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 Dienes
Hydroperoxides
TBARS method
spectrophotometric: lack of specificity, sensitivity, and
reproducibility
HPLC method
Gas compounds
Conjugated Diene
Almost immediately after peroxides are formed, the nonconjugated 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 C
SPME (Solid phase microextraction)
Volatiles Analyses – Dynamic headspace GC/MS
Solatek 72 vial autosampler / Purge & Trap concentrator / GC / MS
Dynamic headspace analysis at 40C