2.2 Concepts and systems for data acquisition
2.2.2 Example: Electronic nose data acquisition for food odor measurement
The process of olfaction in mammals is useful to consider in the development and application of an electronic nose. The fundamental problems in olfaction are establishing the limits of detection, recognizing a particular odorant, coding the response to establish odor intensity and quality, and discriminat-ing between similar odorants. From these problems, the need to understand the nature of the olfactory chemoreceptors, the mechanisms of transduction, the neural coding mechanisms for intensity and quality, and the nature of the higher information processing pathways follows (Persaud et al., 1988).
Figure 2.9 shows a schematic of the olfactory anatomy in humans.
Perception of odor in humans is not well understood. Odorant molecules must reach the olfactory epithelium at the top of the nasal passages where the olfactory receptors are located. Transport to the epithelium is turbulent and results in a nonlinear relationship between concentrations, flow rate, and the number of molecules reaching the membrane. Approximately 5 to
20 percent of the air inhaled will pass through the olfactory cleft, depending on the nasal flow rate. Higher nasal flow rates correspond to increased perceived odor intensity. In addition to transport via the nasal flow, humans possess a retronasal pathway from the mouth to the olfactory epithelium that allows odorant molecules to reach the olfactory receptors by a second pathway. This accounts for much of the sensory perception attributed to flavor when eating. Once odorant molecules reach the epithelium, they must be dissolved into the mucus that covers the olfactory epithelium. The dis-solved molecules are transported to the receptor cells and their cilia where they interact with the receptors and are converted to neural signals. Volatile molecules are also capable of stimulating the trigeminal nerve endings.
While this effect is not fully understood, it is believed that the trigeminal nerves also play an important role in odor perception. Transduction and coding of the receptor response to odorants is not easily reduced and there is no clearly defined theory that covers the perception of the odors (Engen, 1982; Lawless, 1997). The olfactory system responds nonspecifically to a wide variety of volatile molecules. However, most people are able to easily rec-ognize a variety of odors even at very low concentrations.
The electronic nose in Figure 2.10 can be viewed as a greatly simplified copy of the olfactory anatomy, seen in Table 2.1. The receptor cells and cilia are replaced with nonspecific gas sensors that react to various volatile com-pounds. Because there is generally no mucus into which the odorants must dissolve, the molecules must adsorb onto the sensor. There are a variety of sensors that have been employed including those based on metal oxides (Brynzari et al., 1997; Ishida et al., 1996; Egashira and Shimizu, 1993; Nanto et al., 1993), semiconducting polymers (Buehler, 1997; Freund and Lewis, 1997;
Figure 2.9 Schematic of the human olfactory organs. (From Lacey and Osborn, 1998.
With permission.) basal cell
axon
receptor cell
microvilli cilia
olfactory mucous pigment
supporting cell (Bowman's glands)
Nasal epithelium
Yim et al., 1993; Miasik et al., 1986), optical methods (White et al., 1996; Collins and Rose-Pehrsson, 1995), and quartz resonance (di Natale et al., 1996; Matsuno et al., 1995; Moriizumi et al., 1994). Metal oxide and semiconducting polymer sensors are the two most commonly used sensors in commercial instruments.
Generally, steady state sensor response has been used in electronic nose systems (Egashira, 1997; Brailsford et al., 1996), but research indicates that transient response measurements can enhance the ability of the sensors to differentiate between different molecular species (Eklov et al., 1997; Llobert et al., 1997; Nakata et al., 1996; Amrani et al., 1995).
Transduction of the olfactometry receptors is replaced with signal condi-tioning circuits that involve a conversion to voltage (Corcoran, 1994). Coding of the neural signals for odor intensity and odor recognition in humans is replaced with some type of pattern recognition method (Wide et al., 1997;
Table 2.1 Comparison of Human Nose vs. Electronic Nose
Item Human Nose Electronic Nose
Number of olfactory receptor cells/sensors
40 million 4 to 32
Area of olfactory mucosa/sensors 5 cm2 1 cm2
Diameter of olfactory receptor
cell/sensor 40–50 micron 800 micron
Number of cilia per olfactory receptor cell
10–30 0
Length of cilia on olfactory receptor cell 100–150 micron NA Concentration for detection threshold
of musk
0.00004 mgliter air Unknown Adapted From Lacey and Osborn, 1998. With permission.
Figure 2.10 Schematic of generic electronic nose. (From Lacey and Osborn, 1998. With permission.)
Sensors Signal Conditioning
Data
Preprocessing
Artificial Neural Network
Hanaki et al., 1996; Moy and Collins, 1996; Ping and Jun, 1996; Davide et al., 1995; Yea et al., 1994; Nayak et al., 1993; Sundgren et al., 1991).
Despite the limitations of the electronic nose as compared to human olfaction, there have been a number of reports of successful application of the electronic nose to problems in foods (Gardner et al., 1998; Lacey and Osborn, 1998; Osborn et al., 1995; Bartlett et al., 1997; Simon et al., 1996;
Strassburger, 1996; Vernat-Rossi et al., 1996; Pisanelli et al., 1994; Winquist et al., 1993).
There are sources of error in an electronic nose. Many of them are the same as the error sources in olfactometry. These errors include a lack of sensitivity to odors of interest, interference from nonodorous molecules, effects of temperature and humidity, nonlinearity of the sensor response, and errors from sampling methodology. It is beyond the scope of this book to discuss these errors as they affect olfactometry measurements. However, there are several references to quality control in olfactometry (Berglund et al., 1988; Williams, 1986) and published standards for olfactometry measurements (ASTM, 1991; ASTM, 1988).
Osborn et al. (in press) developed an application of a commercial elec-tronic nose for detecting high temperature curing off-flavors in peanuts.
Peanuts were tested in four progressive states of destruction: whole pods, whole kernels with red skins, half kernels without red skins, and ground kernels. Off-flavors in ground kernels were also measured using GC and an OVM for comparison to the electronic nose. The electronic nose sensor array was able to separate good from off-flavored peanuts after some data pro-cessing to remove bias effects from an unknown source. The bias was sus-pected to come from slight water vapor differences between the samples that affected all sensors equally. Further, the electronic nose was able to differ-entiate between the samples nondestructively suggesting that there may be a potential to use this technique to establish quality control points in the processing that could reduce or eliminate off-flavor peanuts.
