Interesting paper [below] as it tends to indicate that oils are more stable
than otherwise have been thought. I would love to read something similar on
olive oil (I stir fry at low temps). The Flax Council of Canada indicates that; 

"Thermal stability was shown in 1992 by the absence of significant changes
in peroxide values and fatty acid composition when both forms of flaxseed
were heated for 60 minutes at either 100°C (212°F) or 350°C (662°F).
Furthermore, gas liquid chromatography showed no signs of new trans isomers
of ALA or of cyclic fatty acid formation in samples subjected to these
degrees of heat."

. . . 

I have deleted the tables and references in order to be able to post this
out - Marilyn

http://www.commonlink.com/~olsen/HEMP/IHA/jiha5108.html

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.

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.

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.

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.

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

[~25 references deleted]

Marilyn