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Chemical composition of olive oil
Olive oil is composed mainly of triacylglycerols (triglycerides or fats) and contains small quantities of free fatty acids (FFA), glycerol, phosphatides, pigments, flavor compounds, sterols, and microscopic bits of olive. Triacylglycerols are the major energy reserve for plants and animals. Chemically speaking, these are molecules derived from the natural esterification of three fatty acid molecules with a glycerol molecule. The glycerol molecule can simplistically be seen as an "E-shaped" molecule, with the fatty acids in turn resembling longish hydrocarbon chains, varying (in the case of olive oil) from about 14 to 24 carbon atoms in length.
Please note that we are dealing here with fatty acids forming part of the triacylglycerol molecule. We will address the free fatty acids in the next section. The fatty acid composition of olive oil varies widely depending on the cultivar, maturity of the fruit, altitude, climate, and several other factors.
A fatty acid has the general formula: CH3(CH2)nCOOH where n is typically an even number between 12 and 22.
If no double bonds are present the molecule is called a saturated fatty acid.
If a chain contains double bonds, it is called an unsaturated fatty acid.
A single double bond makes a monounsaturated fatty acid.
More than one double bond makes a polyunsaturated fatty acid.
The major fatty acids in olive oil triacylglycerols are:
Oleic Acid (C18:1), a monounsaturated omega-9 fatty acid. It makes up 55 to 83% of olive oil.
Linoleic Acid (C18:2), a polyunsaturated omega-6 fatty acid that makes up about 3.5 to 21% of olive oil.
Palmitic Acid (C16:0), a saturated fatty acid that makes up 7.5 to 20% of olive oil.
Stearic Acid (C18:0), a saturated fatty acid that makes up 0.5 to 5% of olive oil.
Linolenic Acid (C18:3)(specifically alpha-Linolenic Acid), a polyunsaturated omega-3 fatty acid that makes up 0 to 1.5% of olive oil.
Triacylglycerols are normally composed of a mixture of three fatty acids. Most prevalent in olive oil is the oleic-oleic-oleic (OOO) triacylglycerol, followed, in order of incidence, by palmitic-oleic-oleic (POO), then oleic-oleic-linoleic (OOL), then palmitic-oleic-linoleic (POL), then stearic-oleic-oleic (SOO), and so on.
Olive oil contains more oleic acid and less linoleic and linolenic acids than other vegetable oils, that is, more monounsaturated than polyunsaturated fatty acids. This renders olive oil more resistant to oxidation because generally, the greater the number of double bonds in the fatty acid, the more unstable and easily broken down by heat, light, and other factors the oil is. It is generally accepted that cooler regions (e.g. Tuscany) will yield oil with higher oleic acid than warmer climates. That is, a cool region olive oil may be more monounsaturated in content than a warm region oil.
Trans Fatty Acids
Note that olive oil has no trans fatty acids. When oil is partially hydrogenated, it can be in the “cis” or “trans” conformation; this refers to which side of the fatty acid double bond the hydrogen is on. Olive oil is not a trans fatty acid because it has not been partially hydrogenated in a factory to make it solid at room temperature like margarine has.
Long Chain Fatty Acids
Long chain fatty acids have from 12 to 20 carbon atoms. The primary fatty acids in olive oil are all long chain fatty acids. Very long-chain fatty acids have greater than 20 carbon atoms. These tend to be more solid at room temperature, such as waxes. There are not appreciable amounts of these in olive oil.
Percentage of Linolenic Acid Allowed in Olive Oil
Regarding the polyunsaturated fatty acids (PUFAs), there is a wide range acceptable for extra virgin olive oil, however the linolenic acid has to be less than 0.9% per the International Olive Oil Council (IOOC) guidelines. Higher levels, e.g. 1.5%, do not present a nutritional problem, but the IOOC uses the linolenic acid level to establish the authenticity of the olive oil. Seed oils like canola oil have higher levels of linolenic acid.
