An electronic tongue (e-tongue) uses an array of liquid sensors that mirror and mimic the human sense of taste, without the intrusion of other senses such as human vision and olfaction that often interfere with perception. E-tongues are used in liquid environments to classify the contents of the liquid, identify the liquid itself, or sometimes to discriminate between samples. Most e-tongues are based either on potentiometric or amperometric sensors. The “tongue” is analytic in nature and “tastes” either liquid samples or solids that have been dissolved in liquid. It has three main components:
- A sampling system
- Detection capability
- Data acquisition and processing system - statistical software that interprets sensor data into taste patterns
E-tongues are designed to meet or exceed the ability of human senses to “taste.” One application is the beverage industry whereby the flavor of wine, for example, is discriminated by using a combination of a taste sensor and an odor sensor array with a conducting polymer.
The electronic tongue uses taste sensors to receive information from chemicals on the tongue and send it to a pattern recognition system. There are five categories of taste:
- Sour – Created by the hydrogen ions in HCl, acetic acid and citric acid
- Salty – Registered as NCl
- Bitter – Includes such chemicals as quinine and caffeine detected through MgCl(2)
- Sweetness – registered by sugars
- Umami (deliciousness) – by MSG from seaweed disodium in meat/fish/mushrooms
Recently, the University of Texas at Austin developed a sensor array functioning as an electronic taste chip. It combines micromachining, photochemical sensing, and molecular engineering of receptor sites and pattern-recognition protocols to detect multi-analyte systems. It effectively rejects other chemical and biochemical species located in the same environment. This solution is becoming popular for several application areas such as:
Human diagnostic testing:
The most compelling news here is that several time-intensive and costly procedures can be replaced with common analysis of blood electrolytes. Several benefits result when replacing costly traditional testing with an e-tongue-based solution. The size of the sample is substantially smaller so that discomfort and sample size are both reduced. Monitoring can be accomplished rapidly, which is important given the short time for toxins to become lethal. There is no accuracy penalty with the new testing. Other medical application areas include pharmaceuticals, health and safety, medicine stability (in terms of taste), and veterinary medicine.
The same multi-sensor array is used to provide digital data on compound class and volume in such applications as radioactive pollutants in water, inorganic and organic pollutants, identification of harmful bacteria or substances, chemical and metabolic breakdown patterns, and potentially destination information on pesticides, oil, dioxin, and more. Additional environmental and chemical applications include waste monitoring and chemical /petrochemical processing.
Beverage industry analysis:
Arrays can be used to monitor results in product development, product purity confirmation, flavor ageing analysis in fruit juice, alcoholic, or non-alcoholic drinks, measure the effect of process control variables, establishing adherence to government standards, quantify levels of spice, flavors, dissolved compounds, and quantify taste masking success.
No matter the application, taste sensors above all else must be consistent in their findings. Simply put, here's how they work: taste sensors have artificial polyvinyl chloride (PVC)/lipid membranes that interact with a target solution such as beverages, blood, caffeine, etc. The membrane potential of the lipid membrane changes – which is the sensor output or measurement. Investigating potential change results in measuring the “taste” provided by the output of the chemical substances. With the array, multiple sensors provide this output and form a unique fingerprint.
Fig. 1: An ST L074 current amplifier was used in an impedance e-tongue instrument for rapid liquid assessment.
E-tongue sensing elements include electrochemical detectors, infrared-based sensors and mass sensitive devices. It is the statistical software , or pattern recognition system, that interprets the sensor data, which is obtained from directly sampled liquids (no preparation) into test patterns.
When compared with the maturity level of such sensor technology as vision, touch, and hearing, both e-tongue and e-nose technology is relatively new. One example of an impedance e-tongue instrument for rapid liquid assessment is the STMicroelectronics T L074 current amplifier (see Fig. 1). While applications are continuing to grow, given that there are multiple sensors involved increases complexity.
The use of e-tongue sensor technology is a great example of sensor fusion--extracting data from multiple sources including sensing devices and measurement methods. Intelligent signal processing then is employed to take the responses and extract the taste information.