Fundamentals of beer and hop chemistry
Denis De Keukeleire
University of Gent - Faculty of Pharmaceutical Sciences - Laboratory of Pharmacognosy and Phytochemistry - Harelbekestraat 72 B-9000 - Gent - Belgium
(www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422000000100019)
Beer brewing is an intricate process encompassing mixing and further elaboration of four essential raw materials, including barley malt, brewing water, hops and yeast. Particularly hops determine to a great extent typical beer qualities such as bitter taste, hoppy flavour, and foam stability. Conversely, hop-derived bitter acids account for an offending lightstruck flavour, which is formed on exposure of beer to light. These various processes are presented in detail, while due emphasis is placed on state-of-the-art hop technology, which provides brewers with efficient means to control bitterness, foam, and light-stability thereby allowing for the production of beers with consistent quality.
Beer is a fermented aqueous drink based on starch and flavoured by hops. This simple definition encompasses the four essential ingredients, which are necessarily used in the brewing of beer....When a slurry of barley malt and brewing water (called 'mash') is heated at a temperature around 60°C, the malt enzymes, mainly amylases but also proteases, degrade starch and proteins, leading to a mixture of sugars and peptides or amino acids. For that purpose, barley must be subjected, prior to mashing, to a controlled germination, during which these enzymes are formed in the barley grain. Such germinated barley is known as barley malt. The starch-to-sugar conversion is stopped by heating. Depending on the conditions (time, temperature), pale or amber-coloured or even dark malts are obtained, the colour being due to caramelization of sugars and to Maillard-type reactions. It is important to notice that the colour of beer is derived from the colour of the malt(s) used. Furthermore, it is obvious that coloured malts exhibited a distinct taste, which often is characteristic of particular dark beers (www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422000000100019).
(Slidesharepresentatie over bierbrouwen)
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(Slidesharepresentatie over maischen)
Denis De Keukeleire
University of Gent - Faculty of Pharmaceutical Sciences - Laboratory of Pharmacognosy and Phytochemistry - Harelbekestraat 72 B-9000 - Gent - Belgium
(www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422000000100019)
Beer brewing is an intricate process encompassing mixing and further elaboration of four essential raw materials, including barley malt, brewing water, hops and yeast. Particularly hops determine to a great extent typical beer qualities such as bitter taste, hoppy flavour, and foam stability. Conversely, hop-derived bitter acids account for an offending lightstruck flavour, which is formed on exposure of beer to light. These various processes are presented in detail, while due emphasis is placed on state-of-the-art hop technology, which provides brewers with efficient means to control bitterness, foam, and light-stability thereby allowing for the production of beers with consistent quality.
Beer is a fermented aqueous drink based on starch and flavoured by hops. This simple definition encompasses the four essential ingredients, which are necessarily used in the brewing of beer....When a slurry of barley malt and brewing water (called 'mash') is heated at a temperature around 60°C, the malt enzymes, mainly amylases but also proteases, degrade starch and proteins, leading to a mixture of sugars and peptides or amino acids. For that purpose, barley must be subjected, prior to mashing, to a controlled germination, during which these enzymes are formed in the barley grain. Such germinated barley is known as barley malt. The starch-to-sugar conversion is stopped by heating. Depending on the conditions (time, temperature), pale or amber-coloured or even dark malts are obtained, the colour being due to caramelization of sugars and to Maillard-type reactions. It is important to notice that the colour of beer is derived from the colour of the malt(s) used. Furthermore, it is obvious that coloured malts exhibited a distinct taste, which often is characteristic of particular dark beers (www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422000000100019).
(Slidesharepresentatie over bierbrouwen)
After filtration, the sugar solution, in brewers' jargon called 'wort', is transferred to the brewing kettle, where it is boiled during at least one hour with the addition of hops (Humulus lupulus L.). The amount of hops needed is only a fraction of the substantial quantities of malt used in the brewery. Usually, a few grams of hops are sufficient as a quantitatively minor, but qualitatively major ingredient with crucial impact on well-defined beer features. Besides the formation of insoluble complexes with proteins and polypeptides, contributing to the colloidal stability of beer, hops sterilize the wort solution, which takes care of the bacteriological stability of beer. The most important asset of hops is the bitter taste conferred to, particularly, blond beers. Furthermore, hops are necessary for the stabilization of beer foam, while, on the other hand, the most precarious off-flavour in beer, called lightstruck flavour, involves degradation of hop-derived components (see below).
