Beer can be classified into two main groups, ale and lager. Ales and lagers differ slightly, mainly due to the fermentation process. Ales are fermented with top fermenting yeasts such as Saccharomyces Cervesiae, where as lagers are fermented with the addition of a top fermenting yeast such as Saccharomyces pastorianus, Hutkins, (2006).
There are two main methods of fermentation, consisting of primary fermentation, and secondary fermentation (otherwise known as lagering or maturation), (Linko et al., 1998). The primary fermentations of ales are for a short period of 3-6 days at a warm temperature of (15-23°C), where lager takes a slightly longer fermentation period of 1-3 weeks at a colder temperature (7-12°C), (Hutkins, 2006). At the end of the primary stage of fermentation, rancid flavours are released such as diacetyl and acetylaldehyde, (Scannell, 2012). However, according to a study conducted on production of volatile sulphur compunds by ale and lager brewing strains of Saccharomyces Cervesiae, the flavour compounds, hydrogen sulphide, methanethiol and methyl thioacetate appear to be stronger in lager than when the than ale when Saccharomyces Cervesiae is added to both beers during fermentation, Walker and Simpson, (1993).
The fermentation process of lager is long, taking up to many weeks, even months. According to Steward, it can take up to nine months for the maturation of beer in Europe, (2002). The maturation stage is also known as lagering, and is generally the longest step during the entire process. Lager is stored at approximately 0-4°C for a few weeks or months, depending on the type of lager produced, (Saito, 2003). During this period of storing the lager in cool temperatures, the lager begins to turn into a somewhat clear colour, and the natural production of esters and other by products is inhibited, which in turn can give the beer a “crispy” taste.
The storage conditions of ale and lager differ significantly during maturation. Ales are stored in casks, where the addition of priming sugars, hops, isinglass finings and potassium or sodium metabisulphite are added as preservatives in the beer to prolong the shelf life of the product by eliminating pathogenic organisms that may be present, (Scannell, 2012). Casks are suitable to store ale, as beer stored in casks tend to be low in carbon dioxide, rich in flavour, and the cost of production is low, which is a major benefit to companies producing the product, (Scannell, 2012). However lagers are generally stored in tanks, where the presence of yeast assists the improvement of the flavour present (diacetyl) and the lager begins to form a clear colour.
This step can be accelerated with the addition of the enzyme a- acetolactate decarboxylase, which changes directly to acetoin, (Rostgaard- Jensen et al., 1987). This method can be commonly used in the food industry where demands are high for swift beer production. Conditioning agents are also sometimes used during the maturation stage. Sugar known as “priming sugar” is sometimes added to ales to provide a sweeter taste and colouring such as caramel can also be added for improvement to the color of the ale or lager. During the maturation stage, carbonation also occurs. This step sometimes involves the injection of carbon dioxide into the beer. Ales require less carbon dioxide than lager; therefore the injection of carbon dioxide is more common in lager products than in ales, (Lea et al., 2003).
References:
Hutkins, Robert, W. (2006). Microbiology and Technology of Fermented Foods. Oxford: Blackwell Publishing. p320.
Lea, A. and Piggott, J.R., (2003). Fermented Beverage Production. 2nd ed. New York: Plenum Publishers. p146-150.
Linko, M., Haikara,A., Ritala,A., Penttila, M. (1998). Recent advances in the malting and brewing industry. Journal of Biotechnology. Vol. 65, p85-88.
Saito, J. and Takamoto, Y. (2003). Thermal Convection in Cylindro-Conical Tanks During the Early Cooling Process. Journal of the Institute of Brewing. Vol. 109 (1), p80-83.
Stewart, G. (2002), `Fermentation of high gravity worts its influence on yeast metabolism and morphology', Proc. 28th EBC Cong., Budapest, 36.
Walker, M. D. and Simpson W. J. (1993). Production of volatile sulphur compounds by ale and lager brewing strains of Saccharomyces cerevisiae. Letters in Applied Microbiology. Vol. 16 (1), p40-43.
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