“Essential” Fatty Acids
In scientific writing the term essential fatty acid refers to all the omega-3 or omega-6 fatty acids. This is a historical convention, not a medical definition. From a medical point of view, essential fatty acids are the ones that the human body cannot make. There are only two, which are the building blocks from which many of the other omega-3 and omega-6 fatty acids are made in a healthy body: linoleic acid and alpha-linolenic acid.
FREE FATTY ACIDS (FFA) AND ACIDITY
The "acidity" in olive oil is the result of the degree of breakdown of the triacylglycerols, due to a chemical reaction called hydrolysis or lipolysis, in which free fatty acids are formed. (In exceptional circumstances, even oils made from fresh, healthy olives can have significant amounts of acidity, caused by anomalies during the actual biosynthesis of the oil in the olive fruit). Oil extracted carelessly and/or from poor quality fruit suffers from a very significant breakdown of the triacylglycerides into fatty acids. These "broken off" fatty acids are called Free Fatty Acids. Sometimes just one of the three fatty acids breaks off, leaving a diacylglycerol. If two fatty acids break off, we are left with a monoacylglycerol. If all three break off, were are left with glycerol.
Factors which lead to a high free fatty acidity in an oil include fruit fly infestation, delays between harvesting and extraction (especially if the fruit has been bruised or damaged during harvesting), fungal diseases in the fruit (gloesporium, macrophoma, etc.), prolonged contact between oil and vegetation water (after extraction), and careless extraction methods. Storing olives in heaps or silos to encourage enzymatic breakdown of cell structure, and thus facilitate oil release (as is the tradition in Portugal and other countries) is certainly not conducive to producing a high quality, low acidity oil.
The free fatty acidity is thus a direct measure of the quality of the oil, and reflects the care taken right from blossoming and fruit set to the eventual sale and consumption of the oil.
Measurement of free fatty acidity is a very simple procedure which can done at a
testing lab or with a CDR tester. The results are presented as grams of oleic acid per 100 grams oil, commonly known as the free fatty acidity of the oil in percent. Freshly pressed oil, made carefully, without the use of excessive heat, from sound, healthy, freshly picked olives, normally has a pretty low "acidity", well under 0.5% FFA. Extra virgin olive oils have less than 0.8% FFA.
The flavenoid polyphenols in olive oil are natural antioxidants that contribute to a bitter taste, astringency, and resistance to oxidation. They have been shown to have a host of beneficial effects from healing sunburn to lowering cholesterol, blood pressure, and risk of coronary disease. Click here to read more about the health benefits of these critical components of olive oil.
Hydroxytyrosol and tyrosol are some of the many phenol compounds in olive oil. There are as many as 5 mg of polyphenols in every 10 grams of olive oil. Many other nut and seed oils have no polyphenols. The polyphenol content is determined by many factors including:
Peroxides are the primary products of oxidation of olive oil. Fats and oils such as olive oil are oxidized when they come in contact with oxygen. Oxygen may exist in the headspace of the container and dissolve in the oil. The oxidation products have an unpleasant flavor and odor and may adversely affect the nutritional value of the oil. Essential fatty acids such as linoleic and linolenic are destroyed, and certain fat soluble vitamins disappear. Fatty acids are oxidized by one of the following mechanisms.
Auto-oxidation occurs in the absence of air by reactive oxygen species or "free radicals". It is temporarily prevented by the natural antioxidants in the oil that absorb these free radicals. When the antioxidants are used up, the oil ages quickly.
Photo-oxidation occurs when the oil is exposed to natural and/or artificial light sources (including halogen lights and store lights). It causes serious deterioration of olive oil, as it can occur up to 30,000 times faster than auto-oxidation.
The more rancid or oxidized the oil, the more peroxides are present. Measurement of the peroxides in olive oil is a very simple procedure which can done at a testing lab or with a CDR tester. High quality extra virgin olive oils have a peroxide value of less than 10meq/kg. In order to be extra virgin, olive oil must have less than 20 meq/kg.
PIGMENTS AND COLOR
The unique color of olive oil is due to pigments like chlorophyll, pheophytin, and carotenoids. The presence of various pigments depends on factors such as the fruit ripeness, the olive cultivar, the soil and climatic conditions, and the extraction and processing procedures.
