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By Ray Cook of Ebortec Ltd., South
Newbald, York, United Kingdom
Isomers are compounds with identical
chemical formulae, but with different
molecular arrangements. In the chem-
istry of oils and fats, isomerization can
take the following forms:
Triglyceride rearrangement isomer-
ization. This type of isomerization
involves fatty acid radicals adopting dif-
ferent positions on the glycerol mole-
cule. The isomerization can be ther-
mally induced but is more commonly
encountered during interesterification
reactions (see Scheme 1).
Rearrangement isomerization can
have a significant effect on the crystal
structure of the solid fat and upset the
melting characteristics of certain com-
pounds such as confectionery fats,
where there also can be a significant
effect on palatability. Also, in certain
circumstances, the way in which the fat
is metabolized can be altered.
Positional isomerization. When dou-
ble bonds occupy different positions on
an unsaturated fatty acid molecule, they
are known as positional isomers. In
polyunsaturated fatty acids (PUFA), the
permutations for different positional
isomers are great, but variations are
uncommon in nature. 
Linoleic acid, with double bonds in the 9
and 12 positions, and linolenic acid, with
double bonds in the 9, 12 and 15 positions,
are classified as essential fatty acids for ani-
mals. These fatty acids, which also are
known as omega-6 (n-6) and omega-3 (n-3)
fatty acids, form an important part of the
human diet. These PUFA are isomerized in
acid or alkaline conditions or by high tem-
peratures, with the double bond migrating
from the 9 and 12 positions to other loca-
tions such as the 9 and 11, 10 and 12, or 8
and 10 positions, resulting in the loss of
health-promoting characteristics. It is there-
fore important that this type of isomeriza-
tion be minimized, or the nutritional value
of the fatty acid can be lost.
Geometric isomerization. A double
bond can have two configurations: cis (c)
and trans (t), as illustrated in Scheme 2.
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Processing
Thermally
induced 
isomerism by
deodorization
Table 1
Comparison of the melting points (°C) of cis and trans isomers
cis Fatty acids Melting point trans Fatty acids Melting point
Linolenic acid (C18:3,c,c,c) –11 Linolenic acid (C18:3,c,t,t) 49
Linolenic acid (C18:3,t,t,t ) 71
Linoleic acid (C18:2,c,c) –5 Linoleic acid (C18:2,t,t) 56
Oleic acid (C18:1,c) 14 Oleic acid (C18:1,t) 44
Scheme 1
Scheme 2
cis trans
The cis form, the one generally found in
natural fats, is the more reactive and
requires a relatively low activation
energy to be changed to the trans iso-
mer. When converted to the trans iso-
mer, the structure permits a tighter
stacking of the molecules, allowing
them to behave in a similar way to sat-
urated fatty acids, with the result that
the trans isomers have much higher
melting points than the cis isomers. This
increase in melting point is illustrated in
Table 1.
The trans isomers have melting
points similar to those for saturated
fatty acids, e.g., stearic acid C18:0,
70°C, and palmitic acid C16:0, 63°C.
In view of the dramatic change in
melting point, and as a result of
numerous dietary studies, most regula-
tory authorities regard trans fatty acids
as similar to saturated fatty acids. Both
can lead to an increase in low-density
liproprotein cholesterol (LDLC) in the
bloodstream, and many dietary guide-
lines specify recommended maximum
daily intakes of these fatty acids. It has
been proposed that labeling laws and
regulations in Europe and the United
States should be changed to take
account of trans fatty acid content and
to forbid misleading low-saturate con-
tent claims when significant amounts
of trans fatty acids are present.
Proposed changes in U.S. labeling rules
could be formally promulgated during
2002.
Formation of geometric isomers
There are two principal conditions
under which geometric isomers are
formed: at very high temperatures and
in hydrogenation reactions. During
hydrogenation, depending on the cata-
lyst and the process conditions, trans
isomers are readily formed as intermedi-
ate compounds. For example, in a typi-
cal partial hydrogenation of soybean or
rapeseed oil, the trans isomer content
can rise to as high as 55%, mainly as
trans oleic (elaidic) acid. In this exam-
ple, trans forms of PUFA will be mini-
mal, as linolenic acid is selectively con-
verted to linoleic acid, which in turn is
selectively converted to oleic acid. 
On the other hand, thermally
induced isomerism specifically affects
linolenic acid and, to a lesser extent,
linoleic acid. This type of isomerism
has become more important as new
products under “zero trans” or “low
trans” descriptions have been intro-
duced.
Development of low trans products
Coronary heart disease is clearly estab-
lished as the No. 1 killer of people. It is
associated with a high level of LDLC,
which in turn is linked to diet, particu-
larly excess consumption of certain sat-
urated and trans isomer fats.
