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
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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.
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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.
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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
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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 ...... 31 4.4 Formation of primary oxidation products ...... 33 4.5 Formation of epoxy fatty acids ...... 38 4.6 Correlations ...... 44
5 Conclusion ...... 47
6 Appendices ...... 50 Appendix 1: Flowchart thesis ...... 51 Appendix 2: IDF method for peroxide determination ...... 52 Appendix 3: Chromatogram of one of the samples ...... 55
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1 Introduction
Lipid oxidation is not only responsible for sensory degradation, it can also be a risk to human health. It is shown that oxidation products are involved in diseases like atherosclerosis and cancer and that they can impair certain metabolic functions (Dobarganes & Marquez-Ruiz, 2003). Recent studies (Mubiru et al., 2013, 2014) optimized the method for detecting and quantifying one of the secondary oxidation products, epoxy fatty acids. This work is a continuation on these studies and focuses on the formation of epoxy fatty acids in oils while oxidizing at an elevated temperature. The thesis revolves on autoxidation of C18:1, C18:2 and C18:3 and is part of a bigger study where photoxidation and thermoxidation are also looked into.
The aim of this work was to determine the stability of the oils by determining the peroxide value, to follow the formation of other primary oxidation products, that is conjugated dienes and conjugated trienes and to monitor the formation of epoxy fatty acids by base-catalyzed- transmethylation and detection and quantification with GC-FID. Finally correlations between peroxide value, conjugated dienes, conjugated trienes and amount of epoxy fatty acids were found.
This work starts with a review of the literature, which gives some background information on the mechanisms and the kinetics of lipid oxidation. It also shows some insights into the measurement of the oxidation level in oils and it talks about the negative effects of lipid oxidation. Next, the material and methods used are explained, including standards and the design of the study. Thereafter the results of the experiments are given and discussed and finally a conclusion is presented.
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2 Literature overview
2.1 Lipid oxidation
Oxidation, a free-radical chain reaction which leads to oxidative rancidity and hydrolysis, the splitting of triacylglycerol into fatty acids by reacting with water, are the two basic reactions of lipid deterioration. Oxidation is more responsible for the deterioration than hydrolysis (List et al., 2005). Oils with high concentrations of polyunsaturated fatty acids (PUFAs) are more susceptible to lipid oxidation. They can autoxidize due to molecular oxygen in the oil reacting with the fatty acids of the triglycerides, they can photoxidize due to light exposure, they can thermoxidize due to exposure to heat and they can also oxidize enzymatically due to enzymatic reactions (Frankel, 2005). Photoxidation can only occur when oils are exposed to direct sunlight or to fluorescent light in presence of a photosensitiser. Enzymatic oxidation is of little concern in commercial oil samples, as the enzymes in the oil are inactivated during the refinement process (List et al., 2005).
The rate and degree of oxidation depends on several factors: fatty acid saturation, amount of molecular oxygen present in the sample, water content and the amount of anti- and pro- oxidants, etc. Antioxidants and pro-oxidants influence the induction period. This is the period where very little oxidation occurs. Pro-oxidants, such as iron and copper, reduce the induction period significantly, whereas antioxidants, such as α-tocopherol, extend the induction period (Steele, 2004). A typical oxidation curve with an induction period is shown in figure 1.
Figure 1: Typical oxidation curve with induction period (IP). (Adapted from Steele, 2004)
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2.1.1 Mechanisms of lipid oxidation
A chemical reaction that occurs at room temperature between molecular oxygen and an organic compound is usually referred to as autoxidation. It is important to note that this reaction is autocatalytic or self-sustaining. Therefore, the rate of oxidation is slow at the start and increases with time. There are three main steps in the autoxidation: initiation, propagation and termination (Porter et al., 1995).
