In contemporary brewing the difference between lagers and ales is more theoretical than practical. 

The German word ‘lager’ translates today as ‘warehouse’ and implies a beer that has been stored for a time to mature.  Allowing beer time to “condition” before going on the market was the norm in the 18th and 19th centuries.  In northern Europe, the best ales would ripen in oak casks for months or years.  The best lagers still condition for a couple of months at 0o to 4o C, while Belgian brewers are adept at developing bottle-conditioned ales by warehousing them in ‘warm chambers’ at 20o C for weeks. 

However, in modern industrial brewing, few beers are allowed to condition for more than a few days – even the lagers. 

The discovery of the nature and actions of brewers’ yeast by French microbiologist Dr Louis Pasteur was followed by the differentiation between ale yeast, or saccharomyces cerevesiae, from lager yeast, or saccharomyces pastorianus. 

Ale yeast work best at room temperature and bring a wide range of volatile flavours, some described as ‘fruity’, to the beers they ferment.  Lager yeast in contrast are most comfortable at around 15o C but can work at any temperatures, having a neutral influence on the beer’s grainy flavour.  Lager yeast at near freezing point are skilled at cleaning up organic by-products of brewing that produce undesirable flavours, which is the rationale behind the lengthy cold-conditioning of traditional lagers. 

Other flavour differences between lagers and ales are accounted for by the choice of hops, some varietals of which are better suited to particular styles of beers. 

Around 90% of the beer drunk in Europe today is made on an industrial scale by producers that ferment their beers as warm as possible to save time, followed by only brief conditioning, whether the beer is technically a lager or an ale.  It follows that for industrial beers, the chasm in character that used to exist between ales and lagers is now reduced to a crack.