Consequently, a large array of
“healthy” margarine and spread prod-
ucts have been developed for an increas-
ingly health-conscious population.
These products are generally low in sat-
urates, low in trans fatty acids, and high
in PUFA. 
There are several definitions of zero
trans fatty acid content, based on serv-
ing size or percentage of total product,
but in general to qualify under the
description of low trans fatty acid, the
product must contain less than 1%
trans acids, with a normally accepted
maximum of 0.8% in the finished prod-
uct. 
Low trans products generally are
manufactured using hardstocks made
from interesterified blends of saturat-
ed fats and nonhydrogenated natural
fats, which can then be blended with
up to 85% soybean, rapeseed, or sun-
flower oil to produce a fat system for
a wide range of products. This method
clearly removes the trans acid content
associated with hydrogenation, but the
oil still has to be deodorized, and this is
where the effect of thermally induced
isomerism becomes important.
Trans isomers 
in nonhydrogenated liquid oils
Thermally induced isomerism of
linolenic and linoleic acids has been
studied by numerous workers in Europe
and the United States, who have sought
to quantify the degree of isomerism
found in normal commercial products,
and also to identify the factors affecting
the rates of isomerism.
S. O’Keefe, now at Virginia
Polytechnic Institute and State
University, and colleagues at the Florida
Agricultural Experimentation Station
published a detailed study on the geo-
metric isomer content of 16 samples of
nonhydrogenated soybean and canola
oils, purchased at random in the United
States. The trans isomer content ranged
from 0.56 to 4.2%, with three-quarters
of the samples having a trans content
above 1%. Most isomers appeared in
linolenic acid, which, in this study,
showed an average of 15.5% conver-
sion compared to 1.1% of the linoleic
acid. The average trans isomer content
from all the samples analyzed was
1.7%.
These results are similar to work car-
ried out previously by R. Wolff in
France, who found linolenic acid iso-
merism in soybean oil in the range of
15.8 to 22.7%, and in rapeseed oil in
the range 10.5 to 26.9%.
O’Keefe and other workers also
observed that the incidence of geometric
isomerism is accompanied by a signifi-
cant degree of positional isomerism.
More recent work in the Czech
Republic, published by the Institute of
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Processing
Chemical Technology in 2000, estab-
lished that, in a series of nine manufac-
turing trials, 30% of the linolenic acid
was isomerized at process temperatures
of 245–257°C, and 37% at tempera-
tures of 265–269°C, equivalent to 2.97
and 3.55% trans isomers, respectively.
Thesetrials were aimed primarily at
comparing rapeseed oil produced by
alkali refining and oil produced by
physical refining methods.
Thus, soybean and rapeseed oils can
be partially isomerized during deodor-
ization to such an extent that the trans
isomer content can exceed the require-
ments specified by the new regulations
defining low-trans products.
Dynamics of trans acid formation
Thermally induced geometric isomerism
is mainly a function of time and tem-
perature. Factors such as amount of
steam used during deodorization or the
method of oil extraction do not appear
to play any part in the isomerism
dynamics, but the rate of isomerism is
related to the location of each double
bond and is further complicated as iso-
mers “cct” or “tcc” isomerize further to
become “tct.” For the purpose of this
article, all trans isomers of linolenic are
treated as a single entity. 
To illustrate this further, data from a
number of published papers have been
collated and extrapolated to produce a
series of graphs. Figure 1 illustrates the
relationship between time, temperature,
and the formation of trans isomers in
rapeseed oil during deodorization. The
graphs are only an approximation, but
they highlight the exponential influence
of operating temperature. It can be
deduced that in order to depress the for-
mation of trans acids to below 1%, it is
necessary to operate at a deodorizing
temperature of 240°C or below for a
maximum of one hour. Alternatively,
higher temperatures can be used for
much shorter times.
Optimal deodorizing conditions for
minimum trans isomerism
“Deodorization” of a product is some-
what of a misnomer as it includes
numerous other functions:
• Distillation of free fatty acids (FFA),
• Distillation of tocopherols,
• Heat bleaching of heat-sensitive
pigments,
• Distillation of pesticides and other
synthetic pollutants, and
• Distillation of aldehydes, ketones,
and other low molecular weight odorif-
erous compounds.
The initial content of each of these
groups of impurities varies widely, from
less than one part per million for com-
pounds that are detectable by taste or
odor, to 5% for FFA.
The optimal conditions for each of
these processes are very different and
create a need for a wide range of oper-
ating conditions within the process.
Odoriferous compounds generally have
higher vapor pressures than the FFA,
which means that their removal can
take place at lower temperatures, and
continue as the oil is being cooled from
the upper operating temperatures, but
they have to be removed to a much
higher degree.