After cooling and removal of spent hops, the liquid, known as 'hopped wort' is pumped to the fermentation vessels and yeast is added under aeration for growth. During the anaerobic phase yeast cells convert sugars to ethanol and carbon dioxide. Depending on the temperature during fermentation and the nature of yeast collection at the end of the fermentation period, beers are distinguished as being produced by 'bottom fermentation' or 'top fermentation'. Yeast strains, appropriate for bottom-fermented beers (Saccharomyces carlsbergensis), are active below 5°C and they settle to the bottom of the fermentor after production of about 5% ethanol. Conversely, yeasts, typical for the production of top-fermented beers (Saccharomyces cerevisiae), operate at ambient temperature and resist higher concentrations of ethanol, up to 12%. When the activity stops, the yeast cells collect to the top as a dense foam (www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422000000100019).
A typical fermentation takes about one week thereby delivering a so-called 'green beer' or 'young beer', which is not drinkable, as a number of offending (bad taste and smell) compounds are formed during fermentation. Consequently, beers need a maturation or lagering period of several weeks at about 0°C, during which the unwanted components are slowly decomposed. High concentrations of diacetyl and pentane-2,3-dione are particularly obnoxious for the quality of lager beers ('pilsner-type') and scrutinous monitoring is required. Only after the content has decreased below critical values (ppb-ranges), beer can be packaged. For prolonged conservation beers may be pasteurized. Special beers often require a slow (several months) second fermentation, usually in oak kegs, to generate sour flavours (www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422000000100019).
The tastes of varying beer types are derived from a judicious choice of raw materials. Hops, in particular, account, in addition to the bitter taste, for a delicate hoppy flavour in beer. Until now, this extremely valuable organoleptic feature has not been fully defined. It is generally accepted that the hoppy aroma in beer is a complex of sensory impressions resulting from many different volatile compounds at low concentrations, many of them acting in synergism1. It appears that volatiles, contained in the hop oil (0.5-3% in hops), and non-volatiles, present in the hop polyphenolic fraction (3-6%), contribute to a full mouthfeel during beer tasting. However, the composition of these hop fractions is very complex and their study has been hampered by the low and varying concentrations normally found in beer. Thus, detailed insights are, at present, elusive (www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422000000100019).
Obviously, during the boiling of hops in wort many constituents are volatilized or oxidized and the hop-derived constituents, present in beer, are to a large extent different from the original molecules, present in hops. It should be interesting to trace the fate of individual volatile hop constituents, particularly terpenes, such as myrcene, caryophyllene, humulene and farnesene, during wort boiling, but the variability due to differing conditions and beers preclude that general statements be made. An inevitable conclusion is that the very intricate composition of the hop oils becomes even more complex during wort boiling (www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422000000100019).
Many brewers try to conserve part of the original composition by adding precious hop varieties ('aroma hops') near the end of the boiling period. This procedure, known as 'late hopping', may be combined with 'dry hopping', a special technique involving addition of hops to beer just before packaging. By doing so, some original hop constituents are directly transferred to the aqueous matrix thereby imparting a discrete hoppy character to the beer. The growing success of small brewers (microbreweries, brewpubs) suggests increasing recognition of the opportunities that the use of hops presents, in particular for developing various hop aromas and flavours.
Hops may contribute up to about one third of the total polyphenols in beer. Little doubt now remains that, amongst the polyphenolics, the low-molecular-weight proanthocyanidins determine the colloidal stability of beer (www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422000000100019).
The hop acids have pronounced bacteriostatic activity; they strongly inhibit the growth of Gram-positive bacteria. This action has been attributed to the interference of the prenyl group, characteristic of the side chains of the hop acids, with the function of the cell plasma membrane. It appears that the more prenyl groups (three in the beta-acids) are present, the stronger the bacteriostatic action is. This remarkable bio-activity is of importance for killing micro-organisms during wort boiling, which ultimately leads to a sterile beer (www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422000000100019).
It has been thought that iso-alpha-acids and phenolic compounds are key components with regard to beer stability. Iso-alpha-acids are partly responsible for production of ageing off-flavours, including stale and cardboard flavours7. Volatile aldehydes, such as trans-non-2-enal, are formed during storage of bottled beer from various precursors, including hop lipids. On the other hand, it has been known for quite some time that beer decomposes on exposure to light, thereby generating an offending 'skunky' flavour. Thus, beer must be stored, either in opaque cans, or in green or brown bottles, in order to prevent light from being transmitted through the glass.
