Tasting with an Electronic Tongue

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Taste buds constitute the taste cells distributed throughout the throat and mouth. These taste buds are separated at the throat and palate surface and clustered in the tongue as specific structures known as papillae. The papillae are classified as circumvallate papillae on the rear surface of the tongue; leaf-shaped foliate papillae at the sides; and mushroom-shaped fungiform papillae in the front. The food particles make contact with a tiny opening called a gustatory pore. The taste bud cells are covered by the nerve endings that are carried by facial nerves and trigeminal nerves. The following video is a brief summary of the physiology of the human tongue.

These nerves play a key role in communicating messages to the brainstem. An insula present in the brain collects the trigeminal messages to identify the flavor, and its messages, in turn, reach the emotional centers in the cingulate gyrus and a temporal lobe. Finally, the messages from all these regions are sent to a frontal lobe of the brain.

Electronic Tongue - Research

An electronic tongue (E-tongue) is composed of various sensory array units that use electrochemical techniques and different artificial membranes for identifying specific substances. These sensory units are characterized by cross-selectivity and other distinct properties; e.g. the analysis of complex liquid samples. As a result, E-tongues are widely used in the food industry. Recent applications of E-tongues include the development of pharmaceutical products.

The primary components of an E-tongue include sensory array, a signal receiving and sending device and a pattern recognition device. Modern E-tongues are developed on the basis of voltammetry, impedance spectroscopy and electrical potential. The principle behind the electronic tasting system is that the sample molecules are allowed to be in contact with the sensors, which induces potential changes. Physiological action potentials are recorded on a computer and compared with potential changes. Finally, the obtained data is analyzed with respect to already available taste patterns of sensor responses. 

Design and development efforts by a team of researchers at Keio University have introduced flavor sensor technology that simulates the human tongue and nerve cells. The system generates an electrical signal when a sweet or savory test sample is measured.  

E-tongue uses low-selective or non-specific sensors on the basis of a biological structure of the human taste system for producing signals while evaluating samples. The human tongue consists of billions of non-specific receptors for receiving taste signals which are transmitted to the brain through a neural network to obtain a sensed object image.

Electronic Tongue for the Beverage Industry

E-tongues employ pattern recognition techniques along with potentiometric/amperometric chemical sensors to detect polyphenols and identify sensory parameters like sour, bitter, sweet, burnt, caramel and fruity. A lipid membrane used in e-tongue sensors acts as a recognition element for converting the substances into electric potential, which is not influenced by the membrane thickness. This ability enables E-tongues to be widely used in the beverage industry for discrimination between various flavors, set out below:

Wine

The E-tongue consisting of a hybrid sensor array with chemically modified voltammetric electrodes has been applied to differentiate 12 Spanish red wines according to vintage, grape variety, origin, and denomination.

Beer

The E-tongue system can identify the polyphenol content, alcohol content, original extract and bitterness in beer. Following this, researchers have carried out experiments to study physiochemical parameters of nearly 50 Dutch and Belgian beers using canonical correlation analysis.

Tea

The E-tongue is capable of detecting the caffeine and catechins of green tea using multivariate calibration. An Ag/AgCl reference electrode and silicon transistor sensors having an organic coating are provided with a taste system of the E-tongue. The difference in electric potential between the reference electrode and coated sensor is used for measuring the response intensity.

Fruit-Based Soft Drinks

Flavors of commercially available fruit juices can be determined using potentiometric E-tongue consisting of cross-sensibility lipid/polymeric membranes.

Advantages of the Electronic Tongue

One of the key benefits of E-tongue is the utilization of voltammetry which is a combination of several analytical techniques like pulsed, stripping and cyclic voltammetry. It enables the identification of various properties of the sample to be studied. In pulse voltammetry, the charged species movements are determined using transient responses. The E-tongue can also provide information related to microbial species classification, liquid properties and the quality of food and water by employing different metals like rhodium, rhenium, platinum, palladium, iridium, and gold as electrodes.

Future Direction   

The E-tongue is an advancing biomimetic measurement technology that is widely being used for various applications. It makes use of sensor arrays that combine both hardware and software developments to exhibit high performance. These sensor arrays are also capable of analyzing the product and process quality. The signals and variations produced by them alone are sufficient enough to analyze the data.

However, the concept of biomimetics should not be exaggerated as they cannot be as effective as the human sensation, which is strongly linked to the signals from the brain and memories about previous taste experiences. Artificially designed sensor arrays give results based only on smell and taste and they do not follow the same mechanism as the human senses. This learning and understanding is important even as we continue to use man-made sensor arrays in biomimetics for a wide range of applications.

With the incorporation of pulse voltammetric measurements, which employs metal electrodes, a large amount of information can be obtained, but with a large redundancy and therefore evaluation is not efficient. Algorithms will in turn need to be further developed to evaluate the measurement data in an efficient manner. Several experiments have also proved that E-tongues integrated with cameras and sensors have the potential to estimate the characteristics of any given sample. This approach can be used to monitor the process as well as the product in a general sense.

Sources and Further Reading

• Krantz-Rülcker C, Winquist F, Lundström I. Electronic tongues: electrode arrays and pattern recognition. The Journal of the Argentine Chemical Society. 2005;93:105–114.
• Baldwin E.A, Bai J, Plotto A, Dea S. Electronic Noses and Tongues: Applications for the Food and Pharmaceutical Industries. Sensors. 2011;11:4744–4766.
• Latha RS, Lakshmi P.K. Electronic tongue: An analytical gustatory tool. Journal of Advanced Pharmaceutical Technology & Research. 2012;3:3–8.

This article was updated on 17^th February, 2020.

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