Electronic nose techniques

For the ripened Asiago strong correspondence exists among the different methodologies and the electronic nose technique the samples considered as outlier are the same. The sample 7 of the summer period and the sample 25 of the winter period, which the electronic nose underlines as anomalous, are recognized as products of fermentation, such as butyric acid, that have evident holes, anomalous taste (bitterness and acid) and are very adhesive the sample 6 of the summer period possesses little fermented odour and is a little bit salty and insipid. 

The origin of coffee can be established by determining its volatile and semivolatile compounds. A reliable technique is the electronic nose technique,120 which is presented in Figure 4.4. It has been established that the composition of volatile and semivolatile compounds depends on such factors as climate and soil temperature. The results obtained by the electronic nose technique are comparable with those obtained by spectrometric techniques. However, spectrometric techniques require a separation step in the sampling process, by GC technique, which results in a decrease of reliability of the analytical information by using the spectrometric techniques for assay of the volatile and semivolatile compounds in coffee. 

Q. Chen, J. Zhao, Z. Chen, H. Lin, Z.D.A. Zhao, Discrimination of green tea quality using the electronic nose technique and the human panel test, comparison of Unear and nonlinetir classification tools. Sens. Actuators B Chem. 159(1), 294—300 (2011)... 

A. Hemandez-Gomez, J. Wang, G. Hu, P. Garcia-Pereira, Discrimination of storage shelf-Ufe for mandarin by electronic nose technique. LWT Food Sci. Technol. 40,681-689 (2007)... 

M.V. Galmarini, M.C. Zamora, R. Baby, J. Chirife, V. Mesina, Aromatic profiles of spray-dried encapsulated orange flavours influence of matrix composition on the aroma retention evaluated by sensory analysis and electronic nose techniques. Int. J. Food Sd. Technol. 43, 1569-1576(2008)... 

G. Zhang, J. Wang, Y. Sheng, Predictions of addity, soluble solids and firmness of pear using electronic nose technique. J. Food Eng. 86, 370-378 (2008)... 

Firmenich US has also performed studies with e-noses, but the results have not been disseminated to our knowledge. Merck used e-noses to discriminate between various flavours used in pharmaceutical formulations, obtaining a good correlation between GC, sensory panel and e-nose. Zhu et al. (2004) of Merck research Laboratories used the electronic-nose technique to qualitatively distinguish raspberry, red berry, strawberry, pineapple, orange, and cherry in placebo formulations. Raspberry flavour samples from different batches made by the same manufacturer, as well as freshly prepared and aged samples, were also distinguished by an electronic-nose. They used not only PCA and CA but also other methods such as Discrimination Function Analysis (DFA). 

A sensor array named the electronic nose is a rapid and relatively simple technique that can be used for monitoring wastewater odors (Stuetz et al., 2000). The electronic nose uses sensors of varying affinities to characterize an odor without reference to its chemical composition. 

Arrays were introduced in the mid-eighties as a method to counteract the cross-selectivity of gas sensors. Their use has since become a common practice in sensor applications [1], The great advantage of this technique is that once arrays are matched with proper multivariate data analysis, the use of non-selective sensors for practical applications becomes possible. Again in the eighties, Persaud and Dodds argued that such arrays has a very close connection with mammalian olfaction systems. This conjecture opened the way to the advent of electronic noses [2], a popular name for chemical sensor arrays used for qualitative analysis of complex samples. 

Buratti et al. (2004) employed an electronic tongue based on ampero-metric detection in a flow injection system (FIA), coupled with an electronic nose, to discriminate wines from vineyard Barbera produced in four Ifalian oenological regions with different denominations Oltrepo Pavese, Piemonte, Asti, and Alba. The chemometric techniques applied were PCA for dafa exploration, and LDA and CART (classification and regression frees) for classification. 

MDGC, and comprehensive two-dimensional GC, or GCxGC), faster separation techniques (fast GG), fast methods for quality assessment or process control in the flavour area ( electronic noses and fingerprinting MS) and on-line time-resolved methods for analysis of volatile organic compounds (VOGs) such as proton-transfer reaction MS (PTR-MS) and resonance-enhanced multi-photon ionisation coupled with time-of-flight MS (REMPI-TOFMS). The scope of this contribution does not allow for lengthy discussions on all available techniques therefore, only a selection of developments will be described. 

Other MS-fingerprinting techniques that are in commercial development are based on atmospheric pressure ionisation (API), resonance-enhanced multiphoton ionisation (REMPI) TOE and proton-transfer reaction (PTR). They are rapid, sensitive and specific and allow measurements in real time and may play an increasingly important role in the future development of electronic noses and tongues. 

