FOOD & DRINK TECHNOLOGY 73
technologies for assessing a wide range of quality and compositional aspects of food products.
FT-IR Infra-red spectroscopy has been an important analytical tool for many years, but recent advances have increased its usefulness. Application of Fourier Transform techniques to the results has lowered the detection limit from the microgram to the nanogram range and from the ppm to ppb level. Meanwhile sample presentation has been greatly simplified with the introduction of Diamond ATR (Attenuated Total Reflectance) sampling.
Infra-red spectroscopy is based on the interaction of specific wavelengths of infra-red light with particular chemical bonds in the material being studied, particularly organic molecules. Individual bonds, such as C-O, C-H or C-N, absorb infra-red light at a particular wavelength. Illumination of a molecule will produce a spectrum of peaks, and each peak can be related to a particular type of bond. Individual spectra thus provide a ‘fingerprint’ of individual molecules. Tis can be used in the identification and verification of incoming product, determination of adulteration (eg, palm oil addition to virgin olive oil, or margarine addition to butter), contamination and origin studies, and quality issues (such as sugar/acid ratio in tomatoes), as well as to identify the chemical composition of foreign bodies.
FT-IR microscopy can be used to study the chemical composition of very small samples (micro-sized), in effect using a microscope to apply FT-IR spectroscopy to those microscopic samples. However, its most valuable application is in the chemical mapping of a sample of varying composition, so that the chemical identity of
Fig. 2. DNA-based tests are routinely used to assess the authenticity meat species, fish species, and the presence of genetically modified material.
particular components can be determined. Tis can be used to study food materials, such as the composition of wheat grains, where chemical mapping shows the distribution of protein, starch, cellulose and phenolics.
FT-IR microscopy can also be used in the analysis of multilaminate plastic packaging materials, which are made up of a number of different, very thin layers, each of which has a specific purpose. When problems are encountered with such materials – for example if a lidding film will not seal adequately to a food tray – it is important to be able to able to analyse the different layers to check them against the manufacturer’s specification. A cross-section is therefore cut from the film and examined under the microscope.
Te traditional approach in identifying the chemical nature of layers (of packaging or food) would be to take spectra from each layer in turn, but the development of a Focal Plane Array detector with up to 128 x 128 separate elements means that spectra can now
be acquired simultaneously across the whole sample to give a chemical map of the various layers. In addition, layers as thin as 1-2 microns can now be analysed, following the development of a Germanium Attenuated Total Reflectance objective, in which a germanium crystal is pressed up against the sample, increasing the resolution four-fold.
Scanning electron microscopy Te scanning electron microscopy (SEM) gives pseudo- three dimensional images with higher magnification and greater depth of focus than a light microscope. Samples can be relatively easily prepared and quickly examined in the SEM, making it an invaluable tool for the rapid examination of the three-dimensional structure of many samples.
Tis technique is put to good use in texture assessments, for example during a product development programme. Different formulations for products such as biscuits or savoury snack foods often have very different textures or
mouth feels. Getting the right texture is a key part of producing an acceptable product. Te SEM can be used to examine the structure of a biscuit, for example, and assess whether specific ingredients are associated with crumbliness or lack of cohesion of the product. Sugar crystals are often associated with these characteristics, and the SEM can reveal whether there is an uneven distribution of crystals or other components and whether this can be related to where and how the product crumbles. Overall crispness of snack products is also related to the relative levels and distribution of individual components. Te SEM can be used to relate the microstructure of the product to this characteristic.
X-ray microanalysis More detailed microanalysis of SEM samples may be carried out using an energy-dispersive x-ray microanalyser. Whereas FT-IR analyses the spectra associated with chemical bonds and is therefore primarily useful for organic materials, x-ray microanalysis is related to the elemental composition of a material.
Te energies of the x-rays given off by a sample irradiated with an electron beam are characteristic of the elements present in the sample, and so the x-ray microanalyser can be used to give a quick non-destructive elemental analysis of the sample. Terefore it has wide application for the food scientist. It can be used to look at the distribution of salt and calcium phosphate crystals in cheddar cheese or of milk solids in chocolate. It can be used to determine such things as the mineral composition of a substance, such as nature of glass found as a foreign object in food. (Different types of glass have different levels of elements such as sodium, aluminium, magnesium, lead and calcium – and so can be distinguished by
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