Potential of Near-Infrared Fourier Transform Raman Spectroscopy in Food Analysis

Abstract

The 1064-nm excited Fourier transform (FT) Raman spectra have been measured in situ for various foods in order to investigate the potential of near-infrared (NIR) FT-Raman spectroscopy in food analysis. It is demonstrated here that NIR FT-Raman spectroscopy is a very powerful technique for (1) detecting selectively the trace components in foodstuffs, (2) estimating the degree of unsaturation of fatty acids included in foods, (3) investigating the structure of food components, and (4) monitoring changes in the quality of foods. Carotenoids included in foods give two intense bands near 1530 and 1160 cm−1 via the pre-resonance Raman effect in the NIR FT-Raman spectra, and therefore, the NIR FT-Raman technique can be employed to detect them nondestructively. Foods consisting largely of lipids such as oils, tallow, and butter show bands near 1658 and 1443 cm−1 due to C=C stretching modes of cis unsaturated fatty acid parts and CH2 scissoring modes of saturated fatty acid parts, respectively. It has been found that there is a linear correlation for various kinds of lipid-containing foods between the iodine value (number) and the intensity ratio of two bands at 1658 and 1443 cm−1 ( I1658/ I1443), indicating that the ratio can be used as a practical indicator for estimating the unsaturation level of a wide range of lipid-containing foods. A comparison of the Raman spectra of raw and boiled egg white shows that the amide I band shifts from 1666 to 1677 cm−1 and the intensity of the amide III band at 1275 cm−1 decreases upon boiling. These observations indicate that most α-helix structure changes into unordered structure in the proteins constituting egg white upon boiling. The NIR FT-Raman spectrum of old-leaf (about one year old) Japanese tea has been compared with that of its new leaf. The intensity ratio of two bands at 1529 and 1446 cm−1 ( I1529/ I1446), assignable to carotenoid and proteins, respectively, is considerably smaller in the former than in the latter, indicating that the ratio is useful for monitoring the changes in the quality of Japanese tea.

Non-destructive descriptions of carotenoids in feathers using Raman spectroscopy

“…Feathers were probed with a 780 nm 150 mW diode laser, through a 50Â Mplan apochromatic objective lens (Olympus, Melville, NY, USA) and 100 mm pinhole aperture (BX51 confocal microscope, Olympus, Melville, NY, USA). Carotenoids have a very broad pre-resonance range, 16,17 and hence the spectra collected with the 780 nm excitation wavelength were analytically useful, and were comparable to spectra collected with 532 nm excitation (532 nm spectra not shown). The green wavelength may be more sensitive to uorescent impurities (i.e.…”

Potential of NIR‐FT‐Raman spectroscopy in natural carotenoid analysis

“…12 Our experiment revealed that this relationship also occurs for carotenoids (i.e., crocetin, ␤-carotene, and lycopene) measured in situ by FT-Raman spectroscopy. The observed 1 shift toward red is correlated with an increasing number of conjugated double bonds in the carotenoid chain, i.e., 1536 cm Ϫ1 (7) 3 1524 cm Ϫ1 (9) 3 1510 cm Ϫ1 (11). Contrary to that, no correlation is observed for v 2 and v 3 modes (see Table I).…”

“…12,13 NIR-FT-Raman spectroscopy also gives a strong enhancement of carotenoids due to the known preresonance effect; furthermore, the disturbing fluorescence effect of biological material usually observed when laser excitation is performed in the visible wavelength range, is avoided. 11 Strong bands of carotenoids are observed in the Raman spectrum within the 1500 -1550 and 1150 -1170 cm Ϫ1 ranges due to in-phase CϭC ( 1 ) and C-C stretching ( 2 ) vibrations of the polyene chain. Additionally, in-plane rocking modes of CH 3 groups attached to the polyene chain and coupled with C-C bonds are seen as a peak of medium intensity in the 1000 -1020 cm Ϫ1 region.…”

“…Their distinctive characteristic is a long central chain with a conjugated double-bond system, which is a light-absorbing chromophore responsible for yellow, orange, or red color of these compounds. Although these natural pigments occur in plants as minor components at the ppm level 11 a very sensitive detection can be achieved by Resonance Raman in the visible region, when the wavenumber of the laser excitation coincides with an electronic transition of the individual carotenoid. 12,13 NIR-FT-Raman spectroscopy also gives a strong enhancement of carotenoids due to the known preresonance effect; furthermore, the disturbing fluorescence effect of biological material usually observed when laser excitation is performed in the visible wavelength range, is avoided.…”

Spectroscopy: Developments in instrumentation and analysis

“…These sensitivities are due to the Raman resonance phenomenon and not to the classical Raman phenomenon that occurs in Raman experiments. Resonance Raman spectroscopy has been applied to analyse selectively trace components such as pigments (Merlin et al, 1994;Ozaki et al, 1992), cholesterol (LeCacheux et al 1996) and vitamin A. Another way to obtain an enhanced Raman signal is to use surface-enhanced Raman spectroscopy (SERS) (Dou et al 1999).…”