2.1.1.1 Initiation
The first step of the lipid oxidation is the formation of a lipid radical, R•. There are three ways from where this lipid radical can be formed. It can be formed due to a thermal or photochemical homolytic cleavage of a RH bond or by the abstraction of a hydrogen atom from a RH bond by a free radical. This free radical can be a hydroperoxide which was formed in the plant oil prior to and during the extraction process. The homolytic cleavage of the RH bond is seen as the main initiation reaction in edible oils (Porter et al., 1995; Steele, 2004).
2.1.1.2 Propagation
The lipid radical, or alkyl radical, formed in the initiation step reacts with molecular oxygen. There are three orbital states in which molecular oxygen can exist, namely two singlet states and a triplet state (Ho et al., 1995), as shown in figure 2.
Figure 2: The partially filled 2p orbitals of respectively triplet oxygen and two singlet oxygens. (Adapted from Ho et al., 1995)
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The singlet states are electronically excited and are therefore highly reactive. It can react directly with an unsaturated fatty acid (Alberti & Orfanopoulos, 2010). The triplet state is less reactive and can react with the previously formed lipid radical first to form a peroxy radical, ROO•. This peroxy radical is highly reactive and will react with a new fatty acid to form a hydroperoxide, ROOH, and another lipid radical. By formation of a new lipid radical, the oxidation process continues. Because of these chain reactions this step is called the propagation. The rate at which this occurs is dependent on the energy needed to break the RH bond of the fatty acid. The dissociation energy of an allylic hydrogen is 10 kcal/mol higher than the dissociation energy of a bisallylic hydrogen. That is why PUFAs are much more susceptible to oxidation than other fatty acids (Porter et al., 1995; Steele, 2004).
2.1.1.3 Termination
When two alkyl radicals react with each other, they form a non-reactive dimer, RR. In case of excess oxygen in the sample, most of the alkyl radicals will react with oxygen to form peroxyl radicals. When there is consequently an excess of peroxyl radicals, the peroxyl radicals will participate in the termination as well. They can react with other peroxyl radicals or with alkyl radicals to form peroxides, ROOR.
Peroxide can decompose to form alkoxy radicals, RO•, which in turn can decompose to form volatile compounds, such as hydrocarbons, alcohols and aldehydes, and non-volatile compounds, such as non-volatile alcohols, ketones and epoxides. These compounds are referred to as secondary oxidation products. The volatile compounds, and more specifically the aldehydes are mostly responsible for the aroma changes in oxidized oils (Steele, 2004) . Figure 3 shows a summary of the mechanism of lipid oxidation.
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Figure 3: Generalized scheme of autoxidation of unsaturated lipids. (Adapted from Shahidi, 1997)
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2.1.1.4 Photoxidation
Light can turn triplet oxygen into singlet oxygen, with aid of photosensitizers, such as riboflavin, chlorophyll, myoglobin, porphyrins, and synthetic colorants (Lee, 2002). Singlet oxygen can start the oxidation reaction due to its high reactivity. Photoxidation depends on the formation of singlet oxygen and it cannot be quenched by free radical antioxidants. It can be inhibited though by singlet oxygen quenchers like carotenoids. Photoxidation is of less concern, unless under direct sunlight or visible light, because light with wavelength less than 220 nm cannot be absorbed by lipids (List et al., 2005).
2.1.1.5 Thermoxidation
Thermoxidation is the oxidation process at high temperatures. At 180 °C and above, oxidation products are being formed which consumers believe to smell and taste good. This can be explained by the difference of solubility of oxygen between low and high temperatures. At low temperatures, solubility of oxygen is high, hence primarily hydroperoxides are formed. At high temperatures, solubility of oxygen is low, although the oxidation process is increased drastically. It is shown that hydroperoxides are almost absent at 150 °C, and high amounts of triacylglycerol monomers and polymers are formed. The formation of compounds at high temperatures is complex, as not only oxidation reactions but also thermal reactions are occurring (Dobarganes & Marquez-Ruiz, 2003).
