The chemical compounds in beer give it a distinctive taste, smell and appearance. The majority of compounds in beer come from the metabolic activities of plants and yeast and so are covered by the fields of biochemistry and organic 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 of brewing: carbohydrates (from malt), hops, yeast, and water.
The carbohydrate source is an essential part of the beer because unicellular yeast organisms convert carbohydrates into energy to live. Yeast metabolize the carbohydrate source to form a number of compounds including ethanol. The process of brewing beer starts with malting and mashing, which breaks down the long carbohydrates in the barley grain into more simple sugars. This is important because yeast can only metabolize very short chains of sugars. [3] Long-carbohydrates are polymers, large branching linkages of the same molecule over and over. In the case of barley, we mostly see polymers called amylopectin and amylose which are made of repeating linkages of glucose. 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 activates enzymes called amylases. The shape of these enzymes, their active 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 a hydrolysis 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 plant Humulus 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 called humulones) and beta-acids (also called lupulones). 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 the glycolysis the simple sugars obtained from the malting process, and convert pyruvate, the major organic product of glycolysis, into carbon dioxide and water via the cellular 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 called fermentation. Fermentation converts pyruvate into ethanol through the intermediate acetaldehyde.
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. The ion species present in water can affect the metabolic pathways of yeast, 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 required cofactors for yeast enzymes. [4]
In aerobic conditions, yeast turns sugars into pyruvate then converts pyruvate into water and carbon dioxide. This process can carbonate beers. In commercial production, the yeast works in anaerobic conditions to convert pyruvate into ethanol, 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 as Ostwald ripening. These larger bubbles lead to “coarser” foam on top of poured beer.
Beers can be carbonated with CO2 or made sparkling with an inert gas such as nitrogen (N2), argon (Ar), or helium (He). Inert gases are not as soluble in water as carbon dioxide, so they form bubbles that do not grow through Ostwald 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 as gas and high performance liquid chromatographs coupled to mass spectrometers, have discovered over 7,700 different chemical compounds in beers. [8]
The beer foam stability depends amongst other on the presence of transition metal ions (Fe2+
, Co2+
, Ni2+
, Cu2+
...), macromolecules such as polysaccharides, proteins, and isohumulone compounds from hops 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 low concentration (1 – 2 ppm) were popular in the sixties, but raised the question of cobalt toxicity in case of undetected accidental overdosage during beer production. As an alternative, organic foam stabilizers are produced by hydrolysis of recovered by-products of beer manufacture, such as spent grains or hops residues. [9]
Amongst the large spectrum of purified, or modified, natural food additives available on the market, soluble carboxymethyl hydroxyethyl cellulose, [10] propylene glycol alginate (PGA, food additive with E number E405), [11] pectins and gellan gum have also been investigated as foam stabilizer.
In 1957, two brewing chemists, Thorne and Helm, discovered that the Co2+
cation was able to stabilize beer foam and to avoid beer overfoaming and gushing. [12] The addition of a tiny amount of cobalt ions in the range 1 – 2 mg/L (ppm) was effective. Higher concentrations would be toxic and lower ones ineffective.
Cobalt is a transition metal whose atomic orbitals are able to interact with ligands, or functional groups (–OH, –COOH, –NH2), attached to organic molecules naturally present in the beer, making macromolecular coordination complexes stabilizing the beer foam. Cobalt could behave as an inter- or intra-molecular bridge between different polysaccharide molecules (changing their shape or size), or cause some conformational 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 being complexed with certain nitrogenous constituents of the beer (e.g., amino acids from malt proteins), might produce surface-active substances inactivating the gaseous nuclei responsible for overfoaming 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 other transition metal ions such as these of nickel and iron, but not by cobalt ions. Isohumulone (an iso-alpha acid responsible for the bitter taste of hops) 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 the US FDA, cobalt sulfate was commonly used at low concentration in the USA as an additive to stabilize beer foam and to prevent gushing after beer is exposed to vibrations during its transport or handling.
Although cobalt is an essential micronutrient needed for vitamin B12 synthesis, excess levels of cobalt in the body can lead to cobalt 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's cardiomyopathy. The first issues mentioned in the literature were reported in Canada in the middle of the sixties after an accidental overdosage in the Dow Breweries in Quebec city.
