Vibrational microspectroscopy of food

FT-IR and Raman spectroscopy are complementary techniques for the study of molecular vibrations and structure. The combination with a microscope results in an analytical method that allows spatially resolved investigation of the chemical composition of heterogeneous foods and food ingredients. The high spatial resolution makes it possible to study areas down to approximately 10 10 mm with FT-IR microspectroscopy and approximately 1 1 mm with Raman microspectroscopy. This presentation highlights the advantages and disadvantages of the two microspectroscopic techniques when applied to different heterogeneous food systems. FT-IR and Ramanmicrospectroscopy were applied to a number of different problems related to food analysis: (1) in situ determination of starch and pectin in the potato cell, (2) in situ determination of the distribution of amygdalin in bitter almonds, (3) the composition of blisters found on the surface of bread, (4) the microstructure of high-lysine barley and (5) the composition of white spots in the shell of frozen shrimps. # 2003 Elsevier Science Ltd. All rights reserved. Introduction Infrared (IR) and Raman spectroscopy are complementary techniques that provide information on molecular structure. By combining spectroscopy with microscopy molecular information can be obtained with great spatial resolution at the microscopic level. Samples of microscopic size can be analysed directly, in air, at ambient temperature and pressure, wet or dry, and in many cases without destroying the sample. Both qualitative and quantitative information can be obtained using microspectroscopy. A number of organic compounds and functional groups can be identified by their unique pattern of absorption, and the intensity of the absorption may be used for the calculation of the relative concentration in the sampled entity (Wetzel & LeVine, 1999). FT-IR and Raman microspectroscopy may be combined with at least three different mapping techniques: point, line and area. With point acquisition several spectra are measured from different places in a sample selected after visual inspection through the microscope, i.e. the spectra are not systematically related to each other spatially. Line mapping defines a series of spectra to be obtained along one dimension (a line) and can be used to investigate changes in a chemical component along a certain direction, i.e. a profile. An area map uses two dimensions, providing a spectroscopic image that can be directly compared to the corresponding visual image, but with an entire spectrum in each pixel instead of a simple colour. By using a series of partly overlapping acquisition areas it is possible to obtain a smoothing of a profile or a map. Using a standard microscope equipped with an XY stage it may take up to several hours to map a few square millimetres, even for an FT-based instrument. However, new instruments that record an entire row (i.e. a line scan) simultaneously by use of a CCD array will make area mapping much less time consuming. Vibrational microspectroscopy offers many possibilities for the study of biological material, including foods systems. In this presentation we demonstrate the potential of the method by relaying examples from our laboratory where IR or Raman microspectroscopy has been successfully applied to potatoes, bitter almonds, bread, barley kernels and shrimp shells to obtain information about microstructure and chemical composition.