Thechemical compounds inbeer give it a distinctive taste, smell and appearance. The majority of compounds in beer come from themetabolic activities of plants andyeast and so are covered by the fields ofbiochemistry andorganic chemistry.^[1] The main exception is that beer contains over 90% water and the mineral ions in the water (hardness) can have a significant effect upon the taste.^[2]

Four main ingredients are used for making beer in the process ofbrewing:carbohydrates (frommalt),hops,yeast, andwater.

The carbohydrate source is an essential part of the beer becauseunicellular yeast organisms convertcarbohydrates into energy to live. Yeastmetabolize the carbohydrate source to form a number of compounds includingethanol. The process ofbrewing beer starts withmalting andmashing, which breaks down the long carbohydrates in thebarley grain into moresimple sugars. This is important because yeast can only metabolize very short chains of sugars.^[3] Long-carbohydrates arepolymers, large branching linkages of the same molecule over and over. In the case of barley, we mostly see polymers calledamylopectin andamylose which are made of repeating linkages ofglucose. On very large time-scales (thermodynamically) these polymers would break down on their own, and there would be no need for the malting process.^[4] The process is normally sped up by heating up the barley grain.^[3] This heating process activatesenzymes calledamylases. The shape of these enzymes, theiractive site, gives them the unique and powerful ability to speed these degradation reactions to over 100,000 times faster. The reaction that takes place at the active site is called ahydrolysis reaction, which is a cleavage of the linkages between the sugars. Repeated hydrolysis breaks the long amylopectin polymers into simpler sugars that can be digested by the yeast.^[4]

Hops are the flowers of the hops plantHumulus lupulus. These flowers contain over 440 essential oils, which contribute to the aroma and non-bitter flavors of beer.^[4] However, the distinct bitterness especially characteristic of pale ales comes from a family of compounds called alpha-acids (also calledhumulones) and beta-acids (also calledlupulones). Generally, brewers believe that α-acids give the beer a pleasant bitterness whereas β-acids are considered less pleasant.^[4] α-acids isomerize during the boiling process in the reaction pictured. The six-member ring in the humulone isomerizes to a five-member ring, but it is not commonly discussed how this affects perceived bitterness.

In beer, the metabolic waste products of yeast are a significant factor. In aerobic conditions, the yeast will use in theglycolysis the simple sugars obtained from themalting process, and convertpyruvate, the major organic product of glycolysis, intocarbon dioxide and water via thecellular respiration. Many homebrewers use this aspect of yeast metabolism to carbonate their beers. However, under industrial anaerobic conditions yeasts cannot use pyruvate, the end products of glycolysis, to generate energy in cellular respiration. Instead, they rely on a process calledfermentation. Fermentation converts pyruvate intoethanol through the intermediateacetaldehyde.

Water can often play, directly or indirectly, a very important role in the way a beer tastes,^[2][4] as it is the main ingredient. Theion species present in water can affect themetabolic pathways ofyeast, and thus the metabolites one can taste. For example, calcium and iron ions are essential in small amounts for yeast to survive, because these metal ions are usually requiredcofactors for yeastenzymes.^[4]

In aerobic conditions, yeast turns sugars intopyruvate then converts pyruvate into water andcarbon dioxide. This process can carbonate beers. In commercial production, the yeast works in anaerobic conditions to convert pyruvate intoethanol, and does not carbonate beer. Beer is carbonated with pressurized CO₂. When beer is poured, carbon dioxide dissolved in the beer escapes and forms tiny bubbles. These bubbles grow and accelerate as they rise by feeding off of nearby smaller bubbles, a phenomenon known asOstwald ripening. These larger bubbles lead to “coarser” foam on top of poured beer.

Beers can be carbonated withCO₂ or made sparkling with aninert gas such asnitrogen (N₂),argon (Ar), orhelium (He). Inert gases are not assoluble in water as carbon dioxide, so they form bubbles that do not grow throughOstwald ripening. This means that the beer has smaller bubbles and a more creamy and stable head.^[6] These less soluble inert gases give the beer a different and flatter texture. In beer terms, the mouthfeel is smooth, not bubbly like beers with normal carbonation. Nitro beer (for nitrogen beer) could taste less acidic than normal beer.^[7]

Beers contain many aromatic substances. Up to now, chemists using advanced analytical instruments such asgas andhigh performance liquidchromatographs coupled tomass spectrometers, have discovered over 7,700 different chemical compounds in beers.^[8]

The beer foam stability depends amongst other on the presence oftransition metalions (Fe²⁺
,Co²⁺
,Ni²⁺
,Cu²⁺
...), macromolecules such aspolysaccharides,proteins, andisohumulone compounds fromhops in the beer. Foam stability is an important concern for the first perception of the beer by the consumer and is therefore the object of the greatest care by the brewers and the barmen in charge to serve draft beer, or to properly pour beer into a glass from the bottle (with a good head retention and without overfoaming, or gushing when opening the bottle).

