When making food or drink commercially it is important to make sure that the product is up to the mark.  This involves making sure that it contains what the packaging says it contains, such as the right alcohol content for whiskey, but also that the product is consistent with what has been sold before.  After all, if you’ve just paid £40 for a bottle of  18 year old Glenlivet you really would like it to taste just like an 18 year old bottle of Glenlivet.

Bearing in mind that usually whiskey is mainly water, with about 40% by volume of ethanol, the compounds responsible for the taste and the colour account for less than 1% of the total.  Measuring all these various compounds accurately and precisely without involving fancy equipment and the technical expertise found in an analytical lab is not easy.  However, a team from the University of St Andrews in Scotland may have the answer.

The team have used a microfluidic chip that was originally designed to analyse bio-chemical samples.  The upside here is that very detailed information can be provided by this portable, re-usable device within 2 seconds, with no prior preparation of the sample and with a sample volume of just 0.00002 litres.

The chip, which is about the size of  a credit card, consists of a polymer called polydimethylsiloxane (PDMS).  It contains a very fine channel that holds the sample after it has been injected into the centre.  Being confined in this microfluidic channel avoids evaporation of the ethanol and so avoids  errors in the percentage alcohol measurement that would be caused by its volatility.

After being injected into the centre, the sample is sucked along the channel until it gets to the detection region.  Running at right angles to this is another channel containing an optical fibre carrying a laser beam in the near infra-red (this is electromagnetic radiation with a wavelength of 785 nanometres, slightly longer than red light).

When the laser hits the whiskey sample it causes the molecules it comes across to become excited.  These molecules will start to stretch and bend as they absorb the energy from the laser.  They will eventually release this energy as photons of electromagnetic radiation as the molecules drop from these excited states to less excited states .  These photons will have either more or less energy than the original photons of the laser depending on what energy levels the molecules fall to.  The various wavelengths given out this way form a spectrum called the raman spectrum and this spectrum is characteristic of the compounds in the sample.  By collecting the photons that are scattered at right angles to the incident signal (the original laser photons) the device ensures that only the raman photons are detected rather than the bulk of the photons that are scattered by the sample.

What makes one whiskey different to another is the mix of trace compounds that produce the colour and the flavour.  Raman spectra can be used to give very detailed information about the sample; the percentage alcohol can be detected to within 1%, but that’s only the beginning.  A lot of these trace compounds are formed in fermentation.  These are known as congeners and would include other members of the alcohol family as well as related compounds such as carboxylic acids, esters and aldehydes.  The raman spectrum gives a profile of all these compounds as well.

Whiskey spends several years maturing in casks, so you would also get compounds that were absorbed from the cask itself.  Traditionally highland malts were matured in old sherry casks and this is where the colour in the scotch came from.  The tannin, acids and colours absorbed from the cask all go together to make up the characteristic taste of each particular scotch, and it all shows up on the raman spectrum.

In fact, the technique is so sensitive that not only can it be used to identify different brands, it can even be used to show that different samples came from the same cask!

This device represents a significant improvement over existing techniques in its power and portability.  And don’t forget that the tiny sample size has also got to be a plus, if only that it means that there is more whiskey left for drinking!

See the original article at Near infrared spectroscopic analysis of single malt Scotch whisky on an optofluidic chip