Formation of 4-hydroxy-2-trans-nonenal during heating of edible oils Bertrand Matthäus1, Dirk Wöhrmann2, 1Federal Research Center for Nutrition and

Food; Institute for Lipid Research; Piusallee 76; 48147 Münster; Germany, email:

matthaus@uni-muenster.de; 2 Institute of Food Chemistry, Westfälische-Wilhelms-

University Münster, Corrensstr. 45, 48149 Münster, Germany

Since more than 40 years 4-hydroxy-2-trans-nonenal (4-HNE) is known as a key

compound for the detection of oxidative stress in cells, but also the formation of 4-HNE

as degradation product of the oxidation of ω-6-fatty acids during heating of fats and oils

is known. This compound is classified as cytotoxic and mutagenic and therefore the

development of the amount of 4-HNE in heated fats and oils could be interesting.

4-HNE was determined by a modified method of H. Esterbauer, which uses the reaction

of αβ-unsaturated aldehydes with 2,4-dinitrophenylhadrazine and the determination of

the reaction products by HPLC. After the reaction 2,4-dinitrophenylhydrazones were

extracted by a mixture of methanol/water to separate fat and then DNPH-products were

reextracted by dichloromethane and separated by HPLC. The results showed that

different cleaning steps used in the original method to separate unreacted DNPH were

not necessary.

The investigation of different oil samples used for continual frying showed that the

amount of soybean oil increased during the first 8 h of frying up to a maximum value of

18 µg 4-HNE/g oil. Afterwards the concentration of 4-HNE continually decreased in

soybean oil and after 24 h of frying a concentration of 5 µg/g was found. A similar result

was found for sunflower oil. During the first 5 h a slight increase of the concentration of

4-HNE was detected, but clearly less pronounced than for soybean oil (5 µg/g). With

continuous frying the amount of 4-HNE in sunflower oil declined. For rapeseed oil and

palm olein, respectively, no increase of 4-HNE frying was found during.

The formation of 4-HNE showed a strong dependence on temperature and time of

heating. Heating between 120 and 160 °C resulted only in a slight increase of 4-HNE

while the maximum concentration of 4-HNE was found for a temperature of 170 °C. At

higher temperatures the formation of 4-HNE was not as pronounced. In all cases the

amount of 4-HNE in the oil decreased after a certain time of heating.