Developments in taint monitoring: electronic noses
4.11 Developments in taint monitoring: electronic noses
In the mid-1990s, electronic noses attracted interest in the packaging indus- try for their potential to control taints arising from packaging. Develop- ments in computer signal processing coupled with the development of new sensors offered the possibility of building sensor arrays that could be trained to recognise the ‘pattern’ or ‘signature’ of a taste or smell. The dis- tinction between existing analytical equipment is that instead of physically
Packaging materials as a source of taints 105
separating a chemical causing a taint from all the others present using a gas or liquid chromatograph, the overall signal from a tainted sample could be recognised as being different to a taint-free sample. In the same way a human brain is able to decide that a taint or odour is different, without any separation of the bad taste or odour causing compounds being required.
Pattern recognition algorithms are used to train a computer to categorise the signals it receives from good (non-odorous) and bad (odorous) samples. Sensors based on conducting polymers andor metal oxides are used to gen- erate the signal. Both types involve a change in the electrical resistance of the sensors. The metal oxides involve oxidative changes of the tainting com- pounds at the sensor surface. The conducting polymer involves binding the tainting compound on the sensor surface which has an effect on electrical conductivity. The metal oxide is generally capable of providing a much larger change in response than conducting polymers. Acoustic wave sensors have been developed where an electric oscillating standing wave is set up in the sensor. Compounds are absorbed onto the sensor surface, resulting in a change in mass and therefore the oscillator frequency. All the sensor types suffer disadvantages: the conducting polymers respond to water; and metal oxides and conducting polymers are poisoned by some compounds.
A review of conducting polymers was carried out by Adeloju and Wallace (1996). Ide et al. (1997) describes a quartz microbalance electronic nose- type sensor.
The availability of very cheap quadrapole mass spectrometers has renewed interest in this technology being able to recognise taint patterns. The approach taken is to use a six-port sample valve to introduce a small volume of air from an equilibrated headspace above the sample directly into the mass spectrometer whilst it is scanning. After a few seconds the mass spectrometer response stabilises and the whole scan of mass fragments is stored in a statistical software package. The six-port valve is then switched back to a constant stream of helium. The mass range scanned is quite narrow, for example between 30 and 200 mz, the reason being that base peaks contribute the bulk of the signal, and the considerably less abundant but more characteristic molecular ions contribute only a tiny proportion of the signal. The significance of a weak molecular ion is lost in the compara- tively high background. The pattern recognition software is used to pick out fragmentation differences between good and bad samples. The same approach can be used for a liquid sample injected into a liquid stream to investigate tastes. In this case the instrument functions as an electronic tongue. Such equipment is more costly because of the interface problems involved in removing water from the sample before it enters the mass spectrometer.
At present, electronic nose technology is not at the stage where it can
be generally applied to quality control tests and it offers no advantages over GC-MS for investigative work. The cost of the equipment is still much too high for it to become widely adopted in packaging production. The current
106 Taints and off-flavours in food state-of-the-art is summarised by Gibson et al. (2000), whose article lists the
main sensor types and suppliers.