Normal commercial deodorization
of edible oils involves heating to
240–260°C, followed by deodorization
and cooling stages. The duration of
these stages depends on the quality of
the undeodorized oil, the product speci-
fication, and the deodorizing equipment
being used, but typically the oil will be
at the elevated temperature for between
1.0 and 2.5 hours.
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Volume 13 • January 2002 • inform
Figure 1. Effect of time and temperature on formation of geometric
isomers (GI) in rapeseed oil.
Upper operating temperatures are
often determined by the need for heat
bleaching, which for soybean and rape-
seed oils is up to 240°C and for palm
oil, up to 250°C. 
The removal of FFA is a much more
complex process than deodorization,
involving the FFA concentration, its
average molecular weight, temperature,
vacuum condition, stripping efficiency,
mass and flow of stripping steam, and
time. All of these factors are interre-
lated. Therefore, if the maximum tem-
perature and time at elevated tempera-
ture are to be minimized, other condi-
tions must be enhanced. For example, if
the deodorization takes place at lower
temperatures, the vapor pressure of the
FFA is reduced, but this can be compen-
sated for by lower vacuum conditions
or an increase in live steam use.
When distilling high concentrations
of FFA, the latent heat of evaporation is
also a factor, as the oil will cool down
approximately 2°C for every 1% FFA
removal. This cooling effect demands a
higher initial temperature, or a sec-
ondary reheat system, or continual
heating of the oil as the FFA is distilled. 
The head space vacuum, oil depth,
and agitation are also major factors in
deodorizer performance. A deep-bed
deodorizer provides poor distillation
characteristics due to the hydrostatic
pressure exerted by the oil, compared to
a shallow-tray design or packed col-
umn, where the oil is flowing over a
large surface area countercurrent to the
stripping steam.
As time is a major factor in the for-
mation of geometric isomers, it is logi-
cal to optimize conditions so that the
period spent at elevated temperatures is
minimized. This includes the time above
230°C while heating and cooling are
taking place. 
Ideally the deodorizer design should:
• optimize heat transfer during heat-
ing stage,
• avoid back-mixing in any of the
trays (semicontinuous flow avoids this
problem),
• use a shallow-tray or packed-col-
umn configuration for maximal FFA
stripping with optimal use of live steam,
• cool rapidly to below 230°C as
soon as FFA stripping is complete,
• allow operation at the lowest vacu-
um consistent with commercially avail-
able equipment, and
• avoid the need to compensate for
the latent heat factor.
One further factor is the molecular
weight of the FFA. Most oils and fats
contain fatty acids in the range
C12–C18, but there is a large differ-
ence in the vapor pressure of the dif-
ferent fatty acids as can be seen from
Figure 2. 
Because one of the primary objec-
tives of processing is to reduce the total
FFA to 0.05% or below, it follows that
the FFA with the largest molecular
weight and the lowest vapor pressure,
which will be the most difficult to
remove, will determine the upper oper-
ating temperature. Conversely, the FFA
with the highest vapor pressure can be
removed at much lower temperatures.
This fact can be used to advantage if
FFA stripping is concurrent with heat-
ing.
Combined heating and stripping tray
As a way to take all of these factors into
account, a combined heating and strip-
ping tray designed to minimize geomet-
ric isomer formation has been invented
by Ebortec Ltd. (United Kingdom
patent pending No. 9927531.5). The
unit (Figure 3) operates in a semicontin-
uous mode and has the following fea-
tures:
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Volume 13 • January 2002 • inform
Processing
Figure 2.Vapor pressure of fatty acids in relation to temperature.
• Stripping of short-chain FFA com-
mences at lower temperatures and pro-
gresses to the higher molecular weight
FFA as the temperature increases.
• Stripping occurs concurrently with
heating, eliminating the need to com-
pensate for the latent heat factor and
reducing the total time at the upper
temperature.
• A special lift pump design utilizes
hydrostatic pressure to induce a high
circulation equal to the whole contents
of the tray once per minute, ensuring
optimal heat transfer and maximal sur-
face area. Note that a lift pump of this
type of design utilizes the fact that 1 kg
of steam at 5 mbar and 240°C will
occupy approximately 500 cubic
meters; therefore, small injections of
steam into the lift pump create a col-
umn of gas/liquid, which has a reduced
average density, thus inducing the flow.
• The unit generates a highly efficient
continuous cascade with the stripping
steam passing countercurrently past the
oil. Following the heating/stripping
stage, the oil is dropped to the deodor-
izing section, where heat bleaching and
final deodorization continue in a shal-
low-tray mode, which is again created
by the use of a lift pump (Figure 4).
Because of the complex nature of
the deodorizing process, the configura-
tion of this tray can be changed during
the cycle. Initially, the circulation rate
ismaximized and the sparge in the
shallow tray is minimal; later in the
cycle the conditions can be reversed.
