The perception of volatile compounds by the human nose is of great importance in evaluating the quality of foods. Therefore, it is not surprising that repeated efforts have been made over the years to introduce instruments operating on a similar principle as the human nose: the electronic nose is an instrument that encloses the human sensitivity to the objectivity of the instrumental response and supplies results similar to the human nose and in short time.
An electronic nose is an instrument which comprises an array of electronic chemical sensors with partial specificity and an appropriate pattern recognition system capable of recognising simple or complex odours. It can be regarded as a modular system comprising a set of active materials which detect the odour, associated sensors which transduce the chemical quantity into electrical signals, followed by appropriate signal conditioning and processing to classify known odours or identify unknown odours.
Research has been carried out into the use of thin and thick film semiconducting (inorganic and organic) materials for odour sensing. Research effort is now centered upon the use of arrays of metal oxide and conducting polymer odour sensors. The latter are particularly exciting because their molecular structure can be engineered for a particular odour-sensing application. The electronic nose finds wide applications in the food industry. Future developments in the use of hybrid micro sensor arrays and the development of adaptive artificial neural networking techniques will lead to superior electronic noses.
IV year, Biomedical Engineering Department
Sahrdaya College of Engineering And Technology,
Kodakara, Thrissur, Kerala.
The perception of volatile compounds by the human nose is of great importance in evaluating the quality of foods. Therefore, it is not surprising that repeated efforts have been made over the years to introduce instruments operating on a similar principle as the human nose: the electronic nose or simply the E-nose is an instrument that encloses the human sensitivity to the objectivity of the instrumental response and supplies results similar to the human nose and in short time.
An electronic nose is Ëœan instrument which comprises an array of electronic chemical sensors with partial specificity and an appropriate pattern recognition system capable of recognising simple or complex odoursâ„¢. It can be regarded as a modular system comprising a set of active materials which detect the odour, associated sensors which transduce the chemical quantity into electrical signals, followed by appropriate signal conditioning and processing to classify known odours or identify unknown odours.
The "electronic nose" is a relatively new tool that may be used for safety, quality, or process monitoring, accomplishing in a few minutes procedures that may presently require days to complete. Therefore the main advantage of this instrument is that in a matter of seconds, it delivers objective, reproducible aroma discrimination with sensitivity comparable to the human nose for most applications. The term "electronic nose" was first used in a jocular sense with sensor arrays in the 1980's. As the technology developed, it became apparent that the animal and human olfactory systems operate on the same principle: A relatively small number of nonselective receptors allow the discrimination of thousands of different odours.
Research has been carried out into the use of thin and thick film semiconducting (inorganic and organic) materials for odour sensing. Research effort is now centered upon the use of arrays of metal oxide and conducting polymer odour sensors.
HUMAN NOSE vs. ELECTRONIC NOSE
ELECTRONIC NOSE INSTRUMENTATION
An electronic nose like human sensory systems, it incorporates:
Â¢ Chemical sensors (10 MOSFET and 5 MOS) as we have human olfactory receptors in our olfactory region.
Â¢ A data-processing system (NST Senstool) as we have our brain.
A MOSFET sensor rely on a change of electrostatic potential. They respond exclusively to molecules that dissociate hydrogen on the catalytic metal surface (such as amines, aldeids, esthers, chetons, aromatics ed alchols) and they work at the temperature of 140-170Ã‚Â°C. When polar compounds interact with this metal gate, the electric field, and thus the current flowing through the sensor, are modified. The recorded response corresponds to the change of voltage necessary to keep a costant present drain current.
A MOS sensor rely on change of conductivity induced by the adsorption of gases. Due to the high operating temperature (300-400Ã‚Â°C) the organic volatiles (such as satured hydrocarbons, NO, CO etc.) trasferred to the sensors are totally combusted to carbon dioxide and water on the surface of the metal oxide, leading to a change in the resistance.
NST Senstool software offers three methods for analyzing sensors input:
Â¢ PCA: Principal Component Analysis
Â¢ PLS: Partial Least Square Regression
Â¢ ANN: Artificial Neural Network
They enable to:
Â¢Get an overview of the data (PCA and PLS)
Â¢Predict properties of the samples (PLS e ANN
PCA is a rotation-projection method that helps visualizing the information contained in a large data set. It is a transformation in which many original dimensions are transformed into another coordinate system with fewer dimensions.
PLS is a regression model which use Principal Components and in which we must give a property of the samples such as class or quantitative value.