According to Apostolos Kiritsakis, one of the premier researchers on olive oil components, fresh olive oil contains between 1 and 10 parts per million chlorophyll. This is miniscule compared to a portion of spinach. Olives are invariably crushed with some leaves still present, so some of the chlorophyll comes from that source. Some producers have been known to deliberately allow leaves in the mill to increase the "grassiness" of the oil.
In the light, chlorophyll and pheophytin will promote formation of oxygen radicals and speed up oxidation, but in the dark chlorophyll acts as an antioxidant. In current physiological studies, chlorophyll is broken down in the body and has no effect as an oxidant or antioxidant.
The color of olive oil can vary from a light gold to a rich green. Green olives produce a green oil because of the high chlorophyll content. Ripe olives yield a yellow oil because of the carotenoid (yellow red) pigments. The exact combination and proportions of pigments determine the final color of the oil.
Vitamins can be divided into the fat soluble and water soluble varieties. Fat soluble vitamins, such as the ones found in olive oil, are generally not broken down by cooking. They are stored in the liver and body fat for long periods so it is not essential to eat them with every meal. Cured whole olives have both water and fat soluble vitamins.
Vitamin E (a natural antioxidant): Olives have 1.6mg, or 2.3 IU (International Units) per tablespoon. One tablespoon provides 8% of RDA for vitamin E.
Vitamin K: The richest sources of vitamin K are green, leafy vegetables. One serving of spinach or collards, for instance, or two servings of broccoli provide four to five times the RDA. The greener the vegetable, the higher the content, say the researchers, because the vitamin is associated with the chlorophyll. According to the USDA , vegetable oils such as olive oil are the second best source.
POLYCYCLIC AROMATIC HYDROCARBONS (PAHs)
Many foods naturally contain small quantities of PAHs. Olive oil, like other vegetable cooking oils, has been found to contain minute amounts of up to 17 PAHs such as benzanthracene and chrysene. Unripe olives tended to have more than ripe olives.
Burning any cooking oil can increase the amounts of PAHs. This is not considered a major risk source in the diet and the oil would have to be heated repeatedly and for extended periods to the smoking point. It is unlikely that, in home use, olive oil or other cooking oils would be a significant source of PAHs.
COOLING AND FREEZING POINT
Olive oil will harden at refrigerator temperatures - around 2-4°C. Determining at what point to call the oil "frozen" is a matter of semantics. The slow increase in hardening as the temperature is lowered is in sharp contrast to a pure substance such as water that switches from a liquid to solid phase at an exact temperature. Olive oil is a complex mixture of oils and waxes. The heavier oils and waxes will form needle-like crystals as the temperature is lowered, then the other oils will start to settle out.
At 4°C most of the oils would not harden or form any crystals. At 2°C most are firm enough that they cannot be poured but are as soft as butter at room temperature. As the temperature is lowered, more components of the oil solidify. At -12°C the oil is hard enough that a fork cannot penetrate it.
Winterisation is the commercial process whereby these waxes are removed to keep some oils clearer when stored on a cold shelf. It is used mostly for aesthetics and to improve mixing when combined into mayonnaise, sauces, and dressings.
Olive oil is a natural product that varies from year to year even from the same producer, so each batch of oil will "freeze" at a different temperature. Freezing olive oil will not harm it; it will actually prolong its nutritional benefits and its flavor. It is a myth that the freezing point of olive oil can be used to predict whether it is pure, virgin or extra virgin.
There are many questions about freezing olive oil, such as: what are the clouds in my olive oil, will olive oil freeze in the refrigerator, is freezing olive oil good or bad for it, and does the way it freezes say anything about its quality?
Most manufacturers preset refrigerator temperatures to around 3°C. Chemistry texts list the freezing point of pure oleic acid at around 4°C. Olive oil manufacturers don't generally list a freezing temperature because it is quite variable depending on the olive variety and ripeness of the olive at processing. Unlike the properties of an element or simple compound like water, olive oil is made up of hundreds of chemicals, many of which change with every extraction.
