Lipid oxidation analysis in food samples is a relevant topic since compounds generated in the process are related to undesirable sensory and biological effects. As the process is complex and depends on the type of lipid substrate, oxidation agents and environmental factors, proper measurement of lipid oxidation remains a challenging task. A great variety of methodologies have been developed and implemented so far, both for determining primary oxidation products and secondary oxidation products. Most common methods and classical procedures are described, including peroxide value, TBARS analysis and chromatography. Some other methodologies such as chemiluminescence, fluorescence emission, Raman spectroscopy, infrared spectroscopy or magnetic resonance, provide interesting and promising results, so attention must be paid to these alternative techniques in the area of food lipid oxidation analysis.

The stability of the saponifiable and unsaponifiable fractions of avocado oil, under a drastic heating treatment, was studied and compared to that of olive oil. Avocado and olive oil were characterised and compared at time 0h and after different times of heating process (180°C). PUFA/SFA (0.61 at t=0) and ω-6/ω-3 (14.05 at t=0) were higher in avocado oil than in olive oil during the whole experiment. Avocado oil was richer than olive oil in total phytosterols at time 0h (339.64; 228.27mg/100g) and at 9h (270.44; 210.30mg/100g) of heating. TBARs was higher in olive oil after 3h, reaching the maximum values in both oils at 6h of heating treatment. Vitamin E was higher in olive oil (35.52 vs. 24.5mg/100g) and it disappeared earlier in avocado oil (at 4 vs. 5h). The stability of avocado oil was similar to that of olive oil.

2-The presence and unsaturation degree of the lipid matrix delayed the COPs formation 3-Significantly higher PV was found when cholesterol was heated in absence of TAG.4-Both cholesterol and triacylglycerol degradation fit a first order kinetic model.3

Cholesterol and phytosterols can suffer oxidation under heating conditions to give Sterol Oxidation Products (SOPs), known by their toxic effects. This paper studied the degradation of cholesterol and three plant sterols during a 360 min heating treatment (180ºC). The formation and further degradation of SOPs was also analyzed by GC-MS.Results revealed a sterol susceptibility to degradation according to the following decreasing order: campesterol ≈ β-sitosterol ≥ stigmasterol > cholesterol. Their degradation curve fit (R 2 = 0.907 -0.979) a logarithmic model. Sterol Oxidation Products increased their concentration during the first 5-10 min and thereafter, their degradation rate was higher than their formation rate, resulting in a decrease over time. Irrespective of the sterol from which they had derived, 7-keto derivatives presented the highest levels throughout the entire process, and also SOPs with the same type of oxidation followed a similar degradation pattern (R = 0.90-0.99).

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Highlights1-Heating sterols at 180 °C produced a high extent of oxidation.2-The presence and unsaturation degree of the surrounding lipids protected sterols from oxidation.3-Plant aqueous extracts protected cholesterol from oxidation.4-Greater amounts of campesterol-derived oxidation products were observed, followed by sitosterol, cholesterol and stigmasterol.5-In 5 out of 7 of our studies, 7-keto derivatives were the most abundant oxysterols formed. AbstractDietary sterols are nutritionally interesting compounds which can suffer oxidation reactions. In the case of plant sterols, they are being widely used for food enrichment due to their hypocholesterolemic properties. Besides, cholesterol and plant sterols oxidation products are associated with the development of cardiovascular and neurodegenerative diseases, among others. Therefore, the evaluation of the particular factors affecting sterol degradation and oxysterols formation in foods is of major importance. The present work summarizes the main results obtained in experiments which aimed to study four aspects in this context: the effect of the heating treatment, the unsaturation degree of the surrounding lipids, the presence of antioxidants on sterols degradation, and at last, oxides formation. The use of model systems allowed the isolation of some of these effects resulting in more accurate data. Thus, these results could be applied in real conditions.

The characteristics of the lipid matrix surrounding sterols exert a great influence in their thermal oxidation process. The objective of this work was to assess the oxidation susceptibility of equal amounts of cholesterol and stigmasterol within a sunflower oil lipid matrix (ratio 1:1:200) during heating (180 ºC, 0 to 180 min). Remaining percentage of sterols was determined and seven sterol oxidation products (SOPs) were analyzed for each type of sterol along the heating treatment. Evolution of the fatty acid profile and vitamin E content of the oil was also studied. Overall oxidation status of the model system was assessed by means of Peroxide Value (PV) and TBARS. PV remained constant from 30 min onwards and TBARS continued increasing along the whole heating treatment.Degradation of both cholesterol and stigmasterol fitted a first order curve (R 2 = 0.937 and 0.883, respectively), with very similar degradation constants (0.004 min -1 and 0.005 min -1 , respectively). However, higher concentrations of oxidation products were found from cholesterol (79 µg/mg) than from stigmasterol (53 µg/mg) at the end of the heating treatment. Profile of individual oxidation products was similar for both sterols, except for the fact that no 25-hydroxystigmasterol was detected. 7α-hydroxy and 7-ketoderivatives were the most abundant SOPs at the end of the treatment. PUFA and vitamin E suffered a significant degradation along the process, which was correlated to sterols oxidation.

Cholesterol oxidation products (COPs) constitute a known health risk factor. The antioxidant effect of a lyophilized aqueous Melissa officinalis extract against cholesterol degradation and COPs formation during a heating treatment was evaluated in a model system (180°C, 0-180 min) at a ratio 2 mg extract / 100 mg cholesterol. Furthermore, the plant extract was subsequently added to beef patties alone or incorporated within an oilin-water olive oil emulsion to assess its effectiveness during cooking. Melisa extract protected cholesterol from thermal degradation in the model system, yielding higher remaining cholesterol and lower COPs values throughout the whole heating process. Maximum total COPs were achieved after 30 and 120 min heating for control and melisacontaining samples, respectively. In cooked beef patties, even though the olive oil emulsion was used as flavour-masking approach, melisa extract off-flavour limited the maximum dose which could be added. At these doses (65 µg/g and 150 µg/g without and with the emulsion, respectively), no additional protective effect of melisa over the use of the emulsion was found. Addition of natural extracts into functional foods should definitively take into account sensory aspects.