Is smoke point a useful determinant for cooking oils?
Is smoke point a useful determinant for cooking oils?
Bad advice when it comes to cooking oils is prevalent on the internet. Look up cooking oils on the LiveScience website, and you’ll be directed to Canola, Grapeseed, Sesame, Sunflower, Soybean, and similar oils for their high smoke points and their “healthy” unsaturated fatty acid content (healthy in quotation marks because the omega 6/omega 3 balance is a thing, that will have to be addressed in another article).
This website is not alone—Time offers similar advice, as do lifestyle and gourmet sites1. One manufacturer of Camelina oil recommends their oil for high heat cooking, due to its high smoke point, as do two manufacturers of modified Flax oil. But is the smoke point of an oil an accurate determination of its suitability for cooking? This article will look at the underlying science of cooking with oils, to provide you with an understanding of why you should avoid cooking with polyunsaturated fats.
Definitions
Lipid is the overall category containing the different kinds of fat molecules including triglycerides, fatty acids, the phospholipids and sphingolipids that make up cell membranes, sterols like cholesterol and phytosterols, and even terpenes.
Figure 1—The glycerol molecule forms the backbone of a fat molecule. Each carbon in glycerol is attached to an OH (alcohol) group. When the OH’s are all substituted with fatty acid chains, the result is a triglyceride; If only two are substituted, a diglyceride, only one, a monoglyceride. The carbons are not specifically shown in these types of drawings, but are assumed at each connection points between lines.
Glycerol is a polyol (a poly alcohol) with a hydroxy (OH) group attached to each of its three carbons. In a dehydration reaction with a fatty acid (alcohol (ROH) + acid (R’OOH) = ester (ROR’) + H2O), each of these bonds is esterified to finally form a triglyceride (or triacylglyceride by another name)—see figure 6. [R and R’ are shorthand notations for unspecified carbon chains].
Figure 2—Saturated Stearic Acid, 18 carbon chain. In a fatty acid chain, the carbon at position 1, the alpha carbon, is bonded to an OH, and a double bonded carbon. This forms a carboxylic acid group (COOH), hence “fatty acid.” In these drawings of molecules, hydrogens bonded to carbons aren’t shown for simplicity, so if there are no double bonds, the carbons are fully saturated with hydrogens, from which we get the term “saturated fats.”
Figure 3—Monounsaturated Oleic Acid (Ωmega 9), 18 Carbon Chain. The double bond, shown between carbons 9 and 10 counting from the alpha carbon, indicates one degree of unsaturation. The positions on either side of the double bond (8 &11) are the allylic positions. Hydrogens bonded to these carbons are allylic hydrogens. The position of the double bond can also be counted from the omega carbon (omega is the last letter in the Greek alphabet, and commonly signifies the end), in this case, an omega 9 fatty acid.
Figure 4—Polyunsaturated Linoleic Acid (Ωmega 6), 18 Carbon Chain. When there are multiple double bonds in a fatty acid, only the position closest to the omega end signifies to which omega class the fatty acid belongs, in this case, omega 6. In this molecule both double bonds share an allylic position (carbon 11). That makes this carbon “bis-allylic.”
Figure 5—Polyunsaturated a-Linolenic Acid (Ωmega 3), 18 Carbon Chain. alpha-Linolenic acid, an omega 3 fatty acid, has two bis-allylic positions, making it twice as vulnerable to damage as the linolenic acid (figure 4)
Fatty acids are the individual chains that make up most lipids. Although we often talk about fatty acids (or classes of fatty acids) such as mono- or polyunsaturated fats, triglyceride fat
molecules rarely have a single kind of fatty acid. Most fat molecules contain a mixture at the three positions. Saturated fatty acids are common in the first position, and unsaturated fatty acids are common in the second position. If an oil is comprised of 70% polyunsaturated fats, many of the triglyceride molecules will require at least two polyunsaturated fatty acids.
