21
Trans-fatty acids in heated hemp seed oil
H. Mölleken
Bergische Universität, Physiologische Chemie der Pflanzen, Gauss-Str. 20, 42119 Wuppertal, Germany
Mölleken, H. 1998 Trans-fatty acids in heated hemp seed oil. Journal of the International Hemp Association 5(1): 21-23. Prompted by the frequently discussed question of whether trans-fatty acids are formed when hemp oil is used for cooking, we have analysed hemp oil and several other oils for their content after exposing them to various stress conditions. The results demonstrate that high temperatures do not change the configuration of the fatty acids.
Figure 1. The cis and trans configurations of a fatty acid with one double bond.
Introduction
Fats and oils have been of
interest to biochemists, nutritionists and health professionals for many years,
and one of the points of discussion is the presence of trans-fatty acids (t-FAs).
The double bonds of essential fatty acids (FAs) are confined to certain
positions on the aliphatic chain and are all of the cis configuration.
Relatively large amounts of t-FAs have been found only in a few plant
species, mainly in the seeds and leaves. The highly polyunsaturated FA 18:2 (10 trans,
12 trans), for example, occurs to the extent of 5-10% in the Mexican bush
Chilopsis linearis , and there is as much as 40% of FA 18:3 (9 trans,
11 trans, 13 trans) in the triglycerides of various Catalpa species
(Belitz & Grosch 1994, Sommerfeld 1983, Steinhardt & Pfalzgraf 1994,
Steinhardt 1996). These are exceptions however, as t-FAs normally occur
to a minimal extent in plants, if at all.
In contrast, t-FAs are widely
found in animal fats. They largely originate from their diet, although in
marsupials and ruminants t-FAs are synthesized in the rumen or stomach by
microbial hydrogenation of the polyunsaturated FAs from plant food. Thus some t-FAs
in the human diet are traced to milk and meat. In human metabolism, they are
also formed during the degradation of unsaturated FAs, though the cis-configuration
is of physiological importance. Polyunsaturated FAs, for example, loosen the
packing of the phospholipids within the hydrophobic zone of biological
membranes.
In the human diet, t-FAs arise
mainly from isomerisation of the natural cis double bonds of vegetable
oils during their industrial hydrogenation to margarine and vegetable
shortenings (Figure 1). The change in the configuration of the double bond means
that the acyl chain more closely resembles that of saturated FAs. The
accompanying hydrogenation of some of the double bonds also means, of course,
that an important property of the food oils - the high content of
polyunsaturated FAs - is sacrificed in favor of increased stability and a higher
melting point (Belitz and Grosch 1994, Steinhardt 1996).
Many foods made with milk or
hydrogenated oils (for example: deep-fry, chips, butter, cheese, margarine)
contain t-FAs. Normally, they cannot be found in products made from
non-hydrogenated vegetable oils. Table 1 compares the content of t-FAs in
various foods. Whereas products of milk and hydrogenated oils have a high
content of t-FAs, the vegetable oils and their products e.g.,
mayonaise, have no more than 0.4 % and often have none.
The physiological effects of these
hydrogenated dietary fats, and especially the influence of t- FAs, on
human metabolism have therefore been of great scientific interest for a long
time. For example, t-FAs have been connected with arteriosclerosis and cancer,
but up to now, these investigations are incomplete and controversial (Ascherio et
al. 1997, Ärzte Zeitung 1994, Christiansen et al. 1997, Kohlmeier et
al. 1997, Katan et al. 1994, Koga et al. 1997, Lichtenstein
1993, VDD-Mitteilungen 1994, Shapiro et al. 1997, Wolfram 1994).
The presence of t-FAs also
becomes a matter for discussion when vegetable oils, such as hemp seed oil, are
used for cooking or frying (Huppertz et al. 1997). We have therefore
heated hemp oil at several temperatures and analysed its deterioration and
isomerisation.
Food Product | t-FAs in % | approx. mean %t-FAs:%FAs |
---|---|---|
margarine | 0.0-10.6 | 4.5 |
chocolate spreads | 0.7-11.1 | 5.5 |
butter | 3.7-5.2 | 4.7 |
cheese | 1.8-5.4 | 3.6 |
diet margarine | 0.0-0.4 | <0.2 |
vegetarian spreads | 0.1-0.4 | <0.2 |
peanut butter | 0.0-0.3 | <0.2 |
(cf. Demmelmair et al. 1996, Fernandez San Juan 1996, Gertz 1996, Mölleken 1996, Ulberth and Henninger 1994, VDD Mitteilungen 1994) |
Materials and Methods
Various samples of a hemp seed
oil were held in an electric heater: (1) for 30 min. at a constant temperature
between 170°C and 350°C and (2) for 16 hours between 200°C and 220°C. A
10-ml sample was then homogenised with 1 ml methylene chloride (CH 2 Cl 2 ). 100
ml trimethylsulfonium hydroxide (TMSH) was added for quantitative hydrolysis of
the triglycerides (Mölleken and Theimer 1997a,b) and conversion of the
resulting FAs to FA methyl esters (FAMEs). The resulting FAMEs were analyzed on
a HP 5890 gas chromatograph equipped with an FID detector, using compound
standards from Sigma (Deissenhofen, Germany) for comparison.
