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Interesterification PDF Print E-mail
Written by Mary G. Enig, PhD   
Friday, 15 February 2008 21:45

Read this article in: Danish

The word is out on the dangers of trans fatty acids and with new laws calling for trans fat labeling, food processors are scrambling for trans-free alternatives. Trans-free liquid vegetable oils burn at too low a temperature and are too unstable to be used for frying and they are not solid enough to use as a shortening in baked goods.

Polyunsaturated oils are liquid because the fat molecules—called fatty acids—contain a bend or twist wherever they are missing hydrogen atoms, which is at the position of the double bonds. Hence, they do not pack together easily and so are liquid, even when chilled. Saturated fats are not missing any hydrogen atoms; they are straight molecules which pack together easily and hence are solid at room temperatures. Monounsaturated fatty acids, which are missing only two hydrogen atoms at the position of the single double bond, are liquid at room temperature but solid when chilled.

The process of partial hydrogenation produces trans fats by straightening out the unsaturated molecules through rearrangement of the hydrogen atoms at the position of the double bond. These altered fats are solid at room temperature and so can be used in baked goods and for frying. But trans fatty acids have been increasingly implicated as contributing to cancer, heart disease, auto-immune disease, tendon and bone degeneration and problems with fertility and growth. trans fatty acids in partially hydrogenated vegetable oils are the main cause of type 2 diabetes, characterized by high levels of both insulin and glucose in the blood, because they inhibit the insulin receptors in the cell membranes.

The obvious solution for the food industry is to use natural saturated fats such as coconut oil, palm oil and tallow (from ruminant animals such as cows and sheep) for frying and for baked goods, as they used to do. But this would involve admitting that the demonization of saturated fats that has been going on for the last fifty years is completely unscientific. And a return to a sensible policy of using natural, traditional fats would bring down the huge and powerful seed oil industry, which is the lynch pin of the American commodity agriculture system.

The "Solution"

The solution is a highly industrial process called interesterification, which rearranges the fatty acids in the triglycerides.

In nature, fatty acids usually are configured as triglycerides, which contain three fatty acids joined to a glycerol molecule. Interesterification moves these fatty acids around with the result that the interesterified fat has different melting and baking qualities.

Interesterification was first applied to natural fats like palm oil and lard. For example, in natural lard, about 2 percent of the triglycerides have three saturated fatty acids and about 24 percent have three unsaturated fatty acids. The remaining triglycerides have a combination of unsaturated and saturated fatty acids. After interesterification, the numbers of triglycerides with three saturates or three unsaturates are increased while the numbers of triglycerides with a combination of saturated and unsaturated fatty acids are decreased. The result is a higher melting temperature and "improved" baking qualities, such as more volume in cakes.

Interesterification of palm kernel oil yields fats with very specific melting properties for candies, such as fats that "melt in your mouth, but not in your hand."

To make low-trans or trans-free margarines and shortenings, manufacturers interesterify a blend of liquid oil with fully hydrogenated oil. Oil that is fully hydrogenated, as opposed to partially hydrogenated, contains 100 percent saturated fatty acids because the unsaturated fatty acids in the liquid oil have been completely saturated with hydrogen. The resulting fatty acids are mostly 18-carbon stearic acid, the same as the demonized fats found in beef and butter! Fully hydrogenated oil is very hard, so only a small amount is needed—about 10 percent—to blend and interesterify with the liquid oil to produce a spreadable fat. Recently in Canada, manufacturers have come up with an interesterified blend of palm oil and/or palm kernel oil and canola oil.


There are two basic methods for producing interesterfied oil blends. The most common uses a chemical catalyst, such as sodium methoxide or ethoxide (dangerous and highly toxic industrial solvents), or hazardous metallic sodium or sodium-potassium alloy. The first three require heat of 80o C to 120oC to produce interesterified fatty acids. The product must then be neutralized (to remove the caustic catalyst), bleached (to get rid of the resultant dark brown color) and deodorized (a process which can actually introduce trans fats into the mix).

Another method uses enzymes to produce the interesterified fats. It is more expensive but results in less loss of oil through the formation of soaps, esters and mono- and di-glycerides.

Whichever method is used, rest assured that this is a highly industrialized process involving heat that begins with oils that have already been subjected to a highly industrialized process. The resulting product may be trans-free, but it will still contain chemical residues, hexanes and many dangerous breakdown products full of free radicals.

Health Effects

Of course, the industry is hoping that these new interesterified blends will not have the negative health effects of trans fats. But a recent study, published in Nutrition and Metabolism, (2007, 4:3) gives great cause for concern. The researchers compared trans-rich and interesterified fats with saturated fat for their relative impact on blood lipids and plasma glucose. Thirty human volunteers participated in the study, which strictly controlled total fat and fatty acid composition in the subjects' diet. Each subject consumed all three diets in random rotation during the four-week diet periods.

