Headspace oxygen techniques

Callus cultures were established with three olive (Olea europaea L.) varieties using a method reported previously (2). Volatiles were collected from the olive cultures (20g) using dynamic headspace sampling techniques described previously (3). Analysis of volatiles was performed using an Automatic Thermal Desorption System linked to a Perkin Elmer Autosystem GC. Lipoxygenase (LOX) activity of the cultures was assayed routinely using an oxygen electrode. 

Rancidity measurements are taken by determining the concentration of either the intermediate compounds, or the more stable end products. Peroxide values (PV), thiobarbituric acid (TBA) test, fatty acid analysis, GC volatile analysis, active oxygen method (AOM), and sensory analysis are just some of the methods currently used for this purpose. Peroxide values and TBA tests are two very common rancidity tests however, the actual point of rancidity is discretionary. Determinations based on intermediate compounds (PV) are limited because the same value can represent two different points on the rancidity curve, thus making interpretations difficult. For example, a low PV can represent a sample just starting to become rancid, as well as a sample that has developed an extreme rancid characteristic. The TBA test has similar limitations, in that TBA values are typically quadratic with increasing oxidation. Due to the stability of some of the end-products, headspace GC is a fast and reliable method for oxidation measurement. Headspace techniques include static, dynamic and solid-phase microextraction (SPME) methods. Hexanal, which is the end-product formed from the oxidation of Q-6 unsaturated fatty acids (linoleate), is often found to be a major compound in the volatile profile of food products, and is often chosen as an indicator of oxidation in meals, especially during the early oxidative changes (Shahidi, 1994). 

Fyhr et al. [201] reviewed several commercially available oxygen analyzers intended for the analysis of oxygen in the headspace of vials. However, preliminary validation revealed insufficient reproducibility and linearity. The authors developed headspace analysis systems. Sample volumes down to about 2.5 ml could be used without significant errors. Sample recovery was in the range 100-102%. It was necessary to measure the head-space pressure and volume in order to be able to present the assay in partial oxygen pressure or in millimoles of oxygen. Up to 40 vials per hour could be analyzed using this technique. 

The analysis of volatile sulphur compounds is difficult as additional compounds may be formed if the sample is heated or exposed to light or oxygen. Headspace analysis by gas-liquid chromatography using a flame photometric detector is the most satisfactory technique although solvent extraction may be necessary for the less volatile compounds. The sulphur compounds which have been identified in beer are listed in Table 22.19. 

Headspace sampling and GC have been used to quantitatively follow the thermal oxidation of a low molecular weight hydroxyl-terminated polybutadiene [26]. Rate studies using this simple, efficient technique revealed an induction period for the oxidation followed by self-catalysed oxidation. The rate for this latter step was quickly controlled by the diffusion of oxygen into the polymer. 

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