The chemistry of beer

It has variously been estimated that there are between 1,000 and 2,000 different chemical species in beer, probably twice as many as are present in wine. It is an extraordinarily complex liquid. Not all of those chemical components make a substantial contribution to the quality of beer, but many do. And brewers strive to control that chemistry, so that every drop of a given brand of beer is fit for purpose – in other words it delivers the same excellent quality glass by glass.

The chemical composition of beer is determined by the raw materials of brewing and by changes that occur during the malting and brewing processes. Quantitatively, by far the major component of all beers (except a couple of latter day products with ludicrous alcohol levels exceeding 50 per cent alcohol by volume, ABV) is water. Most beers are at least 90 per cent water. Ethanol weighs in next, with most beers worldwide being within the range four to six per cent ABV (which equates to approximately 3.2 – 4.7 per cent alcohol by weight, the specific gravity of ethanol being 0.79). And then there is carbon dioxide, CO2, which might be as low as 2g/L in traditional English cask-conditioned ale or in excess of 7g/L in a hefeweissen.

The majority of flavour-determining substances are present in relatively small quantities, but still levels at which a sizeable impact can be made

Foam

It is, of course, the presence of CO2 that leads to bubble formation when beer is dispensed, and the more highly carbonated the beer, the more readily it will generate foam. But there are many carbonated beverages that do not produce stable foam, for the simple reason that producing a huge foam surface is counter to the imperative of surface tension. That beer foam can survive is due to the presence of surfaceactive components, notably hydrophobic (amphipathic) polypeptides originating from the cereal grist and the iso-α-acids, the bittering molecules that are derived from the resins in hops during processing. These molecules migrate preferentially to the surface of bubbles, there to interact to produce flexible films that coat the bubbles. The interaction takes a little time – an interaction that you can witness if you look at the foam on a newly poured beer over a timespan of a couple of minutes. The foam changes from being fundamentally liquid to being more solid in texture.

Some brewers make additions to boost foam stability, for example propylene glycol alginate, a well-known food whipping agent, and nitrogen gas. The latter is especially impactful, as exemplified by the superb stability of Guinness foam. However, even the best of foams is prone to decay introduced by any lipids that survive the brewing process or, especially, are introduced when beer is dispensed into a glass. I would contend that more than 95 per cent of poor head quality in poured beer is due to the presence of either detergent residues or the fact that glass has encountered lipids (e.g. by washing alongside food plates) and these have not been satisfactorily removed by proper washing (a detergent, followed by a thorough water rinse and drying by drainage).

Turbidity

With the exception of a few beers [e.g. hefeweissens with their residual yeast (hefe)], most beers are ‘bright’ and expected to remain free from haze and sediments. Diverse materials can yield turbidity in beer, including polysaccharides (e.g. β-glucan, arabinoxylan and starch) from cereal that have not been adequately degraded during malting and brewing), and oxalates (brewers strive to ensure that oxalic acid from the grain is eliminated in the brewhouse by precipitating it out with sufficient calcium). Particular attention is paid, however, to proteins and polyphenols as the principal culprits when it comes to haze. The relevant proteins are largely different to the ones responsible for foaming: the haze potentiators are rich in the imino acid proline and are derived from the storage proteins in the grain. The polyphenols are extracted in processing from the grain and from hops. If they become oxidised they polymerise into forms that can bridge between proteins and encourage insolubilisation.

Colour

Most of the colour of beer is derived in the Maillard reaction that occurs during the kilning and roasting of malts. Amino acids and sugars that are produced during germination of the grain meld together in the heating process to yield melanoidins. For some of the very pale beers, there is a threat of colour development through the oxidation of polyphenols (cf. apple browning) that can occur both enzymically in the mash and non-enzymically at any stage. Additionally, there are still occasional brewers who will boost the colour of beers through the use of caramels.