Many patents for various types of beer foam stabilizers have been filed by breweries and the agro-chemical industry in the last decades.Cobalt salts added at lowconcentration (1 – 2ppm) were popular in the sixties, but raised the question ofcobalt toxicity in case of undetected accidental overdosage during beer production. As an alternative, organic foam stabilizers are produced byhydrolysis of recovered by-products of beer manufacture, such as spent grains orhops residues.^[9]

Amongst the large spectrum of purified, or modified, naturalfood additives available on the market, solublecarboxymethyl hydroxyethyl cellulose,^[10]propylene glycol alginate (PGA,food additive withE number E405),^[11]pectins andgellan gum have also been investigated as foam stabilizer.

In 1957, two brewing chemists, Thorne and Helm, discovered that theCo²⁺
cation was able to stabilize beer foam and to avoid beer overfoaming and gushing.^[12] The addition of a tiny amount ofcobalt ions in the range 1 – 2 mg/L (ppm) was effective. Higher concentrations would be toxic and lower ones ineffective.

Cobalt is atransition metal whoseatomic orbitals are able to interact withligands, orfunctional groups (–OH, –COOH, –NH₂), attached to organic molecules naturally present in the beer, makingmacromolecularcoordination complexes stabilizing the beer foam. Cobalt could behave as an inter- or intra-molecular bridge between differentpolysaccharide molecules (changing their shape or size), or cause someconformational changes^[13] of different types of molecules present in solution, affecting their absolute configuration and thus the foam molecular structure and its behavior.

Thorne and Helm (1957) also formulated the hypothesis that cobalt, by beingcomplexed with certain nitrogenous constituents of the beer (e.g.,amino acids frommaltproteins), might producesurface-active substances inactivating thegaseous nuclei responsible foroverfoaming and gushing.^[12]

Gushing is a specific problem also studied into more details by Rudin and Hudson (1958).^[14] These authors discovered that gushing is also promoted by othertransition metalions such as these ofnickel andiron, but not bycobalt ions.Isohumulone (aniso-alpha acid responsible for thebitter taste ofhops) and its combinations with Ni, or Fe, also favor gushing, while pure Co ions or their combination with isohumulone do not exhibit gushing and overfoaming. This explains why cobalt salts were specifically selected at a concentration of 1 – 2 mg/L as anti-gushing agent for beer. Rudin and Hudson (1958) and other authors also found that Co, Ni and Fe ions preferentially concentrate in the foam itself.^[14]

In the sixties, after approval by theUS FDA,cobalt sulfate was commonly used at low concentration in the USA as an additive to stabilize beerfoam and to prevent gushing after beer is exposed to vibrations during its transport or handling.

Althoughcobalt is an essentialmicronutrient needed forvitamin B12 synthesis, excess levels of cobalt in the body can lead tocobalt poisoning and must be avoided. It triggered the development of qualitative and quantitative analysis methods to accurately assay cobalt in beer in order to prevent accidental overdosage and cobalt poisoning.^[15]

Too high levels of cobalt are known to be responsible for the beer drinker'scardiomyopathy. The first issues mentioned in the literature were reported in Canada in the middle of the sixties after an accidental overdosage in theDow Breweries inQuebec City.

In August 1965, a person presented to a hospital inQuebec City with symptoms suggestive ofalcoholic cardiomyopathy. Over the next eight months, fifty more cases with similar findings appeared in the same area with twenty of these being fatal. It was noted that all were heavy drinkers who mostly drank beer and preferred theDow brand; thirty out of those drank more than 6 litres (12 pints) of beer per day.^[16]Epidemiological studies found that theDow Breweries had been addingcobalt sulfate to the beer forfoam stability since July 1965 and that the concentration added in the Quebec city brewery was ten times that of the same beer brewed inMontreal where there were no reported cases.^[17][18]

A particular problem with beer is that, unlikewine, its quality tends to deteriorate as it ages.^[20] Acat urine smell and flavor calledribes, named for the genus of the black currant, tends to develop and peak.^[21] A cardboard smell then dominates which is due to the release of2-nonenal.^[22] In general, chemists believe that the "off-flavors" that come from old beers are due to reactive oxygen species. These may come in the form of oxygen freeradicals, for example, which can change the chemical structures of compounds in beer that give them their taste.^[22] Oxygen radicals can cause increased concentrations ofaldehydes from theStrecker degradation reactions ofamino acids in beer.^[23]

