Oil tell you all about it (Part 1 of 2)

A few decades ago, low fat diets were all the craze. From what I can tell, it the trend was based on a partial truth that aimed to eliminate what we now know as saturated fat, but he the negative fallout is still apparent. The resulting demonization of “fat” was due to either partial understanding, poor communication of and/or willingness to understand nuanced points around “fat.” There are summaries online about how this trend came about so I won’t get into the history here. There’s still a lot of confusion around “fat” and there’s a reason I’m putting it in quotation marks. Talking about “low fat” or “high fat” diet, in my opinion, is akin to saying something like “low liquid” or “high liquid” diet. As a vague description, it doesn’t really mean anything without more details.

I’m breaking this topic into two separate but related posts: Here in Part 1, I’ll cover the fundamentals on which the following posts are based. Part 2 will focus on the relevance to the diet and connect it a bit to primary agriculture. But without understanding Part 1, there’s not much point to dive into the other topics.

Here, we’re going to explore plant-derived “fat” in food and you’ll probably also start doing the “air quotes” with your hands every time you talk about “fat.” First, I’m going to break down some naming conventions (and a bit of a chemistry lesson) to better understand labels and nutritional claims when grocery shopping, and for fundamental concepts on which the next parts are based. Then, we’ll take a look at the relevance plant oils have in the diet, as well as a bit of non-food uses. I’ll connect these parts back into primary agriculture when possible. Remember, this is not to say anything is “good” or “bad,” “right” or “wrong;” I want to provide you with fundamental understanding so you can make more informed decisions. Please consult with your health professionals before making any changes that could influence your health.

[Note: A few other causes of confusion around these terms is that “fat” and “oil” are also used to describe some animal-derived products, fossil/petroleum products, and even physiology of the human body. In its most general sense, “fats & oils” can be thought of as: a category of hydrophobic molecules – also called “lipids;” in other words, they do not mix with water (think salad dressing that’s been sitting for a while). Anyways, these are outside the scope of this post, just be aware of the generalized use of the terms.]

I’m going to mention “molecule” a lot…which is just a general word for 2 or more atoms joined in a certain 3D physical arrangement.

 So, let’s start with the source of oils or fats. In this case I’m going to use an avocado, because it looks cool as a cross section and it’s a sexy food these days, right? (I spent some time in Vancouver where I learned about spreading avocado on toast! Ah, you wild West-Coasters!). But, we could just as easily add a flax seed or oat kernel. Remember that pretty much all raw unprocessed food contains SOME proportion of fat, protein, and carbohydrate; relatively, those proportions will differ, as will the qualities of each. (I used BioRender to create the graphics.)

 The sexy avocado has a decent amount of “fat.” (Most food has the nutritional information on the package, but what about things like avocados?? Check out the Canadian Nutrient File, you can find details on almost any food! https://food-nutrition.canada.ca/cnf-fce/index-eng.jsp) So let’s imagine we have some avocado fat squeezed out of it, as a droplet of oil, and zoom in to see what’s actually inside that droplet:

 Notice I’m using “fat” and “oil” almost interchangeably? The difference is basically this: if what we’re talking about is solid, it’s “fat;” if it’s liquid, it’s “oil.” So these are two words for the same broad general category of a major constituent of food. Here’s an analogy: “water” and “ice” are also two terms that refer to the same thing in liquid or solid form, just like “oil” and “fat” refer to the same things in different states (in other words, with different amount of energy added or absent). However, water and ice always ever refer to only one molecule – H₂O – which always has only one melt/solidification temperature, whereas “oil” & “fat” usually refer to a huge, diverse group of complex molecules (and usually a mixture), each having their own unique melt/solidification temperature from one another. Here’s a visual summary:

This just compares the freeze/melt temperature of flax oil, water, and coconut oil along a continuum of temperature. If these three things were side by side and the room was gradually cooled, coconut oil would become solid at +24°C, the water of course would freeze at 0°C, and flax oil would stay liquid down to -24°C. Since room temperature is usually ~23°C, you’ll therefore usually see flax oil as a liquid, and coconut oil as solid. Later, we’ll look at why this happens.

So, back to the diagrams – notice those little structures inside the droplets, that look like little jellyfish?

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Aw, so cute. Actually, these aren’t “inside” the oil, the ARE the oil !!!

 Note: these are NOT mini jellyfish, but rather they are a type of molecule called “lipids.” A more egghead definition of lipid is: “a member of the class of biochemical compounds that are insoluble in water but soluble in non-polar solvents; include fatty acids, triglycerides, and steroids” (drawing a droplet is just much easier than drawing thousands of little jellyfish). 

These “jellyfish” lipids are actually three smaller molecules called “fatty acids” connected at one end to a connector molecule, “glycerol.” Together, these four molecules form the jellyfish-like lipid, called “triacylglycerol” (tri = 3; acyl=the lipid part; glycerol=the connecting molecule) – or TAG for short – and they like to stick together, thus they form together as droplets. 

There are hundreds of structural variations of TAGs that might be present in any “oil” or “fat.” (For simplicity’s sake, only one is depicted here. Confused yet? Wait! – as if it weren’t complicated enough, TAGs are also sometimes called “triglycerides.”) Most fatty acids in plants are found in the TAG structure. Zooming in a bit closer, fatty acids are usually depicted as these zig-zag lines:

Notice we’re getting a bit more granular than simply “something that doesn’t mix in water.” The following terminology refers a bit more to specific structural differences of fatty acids within lipids, which become significant when choosing how to cook with them, how to store them, how to further process for industrial use, and how they might be best used in the diet. 