As soon as the steam is switched off
the lift pump, the contents of the upper
shallow tray are automatically emptied
through drainage ports, making this
design suitable for processes where
stock changes are regularly required.
This design is patented (United
Kingdom patent 2,283,435; U.S.
patent 5,437,714).
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Volume 13 • January 2002 • inform
Figure 3. Combined heater and free fatty acids stripper
Figure 4. Ebortec deodorizing tray
Both this tray and the combined
heating and stripping tray have been
incorporated in a new deodorizer
designed to physically refine degummed
rapeseed oil. This plant is currently
undergoing trials to assess trans acid
formation, and operational data will be
published separately.
Packed column deodorization
Alfa Laval of Sweden has developed a
system for seed oil processing that is
designed to reduce geometric isomer
formation. This design, called “Soft
Column Deodorization,” incorporates a
highly efficient structured packing
inside a column through which the pre-
heated oil flows by gravity. The flow is
countercurrent with steam, which has
been used inside the deodorizer, thus
optimizing the steam usage.
The plant is designed so that the
retention time at elevated temperatures
can be kept as short as possible, mini-
mizing both tocopherol losses and the
formation of thermally induced geomet-
ric isomers. Details of the plant were
published in 1996 in a paper by O.
Stenberg, which also stressed the need
to minimize time transferring oil from
one tray to another using specially
designed drop valves and maintenance
of the heat recovery sections under vac-
uum and steam sparging.
Dual temperature deodorization
De Smet of Belgium has also introduced
a deodorizer specifically to reduce the
incidence of geometric isomers. Their
system is known as “The Dual
Temperature Deodorizer,” and the
design was the subject of a paper pre-
sented by M. Kellens and W. De Greyt
at the PORIM International Palm Oil
Congress in 1999.
The approach adopted by De Smet is
to raise the temperature in two stages:
Initially mild deodorization and deacid-
ification take place at 230°C in a series
of trays, then the oil is passed through a
second heating tray, where the tempera-
ture is raised to 250°C before passing to
the last tray for final stripping and heat
bleaching.
By adopting this approach, De
Smet claims that tocopherol removal
is reduced and the time spent at the
high temperature is minimized,
thus reducing the formation of
trans isomers.
Conclusion
Thermally induced geometric isomer-
ization mainly affects linolenic acid,
which is very different from the trans
isomers of oleic acid formed during
hydrogenation. Thermally induced geo-
metric isomerization is also accompa-
nied by varying degrees of positional
isomerization and, although the jury is
still out on the relative pathological
effects of these different isomers, there
is now a worldwide awareness that
trans isomers exist, and manufacturers
are coming under increasing pressure to
limit the content of geometric and posi-
tional isomers in food products. Clearly,
refiners will be obliged to reduce the
effect of thermally induced isomerism,
which for many years has been regard-
ed as acceptable but will not be in the
future.
NOTE: A deodorizer as described above
has been put into commercial use at a
refinery in Poland where it is now used
in physical refining of crude rapeseed
oil at 250°C, for a maximum time of 30
minutes using 3.0 mbar vacuum. These
conditions are producing a trans isomer
content of 0.5% as predicted in the
above article.
Bibliography
The Institute of Chemical Technology
(Prague, Czech Republic), Effects of
Plant-Scale Alkali Refining and
Physical Refining on the Quality of
Rapeseed Oil, Eur. J. Lipid Sci.
Technol. 1:15–22 (2000).
Kellens, M., and W. De Greyt, New
Concepts in Edible Oil Refining:
Application of Dual Temperature
Deodorization for the Production of
High-Quality Oils, paper given dur-
ing PORIM International Palm Oil
Congress, Kuala Lumpur, Malaysia,
February 1999, and published in cor-
porate literature by De Smet,
Edegem, Belgium.
O’Keefe, S., S. Gaskins-Wright, V.
Wiley, and I-Chen Chen, Levels of
trans Geometrical Isomers of
Essential Fatty Acids in Some
Unhydrogenated U.S. Vegetable
Oils, Florida Agricultural
Experimentation Station, Journal
series RO 3212, Gainesville, Florida,
1993.
Stenberg, O., Improved Deodorizing
Technology from Alfa Laval, Lipid
Technol. 8(5):105–106 (1996).
Wolff, R.L., Heat-Induced Geometrical
Isomerization of Linolenic Acid:
Effect of Temperature and Heating
Time on the Appearance of
Individual Isomers, J. Am. Oil
Chem. Soc. 70:425–430 (1993).
Readers may contact the author at
Ebortec Ltd., Whale Bridge Park, South
Newbald, York YO43 4SU, United
Kingdom (phone: 44-1430-827070;
fax: 44-1430-8207077; 
e-mail: ray.cook@ebortec.co.uk).❏
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