In August 1965, a person presented to a hospital in Quebec City with symptoms suggestive of alcoholic 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 the Dow brand; thirty out of those drank more than 6 litres (12 pints) of beer per day. [16] Epidemiological studies found that the Dow Breweries had been adding cobalt sulfate to the beer for foam stability since July 1965 and that the concentration added in the Quebec city brewery was ten times that of the same beer brewed in Montreal where there were no reported cases. [17] [18]
A particular problem with beer is that, unlike wine, its quality tends to deteriorate as it ages. [20] A cat urine smell and flavor called ribes, 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 of 2-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 free radicals, for example, which can change the chemical structures of compounds in beer that give them their taste. [22] Oxygen radicals can cause increased concentrations of aldehydes from the Strecker degradation reactions of amino acids in beer. [23]
Beer is unique when compared to other alcoholic beveragess 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 the headspace 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 the polymerization of polyphenols 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 Heuberger et al. (2012) concludes that storage temperature of beers affects the flavor stability. They found that the metabolite profile of room temperature and cold temperature stored beer differed significantly from fresh beer. They also have evidence to support significant beer oxidation after weeks of storage, which also has an effect on the flavor of beer. [25]
The off-flavour in beer, such as a cardboard or green apple taste, is often associated with the appearance of staling aldehydes. 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 only amino acid concentration (leucine (Leu), isoleucine (Ile), and phenylalanine (Phe), specifically) and dissolved oxygen concentration caused Strecker aldehyde formation. They also tested carbohydrate and Fe2+ additions. A linear 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 Vanderhaegen et al. (2003), different aging conditions were tested on a bottled beer after 6 months. They found a decrease in volatile esters was responsible for a reduced fruity flavor. They also found an increase in many other compounds including carbonyl compounds, ethyl esters, Maillard compounds, dioxolanes, and furanic ethers. [26] The carbonyl 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 used thiobarbituric acid (TBA) to estimate the staling substances after using an accelerated aging technique. They found that beer staling is reduced by scavengers of the hydroxyl radical (•OH), such as mannitol and ascorbic acid. They also tested the hypothesis that soybean extracts included in the fermenting wort enhance the shelf life of beer flavor. [27]
Beer is one of the oldest alcoholic drinks in the world, the most widely consumed, and the third most popular drink after water and tea. Beer is produced by the brewing and fermentation of starches from cereal grains—most commonly malted barley, although wheat, maize (corn), rice, and oats are also used. The fermentation of the starch sugars in the wort produces ethanol and carbonation in the beer. Most modern beer is brewed with hops, which add bitterness and other flavours and act as a natural preservative and stabilising agent. Other flavouring agents, such as gruit, herbs, or fruits, may be included or used instead of hops. In commercial brewing, natural carbonation is often replaced with forced carbonation.
Glycolysis is the metabolic pathway that converts glucose into pyruvate and, in most organisms, occurs in the liquid part of cells. The free energy released in this process is used to form the high-energy molecules adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide (NADH). Glycolysis is a sequence of ten reactions catalyzed by enzymes.
Hops are the flowers of the hop plant Humulus lupulus, a member of the Cannabaceae family of flowering plants. They are used primarily as a bittering, flavouring, and stability agent in beer, to which, in addition to bitterness, they impart floral, fruity, or citrus flavours and aromas. Hops are also used for various purposes in other beverages and herbal medicine. The hops plants have separate female and male plants, and only female plants are used for commercial production. The hop plant is a vigorous, climbing, herbaceous perennial, usually trained to grow up strings in a field called a hopfield, hop garden, or hop yard when grown commercially. Many different varieties of hops are grown by farmers around the world, with different types used for particular styles of beer.
Lactic acid is an organic acid. It has the molecular formula CH3CH(OH)COOH. It is white in the solid state and it is miscible with water. When in the dissolved state, it forms a colorless solution. Production includes both artificial synthesis as well as natural sources. Lactic acid is an alpha-hydroxy acid (AHA) due to the presence of a hydroxyl group adjacent to the carboxyl group. It is used as a synthetic intermediate in many organic synthesis industries and in various biochemical industries. The conjugate base of lactic acid is called lactate. The name of the derived acyl group is lactoyl.
Wheat beer is a top-fermented beer which is brewed with a large proportion of wheat relative to the amount of malted barley. The two main varieties are German Weizenbier and Belgian witbier; other types include Lambic, Berliner Weisse, and Gose.
Digestion is the breakdown of carbohydrates to yield an energy-rich compound called ATP. The production of ATP is achieved through the oxidation of glucose molecules. In oxidation, the electrons are stripped from a glucose molecule to reduce NAD+ and FAD. NAD+ and FAD possess a high energy potential to drive the production of ATP in the electron transport chain. ATP production occurs in the mitochondria of the cell. There are two methods of producing ATP: aerobic and anaerobic. In aerobic respiration, oxygen is required. Using oxygen increases ATP production from 4 ATP molecules to about 30 ATP molecules. In anaerobic respiration, oxygen is not required. When oxygen is absent, the generation of ATP continues through fermentation. There are two types of fermentation: alcohol fermentation and lactic acid fermentation.