Like most fruit, olives have waxes on their epidermis (epicarp) to protect them from insects, desiccation, and the elements. These natural waxes are what allow an apple to be shined, for instance. If an oil is sent to a cold climate, or if it will be used in a product like salad dressing where it will be stored in the refrigerator, it is often "winterised" (chilled and filtered) to remove the waxes and stearates. A standard test to determine if olive oil has been sufficiently winterised is to put it in an ice water bath (0°C) for 5 hours. No clouding or crystals should occur.
CONGEALED AND PARTIALLY SOLID REFRIGERATED OLIVE OIL
Oil that has not been winterised will clump and form needle-like crystals at refrigerator temperatures as the longer chain fats and waxes in the oil congeal, but the oil will not usually harden completely unless chilled further. Some olive varieties form waxes that produce long thin crystals, others form waxes that congeal into rosettes, slimy clumps, clouds, a swirl of egg white like material, or white sediment that the consumer may fear represents spoilage. These visual imperfections also may form outside the refrigerator during the winter when oil is exposed to cold temperatures during transport. The white color in the hardened oil does not indicate spoilage.
Chilling or freezing olive oil does not harm it, and the oil will return to its normal consistency when it is warmed. The ideal temperature to store olive oil to reduce oxidation but to avoid clouding is around 10°C.
ACTUAL FREEZING TEMPERATURE
To determine the actual freezing temperature, Dr. John Deane put several oils in the freezer with a thermometer. At 40°F, most of the oils had not hardened or formed any crystals. At 35°F, most were firm enough that they could not be poured but were as soft as butter at room temperature. As the temperature lowered, more components of the oil solidified. At 10°F, the oils were hard enough that a fork could not penetrate them. Determining at what point to call the oil "frozen" is a matter of semantics. This slow increase in hardening as the temperature is lowered is in sharp contrast to a pure substance such as water that switches from a liquid to solid phase at an exact temperature.
MYTHS ABOUT FREEZING OLIVE OIL
Myth: Hardening Proves Extra Virgin Status.
There is a rumor that true extra virgin olive oil, placed in a small quantity in a glass bowl and refrigerated for a while, would become crystalline. A chemically refined olive oil with some virgin oil added to it, however, would form a block when frozen.
It is doubtful that this is a valid observation. While refined or pomace oils will usually be stripped of their waxes, thus making them more likely to form a block, and while it is more common for a refined oil to be winterized to be used in a cheap dressing, many excellent extra virgin oils do not form "crystals". It is possible to watch the production process of many premium oils, from the olive to the bottle, which form a solid block when frozen. Unfortunately, detecting fraud is more difficult than just freezing the oil.
Myth: The Fact that Olive Oil Hardens in the Refrigerator Means That It Is Saturated.
Olive oil is not a saturated fat. All fats will harden if they get cold enough, whether they are saturated or not. As above, olive oil often hardens, but not because it is saturated. It has not been refined as seed oils have been, to remove waxes. The presence of waxes does not make the olive oil saturated or unhealthy, it just means it is a natural product.
As a general rule, the more saturated the fat, the more likely it will be hard at room temperature. Beef and pork lard, margarine, butter, and the saturated tropical fats in cookies, packaged foods, and snack foods are all solid at room temperature. This improves their shelf life, makes packaging easier, and improves "mouth feel" but is not necessarily good for your health.
HEATING AND BOILING POINT
HEATING OLIVE OIL AND SMOKE POINT
One of the questions asked most often is what happens when olive oil is heated and/or used for frying. The important thing about cooking with any oil (olive or otherwise) is not to heat the oil over its smoke point (also referred to as smoking point). The smoke point refers to the temperature at which a cooking fat or oil begins to break down. The substance smokes or burns, and gives food an unpleasant taste. But what is the smoke point of olive oil? Depending on where you look for an answer, you may get vastly different ideas.
Relationship between Smoke Point and Quality of Olive Oil
The smoke point of oil varies with its quality. High quality extra virgin olive oils (with low free fatty acids) have a high smoke point. They are an excellent choice, but an expensive one. Mass produced, low quality olive oils have a much lower smoke point.
