Vodka production: Additives
It has long been common practice to add small amounts honey to vodka to increase the vodka's viscosity or mouthfeel and take the edge off the spirit. Many modern vodkas that are highly regarded for their perceived smoothness have been augmented with trace amounts of sugar, honey, glycerine and/or citric acid.
If vodka labels were required to list ingredients in the same manner as food packaging, then many vodkas would be labelled, "Ingredients: rectified potable water, ethanol rectified from edible raw material, natural honey, sugar, food supplements: glycerine, edible polyhydrate citric acid."
A trace amount of honey is still the preferred softener in many modern Russian vodkas, and sugar is the more favoured softener in 'western' vodkas - like honey it serves to mellow the peppery attack of the ethanol alcohol and add mouthfeel. Sugar also amplifies flavour and while it is sometimes found in vodka it is more generally used in cognac, indeed it is difficult to find a cognac without a couple of grams of added sugar per litre.
Glycerine, or glycerol, has the food additive identifier E422. It is colourless, odourless and sweet tasting (60% as sweet as sucrose) and is used to sweeten vodka and, perhaps more importantly, to increase the vodka's viscosity without substantially altering its flavour. Until the late 1980s, UK law made it illegal to sell vodka made from neutral ethanol alcohol merely diluted with water. The alcohol in vodka brands made or imported into the UK prior to the relaxation of this rule had to be adulterated and most did this with trace amounts of glycerine.
Citric acid is a white crystalline powder which dissolves easily in alcohol. As its name suggests, high concentrations of citric acid are found in lemons, oranges, limes and other citrus fruits. It is naturally present in all aerobic organisms (animals and plants) and excess citric acid is readily metabolized and eliminated from the body. This weak organic acid is widely used in beverages as both a natural preservative and to add an acidic/sour taste. It is also routinely added to vodka.
In 1956, U.S. State Revenue Ruling 56-98, 1956-1 C.B. 811, "concluded that citric acid and sugar were not considered to be flavouring ingredients which would materially affect the taste of vodka or change its basic character so long as the sugar did not exceed two-tenths of 1% percent and citric acid was only added in a 'trace amount'."
The ruling did not quantify 'trace amount' and in 1995, the American Bureau of Alcohol, Tobacco and Firearms (ATF) investigated levels of citric acid in vodka and its effect on flavour. To quote from the resulting ATF Ruling 97-1 document, "Available information indicates that the use of citric acid from this time  until the 1980s was in the range of approximately 49 to 150 parts per million (ppm). However, starting in the 1980s, the quantity of citric acid added to vodka increased significantly." In response to the increased usage of citric acid in vodka, a law was briefly introduced limiting the addition of citric acid to 300 ppm (300 milligrams per litre).
A further quote from that informative 1995 document reads, "ATF has historically taken the position that the addition of citric acid in vodka to a level where it could be detected by the consumer would contribute a distinctive character to the product in violation of the standard identity". In other words, ATF has used the approach known as "threshold testing." This type of testing determines the point at which a taste tester can first detect a difference in taste when compared to a reference sample. Since vodka is a product which by definition is without distinctive taste, ATF believes the point at which a taste tester first detects any difference in taste (in this case, citric acid) is the maximum allowable amount of that ingredient. The difference in taste need not be attributable to or identifiable by the taste tester as a specific ingredient.
"In contrast, industry members believe that threshold testing to determine a minimum level at which citric acid becomes detectable has no relevance to the distinctiveness requirements of the vodka standard of identity. In testing conducted by industry, consumers were asked whether samples of vodka containing up to 1,000 ppm of citric acid had a "distinctive character, aroma, taste, or colour." A statistically significant number of participants did not identify the vodka samples containing up to 1,000 ppm of citric acid as having a distinctive character or taste. Therefore, the industry believes that the presence of citric acid at levels up to 1,000 ppm in vodka is appropriate."
The ATF concluded, "The actual presence of critic acid in vodka has not raised any consumer health or safety issues since being introduced in vodka production in 1956, nor has it resulted in any known consumer deception. Moreover, consumers have been purchasing vodka which contains citric acid in varying ranges of up to approximately 1,000 ppm for more than a decade without any complaints being received by ATF. Based on all of the above, ATF has therefore concluded that a level of up to 1,000 ppm citric acid is an industry-standard that will continue to maintain the current standard of identity for vodka, which has been followed for well over a decade while meeting ATF's statutory mandate to protect consumers." Consequently, in the USA vodka may contain up to 1,000 ppm of citric acid (1 gram per litre).
Poland has traditionally frowned upon the use of additives in vodka and Polish legislation which came into effect on 1st January 2013 stipulates that vodka made in Poland may not contain any additives except water. The same legislation allows for flavouring and additives in flavoured vodka but specifies that these must be natural and that no more than 100g of sugar is used per litre. So, if you want vodka free of additives, buy Polish vodka.