Which Cooking Oil?
The supermarkets now offer a bewildering array of cooking oils many of which lay claim to being healthy. So where does the truth lie? Which oils should we choose?
The widespread advice to consume mainly polyunsaturated fats, such as corn, soybean, sunflower, safflower and other vegetable oils, arose from the belief that saturated fats and cholesterol cause heart disease.
Two initial meta-analyses (BMJ 1997, 314 (7074): 112–7 and The American Journal of Clinical Nutrition 2003, 77 (5): 1146–55) found a correlation between high consumption of saturated fats such as coconut and animal fat and coronary heart disease and most advice has been based upon the findings of these studies.
However, more recent meta-analyses have found either no difference or a positive effect when shifting from carbohydrate consumption to animal fats and only a modest health advantage when exchanging saturated for polyunsaturated fats [Jakobsen et al, American Journal of Clinical Nutrition 2009, 89 (5): 1425–32 and Katan et al., 2010, PLoS Medicine 7 (3)].
Polyunsaturated fats have never traditionally been used for cooking. Throughout history, people all over the world have used saturated fats as their main fat for cooking – butter in Europe, ghee in Northern India, pork lard in China, beef tallow in America and coconut and palm oil in the tropics.
And yet, the massive increases in heart disease occurred in the last century in the USA and even more recently in other parts of the world. An increase which has mirrored the widespread introduction of 'healthy' polyunsaturated fats.
The 'French paradox' also gives a lie to much current health advice. The French eat foods high in saturated fat, drink more alcohol, exercise less and smoke more than people practically anywhere else and yet they have less obesity and a fraction of the cardiovascular diseases of comparable countries.
The Japanese people of Okinawa famed for being some of the most long lived and healthy people in the world also eat large amounts of saturated fat and cook mainly with pork lard.
Another anomaly which is hard to explain away with current dietary advice is that during the 12 years of severe dietary restrictions imposed in the United Kingdom during World War II when protein and animal fats in particular where heavily rationed, heart disease rates tripled.
To understand some of the problems inherent in the use of 'healthy' polyunsaturated oils, you first need to know how they are manufactured.
The manufacture of cooking oils
Edible oils have been produced around the world for thousands of years from soy beans, olives, palm kernels and coconuts. The source vegetable or fruit was crushed and boiled and the oil skimmed off. This process probably only yielded 10% of the oil available. The Greeks and Romans introduced mechanical improvements in the production of edible oils and these were employed throughout Europe in the Middle Ages.
Oil refining has now become more efficient by employing solvents in addition to sophisticated mechanisation which mean that up to 98% of the available oil can now be extracted from the source seed or nut. Using these processes, corn oil was one of the first to become commercially available in the 1960s and other sources such as grapeseed can now be used to make oils out of what was, until recently, considered a waste product.
Cold pressing can be used to extract oils from the white meat of the coconut, the pulp of the palm fruit, the flesh of the olive and from peanuts and sunflower seeds. This method involves mechanical means only, does not chemically alter the oil in any way and produces 'virgin' oils full of flavour. These are the highest quality and most expensive oils and only account for a small minority of edible oil production.
Most oil sources, however, do not lend themselves to cold pressing which would leave an oil that was dark, bitter or pungent and these oils undergo processing to produce a bland, clear, and consistent oil.
First, the incoming oil seeds or nuts are cleaned, prepared and ground into a coarse meal which is then heated and fed into a screw press. The pressure progressively increases squeezing the oil out and leaving a solid oil cake.
This oil cake is then treated with solvents such as hexane which dissolve the oil out and the solvents are then recovered by distillation and boiling the oil with steam.
The oil is next refined to remove colour, odour, and bitterness and this involves heating the oil and mixing it with an alkaline substance which causes the undesirable fatty acids to form a soap which is then removed by centrifuge. The oil is further washed to remove traces of the soap and then dried.
The oils are next degummed by treating them with hot water, steam, or water with acid which precipitates out the gums which are also removed by centrifuge.
Oils intended for use in cooking are then bleached by filtering through fuller's earth, activated carbon, or activated clays. And oils intended for use cold are rapidly chilled and filtered to remove waxes so that the oils do not solidify when refrigerated.
The oil is then deodorised by passing steam over the hot oil in a vacuum to cause evaporation of the volatile components. A small amount of citric acid may also be added to inactivate trace metals that might promote oxidation within the oil and hence shorten its shelf-life.
Finally the processed oil is poured into glass or plastic bottles or cans and topped with inert nitrogen to prevent oxidation in the container and the waste oil seed cake may be used to make animal feed and low-grade fertiliser.
The problem is that repeated heating of the oil during processing causes trans fats to form and the oils to become rancid requiring the deodorisation process to remove the smell. One study at the University of Florida at Gainesville found trans fat levels as high as 4.6% in commercial liquid oil (compared to about 40% in margarines). Also, not all of the chemicals and solvents used in processing are recovered and some necessarily remain in the final product.
In the case of olive oils labelled as refined, chemical and physical filters but no solvents have been used to extract the oil. If a product is labelled Olive oil, Light or Extra light the chances are that it is a blend of refined and virgin oils.
In America, the US FDA has allowed Canola oil to make claims on the labels that it supports heart health and reduces the risk of coronary heart disease. However, Canola oil comes from the rapeseed plant which has been genetically engineered and hybridised to breed out its undesirable taste and hazards to health. Rapeseed traditionally required substantial use of pesticides, but many varieties have now been genetically engineered by Monsanto exclusively for use with its herbicide, Roundup.