Fig. 15.14 Analytical techniques for time-resolved headspace analysis. An electronic nose can be used as a low-cost process-monitoring device, where chemical information is not mandatory. Electron impact ionisation mass spectrometry (EI-MS) adds sensitivity, speed and some chemical information. Yet, owing to the hard ionisation mode, most chemical information is lost. Proton-transfer-reaction MS (PTR-MS) is a sensitive one-dimensional method, which provides characteristic headspace profiles (detailed fingerprints) and chemical information. Finally, resonance-enhanced multiphoton ionisation (REMPI) TOFMS combines selective ionisation and mass separation and hence represents a two-dimensional method. (Adapted from [190])...

Pillonel, L Altieri, D Tabacchi, R Bosset, J.O. (2004) Comparison of efficiency and stability of two preconcentration techniques (SPME and INDEx) coupled to an MS-based electronic nose . Mitt. Lebensmittelunters. Hyg. 95 85-98. 

Fenaille, F., Visani, P, Fumeaux, R., Milo, C., Guy, P.A. (2003) Comparison of mass spectrometry-based electronic nose and solid phase microextraction gas chromatography-mass spectrometry technique to assess infant formula oxidation. J. Agric. Food Chem. 51 2790-2796. 

Privat, E., Roussel, S., Grenier, R, Bellon-Maurel, V. (1998) Techniques for ethanol removal before discrimination of alcoholic drinks using electronic noses. Sci. Aliments 18 459-470. 

Several other techniques such as electronic nose (Flodgins, 1997 Schaller et ah, 1998) and ultrasonic methods (Benedito et ah, 2000 Cho et ah, 2001) have also been investigated. Flowever, these methods need... 

Hodgins, D. (1997). The electronic nose Sensory array-based instruments that emulate the human nose. In "Techniques for Analyzing Food Aroma", (R. Marsili, Ed.), pp. 331-371. Marcel Dekker, Inc., New York. 

It must be pointed out that the electronic nose and the electronic tongue methodology are able to show the authenticity of the product only if supported by the multivariate statistical techniques useful not only to classify but also to validate and predict unknown samples. 

The data processing of the multivariate output data generated by the gas sensor array signals represents another essential part of the electronic nose concept. The statistical techniques used are based on commercial or specially designed software using pattern recognition routines like principal component analysis (PCA), cluster analysis (CA), partial least squares (PLSs) and linear discriminant analysis (LDA). 

One area of application is the analysis of flavours in foods in order to ascertain, for example, the ripeness of fruit or the maturity of cheeses [42]. For this purpose dendrimers were used which can distinguish certain carbonyl compounds such as ketones, aldehydes, esters, and amides, also in mixtures. For example, measurement of the concentration of 2-heptanal is of importance specifically for determination of the degree of ripeness of apples. The concentration of this compound increases significantly with increasing ripeness [43]. This technique can also serve as an electronic nose for quality control of high-value products such as saffron, which in powder form may contain undesired contaminants (adulterants) such as curcuma, safflower, or marigold. 

Subsequently, other researchers developed the electronic nose idea with a variety of chemical gas sensor arrays using different pattern recognition techniques for improving the interpretation of responses [2-5]. 

The major conceptual limitation of all regression techniques is that one can only ascertain relationships, but one can never be sure about underlying causal mechanism. The explanation of conclusions with the assistance of other sciences would avoid reaching nonsense conclusions. A hypothetical paradigm can be to use the electronic nose for detecting the adulteration of refined olive oil with refined seed oils when these kinds of oils do not contain volatiles (refined process of vegetable oils includes the deodorization). 

Recent advances in the technology of multisensor arrays and neural computing have made the development of the electronic nose of great interest to the food industry for discrimination between odors (26). Provided the instrument has been calibrated properly, the technique is rapid, nondestructive, and objective. Shen et al. (27) found the electronic nose was capable of measuring changes in volatile compounds associated with lipid oxidation in canola, com, and soybean oils stored under accelerated conditions and Aparicio et al. (28) found the electronic nose could be calibrated to detect rancidity levels in good quahty ohve oil spiked with rancid olive oil. 

Electronic nose technology and analysis of volatiles has long been apphed in the food industry to control the quahty of food products and to determine shelf hves. For example, sensor arrays based on different Sn02 gas sensors can be used to distingiush milk products of different rancidity levels [41]. Standard microbial test prediction of shelf hfe of milk products has a low level of reliability due to relatively poor correlation between microbial counts and actual shelf hfe. Several alternative methods have therefore been developed. One method is based on dynamic headspace capillary gas chromatography analyses of volatiles in mUk followed by MDA analyses. [42]. Principals of this method were later used for development of a faster and simpler test, where the extraction was performed by the SPME technique, the extracts... 

Conducting polymer sensors can be operated either to quantitatively measure the concentration of a target vapor species or to qualitatively analyze a complex mixture of vapors. For single vapors, the detection limits can be in the low-ppm region. Exposure to a mixture of vapors results in a unique pattern of responses, which is usually deciphered using standard chemometric techniques. The pattern can be used like a fingerprint to identify certain products, or to establish the quality of foodstuffs, wines, perfumes, etc. The electronic nose has similar components as the natural nose this is illustrated in Figure 1.15. 

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