The sense of taste consists of five basic tastes: sourness, saltiness, umami, bitterness and sweetness. When tasting a food or beverage, humans perceive each type of taste on sensory organs called taste buds on the tongue.
Taste buds compose approximately 50-100 cells. Research on the mechanism behind the reception of taste substances has a short history. Taste-2 receptors (T2Rs), bitterness receptors present in taste cells, were discovered in 2000, followed by the discovery of sweetness receptors (T1R2 and T1R3) and umami receptors (T1R1 and T1R3).
Each taste receptor receives multiple chemical substances constituting a single taste. Namely, taste receptors exhibit semi-selectivity rather than rigid and high selectivity. High selectivity means one-to-one correspondence to a particular chemical substance.
Taste information perceived by taste buds is transmitted to taste nerves as a result of the release of neurotransmitters, and finally reaches the gustatory area in the brain as a central tissue. It has been clarified that sweetness, umami and bitterness receptors are expressed at not only the taste buds in the tongue, but also digestive organs, kidneys and even the brain.
While research on the molecular and cellular biology of taste reception has only been done recently, sensing technologies for objective evaluation, such as the discrimination and quantification of tastes, have been developed since around 1990, prior to the discovery of taste receptors.
Previously, sensory tests, in which experienced evaluators called sensory panellists actually taste samples to evaluate them, are the main method of evaluating taste for the food industry.
However, these tests display issues such as low objectivity and reproducibility, as well as great stress imposed on the panellists. In order to resolve these issues, a sensory technology for objective discrimination and quantification of the taste of foods, called the electronic tongue, has been developed.
Although the concept of chemical sensors is generally to detect a target chemical substance specifically at a high sensitivity, the taste receptors of humans do not necessarily recognise individual chemical substances.
As mentioned, each of the receptors for the five basic tastes simultaneously receives multiple chemical substances, showing a semi-selective property. Therefore, it is practically impossible to measure the taste of foods containing several hundreds of types of taste substance by chemical analysis methods such as liquid and gas chromatography, although they can be used to measure the concentration of chemical substances.
Moreover, there are interactions between different tastes and between taste substances. For example, the bitterness of coffee is suppressed by adding sugar and a synergetic effect for umami can be obtained by mixing glutamine acid, an amino acid, and nucleotide-derived inosinic acid.
A taste sensor that is equipped with multichannel electrodes using a lipid/polymer membrane as the transducer has been developed. The taste sensor is considered as an electronic tongue with global selectivity.
The term global selectivity is defined as the decomposition of the characteristics of a chemical substance into those of each type of taste and their quantification, rather than the discrimination of individual chemical substance, by mimicking the human tongue, on which the taste of food is decomposed into each type of taste by each taste receptor.
The taste sensor has four main concepts: 1) The taste sensing system must respond consistently to the same taste like the human tongue (global selectivity). 2) The taste sensor threshold must be the same as the human taste threshold. 3) There must be a clearly defined unit of information from the taste sensing system. 4) The taste sensing system must detect interactions between taste substances.
A lipid/polymer membrane comprising a lipid, polyvinyl chloride and a plasticiser is used as the stage for receiving taste substances. The thickness of the membrane is about 200 μm and it can be used about 3,000 times.
The taste sensor has sensor electrodes to which a lipid/polymer membrane is attached and a reference electrode. It measures changes in the membrane potential generated when these electrodes are immersed in a sample solution.
The measurement procedure is as follows. First, the taste sensor is immersed in a reference solution of 30 mM potassium chloride and 0.3mM tartaric acid to obtain the membrane potential, Vr. The reference solution has almost no taste and is used in the system as an alternative to human saliva.
Then, the taste sensor is immersed in the sample solution to obtain the potential, Vs. The difference in potential (Vs-Vr), called the relative value, will approximate the initial taste upon sensory evaluation, including its sourness and saltiness.
After which, the taste sensor will be rinsed slightly with the reference solution before being immersed into the reference solution again to obtain the potential Vr’. The difference in potential (Vr’-Vr), called the change of membrane potential caused by absorption (CPA), will provide data on the absorption of bitter and astringent substances.
Finally, the taste sensor is rinsed properly in alcohol solution to remove the absorbed substances from the membrane before the next sample is measured.
The taste sensor has global selectivity to a taste quality so that its output can be converted to taste information that helps distinguish differences in both taste quality and intensity between samples.
The sensor can be connected to computer terminals to form a management network. Users are able to configure the various settings, such as measurement methods, from the management server.
Input from users is transferred to the sensor through the network and the measurement date obtained will be saved to the database on the server. Analysis of measurement date can be performed from a computer terminal by accessing the database. The analysis results and graphs can be saved as files and edited on the computer terminals as well.
In addition, technologies for detecting pesticide residues to ensure the security and safety of foods have also been developed by applying the taste sensor. Pesticides compose active ingredients (AIs), inert ingredients added to support the AIs and facilitate the formulation, and surfactants. The latter two are referred to as pesticide adjuvants.
The taste sensor detects the changes in the electrical potential of lipid/polymer membranes caused by the physiochemical interaction between the membranes and chemical substances. By applying this principle, a technology that can detect pesticide residues that exceed the acceptable level has been developed.
Recently, the spread of the internet has enabled consumers to easily search for information and products that meet their requirements. In accordance with this, their requirement and expectation for taste is set to increase in the future.
Humans perceive tastes using their tongues and also systematically sense tastes on the basis of odour, texture, visual appearances and factors based on cultural background, such as taste preference, experience and memories. In the future, sensing technologies that can be used to quantify comprehensive tastes or palatability are expected to be realised by advancing comprehensive research on various fields, such as brain science, genetic engineering, psychology and physics.