Figure 6—A fat molecule is technically a triglyceride, three fatty acid chains (in this case, stearic, a-linolenic, and oleic), attached to a glycerin molecule. If you look at the picture of the glycerin molecule (figure 1) and the individual fatty acids (figures 2-5), you’ll notice that to make the ester bonds shown here, 2 hydrogens and one oxygen have to be removed (H2O). That’s a dehydration reaction. Conversely, to remove a fatty acid requires a water molecule in a process called “hydrolysis,” (hydro-lysis, literally “water splitting”). The forming and breaking of these bonds is called esterification and de-esterification, respectively.
Oils are fat molecules, usually triglycerides but also mono- and diglycerides, that are primarily in a liquid state, usually judged at room temperature. If it’s typically solid, it’s usually referred to as fat.
What is the smoke point?
The smoke point of an oil is when organic matter in the oil such as phenols, proteins, or even short chain fatty acids reach their ignition temperature and start burning, though still below the ignition temperature of the oil itself. As these molecules burn, they emit smoke and create hot spots in the surrounding oil. Cooking with the oil at its smoke point will result in off flavours in the food.
While heating oil to the smoke point certainly damages the oil, unsaturated fats are subjected to damage at temperatures well below the smoke point. The more unsaturated a fat is, the lower the temperature required to cause damage. The activation energy to start a reaction in Flax oil (three double bonds) can be as low as 40° C, whereas it is above 70° C for Olive oil (one double bond).2
What is unsaturation?
Unsaturation is when a carbon atom is not bonded to as many hydrogens as it could be. This results in a double bond between carbons. The hydrogens on the carbon next to a double bond are said to be “allylic.” (figure 3). Omega 9 oils have two allylic carbons, Omega 6 vegetable oils have two allylic and one bis-allylic carbon (figure 4), and omega 3 vegetable oils have two allylic and two bis-allylic carbons (figure 5). Omega 3 fish oils have two allylic and four bis-allylic carbons for EPA, five bis-allylic carbons for DHA.
This is important because the amount of energy it takes to replace a hydrogen on a bis-allylic carbon with an oxygenated bond is 1/100th the amount of energy required to replace a fully saturated hydrogen. Allylic hydrogens have a displacement energy 1/10th of a fully saturated hydrogen. Damage to bis-allylic containing fat is minimally 100 times more likely than damage to a saturated fat. The bis-allylic hydrogen is the easiest point for a fat molecule to be damaged, and the weakness of the link ensures the damage happens at relatively low temperatures. Polyunsaturated fats are exponentially easier to damage than saturated fats, due to the bis-allylic hydrogens that are weakest chain in the link.
What causes damage in oils?
Oils are damaged by oxygen. Light facilitates the process, and the resulting chain reactions are accelerated by heat. Consider then the act of frying, where oil is exposed in a thin layer to an excess of oxygen and light, at which point heat is applied. That’s not to say that frying in the dark will avoid damage (it won’t), but merely to say all the elements required of a recipe for disaster are present on your stovetop.
Oil is damaged when a hydrogen, typically next to a double bond (allylic or bis-allylic), is replaced through a series of reactions with oxygen, a process called peroxidation, which then attracts a fatty acid from a neighbouring fat molecule, creating a chain reaction.
As all foods contain some moisture, adding foods to frying oil provides water, which causes the hydrolyzation (literally, water splitting) of fat molecules. This separates the fat molecule into a diglyceride and a free fatty acid (FFA), both polar compounds, and therefore more reactive. Both diglycerides and free fatty acids have been shown to undergo peroxidation faster than a triglyceride.
It makes sense, then, that the degree of unsaturation (how many unsaturated bonds a molecule has) is a direct indicator of the oil’s likelihood to undergo damage. Remember also that the degree of unsaturation is linked to damage at lower temperatures.