Extra virgin olive oil smokes roughly between 400 and 365ºF (204 and 185ºC) depending on its free fatty acid content. Here is what the International Olive Oil Council (IOOC) has to say about frying food with olive oil:
When heated, olive oil is the most stable fat, which means it stands up well to high frying temperatures. Its high smoke point (410ºF or 210ºC) is well above the ideal temperature for frying food (356ºF or 180ºC). The digestibility of olive oil is not affected when it is heated, even when it is re-used several times for frying
As a reference point, the table from the IOOC shows standard cooking temperatures:
How does Olive Oil Compare with Other Cooking Oils?
The table below shows the smoke point of a few other cooking oils. Keep in mind that the smoke point for a vegetable oil will vary according to the variety and growing conditions, and how the oil was produced. Various manufacturers and sources cite different numbers.
Having read all of the above, you may be fairly confused by now. Dr. John Deane wrote the following excellent article about the smoke point of olive oil and various cooking considerations. This is the most comprehensive discussion of smoke point known of.
Smoke Point of Olive Oil by John Deane (updated 09/20/2007)
Pumpkin seed oil, avocado oil, borage and camellia oil; it used to be that a choice of oil for cooking was simple. You used a liquid canola or corn oil for frying or sautéing and a hardened oil such as Crisco for baking. We now live in the age of boutique oils. All seeds have oil in them as the energy source for the growing seedling. Man's ingenuity and desire to create a niche market has led to the extraction of many unusual oils.
The marketing angles on these oils are manifold. Some claim to have health benefits, others to have flavor. Buyers of argan and shea butter oils may be supporting women's cooperatives in developing nations. Hemp seed oil diehards are sticking it to the man. Grapeseed oil has the romance of the vine. JoJoba oil is a earth friendly alternative oil. While it is hard to compare or argue some of these points, there is one point which should be easy for comparison: the smoke point.
A high smoke point is desirable for a cooking oil. When frying, best results occur when the oil is very hot. The food is placed into the hot oil and the natural sugars caramelize and proteins denature into a thin shell which protects the food from soaking up the oil. The outside is crisp and the interior is just cooked. One of the bibles of cooking, Irma Rombauer's The Joy of Cooking recommends frying at 365ºF for best results.
When heated oil smokes, it is not just a nuisance. Besides coating your home interior with a varnish like substance, where there's smoke there's fire. An oil at its smoke point is closer to its flash point - the point where it will burst into flame.
So a high smoke point is one yardstick for a “good oil” If you go to the internet or the market to look for smoke points you will see something interesting. Every oil claims to have the highest smoke point. One website for macadamia nut oil puts their oil at the top of the list with a smoke point of 410ºF. On their chart, olive oil comes in at a measly 190ºF. This is below the temperature of a hot cup of tea! Avocado oil sites say their oil has the highest smoke point and claim nut oils are terrible for frying.
The smoke point for a vegetable oil will vary according to the variety and growing conditions, and how the oil was produced. The smoke you see may be impurities in the oil which are burning. Unfiltered olive oil has small bits of olive in it. When the oil is heated these bits will burn and smoke before the oil itself. A well-filtered or clarified oil will have a higher smoke point generally.
Oil which has oxidized because of exposure to air, heat and light will have a lower smoke point. Using oil repeatedly will also make it smoke sooner. When looking for the smoke point of an oil you should expect a range of values. The Olive Oil Source claims that extra virgin olive oil smokes from 400 to 365ºF, according to its free fatty acid content. But the macadamia nut folk say that olive oil smokes at the temperature of hot water out of the tap. When I suggested to the macadamia people that it seemed unlikely that olive oil smokes at temperature lower than boiling water and that maybe they were confusing centigrade with Fahrenheit they insisted they were right.
