Acrolein (2-propenal) is a toxic aldehyde, classified as a carcinogen and air pollutant. Its high reactivity with proteins and DNA links it to Alzheimer's disease, cardiovascular disease, and cancer. Reducing acrolein exposure is crucial for public health. A common source of acrolein is the thermal degradation of edible oils. Previous studies suggested that linolenic acid (LnA) oxidation contributes significantly to acrolein formation under thermal conditions, primarily through radical oxidation generating fatty acid hydroperoxide (FAOOH) isomers. However, the role of another significant oxidation mechanism, singlet oxygen (¹O₂) oxidation (Type II photo-oxidation), in acrolein formation remained unexplored. This study aimed to investigate the contribution of ¹O₂ oxidation to acrolein formation from the major unsaturated fatty acids in edible oils: oleic acid (OA), linoleic acid (LA), and linolenic acid (LnA). Understanding this pathway could lead to improved oil storage and processing methods to minimize acrolein formation and subsequent health risks. Previous studies primarily focused on radical oxidation mechanisms, neglecting the significant contribution of ¹O₂ oxidation, which produces FAOOH isomers with different hydroperoxyl group positions compared to radical oxidation. This difference in hydroperoxyl group position may significantly influence the subsequent acrolein formation pathway. This study hypothesized that ¹O₂ oxidation of LnA and/or other fatty acids contributes to acrolein formation, representing a novel pathway distinct from the previously known radical oxidation pathways.

Several studies have investigated acrolein formation during edible oil degradation. Early research suggested that glycerol backbone dehydration contributes to acrolein formation. However, isotope labeling studies refuted this, indicating that fatty acid moieties, specifically LnA, are the primary source. One study showed ten times more acrolein formation from LnA compared to linoleic acid (LA) during heating. The prevailing understanding is that acrolein formation initiates through radical oxidation of fatty acids, generating various FAOOH isomers. Subsequent decomposition reactions, such as β-scission, lead to acrolein formation. The fatty acid species and the hydroperoxyl group position within the FAOOH molecule are key determinants. While previous studies largely focused on radical oxidation pathways, recent research highlights the significant role of ¹O₂ oxidation in edible oil degradation. The authors' previous work showed that commercially available edible oils undergo ¹O₂ oxidation even when immediately analyzed after opening, leading to the formation of specific FAOOH isomers. This suggests that ¹O₂ oxidation may significantly influence acrolein formation, warranting further investigation. This research gap concerning the ¹O₂ oxidation pathway in acrolein formation motivated the present study.

The study meticulously prepared high-purity standards of FAOOH positional isomers for OA, LA, and LnA. Radical oxidation of OA was achieved using azobis(4-methoxy-2,4-dimethylvaleronitrile) (MeO-AMVN) as a radical initiator, while ¹O₂ oxidation of LA and LnA utilized rose bengal as a photosensitizer. Purification of the oxidized products involved reverse-phase and normal-phase HPLC. The purity of the obtained FAOOH isomers was confirmed by Q1 mass scan and product ion scan. The hydroperoxyl group position of each standard was determined by collision-induced dissociation (CID) of sodiated FAOOH isomers using TOF-MS, confirming their exact mass values. Quantification of FAOOH isomers was performed using the ferrous oxidation-xylenol orange (FOX) assay. To investigate acrolein generation, the prepared FAOOH standards were individually decomposed thermally at 180 °C for 30 seconds. The volatile compounds produced were collected using solid-phase microextraction (SPME) and analyzed by GC-EI-MS. Acrolein and other volatile compounds were identified by spectral library comparison and standard analysis. The study also analyzed acrolein formation in commercially available edible oils (rapeseed oil, rice bran oil, and soybean oil) with varying LA content. These oils were exposed to light (5000 lux) for 0-7 days, simulating typical shelf-life scenarios, followed by heating at 180 °C for 90 seconds. Volatile compounds, including acrolein, were analyzed by GC-EI-MS. Proposed pathways for acrolein generation were based on the observed volatile compounds and the known chemistry of lipid oxidation, considering β-scission reactions and radical delocalization processes.

