The craft beer renaissance has given us thirsty beer drinkers so much more choice. A mind-boggling array of beer to choose from can make picking the perfect pint quite a dilemma.
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While choice is never a bad thing, in a world of brute IPAs, sours, stouts, imperial ales and milkshake beers it is easy to lose track of the basic but simple truth that almost all beer falls into one of two categories; they are either ales or lager and the difference between lager and ales is the yeasts used in fermentation.
Ales and lagers are not determined by their colour. When it comes to the colour of your pint, that will be determined by the type of malt that is used to brew it. The darker the malt the darker in colour the finished beer will be, so stouts are brewed using lots of dark brown roasted malts.
Yeast are single-celled microorganisms that reproduce by budding. Biologically they are classified as fungi. In brewing they are responsible for converting fermentable sugars into alcohol, carbon dioxide and other by products. While there are hundreds of varieties and strains of yeast, broadly speaking there are two types of yeast brewers care about. They are classified by the conditions they work best in.
Ale yeast is a “top-fermenting” yeast type called Saccharomyces cerevisiae, and lager yeast is the “bottom-fermenting” type, Saccharomyces uvarum.
Saccharomyces uvarum was formerly known as Saccharomyces carlsbergensis as it was first isolated and identified in the laboratory of the Carlsberg brewery. Today, due to recent reclassification of Saccharomyces species, both ale and lager yeast strains are considered to be members of S cerevisiae, meaning beer yeast.
Ale yeast strains work best at temperatures ranging from 13 to 21°C, though some strains will not actively ferment below 12°C. Ale yeasts are generally regarded as top-fermenting yeasts because they rise to the surface during fermentation, creating a very thick foam on the top of the fermenting wort. Fermentation by ale yeasts at these warmer temperatures produces beers high in esters, which gives the beer distinctive characteristics. Top-fermenting yeasts are used for brewing beer styles such as, Kölsch, ales, porters, stouts, Altbier, and wheat beers.
Lager yeast strains, are considered bottom-fermenting yeasts. The work best at temperatures lower than ale yeasts, between 7 to 15°C. Lager yeasts grow less rapidly than ale yeasts with much with less surface foam. They tend to settle out to the bottom of the fermenter as fermentation nears completion and this is why they are often referred to as “bottom” yeasts. The final flavour of the beer depends greatly on the strain of lager yeast used and the temperatures at which the beer was fermented. Pilsners, Dortmunders, Märzen, Bocks, and American malt liquors are examples of beers brewed using lager yeast.
In recent years there has been a growing interest in spontaneous fermentation and beers brewed using wild yeast strains which is reminiscent of the way ancient beer was brewed, before we properly understood the world of microbes from a scientific perspective.
Beer that is exposed to the open air will naturally become infected with wild yeast and bacteria in the environment. These beers are called spontaneous fermented beers because the fermentation just happens naturally without the brewer adding cultivated yeast. One of the typical yeasts which causes this to happen is the Brettanomyces Lambicus strain which gives the beer a complex tart flavour. Craft beers produced in this fashion are sour, non-filtered and inspired by the traditional Belgian lambics.
The strain and style of yeast chosen for brewing impacts greatly on the flavour and aroma of the finished beer. Of course, malt, hops, and water play their part in the flavour of the beer but so does the synthesis of yeast, which forms by-products during fermentation and maturation. A vast array of by-products can arise from this process. The most notable of these are of course ethanol (alcohol) and carbon dioxide (CO2); but in addition, a large number of other flavour compounds are produced such as:
•fruity esters (flavour and aroma of bananas, strawberries, apples, or other fruit)
•solvent (reminiscent of acetone or lacquer thinner)
•sulphur (reminiscent of rotten eggs or burnt matches)
•phenolic (flavour and aroma of medicine, plastic, Band-Aids, smoke, or cloves)
•acetaldehyde (green apple aroma)
•diacetyl (taste or aroma of buttery, butterscotch)
•dimethyl sulphide (DMS) (taste or aroma of sweet corn, cooked veggies)
•medicinal (chemical or phenolic character)
•clove (spicy character reminiscent of cloves)
Depending on what the brewer is trying to achieve these characteristics can be a welcome outcome or an undesired beer fault. We would encourage becoming familiar with beer styles and their various characteristics so you learn what to expect in your glass from particular styles and also what flavours and aromas should not be present.
Judith Boyle is a qualified chemist (MSc) and accredited beer sommelier. Susan Boyle is a playwright, artist and drinks consultant. See www.awinegoosechase.com.
Both sisters are proud to be fifth-generation publicans. Their family business is Boyle’s bar and off-licence in Kildare town