So who do you trust for the real smoke point? The industry group which is advertising and promoting the oil, a random website or a food chemistry text? Here is what some research yielded:
The International Olive Oil Council: 410ºF
Institute of Shortening and Edible Oils: 420ºF
Or why not get some olive oil off the shelf and heat it up in a saucepan with a frying thermometer. This is properly done in a lab with special lighting which shows the first hint of smoke. My stovetop experiment yielded 350ºF for a jug of discount store oil which had been sitting open in the garage for a few years and 380ºF for a premium fresh extra virgin oil. Olive oil is fine for frying.
It is annoying to counter these conflicting claims when most people would not fry with olive oil anyway. A cheap, flavorless oil with a high smoke point is usually recommended - something like canola, soy or peanut oil. Avocado, macadamia and premium olive oils can cost up to a dollar per ounce. It is unlikely that you are going to deep fry that Thanksgiving turkey in 5 gallons of oil at that price.
Besides, if we are so worried about our health, why fry at all? Better to talk up the flavor qualities of olive oil, an area where it shines compared to bland seed oils.
An excellent resource with voluminous bibliography is a monograph entitled Frying Food in Olive oil by Gregorio Varela, Professor of Nutrition, Madrid University. It is available from the International Olive Council (IOC).
The boiling point of olive oil is 299 oC or 570oF
MYTHS ABOUT COOKING WITH OLIVE OIL
There are some myths circulating about olive oil. Following are the two most common.
Myth: Heating Olive Oil Will Make it Saturated or Trans-fatty.
One common myth is that heating olive oil will make it saturated or trans-fatty.
This is not true. As far as making a saturated fat, according to Dr. A. Kiritsakis, a world renowned oil chemist in Athens, in his book Olive Oil from the Tree to the Table -Second edition 1998, all oils will oxidize and hydrogenate to a tiny degree if repeatedly heated to very high temperatures such as is done in commercial frying operations. Olive-pomace oils and virgin olive oils are both highly monounsaturated oils and therefore resistant to oxidation and hydrogenation. Studies have shown oxidation and hydrogenation occurs to a lesser degree in olive oil than in other oils. But in any case, the amount of hydrogenation is miniscule and no home cook would ever experience this problem.
The large refinery-like factories that take unsaturated vegetable oil and turn it into margarine or vegetable lard do so by bubbling hydrogen gas through 250 to 400ºF (121 to 204ºC) hot vegetable oil in the presence of a metal catalyst, usually nickel or platinum. The process can take several hours. You cannot make a saturated product like margarine at home by heating olive oil or any other vegetable oil in a pan. This is a weird notion that has no grounding.
Changing a cis-fat to a trans-fat does not occur on a home stove.
Myth: Cooking in Olive oil Diminishes The Nutritional Value of the Food.
Another myth is that cooking in olive oil diminishes the nutritional value of the food. This is a misconception. The fact is that heating food will break down its nutritional value. High heat such as frying is worse than moderate heat such as steaming, which is worse than eating vegetables raw. It is not the cooking oil per se, but the high heat of frying. There is no known edible cooking oil which by itself diminishes the nutritional value of the food cooked in it. Most nutritionists recommend lightly steaming vegetables or eating them uncooked. A touch of a flavorsome extra virgin olive oil added at the table will add taste and healthful anti-oxidants. Such is the Mediterranean diet which has been shown to help prevent coronary disease and have other health benefits.
pH refers to the hydrogen ion concentration in an aqueous solution. Olive oil and other oils are not water soluble so their acidity cannot be measured in terms of pH.
The information on this page has been edited from the comprehensive resources of The Olive Source.
Apostolos (Paul) K. Kiritsakis: Olive Oil, From the Tree to the Table, Second Edition
Tous, J. and L. Ferguson. 1996. Mediterranean fruits. p. 416-430. In: J. Janick (ed.), Progress in new crops. ASHS Press, Arlington, VA.
Nutritional composition of Mediterranean crops (per 100 g of edible portion). Source: Goulart (1980); Sawaya et al. (1983); Fernandez Diez (1983); IBPGR (1986); Morton (1987); Cantwell (1994).
Guido Costa: A great discussion of olive oil chemistry by Guido Costa in simple terms.
John Deane: Smoke Point of Olive Oil