Measurement of Lipid Oxidation

Measurement of free radicals

Free radicals have a very short half-life, which makes them very hard to measure

Direct measurement:Electron spin resonance: detects the presence of unpaired

electrons

Indirect measurement:Spin Trapping: allow highly reactive radical to react with

exogenous compounds to produce a long-lived radical: Poor sensitivity

Measurement of markers of free radicals

Conjugated DienesHydroperoxidesTBARS method

spectrophotometric: lack of specificity, sensitivity, and reproducibilityHPLC method

Gas compounds

Conjugated Diene

Almost immediately after peroxides are formed, the non- conjugated double bonds that are present in natural unsaturated lipids are converted to conjugated double bonds.

Conjugated double bonds (conjugated dienes) absorb ultraviolet light strongly at 233 nm.

In the later stages of lipid oxidation the conjugated dienes (primary products) are broken down into secondary products (which do not absorb UV-visible light strongly) which leads to a decrease in absorbance

Peroxide value

Peroxides are the main initial products of autoxidation. Can be measured by techniques based on their ability to liberate iodine

from potassium iodide, or to oxidize ferrous to ferric ions. Their content is usually expressed in terms of milli equivalents of

oxygen per kilogram of fat. Although the peroxide value is applicable for following peroxide

formation at the early stages of oxidation, it is, nevertheless, highly empirical.

The accuracy is questionable, the results vary with details of the procedure used, and the test is extremely sensitive to temperature changes.

During the course of oxidation, peroxide values reach a peak and then decline.

TBA assay

TBA is the most widely used test for measuring the extent of lipid peroxidation in foods due to its simplicity and because its results are highly correlated with sensory evaluation scores.

The basic principle of the method is the reaction of one molecule of malonaldehyde and two molecules of TBA to form a red malonaldehyde-TBA complex, which can be quantitated spectrophotometrically (532 nm).

This method has been criticized as being nonspecific and insensitive for the detection of low levels of malonaldehyde.

Other TBA-reactive substances (TBARS) including sugars and other aldehydes could interfere with the malonaldehyde-TBA reaction.

Abnormally low values may result if some of the malonaldehyde reacts with proteins in an oxidizing system.

In many cases, however, the TBA test is applicable for comparing samples of a single material at different states of oxidation.

TBA analysis

Iodine value

Iodine value is a measure of the unsaturated linkages in fat and is expressed in terms of percentage of iodine absorbed.

The decline in iodine value is sometimes used to monitor the reduction of dienoic acids during the course of the autoxidation.

Volatiles

Measurements of various secondary volatile by-products of lipid oxidation such as aldehydes, hydrocarbons, alcohols, ketones

Solatek 72 vial autosampler/Purge & Trap concentrator/ GC/MS

Static headspace analysis at 40 CSPME (Solid phase microextraction)

Volatiles Analyses – Dynamic headspace GC/MS

Solatek 72 vial autosampler / Purge & Trap concentrator / GC / MS

Dynamic headspace analysis at 40C