The cause for this phenomenon is the vulnerability of the iso-alpha-acids to light. The resulting decomposition leads to formation of the so-called 'lightstruck flavour' (www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422000000100019).
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Lichtsmaak= metaalsmaak = pasteurisatie. Verantwoordelijke stof hiervoor: 3-methyl-2-buteen-1-thiol. In isohumulon wordt, onder invloed van licht een keten verbroken. De vrijgekomen zijketen verliest koolmonoxide, waarbij een radicaal ontstaat. Dit radicaal kan reageren met een thiol-radicaal, afkomstig van de afbraak van zwavelhoudende eiwitten. Op deze wijze ontstaat dan 3-methyl-2-buteen-1-thiol. Synoniem voor 3-methyl-2-buteen-1-thiol: prenylmercaptaan (http://members.chello.nl/f.hoedemakers/Techniek_9_L.html).
Beer is an extremely complex drink and several hundreds of constituents have been identified hitherto. Hops are vital to the organoleptic qualities of beer, including taste and flavour. During the multi-stage brewing process many hop components are modified, but even the use of sophisticated separation and analysis techniques has, until now, not allowed to identify unambiguous relations between particular hop-derived compounds and sensory characteristics.
As opposed to the intricate chemistry of hop oils and hop polyphenols, the bitter taste features of beer have been adequately profiled. Alpha-acids or humulones are converted during wort boiling to iso-alpha-acids or isohumulones, which impart the typical bitter taste to beer. Modern hop technology has unlocked the full potential of hops by providing novel hop products, which allow brewers to adjust desired beer properties. Moreover, off-flavours, such as the lightstruck flavour, can be obviated by bittering beers with advanced hop products, based on reduced iso-alpha-acids or reduced isohumulones, thereby rendering beers light-stable (www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422000000100019).
Both bottom- and top-fermented beers, using advanced hop products, have been made in our pilot brewing facilities and sensory analysis proved their superior quality with respect to more traditional brews. Judicious application of advanced analytical methods has been essential to the successful penetration of new hop technologies into the beer world. State-of-the-art analyses involving high-efficient chromatographic variants (e.g. reversed phase HPLC)12 and electro-driven separation methods (e.g. CZE)13 have superseded classical unspecific protocols. Validated quantification of various hop-derived compounds must support beer quality control from a modern brewing perspective (www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422000000100019).
After cooling and removal of spent hops, the liquid, known as 'hopped wort' is pumped to the fermentation vessels and yeast is added under aeration for growth. During the anaerobic phase yeast cells convert sugars to ethanol and carbon dioxide. Depending on the temperature during fermentation and the nature of yeast collection at the end of the fermentation period, beers are distinguished as being produced by 'bottom fermentation' or 'top fermentation'. Yeast strains, appropriate for bottom-fermented beers (Saccharomyces carlsbergensis), are active below 5°C and they settle to the bottom of the fermentor after production of about 5% ethanol. Conversely, yeasts, typical for the production of top-fermented beers (Saccharomyces cerevisiae), operate at ambient temperature and resist higher concentrations of ethanol, up to 12%. When the activity stops, the yeast cells collect to the top as a dense foam (www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422000000100019).
A typical fermentation takes about one week thereby delivering a so-called 'green beer' or 'young beer', which is not drinkable, as a number of offending (bad taste and smell) compounds are formed during fermentation. Consequently, beers need a maturation or lagering period of several weeks at about 0°C, during which the unwanted components are slowly decomposed. High concentrations of diacetyl and pentane-2,3-dione are particularly obnoxious for the quality of lager beers ('pilsner-type') and scrutinous monitoring is required. Only after the content has decreased below critical values (ppb-ranges), beer can be packaged. For prolonged conservation beers may be pasteurized. Special beers often require a slow (several months) second fermentation, usually in oak kegs, to generate sour flavours (www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422000000100019).
The tastes of varying beer types are derived from a judicious choice of raw materials. Hops, in particular, account, in addition to the bitter taste, for a delicate hoppy flavour in beer. Until now, this extremely valuable organoleptic feature has not been fully defined. It is generally accepted that the hoppy aroma in beer is a complex of sensory impressions resulting from many different volatile compounds at low concentrations, many of them acting in synergism1. It appears that volatiles, contained in the hop oil (0.5-3% in hops), and non-volatiles, present in the hop polyphenolic fraction (3-6%), contribute to a full mouthfeel during beer tasting. However, the composition of these hop fractions is very complex and their study has been hampered by the low and varying concentrations normally found in beer. Thus, detailed insights are, at present, elusive (www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422000000100019).