Some countries have now banned the sale of oil made from genetically modified seeds.
Cooking oils for sauteing and pan frying
Whilst the high proportion of monounsaturated fats found in macadamia, almond, avocado and olive oil are thought to offer health benefits, they are not stable when heated. The unrefined, cold-pressed oils are not suitable for use in frying but are excellent for pouring on pasta and for use in salad dressings. However, be aware that there may be an issue for those with nut allergies in using nut derived oils.
Heating an oil changes its characteristics and this means that oils regarded as being healthy at room temperature can become unhealthy when heated above certain temperatures. The smoking point is the temperature at which a particular fat or oil starts to smoke and break down creating acreolein, an obnoxious-smelling compound.
A 2001 review found that polyunsaturated oils like soya, canola, sunflower, and corn oil degrade quickly to yield toxic compounds when heated and that prolonged consumption of these degraded polyunsaturated oils was linked to atherosclerosis, inflammatory joint disease and the development of birth defects (Foodservice Research International 13: 41–55).
The saturated fats and more refined oils tend to be have higher smoking points and to be more stable at higher temperatures. Once an oil has started smoking it is toxic and both the oil and the food being cooked in it should be thrown away.
Advice when shallow frying includes using an eco friendly or non-toxic non-stick pan with a little water added before the oil which prevents it getting too hot.
Some research suggests that shallow frying results in more oil decomposition than deep frying and this may be because pan frying generally takes longer. Shallow frying has also been shown to be more likely to produce carcinogenic compounds called heterocyclic amines (HCAs) when the surface of the meat or vegetable is burnt or overcooked.
Oils suitable for medium-temperature shallow frying (above 374°F/190°C) include:
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Beef tallow (420ºF/215°C)
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Grape seed oil (420ºF/216ºC)
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Cottonseed oil (420ºF/216ºC)
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Almond oil (420ºF/216ºC)
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Semirefined sesame oil (419ºF/215ºC)
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Semirefined walnut oil (400ºF/204ºC)
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Virgin olive oils (391ºF/199°C)
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Pork lard (370ºF/188°C)
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Virgin coconut oil (350ºF/177ºC)
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Duck or goose fat (375°F/190°C)
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Butter (250-300ºF/121-149°C)
Of these only the virgin olive oil, coconut oil, butter and animal fats are not highly refined.
Cooking oils for deep frying
When deep frying, the heat of the oil essentially causes the water present in the foodstuff to steam cook it from the inside. Deep frying temperatures are generally 350°-375°F/177°-190°C. Temperatures much lower than this will result in excessive amounts of oil being absorbed into the food and a greasy end product. Whereas with higher temperatures there will be additional oxidation of the oil and the inside of the food may be uncooked and the outside burnt.
Deep frying requires an oil with a smoke point of at least 350°F/177°C. Palm oil and animal fats such as tallow have a smoke point of 420°F/216°C and Palm oil has been increasingly used commercially for its stability at high temperatures and its high level of natural antioxidants. Although if the food being fried is to be eaten cold, then the animal fats in particular may leave a greasy residue.
The rate of the deterioration of the oil depends upon the number of times the oil has been used, the length of time it has been heated for and the number of times it has been cooled and reheated. With use and deterioration oil becomes dark, viscous, develops a sediment and a rancid or 'off' smell. Most restaurant food is fried in hydrogenated vegetable oils or canola oil and the oils reused.
Oils that are suitable for high-temperature deep frying (above 446°F/230°C) because of their high smoke point include:
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Avocado oil (520ºF/271ºC)
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Safflower oil (509ºF/265ºC)
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Rice bran oil (490ºF/113ºC)
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Ghee (485ºF/252°C)
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Sunflower oil (475ºF/246ºC)
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Refined olive oil (468ºF/242°C)
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Soybean oil (466ºF/241ºC)
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Canola oil (460ºF/238ºC)
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Corn oil (457ºF/236ºC)
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Palm oil (455ºF/235ºC)
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Peanut or groundnut oil (447ºF/231ºC)
Of these, the only fats that are not highly refined are ghee and palm oil and these oils also have a low oxidation index meaning that they are the least likely to become rancid.
The take-home message? Ignore the out-dated advice on the supposed benefits of polyunsaturated fats and cook the way your grandmother used to using the traditional unprocessed oils and fats.
Storing and keeping edible oils
All oils deteriorate on exposure to heat, light, and oxygen. Rancid oil has an unpleasant aroma and acrid taste, and its nutrient value is greatly diminished.
Olive oil should be bought or stored in a glass rather than plastic and ideally a dark glass bottle and kept in a cupboard rather than left exposed to light. Olive oil may be stored at room temperature, but a cool place is ideal. A capsule or two of vitamin E emptied into each bottle of oil will help to prevent oxidation.
Polyunsaturated fats turn rancid very easily. Flaxseed oil, for example, has to be kept in dark bottles and refrigerated and even then most brands sold have some degree of rancidity and other oils such as corn or soybean have agents added to mask any rancid smells. Whilst oils high in monounsaturated fats such as olive, coconut or palm oil will keep for a year or more, oils high in polyunsaturated fats should be used within 6 months.
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