Lipids oxidized at low temperatures show maximum 4-5% oxidized triacylglycerols before being classified as rancid. Frying fats on the other hand display often more than 25% of oxidized compounds, which is the upper limit permitted in most countries (Dobarganes & Marquez-Ruiz, 2003). However, it is believed that many fast food places use fat with even higher amounts of oxidized compounds (Saguy & Dana, 2003) .
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2.1.2 Kinetics of lipid oxidation
2.1.2.1 Induction period
During the induction period or initial phase, the oxidation process proceeds slowly. The length of this phase can be increased by lowering the temperature, by adding antioxidants, by decreasing the oxygen concentration or by decreasing the activity of pro- oxidants.
In theory the oxidation process increases exponentially with the increase of temperature. Practically speaking, this is not completely correct because of the oxygen solubility decreasing with increasing temperature. Antioxidants prolong the initial phase by catching the free radicals in the oil and by forming them into less reactive compounds. By decreasing the activity of pro-oxidants, the induction phase will be extended. Pro-oxidants like iron and copper are very effective in reducing the initial phase, even in concentrations of one part per million or less. They also catalyze the decomposition of hydroperoxides (Steele, 2004).
2.1.2.2 Peroxide formation and decomposition
Initially hydroperoxides are formed faster than they are destroyed, which means their level increases over time. At a certain critical peroxide level the oxidation enters the second phase. In this phase the oxidation rate increases exponentially. From this point on the oil will begin to smell and taste rancid (List et al., 2005). The peroxide levels will keep increasing over time, until the decomposition rate of the hydroperoxides gets higher than the formation rate, as shown in figures 4 and 5. The time required to reach the maximum levels depends on the saturation level of the fatty acids. Highly unsaturated lipids will reach this point sooner (Frankel, 2005). This means that in the early stages of the oxidation process the primary oxidation products will dominate and in the later stages the secondary oxidation products will dominate. The final stages of oxidation, polymerization and degradation, come accompanied with a big increase of viscosity, as shown in figure 5 (List et al., 2005) .
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Figure 4: Development of primary and secondary oxidation products during lipid oxidation. (Adapted from Frankel, 2005)
Figure 5: Peroxide formation and decomposition as function of time. (Adapted from List et al., 2005)
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2.1.3 Negative effects of lipid oxidation
2.1.3.1 Effect on food quality
The main concern of lipid oxidation is rancidification. This process covers the decomposition of lipids, such as fats and oils. It is characterized by off flavors and undesirable odors. In some cases, such as during the frying process, oxidation creates desirable compounds. At present, the emphasis is being given to the use of unsaturated fatty acids in food due to the fact that PUFAs are considered healthy. Consequently, the food sector has problems with rancidity, given that PUFAs are more susceptible to oxidation. Besides the produced flavors and odors, oxidation changes the nutritional quality as well as the fatty acid composition.
To control oxidation, it is advised to minimize the amount of metal in the oil, and to optimize the amount of antioxidants, such as tocopherol. Unfortunately, antioxidants often are destroyed or removed during the refining process (Frankel, 1996). Therefore, antioxidants are often added to the final product (Wsowicz et al., 2004).
2.1.3.2 Effect on health
At present, it is difficult to determine the impact of oxidized lipids on our health. The main reason for this is the fact that it is hard to get trustworthy information on the daily intake of these fats. This is partly blamed on the change of lifestyle in the last decades. The ratio of visible to invisible fats has decreased due to the increasing consumption of convenient foods and fast food. This makes it hard to predict the composition of consumed fats. Secondly, it is difficult to estimate the degree of oxidation of digested fats and thirdly, the oxidation process produces a cocktail of compounds of which the toxicity is not always fully clear yet. Therefore it becomes hard to predict if the amount of oxidized fats in diets is sufficient to express physiological effects (Dobarganes & Marquez-Ruiz, 2003).
