This review provides an overview of the main vegetable oils of different botanical origin and composition that can be used for frying worldwide (olive and extra-virgin olive oil, high-oleic sunflower oil, rapeseed oil, peanut oil, rice bran oil, sunflower oil, corn oil, soybean oil, cottonseed oil, palm oil, palm kernel oil and coconut oil) and their degradation during this process. It is well known that during this culinary technique, oil’s major and minor components degrade throughout different reactions, mainly thermoxidation, polymerization and, to a lesser extent, hydrolysis. If severe high temperatures are employed, isomerization to trans fatty acyl chains and cyclization are also possible. The factors conditioning frying medium degradation are addressed, including oil composition (unsaturation degree, fatty acyl chain length and “free” fatty acid content, and presence of beneficial and detrimental minor components), together with frying conditions and food characteristics. Likewise, this review also tackles how the frying oil and other processing conditions may impact on fried food quality (oil absorption, texture, flavor and color). Finally, potential health implications of fried food consumption are briefly reviewed.

The effect of adding a tocopherol-rich natural extract (TNE) at 0.1 % and 0.5 % on sunflower oil stability under frying and accelerated storage conditions was studied using ¹H NMR and DI-SPME-GC/MS. The impact was more pronounced at the higher enrichment level under both conditions. During frying conditions, oil stability significantly decreased due to accelerated degradation of linoleate and minor components (tocopherols, squalene and sterols), along with a marked increase in oil viscosity. Additionally, the selective generation of aldehydes occurred, with the formation of alkanals, E-2-alkenals and toxic oxygenated α,β-unsaturated aldehydes being lagged. Under storage, TNE initially promoted linoleate degradation and the formation of monohydroperoxy-octadecadienoates but delayed their transformation into secondary or further compounds. Similarly, the degradation of oil minor components occurred more slowly. Tocopherol-derived metabolites varied depending on processing conditions. Pristane was the most abundant during frying, while 4,8,12,16-tetramethylheptadecan-4-olide predominated under storage conditions. Formyl derivatives were identified for the first time.

Although widely consumed, dietary supplements based on Vitamin C contain high doses of this compound, whose impact on lipid oxidation during digestion needs to be addressed. Therefore, the effect of seven commercial supplements and of pure l-ascorbic acid and ascorbyl palmitate on linseed oil during in vitro gastrointestinal digestion was tackled. The advance of lipid oxidation was studied through the generation of oxidation compounds, the degradation of polyunsaturated fatty acyl chains and of gamma-tocopherol, by employing Proton Nuclear Magnetic Resonance. Supplements containing exclusively l-ascorbic acid enhanced the advance of linseed oil oxidation during digestion. This was evidenced by increased formation of linolenic-derived conjugated hydroxy-dienes and alkanals and by the generation of conjugated keto-dienes and reactive alpha,beta-unsaturated aldehydes, such as 4,5-epoxy-2-alkenals; moreover, gamma-tocopherol was completely degraded. Conversely, supplements composed of mixtures of ascorbic acid/salt with citric acid and carotenes, and of ascorbyl palmitate, protected linseed oil against oxidation and reduced gamma-tocopherol degradation. The study through Solid Phase Microextraction-Gas Chromatography/Mass Spectrometry of the volatile compounds of the digests corroborated these findings. Furthermore, a decreased lipid bioaccessibility was noticed in the presence of the highest dose of l-ascorbic acid. Both the chemical form of Vitamin C and the presence of other ingredients in dietary supplements have shown to be of great relevance regarding oxidation and hydrolysis reactions occurring during lipid digestion.

This study evaluated the effect of adding a natural tocopherol extract rich in gamma-isoform (gTE) at 0.1 %, 0.5 %, and 3 % on the stability of sunflower oil when subjected to heating at frying temperatures in the absence of food (170 °C) and to accelerated storage (70 °C). For this purpose, degradation of linoleic acyl groups and formation of several oxidation products were monitored by ¹H NMR and other markers like viscosity and Total Polar Compounds (TPC) were also assessed. Under frying conditions, gTE did not improve oil stability, as linoleic acyl group degradation was enhanced in a dose-dependent manner. In enriched oils, increased viscosity and TPC formation were generally observed but unexpectedly total aldehyde formation was significantly reduced. Under accelerated storage, the behavior of gTE greatly differed, initially promoting the degradation of linoleic acyl groups for a short period of time, but significantly retarding it later. Regarding the underlying mechanism involved, gTE slowed the isomerization of conjugated hydroperoxy-cis,trans-octadecadienoates to trans,trans-isomers, significantly postponing the generation of aldehydes and the progression of polymerisation. Under both thermo-oxidative conditions, the fate of natural and added tocopherols was also studied by DI-SPME-GC/MS. This technique enabled the identification of several degradation compounds, either specific or common to all isoforms. For the first time, 3,4-dehydro-tocopherols were identified among degradation products formed from tocopherol in food matrices during processing. Their formation was notably favoured under frying conditions, whereas 4,8,12,16-tetramethylheptadecan-4-olide was the major product during accelerated storage.