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Raman spectroscopy for food applications

Identification

Key words Raman spectroscopy, scattered light, adulteration, Rayleigh scattering, non-destructive technique
Latest version 2013/02/14
Completed by UTCN, IRTA

How does it work?

Primary objective Non-destructive analysis of food substances, food authentication, traceability and quality control.
Working principle Raman spectroscopy is a usefull technique for determining structural information of solid samples and aqueous solutions [1]. In fact, a Raman spectrum presents well-resolved bands, carrying information on the vibrational band energies of molecules.

In a Raman spectrometer, a laser beam is fired through a window and reflected by a mirror to the filters that guide the photons to the sample. A set of mirrors are gathering the scattered photons reflected by the sample and pass them through the monochromatore that reduces stray light. Scattered light is light transformed in a spectrum by a photomultiplier tube [2]. The spectrum is confronted with databases of reference spectrums for chemical identification.

2 1 1 raman equipment pierre picouet.jpg

Figure 1: Sketch of a Raman spectrometer set-up A small amount of the scattered light is shifted in energy from the laser frequency due to interactions between the incident electromagnetic waves and the vibrational energy levels of the molecules in the sample. Plotting the intensity of this "shifted" light versus frequency results in a Raman spectrum of the sample.

2 1 1 raman mechanism pierre picouet.jpg

Figure 2- Raman and Rayleigh effects. ext frequency of the incoming (excitation) laser.
Spontaneous Raman scattering is weak and therefore the challenge is to separate it from the intense Rayleigh scattered light, which occurs as light passes through substances, being an elastic effect, with no change in the wavelength of the light. Raman scattering is a form of light scattering where the incoming photon excites a molecule, which passes from an initial energy state to a virtual energy state. As the molecule relaxes, a photon is emitted and passed into a different vibrational state, generating Stokes Raman scattering. If the molecule had an already elevated vibrational energy state, then the phenomena is called anti-stokes Raman scattering (see Fig. 2).

Raman spectroscopy can be used for both qualitative and quantitative determinations, since band areas are proportional to concentration. Not all molecules are “Raman active”, since a change in polarisability must be involved. By infrared spectroscopy, on the other hand, only the transitions that cause a change in dipole moment can be observed, leading therefore to different vibrational transitions. This makes the two techniques complementary.

There are different Raman techniques, such as Fourier Transform Raman Spectroscopy (FT-Raman), Surface Enhaced Raman spectroscopy (SERS), Confocal raman Microscopy, Coherent anti-Stokes Raman Scattering (CARS), Resonance Raman Spectroscopy and Raman Sensing. Raman Sensing is related with low-cost fibres optics and miniaturized detectors to be used in remore sensing [3]. For some applications, Raman spectroscopy can be coupled with chemometric analysis (PLS, cluster analysis, etc.) in order to have an appropriate calibration.

Images
Additional effects
  • Fluorescence from impurities of the sample itself can hide the Raman spectrum.
  • Sample heating through the intense laser radiation can destroy the sample or cover the Raman spectrum, since the sample absorbs some of the incident radiation when low laser powers are applied to sensitive samples.
Important process parameters Acquisition time, intensity of laser, calibration solutions
Important product parameters Polarizability, intrinsic fluorescence properties at excitation frequency

What can it be used for?

Products Most food products with weak intrinsic fluorescence at the excitation frequency.
Operations Raw material authentication, quality assessment
Solutions for short comings
  • This technique does not cause chemical decomposition, mechanical disturbance or photo-thermal damage (only in colored samples, and is currently overcame by technical adjustments)
  • High sensitivity towards molecular structure and conformation
  • Potential use as non-destructive and real-time sensor

What can it NOT be used for?

Products A Raman signal is weak and often hidden by the intrinsic fluorescence of the product. Products with a high fluorescence in the visible and NIR range are not suitable for Raman spectroscopy.
Operations It can not be used in harsh environment.
Other limitations Significantly higher costs compared to IR instruments.
Risks or hazards This technology can be risky for operators; risks are related with the use of laser light (optical hazard).

Implementation

Maturity Mainly used on lab-scale. Affordable portable Raman spectroscopy systems are also available on the market.
Modularity /Implementation It can be used along the production line (continuous). Since measurement can take place through fibre optic and an apropiated probe, the Raman equipment can be located separately from the production line.
Consumer aspects No literature available.
Legal aspects Please check local legislation.
Environmental aspects No literature available.

Further Information

Institutes University of East Anglia, University of the Basque Country, University College Dublin - Agriculture and Food Science, Walloon Agricultural Research Centre, CSIC - Instituto de la grasa, AFRC Institute of Food Research
Companies Thermo Scientific, Renishaw, River Diagnostics, HORIBA Scientific, CRAIC Technologies, Ocean Optics
References
  1. Herrero A.M. (2008). Raman spectroscopy a promising technique for quality assessment of meat and fish: A review. Food Chemistry, 107; 1642-1651.
  2. Omar J., Sarmiento A., Olivares M., Alonso I. & Etxebarria N. (2011). Quantitative analysis of essential oils from rosemary in virgin olive oil using Raman spectroscopy and chemo metrics. J. Raman Spectrosc. DOI: 10.1002/jrs.3152.
  3. Das R.S. & Agrawal Y.K.(2011). Raman spectroscopy: Recent advancements, techniques and applications. Vibrational Spectroscopy, 57; 163– 176.
  4. Di Anibal C.V., Marsal Ll.F., Callao M.P. & Ruisánchez I. (2012). Surface Enhanced Raman Spectroscopy (SERS) and multivariate analysis as a screening tool for detecting Sudan I dye in culinary spices.Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 87; 135-141.
  5. Ghosh P.K. & Jayas D.J. (2009). Use of spectroscopic data for automation in food processing industry. Sensing and Instrumentation for Food Quality and Safety, 3(1) ; 3-11.
  6. Graham S.F., Haughey S.A., Ervin R.M., Cancouët E., Bell S.& Elliott C.T.(2012). The application of near-infrared (NIR) and Raman spectroscopy to detect adulteration of oil used in animal feed production. Food Chemistry, 132; 1614–1619.
  7. Kathirvel P.,Ermakov I.V.,Gellermann W.,Mai J. & Richards M. P. (2008). Resonance Raman monitoring of lipid oxidation in muscle foods. International Journal of Food Science and Technology, 43, 2095–2099.
  8. Kizil R. & Irudayaraj J. (2008). Spectroscopic Technique: Fourier Transform Raman(FT-Raman) Spectroscopy, Modern Techniques for Food Authentication, 185-201.
  9. Nikbakht M., Tavakkoli Hashjin T., Malekfar R., & Gobadian B. (2011). Non-destructive Determination of Tomato Fruit Quality Parameters Using Raman Spectroscopy, J. Agr. Sci. Tech., 13: 517-526.
  10. Li-Chan E., Chalmers J.M. & Griffiths P.,(2010). Raman Spectroscopic Imaging, Applications of Vibrational Spectroscopy. Food Science, 1; 167-180. ISBN-10: 0-470-74299-2.
  11. Silva, E. Raman Spectroscopy: A comprehensive review. North Carolina State University: Department of Textile Engineering, Chemistry and science.

Acquisition time, intensity of laser, calibration solutions Polarizability, intrinsic fluorescence properties at excitation frequency Electro-Magnetic equipment 2.1.1 physical other other Literature review, Wiley.com, Web of Knowledge, Amazon.com, Google Books Search terms: Raman spectroscopy, Raman spectroscopy for food applications, Raman and food WikiSysop :Template:Review document :Template:Review status



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Created by Hte irta on 14 February 2013, at 13:38