The Stages of Brewing Beer
Top versus bottom fermentationThere are two types of fermentation called top and bottom. In top fermentation the yeast is able to function at room temperature and it does not get denatured until the concentration of ethanol reaches 12%. When the activity ends, the yeast collects at the top of the fermenter with the consistency of a dense foam. Conversely, in bottom fermentation the yeast is only active below 5 °C and when the ethanol concentration is less than 5%. Once the yeast stops working, it settles on the bottom of the fermenter.
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The final act – conditioningTo allow these unwanted, foul tasting compounds to decompose, the beer has to undergo conditioning. This period can take anywhere from a few days (for ale) to several weeks (for lager). The young beer remains in contact with the yeast to allow the development of a clearer appearance and a more mature flavour. During the conditioning, enzymes within the yeast are able to reduce both butane-2,3-dione and pentane-2,3-dione, converting them into butane-2,3-diol and pentane-2,3-diol respectively, until they fall below critical values (<100 ppb or 0.1 mg per litre of lager). The reduction of butane-2,3-dione occurs sequentially; first to form 3-hydroxybutanone and then butane-2,3-diol. The reduction of pentane-2,3-dione is also sequential; unlike butane-2,3-dione, pentane-2,3-dione is unsymmetrical, therefore, on reduction, two structural isomers are formed depending on which ketone is reduced first. Note that both hydroxy-ketone isomers form the same reduced product (pentane-2,3-diol). Once conditioning is complete the yeast is removed by filtration through porous rock. Yeast that has been used in the beer making process is then sold as baker’s yeast (yeast used as a leavening agent for bread-making and baking), as bedding for growing mushrooms, in ethanol fuel and as a feeding supplement for livestock due to its high protein and vitamin B levels. The beer is then packaged and sold. An important step in the packaging process is the addition of CO2, under pressure, resulting in the formation of beer foam. For the stability of the packaged beer, it is important to ensure there is no oxygen in the beer containers, as we will see later. Historically, beer has faded in and out of popularity. For example, it was a necessity in medieval society due to the practicality of drinking beer over dirty water, while it is currently outlawed in the Middle East. But the current trend in increasing production levels suggests the world’s fondness for beer isn’t going to fade anytime soon. See in the downloads section below for a set of images that summarise the key stages of brewing beer (that include the chemical structures above).
Finally, a toastStarted in 2016 in Yorkshire, Toast Ale is a company that brews beer from unsold bread and the discarded heels of loaves from sandwich makers (apparently, in the U.K. up to 44% of bread is wasted – staggeringly high!). All profits go to fighting global food waste.
Exploring Everyday Chemistry
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