Conductivity, oxidation stability measurement

Li-peroxo transition metal complexes, 1060 Conductivity, oxidation stability measurement, 664... 

Si(Pc)0] (S04)o.09)n> i-s limited by the oxidative stability of the sulfate anion. Thermoelectric power, optical reflectivity, magnetic susceptibility, and four-probe electrical conductivity measurements evidence behavior typical of an [Si(PcP+)0]n compound where p 0.20. That is, there is no evidence that the more concentrated counterion charge has induced significant localization of the band structure. 

Rancimat is an accelerated method to assess oxidative stability of fats and oils. In this test, the sample is subjected to an accelerated oxidative process (by heat in presence of oxygen), where short-chain volatile acids are produced. The acids formed are measured by conductivity. 

A widespread method for determining the induction period for autoxidation of oils and fats consists of passing a continuous stream of air through the heated sample and collecting the volatile acids evolved in a water trap, where they are determined on a real time basis. The time plot usually presents a flat appearance for a certain period and then takes off in an accelerated manner. This test is the basis of several national and international standards (e.g. AOCS Cd 12b-92—oil stability index" ISO 6886—accelerated oxidation test for oxidative stability of fats and oils ) and the design of the Rancimat equipment, where the end determination is based on conductivity measurements . In addition to oxidation stability as determined by the Rancimat method and POV, which negatively affects virgin olive oil stability, other nonstandard properties were proposed for better assessment of the quality of this oil, namely LC determination of Vitamin E (21), colorimetric determination of total polar phenols and UVD of total chlorophyll. ... 

NOTE This method relies on either of two commercially available instruments that consist of a means of controlling sample temperature (maintained at 0.1 °C), an air distribution system, conductivity tubes, stoppers and probes, and electronics or software for measuring conductivity and determining induction period. The following steps are generally applicable to both the Rancimat and the Oxidative Stability Instrument however, some variations exist due to differences in hardware and models available. Consult the manufacturer s instructions for additional detail. 

The oil stability index (OSI) method provides results in a matter of hours instead of months (required for studies done at ambient temperatures). These OSI results are useful as comparative measures of oxidative stability, i.e., to determine the effect of a treatment or antioxidant compared to a control sample. Meaningful predictions of the actual shelf lives of specific commodities require that such shelf life studies be performed at ambient conditions. If only accelerated tests are to be performed, two or more tests based on different principles of lipid oxidation measurement should be conducted the effect of accelerated storage temperature should also be investigated. 

Oa StabUity Index. Two conductivity instruments, Rancimat and The Oxidative Stability Instmment, have been developed as alternatives to AOM Stability analysis. These instruments measure the increase in deionized water conductivity resulting from trapped volatile oxidation products produced when the oU product is heated under a stream of air. The conductivity increase is related to the oxidative stabihty of the products. These instruments provide a more reproducible measurement of oxidation stability with less technician time and attention. 

Fat oxidative stability is measured by the active oxygen method (AOM, AOCS Method Cd 12-57). Oil or fat is held at 97.8°C while air is bubbled through it. The time required to develop a peroxide concentration of 100 meq/kg is the AOM stability of the sample. A closely related method, the oil stability index (OSI, AOCS Method Cd 12b-92), also bubbles air through hot oil. One of the breakdown products is formic acid, which is trapped in a water cell. The machine continuously monitors conductivity of the water, and records the time when it rises sharply. Rancimat times obtained at 110°C are 40-45% of the AOM times, so an OSI stability of 4 h is equal to an AOM stabihty of 10 h. 

The active oxygen method (AOM) is the most common analytical method used to measure oxidative stability of fats and oils products. AOM employs heat and aeration to accelerate oxidation of the oil by continuously bubbling air through a heated sample. Periodic peroxide values are measured to determine the time required for the oil to oxidize to a predetermined peroxide value under the AOM conditions. This method requires close attention to detail to produce reproducible results and even then the variation between laboratories is 25 for a 100 h AOM sample. Conductivity instruments such as the Rancimat and the Oxidative Stability Instrument have been developed as alternatives to AOM stability analysis. These instruments measure the increase in the conductivity of deionized water resulting from trapped volatile oxidation products produced when the oil product is heated under a stream of air. The conductivity increase is related to the oxidative stability of the products. These instruments provide a more reproducible measurement of oxidation stability with less technician time and attention. 

The Oil Stability Index (OSI) is another method to measure oil stability that can be conducted using AOCS Method Cd 12b-92 (AOCS, 2005) with a Rancimat instrument or an Oxidative Stability Instrument. The OSI may be run at temperatures of 100, 100, 120, 130, and 140°C. Although oil processors and food manufacturers are interested in rapid measurements of oxidation, the high temperature at which the procedure is conducted may not be relevant to ambient temperatures used for most oil storage. Frankel (1993) suggested that the variation in results at 110°C with the rapid analysis and ambient temperature storage may be because of differences in the oxidation mechanisms at the two temperatures. 

Depending on the intended end use of the oil, oxidation stability will be measured or expressed in different ways. All of the oxidation stability tests are based on placing a sample of oil under conditions that will greatly increase the rate of oxidation. Buildups of reaction products are then measured. The American Society for Testing and Materials (ASTM) D-943 test is the most widely used. Conducted under prescribed conditions, it measures the time (in hours) for the acidity of a sample of oil to increase a specified amount. The more stable the oil, the longer it will take for the change in acidity to occur. 

Matthaus, B. W. 1996. Determination of the oxidative stability of vegetable oils by Rancimat and conductivity and chemiluminescence measurements. J. Am. Oil Chem. Soc. 73 1039-1043. 

Among the most common methods to measure thermal and oxidative behavior of oils are the classical oxidative stability analysis used by industries, based on the active oxygen method (AOM), which determines the number of hours required for the oil to reach a peroxide value of 100 meq/kg O and the oxidative stability index (OSI), which can be considered as automated AOM with an apparatus that simulates the events under specific atmospheres, usually with the use of high temperatures. The OSI method measures the changes in water conductivity when the oxidation compounds are formed [17]. 

The oxidative stability of oils and fats is an important parameter for the analysis of their quality. It is performed using the Rancimat test equipment or the Omnlon Instrument to measure the changes in the electrical conductivity in water caused by the formation and release of volatile oxidation compounds [13]. 

The most common method used to determine the oxidative stability of fatty compounds is Rancimat (EN14214) also called OSl (Oxidation Stability Index). This test method consists in bubbling air through a sample maintained at llO C. This air is then passed through deionized water while the conductivity is measured. The sudden increase in water conductivity is due to soluble secondary oxidation products (volatile organic acids). The time at which the conductivity drastically increases is the induction time. 

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