The study's key findings demonstrate that ¹O₂ oxidation plays a crucial role in acrolein formation from edible oils. Decomposition of FAOOH isomers revealed that acrolein was not formed from the decomposition of HpOME isomers (oleic acid hydroperoxides) or from the radical oxidation products of LA (9- and 13-HpODE). In contrast, ¹O₂ oxidation-specific HpODE isomers (10- and 12-HpODE) generated significant amounts of acrolein, indicating that LA can contribute to acrolein formation under photo-irradiation, a previously unrecognized aspect. The study also revealed that the decomposition of HpOTE isomers (linolenic acid hydroperoxides) resulted in acrolein formation. Strikingly, ¹O₂ oxidation-specific HpOTE isomers (10- and 15-HpOTE) generated 2-3 times more acrolein than other HpOTE isomers. This significant difference underscores the importance of the hydroperoxyl group position in determining acrolein formation. The proposed mechanism involves initial formation of alkoxyl radicals from the O-OH bond cleavage in FAOOH. Subsequent β-scission, radical rearrangement, and further oxidation lead to the formation of 3-hydroperoxy-1-alkenes, which decompose to form acrolein. This pathway was supported by the detection of intermediate products (e.g., 2-octenal, 2-octen-1-ol, 1-octen-3-one, 1-octen-3-ol, 1-pentanol, pentanal). Analysis of commercially available edible oils under photo-irradiation conditions confirmed that the amount of acrolein formed increased significantly with exposure to light. Oils high in LA, such as rice bran oil, showed a greater increase in acrolein formation upon photo-irradiation compared to oils with lower LA content, such as rapeseed oil. This supports the key role of LA in ¹O₂ oxidation-mediated acrolein formation. The results clearly demonstrate that the amount of acrolein produced, as well as its formation pathway, depends critically on both the fatty acid species and the position of the hydroperoxyl group within the FAOOH molecule. In summary, the significant generation of acrolein from ¹O₂ oxidation-specific HpODE and HpOTE isomers suggests that photo-irradiation significantly contributes to acrolein formation in edible oils. The study's findings highlight the previously underestimated contribution of photo-oxidation in acrolein generation, influencing the overall safety and quality of edible oils.

This study's findings significantly advance our understanding of acrolein formation in edible oils by identifying a novel pathway involving ¹O₂ oxidation. The previously dominant paradigm focused solely on radical oxidation, underestimating the contribution of photo-oxidation. This research explicitly demonstrates that ¹O₂ oxidation-specific FAOOH isomers of LA and LnA are significant sources of acrolein. The observed higher acrolein yield from ¹O₂ oxidation-specific HpOTE isomers (10- and 15-HpOTE) compared to other isomers points to the importance of hydroperoxyl group position in controlling the degradation pathway. The proposed mechanistic pathways, supported by the identification of intermediate products, provide a more comprehensive understanding of acrolein formation than previously available. The findings have practical implications for the food industry and public health. The observation that photo-irradiation enhances acrolein formation in LA-rich oils highlights the need for optimized oil storage conditions to minimize light exposure and consequently reduce acrolein content. The study's meticulous methodology, including the preparation of high-purity FAOOH standards, enhances the reliability and significance of the results. Future research could focus on extending these findings to a wider range of fatty acids and edible oils, investigating the impact of other factors like temperature and oxygen concentration on acrolein formation via ¹O₂ oxidation. Detailed kinetic studies could further elucidate the reaction mechanisms and provide quantitative data to better model acrolein formation under various conditions.

This study identified a novel pathway for acrolein formation in edible oils involving singlet oxygen oxidation. It demonstrated that ¹O₂ oxidation-specific hydroperoxides of linoleic and linolenic acid are significant contributors to acrolein formation, with 10- and 15-HpOTE isomers yielding considerably more acrolein than other isomers. The proposed mechanism, supported by detected intermediate products, clarifies the role of hydroperoxyl group position and β-scission. The findings underscore the importance of minimizing light exposure during oil storage and processing to reduce acrolein content. Future research should explore the broader applicability of this pathway and conduct detailed kinetic studies for more comprehensive modeling.

While the study used high-purity FAOOH standards and carefully controlled experimental conditions, the complexity of lipid oxidation processes means that other parallel reactions, besides those proposed, likely contribute to acrolein formation. The study focused on a limited set of commercially available edible oils, and the results might not be fully generalizable to all types of oils. The influence of other factors like temperature and oxygen concentration on the ¹O₂-mediated acrolein formation pathway warrants further investigation.