The Maillard reaction is a chemical reaction between amino acids and reducing sugars to create melanoidins, the compounds which give browned food its distinctive flavor. Seared steaks, fried dumplings, cookies and other kinds of biscuits, breads, toasted marshmallows, and many other foods undergo this reaction. It is named after French chemist Louis Camille Maillard, who first described it in 1912 while attempting to reproduce biological protein synthesis. The reaction is a form of non-enzymatic browning which typically proceeds rapidly from around 140 to 165 °C. Many recipes call for an oven temperature high enough to ensure that a Maillard reaction occurs. At higher temperatures, caramelization and subsequently pyrolysis become more pronounced.
Zinc sulfate describes a family of inorganic compounds with the formula ZnSO4(H2O)x. All are colorless solids. The most common form includes water of crystallization as the heptahydrate, with the formula ZnSO4·7H2O. As early as the 16th century it was prepared on the large scale, and was historically known as "white vitriol" (the name was used, for example, in 1620s by the collective writing under the pseudonym of Basil Valentine). Zinc sulfate and its hydrates are colourless solids.
Ethanol fermentation, also called alcoholic fermentation, is a biological process which converts sugars such as glucose, fructose, and sucrose into cellular energy, producing ethanol and carbon dioxide as by-products. Because yeasts perform this conversion in the absence of oxygen, alcoholic fermentation is considered an anaerobic process. It also takes place in some species of fish where it provides energy when oxygen is scarce.
Fermentation is a metabolic process that produces chemical changes in organic substances through the action of enzymes. In biochemistry, fermentation is narrowly defined as the extraction of energy from carbohydrates in the absence of oxygen, while in food production, it may more broadly refer to any process in which the activity of microorganisms brings about a desirable change to a foodstuff or beverage. The science of fermentation is known as zymology.
Isohumulones are chemical compounds that contribute to the bitter taste of beer and are in the class of compounds known as iso-alpha acids. They are found in hops.
When drinking beer, there are many factors to be considered. Principal among them are bitterness, the variety of flavours present in the beverage and their intensity, alcohol content, and colour. Standards for those characteristics allow a more objective and uniform determination to be made on the overall qualities of any beer.
Brettanomyces claussenii is a wild yeast of the genus Brettanomyces which has a negative Pasteur effect. It and Brettanomyces anomalus share identical mtDNA. In the wild, it is found on the skins of fruit. It has been shown to be useful for wine and beer fermentation as well as ethanol production.
Lupulone is an organic chemical compound with the molecular formula C26H38O4 and an appearance of a yellow powder which was historically used in beer brewing.
In organic chemistry, the aromatic alcohols or aryl-alcohols are a class of chemical compounds containing a hydroxyl group (−OH) bonded indirectly to an aromatic hydrocarbon group, in contrast to the phenols, where the hydroxyl group is bonded directly to an aromatic carbon atom.
Isovaleraldehyde organic compound, also known as 3-methylbutanal, with the formula (CH3)2CHCH2CHO. It is an aldehyde, a colorless liquid at STP, and found in low concentrations in many types of food. Commercially it is used as a reagent for the production of pharmaceuticals, perfumes and pesticides.
Grodziskie is a historical beer style from Poland made from oak-smoked wheat malt with a clear, light golden color, high carbonation, low alcohol content, low to moderate levels of hop bitterness, and a strong smoke flavor and aroma. The taste is light and crisp, with primary flavors coming from the smoked malt, the high mineral content of the water, and the strain of yeast used to ferment it. It was nicknamed "Polish Champagne" because of its high carbonation levels and valued as a high-quality beer for special occasions.
Symbiotic fermentation is a form of fermentation in which multiple organisms interact in symbiosis in order to produce the desired product. For example, a yeast may produce ethanol, which is then consumed by an acetic acid bacterium. Described early on as the fermentation of sugars following saccharification in a mixed fermentation process.
Ale is a type of beer, brewed using a warm fermentation method. In mediaeval England, the term referred to a drink brewed without hops.
A beerfault or defect is a flavour deterioration caused by chemical changes of organic compounds in beer, either due to improper production processes, or storage. Chemicals that can cause flavour defects in beer are aldehydes, lipids, and sulfur compounds. Small fluctuations within fermentation byproducts can lead to the concentration of one or more of these chemicals exceeding the standard threshold, creating a flavour defect.
