Innovative food companies have shown an increased interest in developing fast analytical techniques to characterize the flavour/aroma of food products that compare well with sensory analysis by human taste panels. Fast sensory-directed techniques can rely on electronic nose (e-nose) or electronic tongue (e-tongue) technology. Both technologies have been developed by research laboratories and commercial companies during the past 10 years, but still have a long way to go before they are applied in an industrial environment. As the human nose is involved in 80–90% of the total flavour perception, digital odour characterization using electronic nose technology can be used for fast classification of food products in good accordance with sensory analysis.
History of Electronic Nose Technology
Conventional techniques for assessing flavour quality, such as sensory analysis and GC–MS, are often too time-consuming and labour-intensive to be used as quality control (QC) methods within the food industry. The ultimate challenge is to develop fast analytical procedures resulting in classifications of food products that correlate well with sensory panel data. Electronic nose technology seemed to be an ideal approach when introduced a few decades ago.1 In the 1990s an 'electronic nose' was defined by Gardner and Bartlett as "an instrument, which comprises an array of electronic chemical sensors with partial specificity and an appropriate pattern recognition system, capable of recognizing simple or complex odours."2These systems generate a sensor array response to a complete volatile pattern, without separating the aroma compounds and use pattern recognition software for data processing. In the same way that humans use the receptors in the nose to smell a mixture of volatiles and the brain to analyse the data, e-nose systems use sensors and a data processor with pattern recognition software for classification of products.3 Currently, two electronic nose technologies exist on the technology market: conventional electronic nose technology based on gas sensors and the more recently developed electronic nose technology based on MS. It is clear that with artificial olfaction systems the operating principle, the number of sensors and their sensitivity and selectivity is very different from the human perception. The indication of such an analytical system as an 'e-nose' system, is perhaps more appropriate when the pattern recognition data interpretation is considered. As is the case with human perception these systems deliver a comparative rather than a qualitative/quantitative composition of the volatile mixture sent to the sensors.
Gas Sensor-based E-noses Versus MS-based E-noses
Conventional electronic noses are based on gas sensors. In the sensor array of a gas sensor-based electronic nose the volatiles interact physically and/or chemically and a dynamic equilibrium develops by adsorption and desorption at the sensor surface. Different types of gas sensors have been used in literature: metal oxide semiconductors (MOS), metal oxide semiconductor field effect transistors (MOSFET), surface acoustic wave sensors (SAW), quartz microbalance sensors (QMB), conducting organic polymers (CP).4–5 Usually 6–12 gas sensors from the same type but with different response characteristics are used in a multi-sensor array.
Furthermore, the first generation of instruments used static headspace to deliver the volatiles to the sensors. Aroma chemists know that this aroma isolation technique is certainly not sensitive enough to deal with the complexity of aroma compounds in food materials and with the odour potency of specific compounds. Also, the literature claimed that the gas sensor array systems have not generally lived up to expectations. Problems with drift, instability as a result of humidity (water vapour) or carbon dioxide, the need for frequent calibration, sensor poisoning and poor sensor-to-sensor and instrument-to-instrument reproducibility (transportability) are some of the major disadvantages.6
More recently, a new generation of e-nose systems based on mass spectrometry was introduced, also referred to as MS-based electronic nose or MS-nose technology and mass fingerprinting or MS-fingerprinting. This e-nose technology was certainly more compatible with the basic knowledge of aroma chemists concerning the complexity and specificity of odour perception and the relationship with food flavour. In our opinion, from a theoretical point of view, the combination of a mass spectrometer and pattern recognition as a fast method to classify food products in relation to their flavour characters should be much more successful compared with the e-nose approach using gas sensor arrays.
The majority of the e-nose papers in the field of food analysis deal with conventional e-nose technology based on gas sensors. Only a small part of the literature is dedicated to the application of MS-based electronic nose technology.5–11