ABSTRACT:—Evaluation of the oxidative stability of food lipids
based on the tendency of formation of radicals is shown to be
possible using electron spin resonance (ESR) spectroscopy and
the spin-trapping technique. Induction time can be determined
for mildly accelerated conditions (50°C for lipid fraction from
mayonnaise enriched with fish oil), and the length of the induc-
tion time decreases during storage and γ-tocopherol depletion.
The protection by ethylenediaminetetraacetic acid against initi-
ation of lipid oxidation is also detected in the new assay. For
more oxidatively stable lipids (butter, rapeseed oil, dairy spread)
the mildly accelerated conditions can be used in the assay, pro-
vided that difference in signal height for fixed times replaces de-
termination of induction time. ESR spin trapping provides a sen-
sitive method for evaluating the oxidative stability of food lipids.
Detection of radicals in the lipid as an early event in oxidation
allows mild conditions to be used, and future experiments
should also include sensory evaluation in relation to determina-
tion of practical shelf life.
Paper no. J9359 in JAOCS 77, 725–730 (July 2000).
KEY WORDS:—Electron spin resonance, ESR, food lipids, lipid
oxidation, peroxide value, radicals, spin trapping.
Especially in highly processed foods, lipid oxidation is one of
the major deteriorative reactions, resulting in undesirable fla-
vors and formation of toxic oxidation products and further af-
fecting the nutritional value negatively (1). Accordingly it is im-
portant to predict the oxidative stability of a given food by rapid
and reliable methods in order to determine shelf life and to eval-
uate the effect of protective antioxidants. Stability testing at am-
bient storage conditions is often too slow for practical use in
quality control, and consequently the oxidative stability is tested
at forced conditions using high temperature or addition of
prooxidants. Several accelerated methods are routinely used to
characterize the stability of food lipids such as the Schaal oven
test, the Rancimat method, and the active oxygen (AO) method.
However, these methods are often criticized for giving wrong
predictions (1). The Rancimat method involves heating of the
sample to a high temperature (100–140°C) and measuring the
levels of volatile acids produced by oxidation. The mechanism
of oxidation changes significantly at elevated temperatures due
to large differences in activation energies for various reactions,
making comparison to oxidation at normal storage conditions
unreliable (1). Furthermore, levels of volatile acids are only sig-
nificant at elevated temperatures and may not be relevant to nor-
mal storage conditions. The Schaal oven test involves a sensory
panel, which periodically examines the sample, heated to
50–60°C, until a definite rancid flavor develops. It requires min-
imal laboratory equipment, but large samples and a trained sen-
sory panel, and furthermore it takes a relatively long time (4–8
d) to complete each evaluation (2).
Electron spin resonance (ESR) spectroscopy is a new tech-
nique in food science but has already proven to be a valuable
tool in the study of the very early stages of oxidation in differ-
ent kinds of products. In beer a rapid method for prediction of
the flavor stability using ESR spectroscopy has been developed
based upon the spin-trapping technique (3,4), and for whole
milk powder (5) and cheese (6) direct ESR measurement seems
promising for prediction of the oxidative stability during subse-
quent storage. Thermal decomposition of grape seed oil has also
been studied using the spin-trapping method (7). For food emul-
sions like mayonnaise, it became possible to quantify the rate of
production of radical precursors for hydroperoxides and sec-
ondary lipid oxidation products (8). In contrast to most other
methods used in food science, ESR spectroscopy provides the
possibility of studying the very early stages of lipid oxidation.
At this stage the sensory attributes of foods are still unchanged
and by evaluating the oxidative status of a given food at this
early stage, action can be taken to change process parameters
before the consumers experience a deteriorated product.
The objective of this study was to evaluate whether it is
possible to develop a rapid test in which the resistance to oxi-
dation can be expressed as an induction time for formation of
radicals using only mildly accelerated conditions with a sig-
nificantly lower temperature compared to the existing alter-
natives for determination of the oxidative stability of pure
food lipids or foods containing high amounts of fat. Rapeseed
oil, oil separated from mayonnaise containing fish oil, and the
lipid fraction separated from butter, and dairy spread as four
*To whom correspondence should be addressed at Food Chemistry, Depart-
ment of Dairy and Food Science, The Royal Veterinary and Agricultural Uni-
versity, Rolighedsvej 30, DK-1958 Frederiksberg C, Denmark.
E-mail: ls@kvl.dk.
Electron Spin Resonance Spectroscopy
for Determination of the Oxidative
Stability of Food Lipids
Marianne K. Thomsen, Dorthe Kristensen, and Leif H. Skibsted*
Food Chemistry, Department of Dairy and Food Science, The Royal Veterinary
and Agricultural University, DK-1958 Frederiksberg C, Denmark
Copyright © 2000 by AOCS Press 725 JAOCS, Vol. 77, no. 7 (2000)