Lipids oxidized at low to moderate temperature (below 20 °C) are not believed to be a problem for human health, as these products show sensory degradation and would be discarded before consumption. It is still important to keep in mind that small amounts of oxidation products are ingested through fats which do not show degradation. On the other hand, lipids that are oxidized at high temperatures (above 180 °C) contain several oxidation products that humans seem to appreciate. Therefore these products are being eaten and are a potential risk to our health. That is why oxidation processes at high temperature are believed to be the primary source of oxidation products in our diets (Dobarganes & Marquez-Ruiz, 2003).
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In addition, it is important to notice that human gastric fluid is a perfect medium for further oxidation of the lipids. This suggests an underestimation of the dietary intake of oxidation products (Kanner & Lapidot, 2001). High intake of oxidation products can attribute to atherosclerosis, cancer and impairment of metabolic functions, but more data are needed on the intake of these compounds to see if the intake is sufficient to produce these effects. Literature suggests that a moderate intake of oxidized fats is safe. However, ingestion of highly oxidized fats, such as overused frying fats, can be harmful in the long term (Dobarganes & Marquez-Ruiz, 2003).
2.1.4 Epoxy fatty acids
Epoxides are molecules with a ring structure of three atoms containing an ether group, as can be seen in figure 6b. Epoxides can be used as biomarkers of lipid oxidation since they are secondary oxidation products. They are believed to be formed by two different mechanisms. The first mechanism consists of the addition of a peroxy radical to a non-conjugated double bond. The double bound will undergo a 1,3- cyclization which forms an epoxide and eliminates an alkoxy radical (figure 6a). The second mechanism is a rearrangement or a cyclization, involving a 1,2- addition to an adjacent double bond and formation of an epoxyallylic radical (figure 6b) (Mubiru et al., 2014).
Figure 6: Mechanism of epoxy fatty acid formation. (adapted from Schaich, 2005)
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Recent studies have shown that especially the C18 epoxy fatty acids appear in high amounts in oxidized oils and foods (Mubiru et al., 2013, 2014). This may have consequences on human health, as epoxides have been found to be leukotoxic and some isomers are implicated in acute respiratory distress (Greene et al., 2000; Hayakawa et al., 1990).
There are 12 epoxy fatty acids from the dominant C18 fatty acids, that is oleic, linoleic and linolenic acid. Two from oleic acid (c18:1) namely methyl trans-9,10-epoxystearate and methyl cis-9,10-epoxystearate. Four epoxy fatty acids from linoleic acid (c18:2) namely methyl trans-12,13-epoxyoleate, methyl cis-12,13-epoxyoleate, methyl trans-9,10- epoxyoleate and methyl cis-9,10-epoxyoleate and six from linolenic acid (c18:3) namely methyl trans-12,13-epoxy-9,15-octadecadienoate, methyl cis-12,13-epoxy-9,15- octadecadienoate, methyl trans-15,16-epoxy-9,12-octadecadienoate, methyl cis-15,16-epoxy- 9,12-octadecadienoate, methyl trans-9,10-epoxy-12,15-octadecadienoate and methyl cis-9,10- epoxy-12,15-octadecadienoate (Mubiru et al., 2014). Their structures can be seen in figure 7 on the next page.
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Figure 7: Structure of formed epoxy fatty acids.
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2.2 Measurement of lipid oxidation level
2.2.1 Peroxide value
Free radicals, abstracted from fatty acids, react with molecular oxygen to form peroxide radicals (primary oxidation products). These peroxide radicals react with unsaturated fatty acids, forming other free radicals and hydroperoxide. Hydroperoxides are very unstable and will form secondary oxidation products. The peroxide value is therefore correlated with the degree of oxidation of fats.
2.2.1.1 Iodometric titration
The AOCS Official Method 965.33 is commonly used to determine the peroxide value of oils and fats. This method is based on the reaction of peroxides in the oil sample with iodide ion present in a saturated potassium iodide solution. The reaction will form iodine which is an indicator of the initial quantity of peroxides present in the sample. The amount of iodine can be measured by titration with sodium thiosulphate with starch as end point indicator. The peroxide value is expressed in milliequivalents of peroxide per kilogram oil. The reaction mechanism is illustrated below .