in wooden casks, especially American oak and French oak casks, whisky undergoes six processes that contribute to its final flavour: extraction, evaporation, oxidation, concentration, filtration, and colouration. Extraction in particular results in whisky acquiring a number of compounds, including aldehydes and acids such as vanillin, vanillic acid, and syringaldehyde. The casks used for aging bourbon whiskey are required to be new (and charred); after being used for this purpose, these casks are typically exported for use in the aging of other whiskies elsewhere. Distillers will sometimes age their whiskey in barrels previously used to age other spirits, such as port, rum or sherry, to impart particular flavours. Packaging Most whiskies are sold at or near an alcoholic strength of 40% abv, which is the statutory minimum in some countries – although the strength can vary, and cask-strength whisky may have as much as twice that alcohol percentage. Exports Whisky is probably the best known of Scotland's manufactured products. Exports have increased by 87% in the decade to 2012 and it contributes over £4.25 billion to the UK economy, making up a quarter of all its food and drink revenues. In 2012, the US was the largest market for Scotch whisky (£655 million), followed by France (£535 million). It is also one of the UK's overall top five manufacturing export earners and it supports around 35,000 jobs. Principal whisky producing areas include Speyside and the Isle of Islay, where there are nine distilleries providing a major source of employment. In many places, the industry is closely linked to tourism, with many distilleries also functioning as attractions worth £30 million GVA each year. In 2011, 70% of Canadian whisky was exported, with about 60% going to the US, and the rest mostly to Europe and Asia. 15 million cases of Canadian whisky were sold in the US in 2011. Types Whisky or whisky-like products are produced in most grain-growing areas. They differ in base product, alcoholic content, and quality. Malt whisky – made primarily from malted barley Grain whisky – made from any type of grain Malts and grains are combined in various ways: Single malt whisky is whisky from a single distillery made from a mash that uses only one particular malted grain. Unless the whisky is described as single-cask, it contains whisky from many casks, and different years, so the blender can achieve a taste recognisable as typical of the