Flavour

Flavour impacting molecules in beer arise from the diversity of raw materials but are also substantially impacted by processing. As alluded to above, many of these molecules are present at dramatically low levels but even then some of them can impact aroma and taste if they possess very low flavour thresholds (the concentration at which they can be detected by the human). Ethanol makes a direct contribution to flavour (perceived as burning / warming) but also impacts flavour by influencing the extent to which other substances enter into the headspace of beer to be detected by the nose (most of the flavour of beer is detected there rather than on the palate). Carbon dioxide reacts with the trigeminal system and is detected as pain just like capsaicin from chilli peppers. Thus, CO2 registers as tingle, a major determinant of the mouthfeel of beer. Interestingly, N2 delivers smoothness to beer texture, even at the very low levels in which it is present on account of its low solubility. It also reduces the impact of other flavours, notably hop aroma, for unknown reasons. The next most abundant entities in beer, non-fermentable digestion products of starch called dextrins as well as glycerol produced in yeast metabolism, are touted by some as contributors to mouthfeel, but there is no good evidence for this. Any residual unfermented sugars, as well as any added sugars (‘primings’), will afford sweetness, of course. This may be desirable to counter sourness, which is due to a range of acids primarily produced by yeast. The pH of most beers is in the region of 4-4.5, although there are some with rather lower pHs and these are beers (e.g. the lambics) whose production involves a plethora of microorganisms, including lactic acid bacteria. Salt character in beer is afforded by potassium and sodium, which may originate in several of the raw materials. The bitterness is due to the iso-α-acids referred to earlier. These are not especially soluble and some of the claims made for phenomenally bitter beers are ludicrous. Realistically, 100 mg/L is about as high as can be routinely achieved with conventional processing. The downside to these acids is that they are susceptible to light in the range 350-500 nanometres (the light is captured and transferred by riboflavin in the beer) and degradation products react with sulphurcontaining substances to produce 3-methyl-2- butene-1-thiol (MBT), which has the aroma of skunk and is an example of a molecule that can be detected at extremely low levels (less than 1 ng/L). Brown glass greatly suppresses all this by trapping much of the light, but green, blue and clear glass offer no protection. Some brewers use hop processing in which the resins are converted into reduced forms that do not give this unappealing reaction. It is estimated that there are more than 300 constituents in the essential oil fraction of the hop, divisible into oxygen-containing, sulphurcontaining and hydrocarbon fractions. There is not a simple understanding of how a given balance of components affords the specific characteristics of a given hop varietal and control of hop aroma is as much art as science. In much the same way, the flavour contributions of malt is less than perfectly understood and is controlled on the basis of grain variety, the extent of its germination and how it is kilned. There is a rather better appreciation of the individual flavour contributors from yeast. Most attention is paid to the vicinal diketones, namely diacetyl (butterscotch aroma) and pentanedione (honey). For the majority of beers, these are considered undesirable (although some argue that a little goes a long and beneficial way in some ale). These molecules are produced spontaneously by chemical degradations occurring in fermenting wort, but given time, healthy yeast will scavenge these substances and this represents the true limiting stage in the brewery fermentation cellar. Diacetyl can also be produced by contaminating bacteria, including those thriving in poorly cleaned pub dispense systems. Yeast produces a range of esters, of most importance being ethyl acetate (pear aroma) and isoamyl acetate (banana). The latter is especially important in hefeweissen, the authentic ale yeast used to produce that style of beer being especially replete in the necessary enzyme. This yeast also produces an enzyme that can decarboxylate grain-derived ferulic acid to produce 4-vinylguiaicol, which affords a clove aroma to a genuine hefeweissen. Several sulphur-containing molecules can impact beer aroma. We have already encountered MBT but there are others that are usually (though not always) considered undesirable, including hydrogen sulphide and the mercaptans. There are mixed emotions about the importance of dimethyl sulphide (DMS) which has a sweet corn aroma and has been deemed the key marker for an authentic European lager and yet is deemed an off flavour by many a brewer of such styles. It originates from a precursor that develops during the germination of barley, S-methylmethionine, and which is degraded to DMS whenever significant heat is introduced in the process. The DMS may also be derived by yeast reducing dimethyl sulfoxide, which is found in all malts. Other flavour-active components of beer include short chain fatty acids and higher alcohols. The flavour of beer is not stable with time. Whilst some of the stronger beers may benefit from aging in a manner akin to wine, through the diminution of astringent polyphenols and interactions between alcohols and acids to produce desirable flavoursome molecules, for most beers to age is to deteriorate. This primarily occurs through oxidation reactions leading to a myriad of unsaturated carbonyl substances such as E-2-nonenal, with aromas likened to cardboard and wet paper. They arise from precursors that include unsaturated fatty acids, iso-α-acids, higher alcohols and amino acids. Brewers therefore strive to maintain as low an oxygen content as possible in beer and also stress the importance of storing beer cold (but not frozen) to greatly lengthen shelf life. Also, they will ensure minimal levels of metal ions such as iron and copper in beer as they activate the oxygen.