Artificial neural networks are the most powerful type of data processing technique being employed in Electronic Nose instruments. ANNs are self-learning; the more data presented, the more discriminating the instrument becomes. By running many standard samples and storing results in computer memory, the application of ANN enables the Electronic Nose to "understand" the significance of the sensor array outputs better and to use this information for future analysis.
ANNs allow the Electronic Nose to function in the way a brain functions when it interprets responses from olfactory sensors in the human nose. The ANN's processing elements (or nodes) can be compared to the neurons in the brain. "Learning" is achieved by varying the emphasis, or weight, that is placed on the output of one sensor versus another. ANNs also can be trained to compensate for small response changes that occur when sensors degrade over time. Ideally, a sensor array would respond to a specific sample with the same precision over a long period of time. However, sensors can degrade with prolonged use and the output can vary. ANNs can correct for this problem.
As the organic vapors pass over the sensor array each sensor responds with a certain selectivity. These patterns need to be further processed. Electronic Nose Technology (ENT) is the combination of sensor arrays linked with advanced statistical and neural network software that provides a visual image of an odour, or how an odour relates to other odours. This relationship could represent good â€œ bad, pass â€œ fail, new â€œ old, or the system can be trained to recognize attributes such as green, fruity, floral or spoiled. ENT correlates exceedingly well with both sensory and tradition analytical techniques and ENT can combine both elements in a single analysis.
A number of prototype electronic noses have been developed by the electronic nose research group. There are several laboratory-based instruments, one employing an array of metal oxide sensors, and another employing an array of conducting polymer sensors. This research has led to the production of two desk-top sized electronic nose instruments. Several portable instruments have also been designed and built. These include a 4-element tin oxide electronic nose, a 6-element tin oxide electronic nose, and four 12-element polymer electronic noses.
The E-Nose is best suited for matching complex samples with subjective endpoints such as odor or flavor. For example, when has milk turned sour? Or, when is a batch of coffee beans optimally roasted? The E-Nose can match a set of sensor responses to a calibration set produced by the human taste panel or olfactory panel routinely used in food science. The E-Nose is especially useful where consistent product quality has to be maintained over long periods of time, or where repeated exposure to a sample poses a health risk to the human olfactory panel. Although the E- Nose is also effective for pure chemicals, conventional methods are often more practical.
The electronic nose finds wide applications in the food industry. It is used to detect the bacterial growth on foods such as meat and fresh vegetables. It can be used to test the freshness of fish. It is used in the process control of cheese, sausage, beer, and bread manufacture. Other applications include Identification of spilled chemicals in commerce (for U.S. Coast Guard), Quality classification of stored grain, Diagnosis of ulcers by breath tests, Detection and diagnosis of pulmonary infections (e.g., TB or pneumonia), Identification of source and quality of coffee, Monitoring of roasting process, and so on.
There are numerous potential applications of electronic noses from the product and process control to the environmental monitoring of pollutants and diagnosis of medical complaints. However, this requires the developments of application-specific electronic nose technology, that is electronic noses that have been designed for a particular application. This usually involves the selection of the appropriate active material, sensor type and pattern recognition scheme. The work has led to several commercial instruments, one employing commercial tin oxide sensors (Fox 2000, Alpha MOS, France) and another employing conducting polymer sensors (NOSE, Neotronics Ltd, UK). Future developments in the use of hybrid microsensor arrays and the development of adaptive artificial neural networking techniques will lead to superior electronic noses.
The major areas of research being carried out in this field are:
1. Improved sensitivity for use with water quality and sensitive microorganism detection applications.
2. Identification of microorganisms to the strain level in a number of matrices, including food.
3. Improvement in sensitivity of the E-Nose for lower levels of organisms or smaller samples.
4. Identification of infections such as tuberculosis in noninvasive specimens (sputum, breath).
5. Development of sensors suitable for electronic nose use, and evaluation of unexploited sensors.
Advantages of the electronic nose can be attributed to its rapidity, objectivity, versatility, non requirement for the sample to be pretreatment, easy to use etc. And now scientists at the University of Rome have developed a sensor, which, they claim, can detect those chemicals flowing out of a cancerous lung. Their tests, on a group of 60 people - half with lung cancer - pinpointed every single cancer patient. They suggested that an 'e-nose' could one day form the basis of a screening test for smokers and others at risk of lung disease. The only way of doing this reliably at the moment is to use a bronchoscope to look directly at the insides of the lungs for signs of cancer.