But wait, it gets worse. A peroxidized molecule has a lower energy barrier to secondary damage than an undamaged molecule has to peroxidization. Secondary damage occurs when the peroxidized oil undergoes further reactions that split the fatty acid into two products. The peroxidized fat molecule is subject to further degradation into aldehydes, dialdehydes, and cyclic molecules, some of which are quite toxic to DNA or cells (genotoxicity and cytotoxicity).
This likelihood for damage has been demonstrated again and again in oxidative stability studies. The amount of time required to initiate damaging chain reactions (negatively correlated), the temperature required to begin initiation (negatively correlated), and the amount of damage (positive correlation) are all related to the degree of unsaturation. The higher the degree of unsaturation, the less time it takes, at lower temperatures, to do more damage.
Figure 7—Oxidative stability test of various oils commonly used for cooking. Two varieties of Camelina oil, consisting of 38% Omega 3, 18% Omega 6, and 17% Omega 9, are the least stable oils tested. Red Palm oil, with 44% Omega 9 and only 12% Omega 6, is the most stable oil tested. Unpublished study, Alpha Health Products Ltd, June 11, 2016.
How are damaged fats unhealthy?
The secondary products of oxidation have been most widely studied for the omega 6 and 3 polyunsaturated fats. While there is some overlap, each class forms their unique secondary products, with different actions of toxicity.
Omega 3 oils form a high amount of acrolein. This molecule is capable of binding DNA, causing genetic damage, and of binding to proteins, particularly in a crosslinking manner. Some of these crosslinked proteins have been found in the brain tissue of Alzheimers patients. While there is a metabolic pathway for the elimination of acrolein involving glutathione, an excess of dietary acrolein can overwhelm the clearance pathway.
Omega 6 oils yield 4-hydroxy-2-nonenal (4HNE, or HNE) as a secondary product which has received a lot of attention of late. 4HNE has been implicated in insulin resistant obesity, rheumatological diseases, neurological diseases generally, Alzheimers and Parkinsons specifically, Lupus, atherosclerosis, and gastrointestinal diseases.3
These are not the only toxic byproducts of omega 3 and 6 cooking oils, but they are perhaps the most significant ones to date. More studies are ongoing, and indications are an increase of bad news is on the way regarding the suitability of seed oils for cooking.
Conclusion
Smoke point is of lower importance in a cooking oil than the level of unsaturation. In looking for a good cooking oil, the first requirement is a low quantity of polyunsaturated fatty acids, preferably comprising less than 10% of the oil. The best value for the omega 3 component is zero. The safest oils for cooking are those with the lowest percentage of unsaturated bonds—coconut and red palm oil, both of which are available as organic and fair trade. Ghee, which is butter with the dairy component removed, has a good profile for cooking. Rendered pork, chicken, or beef fat are also recommended, with a slightly lower unsaturated fatty acid percentage than olive or avocado oil. For deep frying, refined coconut or refined red palm oil have the best fatty acid profiles combined with smoke points that are high enough to withstand the required temperatures.
http://time.com/5342337/best-worst-cooking-oils-for-your-health/
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accessed Nov 12, 2018https://www.bistromd.com/healthy-eating/the-top-10-best-cooking-oils
accessed Nov 12, 2018https://www.livescience.com/59893-which-cooking-oils-are-healthiest.html
accessed Nov 12, 2018↩Litwinienko, G., & Kasprzycka-Guttman, T. (2000). Study on the autoxidation kinetics of fat components by differential scanning calorimetry. 2. Unsaturated fatty acids and their esters. Industrial and Engineering Chemistry Research, 39(1), 13–17. https://doi.org/10.1021/ie990552u↩
Castro, J. P., Jung, T., Grune, T., & Siems, W. (2017). 4-Hydroxynonenal (HNE) modified proteins in metabolic diseases. Free Radical Biology and Medicine, 111(November 2016), 309–315. https://doi.org/10.1016/j.freeradbiomed.2016.10.497↩