Safety and wholesomeness

Whereas the wine industry is not shy to tout their product on a health-beneficial platform, brewers have largely been reluctant to do this, although it can be argued that they have an even firmer foundation on which to boast. It is now generally recognised that the key component in alcoholic beverages that counters atherosclerosis is alcohol rather than any specific minor metabolite, such as resveratrol (which, incidentally, has been located in hops). Beer is certainly the more nutritionally replete drink. It is perhaps the most significant source of assimilable silicon in the diet, the silicon absorbed into beer from the husk of barley and from hops. Beer contains B vitamins in significant quantities (with the exception of thiamine). Antioxidants include ferulic acid and a series of polyphenols. Beer contains soluble fibre in the shape of arabinoxylans and the degradation products of the β-glucans may be prebiotics. And the chemistry of hops is being mined as diverse pharmacologically active molecules are attracting interest. The closest botanical relative of the hop is Cannabis sativa: might it be that some of the same desirable relief to suffering that have been linked to the use of marijuana by certain patients might also be alleviated by hops and products in which hops are used, which at the moment is … beer? Despite the opportunities for boasting about this meritorious composition (small wonder that beer has long been known as liquid bread), most brewers have focused on ensuring that their products do not contain materials overtly deleterious to health. Dedicated attention in the 1970s eliminated malt-derived nitrosamines as a concern and many brewers demanded organic or close to organic raw materials and eschewed the use of pesticides. Owing to the labelling laws in the US that demand that more than 10 mg/L sulphur dioxide must be declared on the label, brewers avoid the use of a material that could ‘buy’ them flavour life as they constitute one beverage industry that does not want to see the words ‘contains sulphites’ on the label. It has generally become accepted that people with celiac disease should avoid most beers, as the storage proteins from wheat and barley contain the peptide sequences that they are sensitive to. Recent studies have suggested that, for many beers, the malting and brewing processes have largely de-nuded the beers of the problematic materials. Use of a recently available commercial enzyme, prolyl endoproteinase, seems to ensure that levels of the damaging species in beers are rendered minuscule.  

About the author

Dr. Charles ‘Charlie’ Bamforth is Anheuser-Busch Endowed Professor of Malting & Brewing Sciences at UC Davis. He has been part of the brewing industry for over 34 years. He is formerly Deputy Director-General of Brewing Research International and Research Manager and Quality Assurance Manager of Bass Brewers. He is an Honorary Professor in the School of Biosciences at the University of Nottingham and was previously Visiting Professor of Brewing at Heriot-Watt University in Scotland. Charlie is a Fellow of the Institute of Brewing & Distilling, Fellow of the Society of Biology and Fellow of the International Academy of Food Science and Technology. Bamforth is Editor in Chief of the Journal of the American Society of Brewing Chemists, is on the editorial boards of several other journals and has published innumerable papers, articles and books on beer and brewing – and also written prolifically on soccer. His recent contributions have included The Brewmaster’s Art (A seven CD recording in The Modern Scholar series), Beer is Proof God Loves Us: Reaching for the Soul of Beer and Brewing (FT Press) and the first of a five-part series on beer quality called simply Foam (ASBC). In October 2010 he was on The Honor Roll as one of the 20 professors who are changing the classroom in the US (Playboy magazine). Charlie has been featured extensively in the media, including by the BBC, Discovery Channel, NPR’s Science Friday, PBS, New Scientist, Popular Mechanics, Los Angeles Times, San Francisco Chronicle, Popular Science and many more. He has presented at innumerable venues including Google, the New York Academy of Sciences, the Bohemian Club, the National Press Club and Xerox Parc Forum. In 2011, Charlie was honoured by the Award of Distinction from the American Society of Brewing Chemists for ‘exceptional contributions and long diligent service to brewing science and the brewing industry’. He has been honoured by the students of UC Davis as one of the top three educators in his college. In July 2012, he was elected Vice President of the Institute of Brewing and Distilling and will become President for a two year stint in July 2014.