Lipid Oxidation Analysis

Foods which contain high concentrations of unsaturated lipids are particularly susceptible to lipid oxidation. Lipid oxidation is one of the major forms of spoilage in foods, because it leads to the formation of off-flavors and potentially toxic compounds. For examples, some lipid oxidation products, such as malondialdehyde, acrolein, 4-hydroxy-trans-nonenal, 4-hydroxy-trans-hexanal, and crotonaldehyde-like compounds, generate not only off-flavor but also cause diseases, such as inflammation, ageing, cancer, and atherosclerosis, in humans. In addition, they can interfere with the signaling pathways, leading to significant biomolecular damages ^[1]. Therefore, lipids play a crucial role in food quality and lipid oxidation analysis is very necessary. Alfa Chemistry has developed a number of methods to characterize the extent of lipid oxidation in foods, and to determine whether or not a particular lipid is susceptible to oxidation.

The Mechanism of Lipid Oxidation

Lipid oxidation depends on many external and internal factors. Fatty acid profile, lipid class, and fatty acid composition are the main internal factors. Temperature, light, moisture level, atmospheric oxygen, irons, activators, and inhibitors are the main external factors. Depending upon the reaction mechanisms and factors involved, lipid oxidation can be divided into autoxidation, photo-oxidation, and enzyme-catalyzed oxidation. The whole oxidation process of lipids are that firstly yields to primary products (peroxides), which once exposed to extended oxidation conditions gives rise to secondary oxidation products, including aldehydes, ketones, epoxides, hydroxy compounds, oligomers and polymers ^[1-2].

Fig.1 The mechanism of lipid oxidation in food (take vegetable oils for example)

Our Common Analysis Methods

Alfa Chemistry has various methods to analyze lipid oxidation. The methods analyze lipid oxidation degree by measuring the primary and secondary oxidation products of lipids to assess oxidation level.

• Peroxide value (PV) method: Iodometric and ferric thiocyanate. The methods is used to detect the primary oxidation products. The principle is that oxidation of iodide by hydroperoxides or oxidation of Fe²⁺ to Fe³⁺ by hydroperoxides. And then use a spectrophotometer to obtain the final reading to give an indication of the progress of lipid oxidation.
• Conjugated diene method. The methods is used to detect the primary oxidation products. Almost immediately after peroxides are formed, the non-conjugated double bonds (C=C-C-C=C) that are present in natural unsaturated lipids are converted to conjugated dienes and trienes. Conjugated dienes absorb ultraviolet radiation strongly at 233nm, whereas conjugated trienes absorb at 268nm. Their absorption can be measured by spectrophotometric techniques to assess oxidation level.
• Thiobarbituric acid reactive substances (TBARS) method. The methods is used to detect the secondary oxidation products. The secondary oxidation product, malonaldehyde (MA), can react with thiobarbituric acid when heated in an acidic solution to form a unique Schiff base compound that gives a pink color, and the color can be measured using a visible (at 532–535 nm) or fluorescent (excitation 515 nm, emission 553 nm) spectrophotometers to assess oxidation level.

Fig.2 The reaction of thiobarbituric acid (TBA) and MA

• Chromatographic method. The methods is used to detect both primary and secondary oxidation. It is possible to monitor the loss of reactants (e.g. unsaturated lipids) and the formation of specific reaction products (e.g., aldehydes, ketones or hydrocarbons) using chromatography, and then the oxidation level can be assessed.
• Fluorometric method. The methods is used to detect the secondary oxidation products. The principle is that different fluorescent porphyrins are used to interact with MDA produced during oxidation, and then the reaction progress is observed using a fluorimeter to determine the level of oxidation in foods.

How to Choose the Analysis Methods

Each foods have the optimal method for lipid oxidation analysis, and one method cannot be used for all foods. Therefore, selecting proper methods for specific foods is essential for accurately evaluating lipid oxidation. Alfa Chemistry gives some suggestions listed in following table, or you can contact us for detailed information.

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References

1. Abeyrathne E. D. N. S., et al. Analytical Methods for Lipid Oxidation and Antioxidant Capacity in Food Systems[J]. Antioxidants, 2021, 10(10): 1587.
2. Noemí E., et al. Chapter 7 - Lipid oxidation of vegetable oils[M]. Food Lipids, 2022, 127-152.