AL1503
OLIVE OIL ADULTERATION continued
The most expedient way to distinguish EVOO and soybean oil is the bis-allylic region. EVOO is composed primarily of monounsaturated fatty acids and the polyunsaturated, bis-allylic-containing fatty acids repre- sent only a small component of EVOO. In contrast, soybean oil comprises 50–60% polyunsaturated fatty acids, with linoleic acid (ω-6) represent- ing the major component. As a result, the bis-allylic region in pure soybean oil, or soybean-adulterated EVOO, is much more intense than genuinely pure EVOO samples. Determining the absolute integration of all components relative to the amount of bis-allylic protons provides a direct indication of adulteration; larger-than-expected composition signifies the presence of polyunsaturated fatty acids.
Alternatively, we can observe and quantify the amount of adulteration. Stock solutions of EVOO and soybean oil were prepared (~0.5 M in CDCl3
)
and used to make EVOO samples doped at 5, 10, 20, 25, 30, 35, 40, 45, 50 and 60 (vol/vol)% soybean oil. Three sets of stock solutions were prepared for analysis (these will be referred to as ”trials” throughout the text). An NMReady benchtop spectrometer (Nanalysis Corp., Calgary, Alb.) was used to acquire 60-MHz 1
H NMR spectra in triplicate for all
samples to ensure measurement repeatability. Spectra were measured with eight scans at a 12-ppm spectral window, 2k points, 0.1-Hz apodiza- tion and 5-second relaxation delay.
(δ = 7.24 ppm). A stacked plot example is shown in Figure 3a. Once referenced, the spectra were integrated with the predefined regions.
Using the MNova spectral processing program (Mestrelab Research, Escondido, Calif.), the same phase and baseline corrections were ap- plied to each spectrum. The spectra were then δ-referenced to residual CHCl3
The percentage of bis-allylic protons was calculated for each spectrum using the absolute integration of the bis-allylic region relative to the total sample absolute integration. For each trial, the values measured in triplicate were averaged. These three averages, along with the overall average of all data obtained, were plotted versus the known amount of soybean oil adulteration to establish accuracy and linearity of the method (Figure 3b).
a b
Figure 3 – a) Stacked 60-MHz 1
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H NMR spectra of olive oil samples manu-
ally adulterated with soybean oil. b) Amount of soybean oil adultera- tion as determined by the percentage of bis-allylic protons in each EVOO sample.
AMERICAN LABORATORY • 18 • MARCH 2015
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