The Characterization of Olive Oils by DSC

Can olive oils be used for frying? What are the optimal storage conditions for these products? These questions are discussed in the following article based on the results from different DSC experiments. OIT and OOT measurements were performed to characterize oxidation stability. Crystallization and melting behavior was also investigated.

Introduction

The advantage of OIT is that it offers better reproducibility than OOT. Furthermore, the values obtained at particular temperatures can be more meaningfully compared. High-pressure DSC can be used to increase the oxygen concentration. This allows reactions to be performed more rapidly at lower temperatures. OIT and OOT measurements can however be quite easily performed with standard DSC instruments.

Oxidation stability is an important criterion for assessing the shelf life and quality of different products. Oxidation stability can be characterized by the OIT (Oxidation Induction Time) [1] or the OOT (Oxidation Onset Temperature) [2]. The OIT of a material is measured in an oxygen atmosphere at a particular isothermal temperature. It is the time from when the material is first exposed to oxygen up until the onset of oxidation. In contrast, OOT experiments can be performed more rapidly. The OOT of a material is measured in a dynamic measurement in oxygen. The OOT is defined as the temperature at which oxidation of the material begins, i.e. the onset temperature of oxidation. The advantage of OIT is that it offers better reproducibility than OOT. Furthermore, the values obtained at particular temperatures can be more meaningfully compared. High-pressure DSC can be used to increase the oxygen concentration. This allows reactions to be performed more rapidly at lower temperatures. OIT and OOT measurements can however be quite easily performed with standard DSC instruments.

Experimental Details

Three different olive oils (Extra Virgin) were investigated. The OIT and OOT measurements were performed using a METTLER TOLEDO STARe System consisting of a DSC 1 with gas controller and air cooling. Samples of 0.5 to 3 mg were measured in 40-µL aluminum crucibles without a lid.

The program used for the OIT measurements was as follows: Heating from 30 °C to the isothermal measurement temperature at 40 K/min with a purge gas flow of 50 mL/min nitrogen; isothermal for 5 min at the measurement temperature with a gas flow of 50 mL/min nitrogen; then switching to oxygen to enable the oxidation reaction. The gas was automatically switched by the gas controller. The sample was held at the isothermal temperature until the oxidation reaction began.

The OOT measurements were performed by heating the samples from 30 to 300 °C at 5 K/min using oxygen as purge gas. The crystallization and melting experiments were performed using a DSC 1 equipped with an intracooler. Samples of 2 to 3 mg were sealed in 40-µL aluminum crucibles. These were cooled from 30 °C to −75 °C at 5 K/min and then heated from −75 °C to 150 °C at 5 K/min.

Results

OIT/OOT

Figure 1 shows the results of the OOT measurements of the three olive oils. The OOT temperatures lie between 171 and 185 °C. Although the oils belong to the same quality class, their stability is clearly different. Based on these results, an isothermal temperature of 140 °C was chosen for the OIT measurements.

_{The Characterization of Olive Oils by DSC | Thermal Analysis Application No. UC 282 | Application published in METTLER TOLEDO Thermal Analysis UserCom 28}