The chemistry of beer

As we shall see in Chapter 6, there is increasingly good evidence for the bene cial impact of moderate levels of ethanol on the body. There are several other effects of alcohol on the quality of beer. It contributes directly to avour, by impacting characters variously described as warming and sweet as well, of course, as alcoholic. It also moderates the contribution of other components to avour by in uencing their partitioning between the body of the beer and its headspace ('the nose'). Ethanol also in uences the foaming properties of beer (Brierley et al. 1996). It lowers surface tension, and so aids bubble formation, but it also competes with other surface-active molecules (notably proteins) for places in the bubble wall, thus detracting from stability of the head.
Beer strength is usually de ned in terms of alcohol by volume (ABV), i.e. the number of cm3 of ethanol per 100 cm3 of beer. Sometimes alcoholic strength is described in terms of weight per volume. As the speci c gravity of ethanol is 0.79, this means that a beer that contains 5% alcohol by volume has approximately 4% alcohol by weight. One of the most relevant examples to use by way of illustration is the so-called '3-2 beer' in Utah. Most of the beer in that US state is in this category, which refers to the fact that it contains no more than 3.2% by weight. This is of course 4% when quoted on the basis of volume.
Another way of describing the strength of a beer is on the basis of its 'original gravity' (known as 'original extract' in the US). This is variously quoted on the basis of speci c gravity or, increasingly commonly, degrees Plato. It is basically a measure of the strength (approximating to the sugar content) of the wort prior to fermentation. During fermentation, the fermentable sugars are converted into alcohol, leaving behind that proportion of the solubilised starch that is not fermentable. Sugar solutions have a high speci c gravity (weight per unit volume), as compared to water (1 mL of which weighs 1 g - i.e. the speci c gravity is 1.00) and to ethanol (speci c gravity 0.79). Thus there is a fall in speci c gravity during fermentation and the nal speci c gravity of a beer re ects the balance between ethanol and the residual unfermentable 'dextrins' (see later). By measuring the speci c gravity and ethanol content and putting the values into an equation, the brewer can calculate the original extract, that is, the original strength of the wort.
One degree Plato basically represents a 1% by weight solution of sugars. Thus a wort that is 10° Plato is the equivalent of a 10% sugar solution. A 12°P wort is a 12% sugar solution. If they contain the same proportion of fermentable sugars, then the latter would go on to give a more alcoholic beer. For most beers the sugars originate from malted barley, but some brewers use adjuncts. Thus, for instance, if the grist comprised 70% malt : 30% corn syrup, then, when compared to one of the same strength in degrees Plato derived from an all-malt grist, the former would contain less of the other components that are derived from malt (protein, vitamins, polyphenols, bre, etc.). Thus, although a knowledge of the original gravity of a beer is useful for 'normalising' analytical data on beers, it is important to bear in mind that the exact nature of the grist has a key role to play.