Food Lipids: Chemistry, Nutrition, and Biotechnology

ated fatty acids during oxidation (11). Therefore, oxidative changes in marine oils
and highly unsaturated vegetable oils may be monitored using this indicator. Similarly,
changes in iodine value due to loss of unsaturation during accelerated oxidation
studies may be used as an index of lipid oxidation [12].
2. Weight Gain
It is generally accepted that addition of oxygen to lipids and formation of hydroperoxides
is reasonably quantitative during initial stages of autoxidation. Therefore,
the measurement of induction period from weight gain data is theoretically sound.
In this method, oil samples (about 2.0 g) are weighed into Petri dishes; then traces
of water are removed by placing the samples overnight in a vacuum oven at 35�C
and over a desiccant. Samples are then reweighed and stored in an oven at a set
temperature. The weight gain of the samples may be recorded at different time
intervals.
Olcott and Einset [13] reported that marine oils exhibit a fairly sharp increase
in their weight at the end of the induction period and are rancid by the time they
gain 0.3–0.5% in weight (at 30–60�C). Ke and Ackman [14] reported that this
method is simple, has a satisfactory reproducibility, and may be used to compare
oxidation of lipids from different parts of fish. Recently, Wanasundara and Shahidi
[15,16] used this method to compare storage stability of vegetable (Fig. 1) and marine
oils as affected by added antioxidants and were able to compare relative activity of
antioxidants employed. However, surface exposure of the sample to air is an impor-
Figure 1 Effect of canola extracts (CEs) and commercial antioxidants on the weight gain
of canola oil stored at 65�C. BHA, butylated hydroxyanisole; BHT, butylated hydroxytoluene;
MGC, monoacylglycerol citrate; TBHQ, tert-butylhydroquinone.
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