The human nose and taste buds can make out a great number of aromas and flavours, says S.Ananthanarayanan.
The taste buds can tell the difference between levels of salt, sweet, sour, bitter and a broth-like feeling, called umami. The great many combinations that are possible make for the variety of cuisines and the wide canvas for the chef’s palette. The nerve endings in the nose, in comparison, are believed to differentiate well over a trillion kinds of odours. Smell is hence a sensitive medium of communication in all the species.
Zheng Li, Ming Fang, Maria K. LaGasse, Jon R. Askim, and Kenneth S. Suslick, from University of Illinois at Urbana-Champaign and National Institute of Standards and Technology Gaithersburg, USA, report in the journal, Angewandte Chemie (International Edition), their success in building a laboratory version of the nose – an array of sensors which can rapidly analyse vapours and identify the sources by comparison with an odour dictionary. The method was found to work with ten different kinds of alcoholic drinks and promises wider application.
The way taste buds and olfactory (related to smell) nerves of the body function is by being sensitive to chemicals that cause tastes or smells. The smell-sensitive tissue in the lining of the nose has six or more different cell types which react to classes of molecules. Substances that produce odours dissolve in the mucus in the nose and their molecules attach to specific shapes presented by smell-sensitive nerve cells. This brings about re-organisation of electric charges and the nerve cell sends out a message that is received by the brain. The body keeps renewing the lining so that the nose stays sensitive to continuing or fresh smells that come to it.
Smell is perhaps the most sensitive of the senses that animals have evolved to possess. Eyes and the sense of sight, or ears and sensitivity to sounds, mainly found in larger animals, work at fineness not much better than the millimetre. Smell, however, depends on the detection of single molecules and can hence be extremely sensitive and tuned to be highly specific. Single celled animals, like bacteria, communicate only by exchange of chemical signals and millions of small organisms, insects, animals, conserve energy by using smells to pass messages. Moths use smells to attract mates from kilometres away. Ants are well known to navigate or follow trails created by smells. The legendary capacity of the dog, which has thousands of times the number of nasal nerve endings of humans, may not rival that of many other animals in nature.
The nose and wine
Different congeners arise from differences in the strain of the fruit used and of depend on the yeast cells that bring about fermentation. It is the strain of yeast, in fact that decides the main mix of alcohols and some other substances produced. The fruit is also important and so is the process of fermentation and bottling. Countries like France and Italy have cultivated vineyards and processes that consistently produce well appreciated wines.
As in many arts, what the artist creates is fashioned by the consumer and the critic. Over the centuries, wine tasting has specialised and vineyards employ highly paid experts to savour the wine at all stages of production, so that the result has the right mix of congeners. A connoisseur can thus identify a wine, maybe the group of vineyards it came from, the climate in the year of vintage and so forth.
While the wine-taster relies on the flavours, the acid of the alcohol, the astringency of the tannin, the smoothness of glycerol and many other substances, a great part of the character of wine is in the aroma, or the vapours that are released when wine is swirled, first in the glass and then in the mouth. It is with the help of the nose, or smell sensitive areas in the whole nasal cavity, that fine wines are appreciated and judged.
Smells in industry
Identifying volatiles is generally an efficient way of analysing or detecting different substances. While one application is in analysis, to find out the proportions of the components of unknown mixtures or the intermediates in a process, another is to detect leaks or toxins or to detect concealed substances, like drugs in airline baggage. There are even applications in medicine, of detecting things that point to diseases, or in pathology.
Mechanised or laboratory arrangements for these purposes mimic the natural method by creating an array of detectors of different target volatiles. In the place of the effect of volatile molecules on cells, the method of detection is by recording changes in colour of reagents when an array of them is exposed to the vapour. The changes may happen spontaneously or on ‘developing’, like when viewed in UV light. The patterns of colour changes, to the extent that they are unique to the volatiles, then become their ‘fingerprints’. The ‘electronic nose’, as these arrangements, which can print out the name of the substance, are called, however, need to integrate the sensors of volatiles with the remaining circuitry and the sensors need to be reusable. This places a limit on what reactions are permitted and hence on the sensors’ sensitivity. The arrangements are also affected by the humidity, etc. in the environment and often fail to detect closely related volatiles, the paper in Angewandte Chemie says.
To improve the available discriminatory power, the team explored the vista of existing reagents and zeroed in on compounds that changed colour in characteristic ways when exposed to volatiles, at levels which went down to parts per million. An array of chemically sensitive inks was then exposed to vapours containing groups of common fermentation congeners, called aldehydes and ketones. A sensor array with 21 plates threw up, within a two-minute exposure, patterns that identified the different aldehydes and ketones. Over 155 trials with random volatiles present only at parts per million, and even down to 40 parts per billion, the identification success was over 99.4 %.
To examine the value of the method for the food and beverage industry, it was tried out with commonly used alcoholic drinks. In the same way as vapours rise from wine, distilled spirits, like Scotch whiskey, also have distinctive vapour signatures. “Each liquor has its own distinct aroma produced during production and aging in wood barrels that arise from a highly diverse set of compounds….,” the paper says. The colour detection array used was expanded to 36-elements, with the capacity to detect more than just aldehydes and ketones and it was tried with a set of different whiskies, brandy and vodka. The successful results showed that the method had “promising applications in the food and beverage industry for quality control and assurance,” the paper says.
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