed.
It is therefore important to observe the bottle that the oil you purchase comes in. Avoid buying oil in clear see-through bottles as photo-oxidation will have been acting upon this oil since bottling. Try to buy oils in darker coloured bottles or in stainless steel containers. Also avoid placing your oil in direct sunlight - store in a cool, dark area. Avoid keeping it next to the oven or hobs. Avoid leaving oil open for any amount of time that you are not using it.
Tips for avoiding oxidised rancid oil
In order to best avoid any further oxidation of oils you may have purchased there are some hints to follow when purchasing oils for use at home/eating out
- Avoid purchasing seed oils/vegetable oil in bottles for use at home - instead buy fruit oils (avocado, olive, coconut) or fats (butter, lard etc) to cook with and dress salads, or drink, or whatever you want to do with your oils/fats.
- Extra virgin, cold-pressed olive or avocado oil are the finest oils you can buy with the least potential for oxidation. These can both be used for roasting. Olive oil for frying on a medium/low heat and avocado can be used to fry on high heat. Or both can be used as a nice salad dressing.
- Don't forget about using fats for cooking rather than oils. Butter, lard, duck fat, goose fat are all delicious, healthy and much more stable than seed oils so oxidation is not such a worry. They can be used for roasting and frying at low/medium heats.
- Avoid eating fried foods in restaurants and especially in fast food places - in fact avoid fast food places all together (duh). This is a big one because the oils used and reused in fryers are likely going to be the most toxic, rancid and disease inducing things you could eat.
- Check the packets of anything you buy and avoid anything which has a seed oil (vegetable oil, rapeseed oil, sunflower oil being the most common) listed in the ingredients. You may be surprised at the breadth of food items which contain seed oils.
- Purchase your fruit oils in dark coloured non-clear bottles or stainless steel containers. Store in a cool dark place out of direct sunlight and away from the oven. To avoid light and heat exposure.
- Don't leave the oil open for any amount of time except when using it in order to avoid air exposure.
Summary
This was a long one, but a very important one nonetheless.
- Seed oils are oils extracted from the seed of plants - rapeseed and sunflower being the most common.
- Fruit oils are extracted from the fruit of the plant - olive and avocado being the most common.
- Seed oils contain a much higher proportion of PUFAs - these polyunsaturated fatty acids are more reactive and unstable than other kinds of fatty acids and are therefore more prone to oxidation.
- Oxidation of oils makes them rancid, produces volatile and toxic products within them, reduces the quality of the oil and depletes nutritional elements such as vitamins, antioxidants and polyphenols.
- Oxidation also creates free radicals in the oils, free radicals wreak havoc in the human body, increasing risk of cancer, heart/kidney disease, diabetes and Alzheimer's.
- Seed oil extraction methods involve either a lot of heat or a bit less heat and petroleum chemical washing. The heat during the extraction process causes high levels of oxidation within the oils rendering the toxic and rancid.
- The oils can then be further oxidised through home cooking, exposure to light and exposure to air.
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