EVALUATION OF NEW THERMO-OXIDATION MARKERS FOR OILS SUBMITTED TO PROLONGED AND DISCONTINUOUS FRYING.
GIORDANO, ANELLA (2011) EVALUATION OF NEW THERMO-OXIDATION MARKERS FOR OILS SUBMITTED TO PROLONGED AND DISCONTINUOUS FRYING. [Tesi di dottorato] (Inedito)
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Deep-fat frying is an important, common and highly versatile process, which has been used from antiquity to cook a wide variety of products. During deep-fat frying, hydrolysis, oxidation and polymerization reactions cause a spectrum of physical and chemical changes, leading to the formation of decomposition products posing a direct impact damaging both the oil quality and the fried food nutritional value. The amount and type of degradation products formed in frying oils is primarily dependent on the fatty acid composition of the frying oil, so it is important to keep this in mind in selecting frying oils. Therefore, the choice of medium frying remains of great interest and of crucial importance, not only for its potential nutritional value but also for its ability to withstand the drastic conditions of this cooking technique. The objective of this study was to evaluate the effects of discontinuous and prolonged heat treatment on five oils, characterized by different ratios of unsaturated/saturated fatty acids. In addition, we attempted to identify possible markers of heat treatment correlated with total polar compounds (TPC) in assessing the quality of used frying oils. The determination of TPC has been adopted as a unique official method to test for excessive use of fats for frying. But because there are many variables that affect oil degradation, a specific marker may be ideal for one oil but completely useless in another. In fact, the work wanted to show that it is not correct to use the same indicator for all fatty matrices but, on the basis of the fatty acid composition of the oil or fat used in frying, the indicator that best suits it. The experimentation was carried out by a prolonged and discontinuous deep-fat frying process with frozen pre-fried potatoes (McCain) and by a simple thermo-oxidation process of oil without food matrix. Both thermal treatments were conducted over a period of 8h per day for 6 days. Every hour, 200 g of frozen pre-fried potatoes were deep fried for 4min (based on preliminary trials). The batch volume of deep fat fryer was 3L. Fresh oil was added every 8h after oil samples were collected, to maintain a constant level in the fryer. The oils used were: bi-fractionated palm oil, olive oil, sunflower oil, lard, and a mix oil. After frying process, the French fries were subjected to fat extraction. Three sets of samples were collected: • thermo-oxidized oil samples • frying oil samples • fat samples extracted from French fries So, the samples analyzed correspond to following treatment hours: 8, 16, 24, 32, 40, 48 and the oil and French fries samples of time “0”. All samples were subjected to the following determinations: • free fatty acids (FFA) determination by alkaline titration; • peroxide value (PV) determination by iodometric titration; • TPC determination; • fatty acids (FA) composition determination by gas-chromatographic (GC) analysis of fatty acid methyl esters (FAME); • volatile organic compounds (VOC) analysis by dynamic head-space system (DHS) coupled with GC and mass spectrometer (MS). • Statistical analysis. About performances of three sets of samples (thermo-oxidized, frying and extracted from potatoes), about FFA values, the best fat frying has proved to be the sunflower oil. About PV, during thermo-oxidation process the mix oil, followed by bi-fractionated palm oil, olive oil and sunflower oil, showed the best results while the lard showed the highest values. During frying process the worst oil appeared to be the mix oil and about fats extracted from French fries the worst oil appeared to be the sunflower oil. The sunflower oil appeared to be the best during frying, while about extraction from potatoes, lard was showed to have the lowest values. Considering the performance recorded in three sets of samples, about TPC determination, the best fat turned out to be the frying lard. About changes of fatty acids occurred during the heat treatment possible markers of oil degradation were: • For the bi-fractionated palm oil C8:0 and Σtrans • For the olive oil UFA/SFA • For the sunflower oil C18:2 n6cis, C18:2 n6cis/C16:0 and UFA/SFA • For lard UFA/SFA • For the mix oil C8:0 Among VOCs, possible markers could be the Σalkanes for bi-fractionated palm oil, the Σalkadienals for olive oil samples, the octanal and butyl pyrrole for sunflower oil and 2-E-nonenal, 2-heptanone and Σketones for mix oil. The highly variable pattern of VOCs in lard is not allowed to get any good correlation. In conclusion a specific marker may be ideal for one oil but completely useless in another.
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