HDL-cholesterol dropped slightly with both the trans fat and interesterified blends but the real problem concerned blood glucose and insulin levels. Insulin levels dropped 10 percent on the partially hydrogenated soybean oil diet but dropped more than twice as much on the interesterified fat diet, causing blood sugar to rise by an alarming 20 percent. Thus it seems that these interesterified blends affected the production of insulin by the pancreas rather than the receptors for insulin in the cell membranes.

The trade-off of type 2 diabetes for type 1 diabetes does not seem like a good one.

Demonizing the Competition

The uproar over trans fats presents the perfect opportunity to bring natural saturated fats back into our diets, but the vegetable oil industry has been working overtime behind the scenes to make sure that doesn't happen. They have hired PR firms to get articles published in journals and the popular media warning the public against the evils of saturated fats. A good example is the following, published at "But palm oil is a horrible alternative... all these tropical oils are highly saturated fats. Like butter, cheese and meat, tropical oils raise LDL-cholesterol and clog arteries with plaque, increasing your risk of a heart attack... . We're trading one artery-clogger for another?"

Another victim is butter, which contains a small percentage of natural trans fats that are not harmful; in fact the body transforms some of the natural trans in butter into CLA, a compound that has anti-cancer properties. In Europe, government agencies have been careful to distinguish between the natural trans fats in butter and meat fat from the artificial trans in partially hydrogenated vegetable oil; the natural trans have not been singled out for pejorative labeling. But according to the US Food and Drug Administration, there is no difference between natural and artificial trans fats. As a result, retailers like Starbucks are banning butter in baked goods in order to claim that their baked goods are 100 percent trans-free. Thus, once again, natural fats are being tarred with the black brush of their artificial substitutes.

Other Solutions

TALLOW BLENDS: A company called Source Food Technologies, based in North Carolina, is making a patented proprietary blend of corn oil and tallow. Tacitly admitting to the virtues of tallow, the company notes that the blend reduces cholesterol (yes, the main fatty acid in tallow reduces cholesterol!), has decreased levels of trans fats (implying that at least some of the corn oil has been partially hydrogenated) and results in less oil absorption in fried foods. This begs the question of just using a blend that is mostly tallow!

RICE SYRUP SOLIDS: A company called California Natural products has developed "an all-natural rice syrup solid that provides the functionality of trans fats and can replace 100 percent of the shortening in baked goods" as well as the fat in ice cream. The compound is similar in size to fat globules and has "unique carbohydrate structures due to the structure of the rice starch molecule. So it's a carbohydrate that acts like a fat."

LOW-LINOLENIC OILS: Biotech companies have begun commercializing low-linolenic soybeans (soybeans low in the very unstable omega-3 fatty acids) produced either through genetic modification or conventional breeding in order to reduce or eliminate trans fats in processed soybean oil. (During processing, it is the omega-3 fatty acids that are preferentially converted to trans fats.)

TROPICAL OILS: In Europe, companies are using more of the tropical oils, such as palm oils, which "provide the body and texture to products such that no further modification of the oil is necessary, resulting in a natural trans-free choice." Unfortunately, in the US, this sensible move has met vehement resistance from the soybean industry and their agents, such as Center for Science in the Public Interest.

Source: Todd Runestad. How to live without trans fats. Functional Foods & Neutraceuticals, December 2004.

Good Reasons to Avoid Potato Chips... .

... whether the oil they're fried in is liquid, partially hydrogenated or interesterified. The following letter from Dennis Meizys of Maryland Green Power Co. is posted at

My company is researching the production of biodiesel from used vegetable oil, and has contacted manufacturers which we suspected would produce the most waste oil. What comes to mind? Well, greasy potato chips (just look at your fingers after you eat them!) and donuts came to mind, after contacting the obvious home-run hitters, McDonald's and KFC. Contrary to what you might think, it seems the worst abusers of vegetable oils were not McDonald's, but potato chip and donut manufacturers.

One manufacturer replied to my offer to purchase their used oil with the explanation that they hardly have any used oil left-over after the process. Tens of thousands of gallons come in, barely hundreds come out. The reason? This manufacturer recycles the oil until it is entirely absorbed by the food. All that dirty oil eventually ends up in the potato chips themselves.

One problem that occurs after re-using vegetable oils is that FFAs (free fatty acids) concentrate. The manufacturer volunteered this fact and noted that their solution is to chemically treat the oil to reduce the FFA's, after which it is sent back to produce more potato chips. Mmmm—re-used vegetable oil treated with chemicals to reduce free fatty acids!

It turns out that these oils are so bad that biodiesel manufacturers shun them! In other words, they are difficult to catalyze into methyl-esters (biodiesel) and producers are reluctant to use them for engine fuel, yet people still eat the potato chips!

That brings us to the last time I ate a donut, those nicely-colored sweet confections. If you only saw the waste products. My offer to pick up one donut shop's used oil for free was met by much enthusiasm by the management, and they told me that I could pick up a 55-gallon drum once every 6 months. Did you ever go inside the donut shop and look at how much oil they have in those vats? Now consider the fact that they only dispose of 55 gallons every six months!

