Electronic Senses
Can miniaturization and microfluidic advances take ‘taste’ and ‘smell’ to the next level?
In food analysis, arrays of gas sensors are termed “E-Noses” while arrays of liquid sensors are referred to as “E-Tongues”. The first scientific literature on these systems appeared in the 1980s but it has only been in the last decade, due to the food industry’s progressive interest in rapid at- and on-line analysis of product quality and safety, that special attention has been given to emerging technologies in electronic senses.
Taste in humans can be classified into five basic categories: sweet, sour, salty, bitter and umami (borrowed from the Japanese and translated as “pleasant savory”). Unlike taste, smell cannot be easily classified into various groups. Humans can distinguish around 10,000 chemicals, with olfactory receptors being stimulated by different combinations of a limited number of primary odors.
E-Noses and E-Tongues, as their names suggest, are inspired by the neurophysiology of smell and taste and attempt to mimic the abilities of their human counterparts. These technologies automate the evaluation of samples with complex composition and are able to recognize specific properties and characteristics. In animals, sensory information is processed by the neural system. Likewise, data collected through selective sensor arrays must be analyzed by pattern recognition tools that employ various mathematical and statistical processing techniques. Such systems can provide quantitative results and, in some cases, are even able to detect differences that a human sensory panel cannot distinguish.
E-Nose instruments typically exploit four main sensor types: conducting polymers (CP), metal oxide semiconducting (MOS), metal oxide semiconducting field-effect transistors (MOSFET), and oscillating sensors, such as quartz crystal microbalances (QCM). E-Tongue instruments generally use the following analytical solutions: mass sensors, which are miniaturized solid-state devices that exploit the piezoelectric effect; potentiometric methods, for example, ion-selective electrodes; and voltammetric or optical sensors, in which an indicator molecule changes its optical properties when exposed to a target analyte. Hybrid E-Tongues, based on a combination of potentiometry, voltammetry and conductimetry, offer great potential and are the subject of an increasing number of papers.
Concrete applications of the discriminating power exhibited by E-Noses can be found in the analysis of meat flavors, volatile organic compounds (VOCs) formed during post-harvest ripening of fruits, ham product evolution during storage, packaging off-flavors, olive oil defects and the identification of geographical origin of foodstuffs. In the last 10 years, for example, Barilla has been successfully implementing MOS-based E-Noses in different quality control labs to recognize residual solvents and to continuously monitor the various plastic food-packaging materials adopted within bakery production sites. At Barilla, E-Noses are, in fact, used as the first appraiser of packaging quality, which limits the number of gas chromatographic confirmatory analyses required solely to the samples that are marked as uncertain or bad by the MOS instrument.
E-Tongues can be used to monitor and discriminate among mineral water, coffee and soft drink samples. Reported applications of E-Tongues in food analysis cover process monitoring, foodstuff recognition/characterization, evaluation of ‘freshness’, quality control and authenticity assessments.
Challenges that remain with both E-Nose and E-Tongue technologies are the needs to improve sampling procedures (by reducing clean-up or extraction before analysis, for example) and to reduce carry-over/environmental noise (for instance, in the form of moisture contamination), which affect sensor drift and sensitivity.
It is realistic to imagine that within the next few years, thanks to significant advances in microfluidics and electronics, that E-Nose and E-Tongue technology will evolve both in terms of robustness and reduction in the current need to optimize each application – a process that requires significant investment of time and resources. Miniaturization will further extend flexibility. Beyond food, E-Sense technology may find applications in other industries, for instance in environmental analysis to detect water contamination or illicit drugs; in clinical diagnostics to monitor saliva, sweat or urine; and in agriculture to detect fungal contamination in feed. There is great potential in these applications, in spite of the fact that the term ‘E-Tongue’ doesn’t conjure up a wonderful image in some instances.
Michele Suman is food chemistry & safety research manager in the Research, Development and Quality Department at Barilla, Parma, Italy.
Please read the other articles in this series:
Non-Targeted Analysis
Foodomics
Regulating Food Allergens
Emerging Contaminants
“When my mother mentioned the possibility of me becoming a chemist at primary school age, my response was, ‘No way!’” Despite his initial skepticism, Michele Suman became a chemist and then took a masters and doctorate in chemistry and materials science before eventually landing the role of Food Chemistry & Safety Research Manager at Barilla SpA. There since 2003, he has been working in an international contest on research projects within the field of food chemistry, food contact materials, sensing and MS applications for food products.
