Advances in brewing science have allowed brewers to reduce the time required to produce beer. Most breweries carbonate their beer in the fermenter, then filter the beer to remove sediment, and bottle the beer under pressure. This is known as forced carbonation, when CO2 is actually injected into the beer. This allows the beer to be consumed as soon as it's packaged. Because no live yeast is left in the beer the quality begins to fade within months. It is not uncommon for manufacturers (or their representiatives) to rotate stock and remove old beer since it usually goes off in flavor within three months.
A more time-intensive process known as krausening is another way to carbonate beer. By adding yeast and new beer to finished beer, a refermentation occurs in the bottle. The yeast sediment acts as a preservative and the beer continues to age and mature. This is also known as "bottle-conditioning". Hair of the Dog Brewing Company uses the krausening technique to create bottle-conditioned products. The beers we create will be made to last quite well in the bottle, and some of our beers will be collectable the way wine is, coming into its own in 2 to 10 years. We build shelf life into every bottle of beer we make.
Recently in a magazine called the Malt Advocate there was a review of a tasting that occurred with aged beer. They were actually tasting beer that was 25 years old! The tasting was for Thomas Hardy's Ale, which has been produced since 1968. All the beer they make is meant to be aged for a time, and reading from the back label of a 1978 vintage, produced just ten years after the first vintage, "This beer will improve if kept at 55F and will last at least 25 years. Do not open before 1988." That is quite a confident statement to put on a bottle of beer. But bottle-conditioned beer is very special beer. Over time, the yeast interacts with the beer, consuming residual sugars and changing the beer's ``personality''. It changes quite a bit in the first six months, the richness of the hops fusing and blending with the malt flavors to mellow the overall taste profile. Then the real secret ingredient, time, is applied. The longer the maturing, the more fine and mellow the flavor.
Here are a few extracts from "The Great Beers of Belgium" by beer expert Michael Jackson which refer to aging and bottle conditioned beers:
"This appears in the large bottle as Chimay Grande Reserve. At this gravity and strength (1081, 7.1w, 9.0v) it has a massive character, especially in its spiciness (a hint of pepper too?). It is the beer world's answer to Zinfadel or Port. This beer definitely develops with some bottle age. The monastery considers two years sufficient, but I prefer it older than that."
"The beer has a secondary fermentation in the bottle. It is held for 10 days at 77F and three months at 57--59F, and emerges with an alcohol content of between 5.6 and 6.2 by weight, 7.0 and 7.8 by volume. Some customers keep the beer for between one and two years, ideally at 57--59F. Mr van Breedam likes to keep his own personal stock for three to six months, to let the flavours marry. I once tasted a 22-year-old bottle that had not only the armona but also the palate of an Oloroso sherry. "