Saturday, February 2, 2013

Bottle maturation (OBE)

Laphroaig 1887
Scotch whisky ages in oak wood casks for a minimum of three years, usually longer. After wood maturation it is usually diluted and colored with caramel E150a prior to bottling. After that the changes of whisky in a closed bottle are not fast or drastic, at least compared to for example the bottle maturation of wines. Some bottle maturation changes or "old bottle effects" are nevertheless possible.

The bottle maturation of wines depends mainly on the closure of the bottle. If there is enough oxygen present, for example through a porous or leaky closure, the wine becomes oxidized. If the closure tight, restricting the oxygen permeation, the wine becomes reduced. Screw caps and cork stoppers are usually the most tightest, synthetic corks are very permeable to oxygen and the permeability of natural corks is somewhere in between, although there are variations, especially among natural corks. The fastest oxidation happens in the beginning of bottle maturation as there is bound to be some residual oxygen in the wine (or spirit), the headspace between the closure and liquid and additionally the porous closures release some air into the bottle. Oxygen ingress in a screw cap sealed bottle is below 1 µL/day, a natural cork sealed bottle gets 2-6 µL/day for the first year and then 0,1-2,0 µL/day depending on the cork quality and a synthetic cork sealed bottle about 6-13 µL/d depending on the material. The main route for the air into the bottle is from between the glass and the cork. Practically all modern commercial wines are protected from excess oxidation by adding sulphur dioxide and sometimes ascorbic acid.
Oxidation affects wines

The oxidation usually decreases the amount of esters and several thiols, resulting in less citrus, grapefruit, boxtree and fruity notes. The amount of H2S (rotten eggs, bad sulphur) increases slightly during bottle maturation, but less so in an oxidative environment. If there are sugars available in the liquid, the amount of furfural (nutty, rancio) usually increases, but furanone (strawberry, fresh pineapple) usually decreases. Unknown reactions produce notes of wet wool, toasted bread and caramel. Most phenols oxidize slowly, usually forming polyphenols, resulting in diminished astringency and probably less peaty whisky over years of bottle storage. An exeption in the phenol group is vanillin, which increases slowly independently of the oxidation/reduction state. Serious over-oxidation in wines creates vinegar and in considerable evaporation of alcohol out of spirit proof liquids.

In the absence of oxygen, the wine is reduced, forming significantly more esters (fruits), higher alcohols (floral, aetheral), abhexon (peach) and sulphur volatiles (struct flint, rubber), especially H2S (rotten eggs), but also thiols and polythiols (petrol, kerosine), apparently from sulphates, sulphites and phenols.

Independently of the oxidation, tannins and antocyanins form bigger molecules, which stabilize the colour and usually turn reddish colours into orange, bricklike hues. Oaklactones tend to partially transform from trans- (spicy, incence) to cis-isomers (coconut, vanillin) in the bottle.
The cork stoppers act as a sorbtive material, especially if they are coated with polyethene and allowed to soak with the liquid. As a result the sulphur odors, especially H2S and small thiols are reduced. On the other hand uncoated corkstoppers are more likely to leak H2S and other volatiles out of the bottle. Screw caps do not have a significant effect on the H2S. A faulty bottle closure or prolonged storage of  opened bottle with a low amount of spirit left leads to evaporation of alcohol and oxidation of the spirit. In that case the filling level is likely to drop and the amount of esters, small thiols and other volatiles to diminish. Bottle breathing, ie leaving the bottle open overnight or half-empty with the cork for months, might therefore cure some sulphur taints, at the cost of reduced fruit and body. 

The glass bottle itself is not completely inert. Especially alkaline high-alcohol solutions (vodka for example) increase the leaching of glass. This happens especially if the bottles are stored for a long time (months) in a humid environment before bottling, allowing water to attach to the inside of the bottle. Bottle glass is composed mainly of silica (SiO2, 75%), sodium oxide (Na2O, 15%), calcium oxide (CaO, 12%) and aluminum salts (2%). First, the water condensed from the humid air causes mainly sodium to leach out of the glass (a damp bottle storage before bottling increases the corrosion) and the increased alkalinity increases the leaching of silica, forming salts of silicic acids. Then ethanol in turn increases the solubility of  inorganic acids. As a result a deposit is created in the bottle, it consists first of sodiumhydroxide (NaOH) and sodiumhydrosilicate (Na2O x SiO2), but it polymerizes into an amorphic gel-like structure (for example H2SiO3 + CaO SiO3 + H2O) consisting in average of CaO (43%), SiO2 (43%) and Na2O (14%). At the process also some metals from the glass structure are leached out. The glass leaching increases the pH of the spirit, for example in one study with vodka the pH increased from 8,85 upto 9,50, which is probably significant for flavour release, too. In the same study the weight of dried crystals was 0,5-2,1 mg per 0,7 litre bottle. As the surface/spirit ratio increases as the bottle size dimishes, the miniature bottles are certainly more prone to impart sediments. As whisky is more acidic than vodka, this is most likely a very slow reaction, happening in older lowgrade and/or miniature bottles during decades rather than months and especially if they have been stored for a while before bottling.

Lagavulin 1875
In a bottle of whisky, the same reactions are likely to happen, although the higher ethanol strength diminishes the oxidation, as ethanol is an oxygen scavanger itself. The oak extracts and the ethanol micelles diminish the extraction of volatiles from the spirit by increasing the surface tension. Most likely the bottle maturation of whisky is more reductive than oxidative, producing more fruity, aetheral, peachy, vanilla, petrol, rubbery and metallic notes and less phenolic, bitter spicy and citrus notes. Rancio flavours might arise from pentose sugars derived from caramel colouring and/or a very extractive charred cask. Some oxidation reactions are bound to happen between the spirit and the air of the bottle headspace, but they are hardly significant. Long chain fatty esters and glass silica can both flocculate in the bottle, especially if the whisky is not filtered and it is diluted and/or colored with caramel before bottling. Light usually increases the speed of reactions, whether reductive or oxidative. In any case, organoleptically significant changes in bottled whisky are likely to occur during decades, if at all.

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