FAMEs | native | 170ºC 30 min. [%] |
220ºC 30 min. [%] |
250ºC 30 min. [%] |
350ºC 30 min. [%] |
---|---|---|---|---|---|
C16:0 | 8.94 | 8.86 | 8.41 | 9.21 | 14.92 |
C18:0 | 3.52 | 3.45 | 3.47 | 3.51 | 12.25 |
C18:1, 9c | 11.00 | 10.75 | 10.95 | 11.07 | 22.53 |
(C18:1, 9t) | - | - | - | - | 2.95 |
C18:1, 11c | 1.00 | 1.01 | 1.02 | 1.08 | - |
C18:2, 9, 12c | 53.64 | 53.54 | 53.74 | 53.39 | 20.87 |
(C18:2, 9, 12t) | - | - | - | - | 10.98 |
C18:3, 6, 9, 12c | 2.15 | 2.10 | 2.10 | 2.03 | - |
C18:3, 9, 12, 15c | 17.44 | 17.92 | 17.87 | 17.26 | 2.33 |
(C18:3, 9, 12 ,15t) | - | - | - | - | 13.17 |
C18:4, 6, 9, 12, 15c | 0.79 | 0.79 | 0.86 | 0.75 | - |
C20:0 | 0.94 | 1.06 | 1.03 | 1.14 | - |
C20:1, 11c | 0.58 | 0.52 | 0.55 | 0.56 | - |
* The data represent the mean of about three analyses. The standard deviation of all data lies between 0.00 and 0.19. |
Results
Table 2 compares the influence of
various temperatures on the t-FA content of hemp seed oil. It is clear
that cooking temperatures of about 170-250°C do not lead to an increase of the t-FAs.
Similar investigations with other hemp seed oils underscore these results. Only
two oils had a t-FA content of 0.85% after being heated at 220°C for
half an hour (Mölleken and Melchior 1998). In contrast, a temperature of 350°C
deteriorates the hemp oil, and leads to the formation of significant amounts of t-FAs.
This fact is confirmed by the results
in Table 3. Some t-FAs can be detected in hemp oil that is held at 200 to
220°C for a long period (16 hours). Thus, isomerisation does take place under
prolonged moderate stress, so that t-FAs are formed and the amounts of
the unsaturated FAs decrease.
FAMEs | 200ºC 16 hrs. [%] |
220ºC 16 hrs. [%] |
---|---|---|
C16:0 | 8.56 | 12.78 |
C18:0 | 3.40 | 4.95 |
C18:1, 9c | 11.17 | 15.45 |
(C18:1, 9t) | 0.85 | n.d. |
C18:1, 11c | 1.10 | 1.43 |
C18:2, 9, 12c | 53.09 | 49.67 |
(C18:2, 9, 12t) | 0.82 | 0.69 |
C18:3, 6, 9, 12c | 2.14 | 1.61 |
C18:3, 9, 12, 15c | 17.46 | 10.15 |
(C18:3, 9, 12, 15t) | n.d. | 1.00 |
C20:0 | 0.90 | 1.58 |
C20:1, 11c | 0.46 | 0.69 |
* The data represent the mean of about three analyses. The standard deviation of all data lies between 0.00 and 0.28. n.d. = not detected |
Discussion and Conclusion
Both experiments demonstrate that
isomerisation to t-FAs does not occur when native hemp oil is used under
normal cooking conditions, though the contrary seems to be generally accepted (Huppertz
et al. 1997). Experiments with various vegetable oils (for example from
sunflower, safflower, soy bean or walnut) give similar results (Mölleken and
Melchior 1998). The comparison of Table 1 and Table 2 also supports this
conclusion. It is obvious that hydrogenated vegetable oils like margarine and
deep-fry fats contain t-FAs in higher amounts than heated hemp oil.
This does not mean that the
polyunsaturated FAs of hemp oils are stable towards all negative influences.
Under especially stressful conditions they do change their configuration or get
destroyed, and then t-FAs, hydroxy FAs, hydroperoxides, aldehydes and
ketones can be formed (Mölleken and Melchior 1998).
Heated native hemp oils are quite
stable under high-temperature conditions. Temperatures of 170-250°C do not
stress this oil, so that high concentrations of t- FAs are scarcely
formed. Presumably antioxidants, such as tocopherols, stabilise the oil, since
hemp oils contain enough gamma-tocopherol to have a strong antioxidant
effect (Belitz and Grosh 1994, Gordon and Kourimská 1994, Mölleken and
Andersen 1998, Pryzybylski et al. 1997). Further investigations show that
the influence of temperature on various vegetable oils leads to degradation of
the tocopherols (Mölleken & Melchior 1998).
While as much as 6-10 g of t-FAs
per person per day is thought not to affect health adversely (Ärzte Zeitung
1994, Wolfram 1994), the current mean human uptake through normal nutrition
(milk, cheese, margarine and deep-fried products, cf. Table 1) has been
estimated at only 3-4 g per day (Gertz 1996). It seems reasonable, nevertheless, to keep the amounts of t-FAs in the diet to a minimum
for general health purposes, and in accordance with this, many people, including
scientists, recommend: "Use high-quality vegetable oil for frying,
deep-frying and salads" (Ärzte Zeitung 1994, Wolfram 1994). Hemp oil
fulfills this requirement, since like other vegetable oils, as we show here, it
does not form significant amounts of t-FAs on being heated at usual
cooking temperatures.
References