Vodka is definitely NOT a neutral spirit (even when some brands claim to be neutral...).
Three main elements influence the flavour of vodka: the raw ingredient, the water used for the dilution of the raw spirit and the methods of production.
The raw ingredient (carbohydrates)
Historically local ingredients are used to product the 'little water', hence the tradition of rye & potato in Poland, wheat in Russia, barley & wheat in Scandinavian countries.
However vodka is no longer the jewel of Eastern Europe.It is now produced all over the world and the past decade has seen a huge increase in cheaply produced domestic brands, employing the most inexpensive ingredients available (molasses, corn, sugar cane).
As the vodka market has become competitive, brands have had to use innovative marketing and use new raw ingredients such as grapes, organic corn, soya...
The current fashion for super-premiums is to blend together various ingredients to create a ‘unique product’ (U'Luvka, Ultimat).
Rye, wheat, potatoes and barley each have distinctive characters & flavours.
Cheaper ingredients (molasses = sweet residue from sugar production; corn...) will impart no depth, only unpleasant sweetness.
The water source
Much is made of the importance of water in the production of vodka and it is a frequent feature in glossy advertising campaigns for many brands.
Vodka comes out of the distillation still at approximately 96%ABV and the regular bottling strength is 40%, which means that over half the content of a bottle is water. Its importance should not be neglected.
However all vodkas use de-mineralised water, as the use of minerals would lead to an unstable product, cloudiness and certain off tastes.
Vodka by definition should be clear & bright and therefore water must be extremely clean.
Different brands use different methods to purify the water they use in the dilution process, but as a basic guide the more polluted the water is to start with, the more chemicals and processing will need to be employed to clean it to a satisfactory level.
Since mineral content is being completely removed anyway, the use of natural springs and arctic melt icebergs can be perceived as fairly inconsequential.
But the purer the source initially is, the less processed the water will be so there is an element of truth in claims that purer water produces purer vodka.
The methods of production
The production methods are a key influencing factor in the taste & flavour of vodka.
Except for a few minor styles, vodka is not put in oak barrels and aged in any way (see Starka) – it is simply made by fermenting and then distilling sugars from mash or grains or vegetal matter and then filtered to remove congeners and fusel oils, in order to create a pure spirit.
However different methods and processes employed to carry out this fairly simple production shape the final product to a huge extent.
The choice of pot or column still has a fundamental effect on the final character of the product.
All vodkas come out of still as a clear spirit but vodka from pot stills (traditionally still used to produce whiskey and cognac) will contain a higher percentage of impurities, called congeners or fusel oils.
These congeners contain aromatics and flavour elements, from the raw ingredient and give the vodka its distinctive character, taste and flavour.
Achieving a balance between positive and negative fusel oils is a complex process and pot stills are relatively inefficient so the resulting spirits are usually re-distilled (rectified) to increase proof.
A spirit which is re-distilled twice or 3 times using pot stills can be called double or triple distilled.
Confusingly however these terms are frequently applied to market brands produced in continuous column stills through 2 or 3 columns.
Column stills are a more efficient way to produce vodka as they benefit from advances of modern technology and by application of computer generated pressure and heat, the final result is a precise tasting spirit.
Although continuous column distillation is a more expensive system to set up, it is in a long term perspective a more economical way to produce alcohol. It is for this reason that only a handful of specialty brands use pot still distillation.
It is worth to mention that in fact 95% premium brands create their vodka from pre-produced un-rectified spirit, purchased from local agricultural distilleries and then rectified to individual specifications.
Following the rectification, the obtained raw spirit is diluted to bottling strength (for a spirit to be classified as vodka, the ABV must be a minimum of 37.5% - Smirnoff Red in the UK).
Once diluted, it is filtered. The filtration process can have 2 distinct purposes: either to change aromas and flavours, or simply to remove tiny particles or sediments and brighten the liquid.
Charcoal and activated carbon are common substance: they actually add elements and can change the taste of vodka, whereas fine mesh and cellulose are common barrier style filters which will not change the flavour and aroma of the spirit.
Mesh and cellulose are used alone and in addition to charcoal filtration to ensure removal of any hard particles before bottling.
Although in Poland it is illegal to add sugars (or else the spirit is called liqueur) nor other ingredients to clear vodka, many other brands don’t have such restrictions and often add sugars, aniseed, almond and other flavourings to mask badly produced spirits.
It is for this reason that cheaper brands carry with them an often unpleasant sweetness.