One closed-down shop asked me to pick up their barrel of used vegetable oil from their parking lot because it was leaking and causing environmental damage. I tried to drain the oil out, but it was so thick and sludgy that it clogged my pump. I was considering using a heavy-duty sewage pump to drain it, but decided not to, because the thick, smelly contents of that barrel were not usable as an ingredient for fuel, and refining it would be too expensive. The material had an uncanny resemblance to sewage. The only reason I knew it wasn't, was that it had a sweet, donut-like smell to it, but entirely unpleasant.

Scientific facts like knowing the carcinogen content of these "foods" is interesting, but if you want real motivation to avoid junk foods, go to the back of the "restaurant" were they dispose of their environmentally-harmful by-product and take a look. Also, you can ask them why they have to keep the stuff in barrels and wait for an expensive disposal service instead of just sending it down the drain? The reason: the Environmental Protection Agency does not allow it!


This article appeared in Wise Traditions in Food, Farming and the Healing Arts, the quarterly magazine of the Weston A. Price Foundation, Summer 2007.

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Comments (8)Add Comment
Hydrogenation of v. oils
written by Hemal de Silva, Nov 23 2013
Pentadesma fat, extracted from the seeds of the species, Pentadesma butyracea produces a good yield of solid fat rich in C-18 acids (stearic and oleic). The oil is of interest in that it contains only very low levels of palmitic and linoleic acids. This should make it oxidatively stable (low linolenic) and nutritionally useful (low plamitic, high C-1smilies/cool.gif. It is of interest for confectionery fats. At a time when it is necessary to avoid partial hydrogenation which produces undesirable trans-fatty acids, there is an interest in solid fats for use as hardstock in spreads and the presence of stearic acid rather than palmitic acid could be an advantage.

Is this information of interest to you?

With best wishes

Hemal de Silva -

written by jane, Sep 30 2013
Some of this information is incorrect. I used to work in a donut shop as a baker. We changed our oil vat fryers weekly. NOT all places are equal...maybe some change every six months but where I worked...once a week. If you say 'the donut shop' then your are making reference to one place not ALL. This skews the article to appear ALL not 'the one you went into'
written by Danielle Johansson, Nov 07 2012
I contacted a PhD from our university's department of epidemiology and asked for his thoughts regarding trans fats and diabetes and this was his reply: "We have reviewed the literature thoroughly on trans fats and diabetes risk and there is no clear consistent evidence to support Mary Enig’s claim."
written by Danielle Johansson, Nov 06 2012
This journal article seems to imply that one should not consume any form of saturated fats (natural or unnatural e.g. hydrogenated veg fat):
Saturated and trans fatty acids decrease insulin concentration leading to insulin resistance. In contrast, polyunsaturated fatty acids increase plasma insulin concentration and decrease insulin resistance.
written by Danielle Johansson, Nov 06 2012
Mary, Regarding your statement, "trans fatty acids in partially hydrogenated vegetable oils are the main cause of type 2 diabetes, characterized by high levels of both insulin and glucose in the blood, because they inhibit the insulin receptors in the cell membranes," - this is very intriguing and I've never read it anywhere else, but would like to see some references of studies done that would support it - can you add one/some? Thanks.
written by S, Apr 29 2012

There is no need to apologize for your comment. What you said is a fact.
Europeans care about where their food come from, and how ingredients are treated.
Their elected governments are really stringent in what is allowed to enter into the whole food chain.

If one had travelled to Europe by oneself, and eat where the local folks eat, visit the local markets to mingle with local people, you would tell Europeans'
general attitude towards cuisine and treatment of food ingredients is vastly
different from that of North Americans.

For example, I did not have to ask whether the sausage roll I am buying in
London contains any margarine. Nor do I have to ask the server in a boulangerie in Paris not to warm my pain au chocolat and croque monsieur in the microwave, because they simply do not use one. When in Rome, it was so easy to buy fresh fruits and snacks as I walked along the streets. I did not have to drive a long way like when I was in Florida to find a supermarket just to buy a few apples.

It is indeed way more expensive to cook with unadulterated oils here in NA.
But if you took into account the eventual toll on your health, and the $$$$ needed to buy drugs for the rest of your life, it is small change.
written by Ronny, Apr 09 2011
I want to partially apologize for my earlier comment on this article. I realize that it was wrong to generalize about how countries compare based solely on this information. However, I stand by my statement about how American industries do not care about our health. The fact that the soybean industry has not wellcomed the use of healthy tropical oils is enough to prove that.
written by Ronny, Nov 30 2010
I'm sure we know why Europe and America are so different in their attitudes toward food manufacturing. European governments are more concerned with the health of the community whereas in America, economists are free to fulfil their own selfish, greedy desires with no thought of public health. It is no wonder America is lagging behind other developed countries in about every aspect you can think of.

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Last Updated on Tuesday, 09 June 2009 20:38