The Formation of Methyl Ketones during Lipid Oxidation at Elevated Temperatures

Abstract

Lipid oxidation and the resulting volatile organic compounds are the main reasons for a loss of food quality. In addition to typical compounds, such as alkanes, aldehydes and alcohols, methyl ketones like heptan-2-one, are repeatedly described as aroma-active substances in various foods. However, it is not yet clear from which precursors methyl ketones are formed and what influence amino compounds have on the formation mechanism. In this study, the formation of methyl ketones in selected food-relevant fats and oils, as well as in model systems with linoleic acid or pure secondary degradation products (alka-2,4-dienals, alken-2-als, hexanal, and 2-butyloct-2-enal), has been investigated. Elevated temperatures were chosen for simulating processing conditions such as baking, frying, or deep-frying. Up to seven methyl ketones in milk fat, vegetable oils, and selected model systems have been determined using static headspace gas chromatography-mass spectrometry (GC-MS). This study showed that methyl ketones are tertiary lipid oxidation products, as they are derived from secondary degradation products such as deca-2,4-dienal and oct-2-enal. The study further showed that the position of the double bond in the precursor compound determines the chain length of the methyl ketone and that amino compounds promote the formation of methyl ketones to a different degree. These compounds influence the profile of the products formed. As food naturally contains lipids as well as amino compounds, the proposed pathways are relevant for the formation of aroma-active methyl ketones in food.

Formation of Secondary and Tertiary Volatile Compounds Resulting from the Lipid Oxidation of Rapeseed Oil

“…VOC from rapeseed oils were analysed by headspace gas chromatography-mass spectrometry (HS-GC-MS) as described previously [11]. Briefly, 1 g of oil sample was weighed into a 20 mL-headspace vial and incubated at 160 • C for 5-120 min.…”

“…After a heating time of 120 min, up to 65 compounds were detected. In addition to secondary degradation products, tertiary degradation products such as alkyl furans, organic acids, and ketones were found [11]. A detailed overview of all volatile compounds is listed in Tables S1-S3.…”

“…For this reason, individual compounds from the different compound classes were tested in model systems. In this study, focus was on the compound classes of saturated aldehydes and monounsaturated 2-alkenals, as previous studies have shown that double unsaturated secondary degradation products (2,4-alkadienals) can react to form methyl ketones as tertiary lipid oxidation products [11].…”

“…However, as these secondary compounds are still reactive due to their chemical structure, they can react independently or with other compounds. Accordingly, tertiary VOCs such as the methyl ketones of 2,4-alkadienals can also form during lipid oxidation [11]. Most of these VOC are aroma-active and can cause significant sensory changes [12,13].…”

“…Here, it seems to be important to take possible interactions with other food compounds into account. Nucleophilic nitrogen containing compounds (e.g., phospholipids) can affect composition of the VOC, as well [11,26,27]. For these reasons, the monitoring of tertiary compounds from lipid oxidation can be a superior and simple method to the traditional methods used in routine analysis as described above.…”