Tim Hutton looks at a range of imaging technologies for assessing a wide range of quality and compositional aspects of food products.
The recent issue of horse meat in the UK meat supply chain has highlighted the need for rapid methods for testing food products and their raw materials. The quest for methods that are simpler, faster, easier, cheaper, more reliable, more sensitive and robust is unrelenting.
Techniques that were once the preserve of forensics labs - like DNA profiling methods and mass spectrometry - are now equally well established in food and drink laboratories. DNA based tests are routinely used to assess the authenticity of many materials – meat species, fish species, and the presence of genetically modified material, for example – as well as to ‘type’ problem bacteria such as Salmonella or Listeria, to trace the source of such contaminants. Meanwhile, mass spectrometry enables screening for a wide range of contaminants such as mycotoxins and for trace components such as vitamins.
What is perhaps less well known is that these analytical techniques are complemented by a wide range of sophisticated imaging technologies for assessing a wide range of quality and compositional aspects of food products. This article illustrates some that we use at Campden BRI.
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. This 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 can be used to study the chemical composition of very small samples
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 particular components can be determined. This 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.
The 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
The 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.
This 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. The 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. The SEM can be used to relate the microstructure of the product to this characteristic.
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. The 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. Therefore 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 their ‘elemental fingerprint’).It can also rapidly identify samples erroneously reported as glass such as struvite (magnesium ammonium phosphate), salt and silica minerals.
Colour is a familiar basis for recognising different components, for example to discriminate lean regions from fat in meat for assessment of marbling. Chromameters and spectrophotometers are widely used in the food industry for accurate measurement of average colour over an area of a sample. A common measurement scale is the CIELAB system, which provides a standardised description of colour by three parameters L*, a* and b*:
L* Lightness (0-100)
a* Variation from green (-100) to red (+100)
b* Variation from blue (-100) to yellow (+100)
Methods are available to calibrate colour imaging devices against such scales, enabling accurate colour measurements to be made for individual regions. Approaches include hyperspectral imaging using spectral cameras or a series of narrow-band illumination sources (eg, Videometer), or calibration of a digital camera.
At Campden BRI, we use a DigiEye imaging system (Verivide Ltd) for this purpose. The system includes an imaging cabinet with standardised lighting. Images are taken with a high resolution digital camera and the system is calibrated against a reference colour chart. The system can be used for colour measurement, and provides accurate images for documentation of process trials and production of specification images.
We use a hyperspectral near infra-red imaging system to measure distributions of composition in a range of food samples. Applications have included:
* Moisture distribution in bread, biscuits and multi-component products such as custard tarts.
* Fat distribution in fried chips and doughnuts;
* Detection of crystalline sucrose in biscuits;
* Measurement of components in powders, for example to detect particles of added dried gluten in wheat flour;
* Detection of damage in fruit;
* Measurement of fish and beef quality.
Samples are presented on a moving translation stage and imaged with a SWIR spectral camera (Specim Ltd.) using a pushbroom approach. A HgCdTe detector provides a wavelength range of 970 to 2500nm and a selection of lenses provide a field of view of 10 to 300mm with a minimum pixel size of 30µm.
The method is rapid, requiring a few seconds to scan a sample. The system at Campden BRI is designed for laboratory and at-line use. The same approach can also be adapted for on-line measurement of samples on moving production lines.
Tim Hutton is Communications Manager with Campden BRI, Chipping Campden, Gloucestershire, UK.