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Formation of Epoxy Fatty Acids During Autoxidation of Oils at Elevated Temperature

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Formation of Epoxy Fatty Acids During Autoxidation of Oils at Elevated Temperature Faculty of Bioscience Engineering Academic year 2013 – 2014 Formation of epoxy fatty acids during autoxidation of oils at elevated temperature Ziggy Buyle Promotor: Prof. dr. ir. Bruno De Meulenaer Tutor: MSc. Edward Mubiru Master thesis submitted to obtain the degree of Master of Science in Bioscience: Food Industry Faculty of Bioscience Engineering Academic year 2013 – 2014 Formation of epoxy fatty acids during autoxidation of oils at elevated temperature Ziggy Buyle Promotor: Prof. dr. ir. Bruno De Meulenaer Tutor: MSc. Edward Mubiru Master thesis submitted to obtain the degree of Master of Science in Bioscience: Food Industry The author and the promoter give the permission to use this thesis for consultation and to copy parts of it for personal use. Every other use is subject to the copyright laws, more specifically the source must be extensively specified when using the results from this thesis Ghent, 8 th of June 2014 II Preface The practical work of this thesis was carried out at the department of Food Safety and Food Quality at Ghent University, located in Ghent, Belgium. Tutor MSc. Edward Mubiru is thanked for the great guidance and always being available to answer questions and to discuss results, no matter the time. Without you this work would have never been completed. Prof. dr. ir. Bruno de Meulenaer is thanked for his advice and for proofreading my thesis. It was very valuable to have a second opinion on my writing. Prof. dr. ir. Mia Eeckhout is thanked for giving me good advice when starting with my thesis work. It helped me to organize my work and your deadlines helped me to get the work done in a timely fashion. To my colleagues in the lab, thank you for showing me around and for always helping me when I had a question or a problem. A special thank you to the other master students for the companionship and the shared laughter and pains. Last but not least, special thanks to my friends and family for the continuous support and help. III Summary Lipid oxidation is a free-radical chain reaction which leads to oxidative rancidity. During lipid oxidation many oxidation products are formed. Some of these oxidation products can be harmful to human health. That is why it is important to understand the formation mechanism of these oxidation products and to know how fast these compounds are being formed. This work focuses on epoxy fatty acids, which are secondary oxidation products. To monitor the oxidation process and the formation of epoxy fatty acids, stripped and non- stripped oil models containing primarily oleic, linoleic or linolenic fatty acids were stored at an elevated temperature of 70 °C for 0 to 5 days. After 0, 1, 3 and 5 days sampling was done and the peroxide value, the conjugated dienes, the conjugated trienes and the amount of epoxy fatty acids were determined. The stripped oil models showed a very fast oxidation process. The models containing mostly unsaturated fatty acids were oxidizing at a higher rate than the oil models with less unsaturated fatty acids. The models containing a significant amount of linolenic acid oxidized faster and formed the highest amount of epoxy fatty acids. In only five days, up to 5 mg/g oil of epoxy fatty acids were formed, which is very high. The non-stripped oil models showed the same oxidation kinetics but were oxidizing much slower. Only the oil models with linolenic acid are showing a significant increase of epoxy fatty acids in such a short time. IV Table of contents Preface .................................................................................................................................... III Summary ................................................................................................................................. IV 1 Introduction ..................................................................................................................... 3 2 Literature overview ......................................................................................................... 4 2.1 Lipid oxidation .............................................................................................................. 4 2.1.1 Mechanisms of lipid oxidation .............................................................................. 5 2.1.1.1 Initiation ........................................................................................................ 5 2.1.1.2 Propagation .................................................................................................... 5 2.1.1.3 Termination ................................................................................................... 6 2.1.1.4 Photoxidation ................................................................................................ 8 2.1.1.5 Thermoxidation ............................................................................................. 8 2.1.2 Kinetics of lipid oxidation ..................................................................................... 9 2.1.2.1 Induction period ............................................................................................ 9 2.1.2.2 Peroxide formation and decomposition ......................................................... 9 2.1.3 Negative effects of lipid oxidation ...................................................................... 11 2.1.3.1 Effect on food quality .................................................................................. 11 2.1.3.2 Effect on health ........................................................................................... 11 2.1.4 Epoxy fatty acids ................................................................................................. 12 2.2 Measurement of lipid oxidation level .......................................................................... 15 2.2.1 Peroxide value ..................................................................................................... 15 2.2.1.1 Iodometric titration ...................................................................................... 15 2.2.1.2 Ferric thiocyanate method ........................................................................... 16 2.2.2 Conjugated dienes and trienes ............................................................................. 16 2.2.3 Malondialdehyde value ....................................................................................... 17 3 Material and Methods ................................................................................................... 18 3.1 Chemicals and reagents .............................................................................................. 18 3.2 Samples ........................................................................................................................ 18 3.2.1 Preparation of the oil models .............................................................................. 19 3.3 Sample preparation ..................................................................................................... 20 3.3.1 Stripping of the oils ............................................................................................. 20 3.3.1.1 First step ...................................................................................................... 20 3.3.1.2 Second step .................................................................................................. 20 3.4 Treatment and storage of the samples ......................................................................... 21 3.5 Base-catalyzed-transmethylation ................................................................................ 22 3.6 Solid phase extraction (SPE) ....................................................................................... 22 3.6.1 Activation of the silica gel .................................................................................. 22 3.6.2 Preparation of the column ................................................................................... 22 3.6.3 Separation of the polar fraction ........................................................................... 23 3.7 Gas chromatography and flame ionization detection ................................................. 24 1 3.8 Peroxide value ............................................................................................................. 25 3.8.1 External calibration curve and equations ............................................................ 25 3.9 Conjugated dienes and conjugated trienes ................................................................. 27 3.10 Determination of fatty acid profile using acid-catalysed esterification and transesterification ................................................................................................................. 27 3.11 Determination of chlorophyll pigments .................................................................... 28 3.12 Data processing ........................................................................................................ 28 4 Results and discussion ................................................................................................... 29 4.1 Fatty acid composition of the models ........................................................................... 29 4.2 Chlorophyll level .......................................................................................................... 30 4.3 Stability of the oils .......................................................................................................
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