Beer is unique when compared to otheralcoholic beverages because it is unstable in the final package. There are many variables and chemical compounds that affect the flavor of beer during the production steps, but also during the storage of beer. Beer will develop an off-flavor during storage because of many factors, including sunlight and the amount of oxygen in theheadspace of the bottle. Other than changes in taste, beer can also develop visual changes. Beer can become hazy during storage. This is called colloidal stability (haze formation) and is typically caused by the raw materials used during the brewing process. The primary reaction that causes beer haze is thepolymerization ofpolyphenols and binding with specific proteins. This type of haze can be seen when beer is cooled below 0 degrees Celsius. When the beer is raised to room temperature, the haze dissolves. But if a beer is stored at room temperature for too long (about 6 months) a permanent haze will form.^[24] A study done by Heubergeret al. (2012) concludes that storage temperature of beers affects the flavor stability. They found that themetabolite profile of room temperature and cold temperature stored beer differed significantly from fresh beer. They also have evidence to support significant beeroxidation after weeks of storage, which also has an effect on the flavor of beer.^[25]

Theoff-flavour in beer, such as a cardboard or green apple taste, is often associated with the appearance of stalingaldehydes. The Strecker aldehydes responsible for the flavor change are formed during storage of the beers. Philip Wietstock et al. performed experiments to test what causes the formation of Strecker aldehydes during storage. They found that onlyamino acid concentration (leucine (Leu),isoleucine (Ile), andphenylalanine (Phe), specifically) and dissolvedoxygen concentration caused Strecker aldehyde formation. They also testedcarbohydrate and Fe²⁺ additions. Alinear relationship was found between Strecker aldehydes formed and total packaged oxygen. This is important for brewers to know so that they can control the taste of their beer. Wietstock concludes that capping beers with oxygen barrier crown corks will diminish Strecker aldehyde formation.^[23]

In another study done by Vanderhaegenet al. (2003), different aging conditions were tested on a bottled beer after 6 months. They found a decrease involatileesters was responsible for a reduced fruity flavor. They also found an increase in many other compounds including carbonyl compounds, ethyl esters, Maillard compounds,dioxolanes, andfuranic ethers.^[26] Thecarbonyl compounds, as stated previously in the Wietstock experiments, will create Strecker aldehydes, which tend to cause a green apple flavor.Esters are known to cause fruity flavors such as pears, roses, and bananas.Maillard compounds will cause a toasty, malty flavor.

A study done by Charles Bamforth and Roy Parsons (1985) also confirms that beer staling flavors are caused by various carbonyl compounds. They usedthiobarbituric acid (TBA) to estimate the staling substances after using an accelerated aging technique. They found that beer staling is reduced by scavengers of thehydroxyl radical (^•OH), such asmannitol andascorbic acid. They also tested the hypothesis that soybean extracts included in the fermenting wort enhance theshelf life of beer flavor.^[27]

• Barm
• Beer head
• Beer faults
• Brewing
• Bright beer
• Carbonation
• Lightstruck beer
• Mouthfeel
• Isohumulone
• Yeast in Beermaking

• Barth, Roger (2013),The Chemistry of Beer: The Science in the Suds,John Wiley & Sons,ISBN 978-1-11873-379-0
• Baxter, Denise; Hughes, Paul (2001),Beer: Quality, Safety and Nutritional Aspects,Royal Society of Chemistry,ISBN 978-0-85404-588-4
• Hopkins, R (2011),Biochemistry Applied to Beer Brewing – General Chemistry of the Raw Materials of Malting and Brewing, Tobey Press,ISBN 978-1-44654-168-5
• Hornsey, Ian (2003),A History of Beer and Brewing, Royal Society of Chemistry,ISBN 978-0-85404-630-0
• Janson, Lee (1996),Brew Chem 101,Storey,ISBN 978-0-88266-940-3
• Verhagen, Leen (2010), "Beer Flavor",Comprehensive Natural Products II: Chemistry and Biology, vol. 3: Development & Modification of Bioactivity,Newnes, pp. 967–998,ISBN 978-0-08045-382-8
• Verzele, M; De Keukeleire, D (2013),Chemistry and Analysis of Hop and Beer Bitter Acids,Elsevier,ISBN 978-1-48329-086-7

• Tapping into the Chemistry of Beer and Brewing—an online lecture by Charles Bamforth, Professor of Malting & Brewing Sciences at the University of California
• Chemistry of BeerArchived 2015-07-17 at theWayback Machine—an online course in the subject at the University of Oklahoma

• Beer
• Alcohol chemistry

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