Preučevanje oksidacije oljčnega in arašidovega olja pri segrevanju z metodami termične analize

Simonič, Nika (author), Cerc Korošec, Romana (mentor)

V magistrski nalogi sem raziskovala termično stabilnost oljčnega in arašidovega olja med segrevanjem v pretoku zraka oziroma dušika. Olja so v glavnem sestavljena iz lipidov trigliceridov, ki so estri glicerola in treh maščobnih kislin. Od vrste in deleža maščobnih kislin v trigliceridih so odvisne fizikalne in kemijske lastnosti olja. Termično stabilnost sem proučevala s pomočjo TGA in DSC analize; meritve sem izvajala na simultanem TGA–DSC instrumentu, ki hkrati meri maso vzorca in toplotni tok. Primerjala sem TGA krivulje, dobljene pri štirih različnih hitrostih segrevanja – 5 K/min, 10 K/min, 20 K/min in 50 K/min. V zračni atmosferi je termičen razpad potekal večstopenjsko; v prvih stopnjah razpadajo spojine, ki se lažje oksidirajo, v zadnji stopnji pa tiste, ki so bolj odporne na oksidacijo. Z večanjem hitrosti segrevanja se zaradi toplotnega gradienta v peči krivulje pomaknejo k višjim temperaturam, v zraku pa je pri oljčnem olju pri 50 K/min, pri arašidovem olju pa pri 20 K/min in 50 K/min, prišlo tudi do prekrivanja prvih dveh stopenj termičnega razpada. V dušikovi atmosferi poteče popoln razpad enostopenjsko pri vseh hitrostih, temperatura začetka razpada je višja kot v zračni atmosferi. V magistrski nalogi so predstavljene tudi DIFTG krivulje, ki predstavljajo razliko med TGA krivuljami v zraku in dušiku. Arašidovo in oljčno olje sta si po sestavi podobna, zato je podoben tudi potek njunih TGA–DSC krivulj. Iz primerjave poteka termičnega razpada obeh olj v pretoku zraka sem pri segrevanju s hitrostjo 5 K/min opazila, da arašidovo olje počasneje termično razpada. Vzrok za to je v večjem deležu večkrat nenasičenih maščobnih kislin, ki se med razpadom oksidirajo, produkti oksidacije pa nekaj časa ostanejo raztopljeni v tekoči fazi. S pomočjo DSC meritve sem določila čas inducirane oksidacije (OIT) pri 160 °C, ki znaša 18,6 min za oljčno olje in 17,3 min za arašidovo olje. Določila sem tudi jodovo število (IV), ki je merilo za stopnjo nenasičenosti olja in predstavlja maso joda v gramih, ki se veže na 100 g olja. Vrednosti IV, določene v magistrski nalogi, znašajo 88,8 ± 0,3 za oljčno olje in 88,6 ± 0,6 za arašidovo olje in se ujemajo s podatki iz literature.

• Language: Slovenian
• Keywords: termična analiza, termična obstojnost, olja in maščobe, oljčno olje, arašidovo olje, OIT (čas inducirane oksidacije), jodovo število
• Work type: Master's thesis/paper
• Typology: 2.09 - Master's Thesis
• Organization: FKKT - Faculty of Chemistry and Chemical Technology
• Year: 2019
• PID: 20.500.12556/RUL-111963
• COBISS.SI-ID: 1538480323
• Publication date in RUL: 17.10.2019

Secondary language

• Language: English
• Title: Study of the oxidation of olive and peanut oil during heating using thermoanalytical methods

Abstract

This master's thesis presents a study of the thermal stability of olive and peanut oil, obtained during heating under air and nitrogen atmosphere. Oils are mainly composed of triglycerides, esters of glycerol and three fatty acids. Physical and chemical properties of oil depend on the fatty acid composition. Thermal stability of the oils was studied with the help of thermogravimetry (TGA) and differential scanning calorimetry (DSC). Measurements were carried out on simultaneous TGA–DSC instrument, which measures the mass of the sample and the heat flow at the same time. A comparison between TGA curves measured at different heating rates – 5 K/min, 10 K/min, 20 K/min and 50 K/min, is described. Oxidative decomposition took place in several steps; compounds which oxidize more easily, decomposed in the first two steps while in the last step thermal decomposition of the species which are not prone to oxidation, takes place. Higher heating rates lead to the shift of TGA curves to higher temperatures. In air, olive oil at 50 K/min and in peanut oil at 20 K/min at 50 K/min, decompose in only two steps. The TGA of olive and peanut oil in nitrogen shows that thermal decomposition took place in a single step. In addition, DIFTG curves, which represent the difference between TGA curves obtained in air and nitrogen atmosphere, are shown. Peanut and olive oil have a similar composition and therefore a similar profile of thermal decomposition. The comparison of the thermal decomposition of both oils in air and nitrogen atmosphere at 5 K/min shows slower decomposition of the peanut oil sample, because peanut oil sample contains more polyunsaturated fatty acids, which oxidize in the first step of thermal decomposition. The products of this oxidation dissolve in the liquid phase and decompose later. Using DSC, the induced oxidation time (OIT) was determined at 160 °C to 18.6 min and 17.3 min for olive oil and peanut oil, respectively. Iodine value (IV) was also determined, which refers to the mass of iodine in grams absorbed by 100 grams of oil. The IV values obtained in this thesis are 88.8 ± 0.3 and 88.6 ± 0.6 for olive and peanut oil, respectively and correspond to the literature data.

• Keywords: thermal analysis, thermal stability, fats and oils, olive oil, peanut oil, OIT (oxidation induction time), iodine value