Let’s zoom in a bit closer on this TAG to understand the chemistry of fatty acids a bit more, and how it relates to your groceries and diet. We can now see the individual atoms in the molecules. These molecules are mostly comprised of carbon (C ) and hydrogen (H) linked in a linear chain, called a “hydrocarbon” (for obvious reasons):

But remember the fatty acids are connected to the glycerol as a “jellyfish”? A few oxygen (O) atoms at one end, called a carboxyl group, act like the velcro to attach them; O atoms on a linear hydrocarbon like we see here makes it a “fatty acid;” remove them, and it’s just a boring old hydrocarbon. Here’s something kind of neat: a hydrocarbon in a plant can be very similar or even chemically identical (by removing the carboxyl group) to hydrocarbons that come from petroleum…! This is the origin of diesel-compatible biofuel (search “diesel hydrocarbon structure” and check it out). But I digress.

Back to understanding the fatty acids, there are three important things to note here: 

1) The chains can vary in length (in other words, number of carbons in the chain) but in plants the majority will be in the range of 16 to 20 carbons in length. Generally, chains with <6 carbons are “short-chain,” 6 to 12 carbons are “medium-chain,” 14 to 20 carbon chains are “long-chain,” and 22+ carbon chains are “very long-chain.” (there does appear to be some discrepancy in the literature about cutoffs of some of these categories.)

2) Most of the carbons are joined through single bonds and thus have as many hydrogen atoms connected along the chain as possible – in other words, they’re “saturated” with hydrogens. Remember what type of molecule we’re talking about? Fatty acids. Exactly, these are saturated fatty acids, and they are always a straight chain. But “double bonds” can exist when at least one hydrogen is removed from that chain; double bonds result in a kink in the chain, since it is no longer saturated with hydrogens. 

3) These unsaturated fatty acids always have this kink in the chain, and actually there can be multiple double bonds on one fatty acid. When only one double bond is present, this is a mono-unsaturated fatty acid. More than one, and it’s called a poly-unsaturated fatty acid (PUFA). 

There’s something important to notice withe these kinked fatty acids. With any matter, when energy is added, molecules start moving apart and bouncing around; as energy is removed (when something is cooled), the molecules won’t move around as much and tend to pack together more closely, and more easily, until they “stop” and become “solid” (in other words, “frozen”). Now imagine there are a bunch of fatty acids in a straight line, and then a bunch of bent ones; the group of straight ones are going to be able to fit much more closely together, simply due to the physical shape. The bent ones won’t be able to. Think of a stack of papers that you’re loading into a printer: hundreds stack up in a few cm, but what would happen if you creased each piece of paper and tried to stack them? there would be physically more spaces in between each sheet, the stack would be higher, and you’d need way more pressure to push the stack down to the same height as the flat ones. What does this look like zoomed out? Take a look at the diagram comparing freeze temperatures of flax oil and coconut oil: flax oil contains only about 9% saturated fat, while coconut oil has about 87% saturated fat. So, it doesn’t take much energy removal from coconut oil for all those saturated fatty acids to pack together and become solid (in other words, it’s solid at much warmer temperatures and would need to be warmed much more than flax oil to get the fatty acids to break apart and become liquid).

While most lipids follow these naming rules, here’s an anomaly that is worth pointing out; notice the molecule on the far right is linear, but DOES have a double bond half way down?

There are only two ways the double bond can occur on a fatty acid: “up” or “down” (technically called “cis,” which creates the bend, or “trans,” which creates the linear chain). These trans double bonds can occur naturally, but are uncommon. 

More often such a structure is the result of a process called hydrogenation. The intent is to saturate the unsaturated fatty acids with hydrogen to induce linear saturated fatty acids (to modify texture of food, or prolong shelf life, for example), but partial hydrogenation results in these weird linear but double-bond containing fatty acids, called trans-fatty acids. Apparently, the safety of ingesting trans fat dates back to the 1940s, and pretty clear evidence that trans fats have adverse health effects published in 1990. Despite this, mandatory labelling wasn’t required until 2005, and finally, decades after researchers identified this dietary health risk, it became illegal to add trans fats to foods sold in Canada.

So, now we have a good idea about the physical structures of oil/fat, so let’s recap by putting the labels on each group. We’ve already covered some common generic names:

And remember, “lipids” can be categorized further:

“Saturated” is also a group of lipids that can vary in length; those in the “unsaturated” group also vary in length, as well as in number of, and position of, double bonds. The naming gets a bit more complicated here:

To differentiate unsaturated fatty acids from one another, a naming convention is used to specify the location of the double bond(s). A fatty acid, when not connected to glycerol, has 2 free ends with slightly different chemistry; one end is called omega (shaded with a pink circle on the diagrams) and one is carboxyl (the oxygen-containing end, depicted with the red oxygen atoms). Naming double bonds is done relative to the simpler end, the omega end, simply by counting which carbon atom the bond starts at! Thus, we have “omega fatty acids.”

You can probably already see what the numbers refer to after the omega label; check out the bent molecule in the diagram and count up to where the double bond is; which PUFA subcategory would it be?

Ok, this is a lot to process so enough chemistry lesson for now. But I’m going to reference these concepts in Part 2 and Part 3… stay tuned!

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Additional reading (no affiliation):

• https://lipidlibrary.aocs.org/
• https://www.amazon.com/Plant-Biochemistry-Caroline-Bowsher/dp/0815341210
• https://www.amazon.ca/Chemistry-Molecular-Approach-Canadian-Version/dp/0134894820/ref=sr_1_2?dchild=1&keywords=tro+chemistry&qid=1614189915&sr=8-2