RAMAN APPLICATIONS IN FOOD ANALYSIS - Raman Spectroscopy for Soft Matter Applications - page 202

crystallinity of lactose did not perform as well as the corresponding NIR
models (RMSECV = 0.63% using six PLS factors; Nørgaard et al. [13]).

8.4.2 Edible Oils

Another important fi eld for the application of Raman spectroscopy to
foods is the edible oil area, in which the high sensitivity to the C

C
bond (Figure 8.3) early on proved useful in determining the total level
of unsaturation, the degree of conjugation, cis / trans isomer ratios, and
the total number of double bonds in the hydrocarbon chains [4,14].
Later, we investigated the deterioration of frying oil by various spec-
troscopic and chemical/physical techniques including FT-Raman spec-
troscopy [15]. Daily oil samples were collected during a 4- week period
of frying spring rolls, completing one round before the process was
renewed with fresh oil. The Raman spectra displayed an increasing
fl uorescence toward low wave numbers as a function of increasing
frying time. The Raman spectra included peaks at 1302 and 1440 cm − 1
due to polymethylene twisting and scissoring, respectively. Carbon-
carbon double-bond stretching was found as a sharp and relatively
strong peak at 1656 cm − 1 , characteristic of cis olefi ns (Figure 8.3 ; note
that conjugation lowers

=

C) by 20 - 30 cm − 1 ), and only a broad and
weak peak centered around 1748 cm − 1 corresponding to the ester C

σ (C =

= O

stretch. In the aliphatic C H stretching region, the two main peaks at

2852 and 2925 cm − 1 correspond to polymethylene symmetric and asym-
metric C

−

H stretchings, respectively, and a peak at 3009 cm − 1 is due to

−

H stretchings in cis confi gurations. The quantitative models
computed from the Raman spectra yielded good calibration models to
triglycerides, free fatty acids, and iodine value but were inferior to the
models obtained from IR and NIR, except for a model predicting
the age of the frying oil expressed in days. The good performance of
the latter Raman model was attributed to the increasing fl uorescence
in the signals at low wave numbers in the samples, originating from
complex fl uorescence phenomena in the frying oils correlated with
time [15] .

olefi nic = C −

8.4.3 Cyanogenic Compounds

The content of triple bonds in food and food ingredients has also been
the subject of numerous Raman studies. In one example, the content
and distribution of the cyanogenic glucoside amygdalin in bitter almond
cotylodons was investigated using the characteristic strong and narrow
Raman active peak from the cyanide functional group (C ≡ N; Figure