The mechanism of 2-alkanone and lactone formation by thermal oxidation of saturated fatty acids

It has long been known that even saturated fatty acids (SFAs), which lack double bonds in their alkyl chains, are oxidized at high temperatures to form unique aroma compounds such as 2-alkanones and lactones. The hypothesis that certain positional isomers of SFA hydroperoxides (SFA;OOHs) play a role in the formation of these aroma compounds was proposed as far back as 50 years ago (Crnjar et al., 1981). However, due to the challenges in preparing these isomers, this hypothesis has remained unproven. In this study, we therefore attempted to test this half-century-old hypothesis using the following novel methods we have developed.

Glyceryl trioctanoate (C8TG) was selected as an SFA, and a crude sample was prepared via thermal oxidation at 170 °C for 1 hour. Using derivatization reactions, the crude sample was purified through reversed- and normal phase liquid chromatography to isolate hydroperoxide positional isomers (C8TG;n-OOHs). Each isomer was analyzed by LC-MS/MS, and positional information was determined based on a fragmentation pattern specific to the bonding position of the hydroperoxy group. Ultimately, six isomers of C8TG;2-, 3-, 4-, 5-, 6-, and 7-OOH (15 isomers when considering α-/β-isomerism) were successfully prepared. Each isomer was thermally decomposed individually, and the volatile decomposition products were analyzed using SPME/GC-MS. The results showed that 2-heptanone was selectively formed from C8TG;3-OOH, and γ- and δ-octalactone were mainly formed from C8TG;4- and 5-OOH, respectively. These results show that 2-alkanones and lactones are produced via the specific SFA;OOH isomers, and demonstrate the half-century old hypothesis.

2-Alkanones and lactones are aroma compounds found in animal products such as dairy products and beef. Therefore, the mechanisms elucidated in this study could be used in the technology for creating aromas of animal products from SFA-rich vegetable oils such as palm and coconut oil. In this presentation we will also discuss these latest findings.