Obviously, during the boiling of hops in wort many constituents are volatilized or oxidized and the hop-derived constituents, present in beer, are to a large extent different from the original molecules, present in hops. It should be interesting to trace the fate of individual volatile hop constituents, particularly terpenes, such as myrcene, caryophyllene, humulene and farnesene, during wort boiling, but the variability due to differing conditions and beers preclude that general statements be made. An inevitable conclusion is that the very intricate composition of the hop oils becomes even more complex during wort boiling (www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422000000100019).
Many brewers try to conserve part of the original composition by adding precious hop varieties ('aroma hops') near the end of the boiling period. This procedure, known as 'late hopping', may be combined with 'dry hopping', a special technique involving addition of hops to beer just before packaging. By doing so, some original hop constituents are directly transferred to the aqueous matrix thereby imparting a discrete hoppy character to the beer. The growing success of small brewers (microbreweries, brewpubs) suggests increasing recognition of the opportunities that the use of hops presents, in particular for developing various hop aromas and flavours.
Hops may contribute up to about one third of the total polyphenols in beer. Little doubt now remains that, amongst the polyphenolics, the low-molecular-weight proanthocyanidins determine the colloidal stability of beer (www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422000000100019).
The hop acids have pronounced bacteriostatic activity; they strongly inhibit the growth of Gram-positive bacteria. This action has been attributed to the interference of the prenyl group, characteristic of the side chains of the hop acids, with the function of the cell plasma membrane. It appears that the more prenyl groups (three in the beta-acids) are present, the stronger the bacteriostatic action is. This remarkable bio-activity is of importance for killing micro-organisms during wort boiling, which ultimately leads to a sterile beer (www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422000000100019).
It has been thought that iso-alpha-acids and phenolic compounds are key components with regard to beer stability. Iso-alpha-acids are partly responsible for production of ageing off-flavours, including stale and cardboard flavours7. Volatile aldehydes, such as trans-non-2-enal, are formed during storage of bottled beer from various precursors, including hop lipids. On the other hand, it has been known for quite some time that beer decomposes on exposure to light, thereby generating an offending 'skunky' flavour. Thus, beer must be stored, either in opaque cans, or in green or brown bottles, in order to prevent light from being transmitted through the glass.
The cause for this phenomenon is the vulnerability of the iso-alpha-acids to light. The resulting decomposition leads to formation of the so-called 'lightstruck flavour' (www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422000000100019).
Lichtsmaak= metaalsmaak = pasteurisatie. Verantwoordelijke stof hiervoor: 3-methyl-2-buteen-1-thiol. In isohumulon wordt, onder invloed van licht een keten verbroken. De vrijgekomen zijketen verliest koolmonoxide, waarbij een radicaal ontstaat. Dit radicaal kan reageren met een thiol-radicaal, afkomstig van de afbraak van zwavelhoudende eiwitten. Op deze wijze ontstaat dan 3-methyl-2-buteen-1-thiol. Synoniem voor 3-methyl-2-buteen-1-thiol: prenylmercaptaan (http://members.chello.nl/f.hoedemakers/Techniek_9_L.html).
Beer is an extremely complex drink and several hundreds of constituents have been identified hitherto. Hops are vital to the organoleptic qualities of beer, including taste and flavour. During the multi-stage brewing process many hop components are modified, but even the use of sophisticated separation and analysis techniques has, until now, not allowed to identify unambiguous relations between particular hop-derived compounds and sensory characteristics.
As opposed to the intricate chemistry of hop oils and hop polyphenols, the bitter taste features of beer have been adequately profiled. Alpha-acids or humulones are converted during wort boiling to iso-alpha-acids or isohumulones, which impart the typical bitter taste to beer. Modern hop technology has unlocked the full potential of hops by providing novel hop products, which allow brewers to adjust desired beer properties. Moreover, off-flavours, such as the lightstruck flavour, can be obviated by bittering beers with advanced hop products, based on reduced iso-alpha-acids or reduced isohumulones, thereby rendering beers light-stable (www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422000000100019).
Both bottom- and top-fermented beers, using advanced hop products, have been made in our pilot brewing facilities and sensory analysis proved their superior quality with respect to more traditional brews. Judicious application of advanced analytical methods has been essential to the successful penetration of new hop technologies into the beer world. State-of-the-art analyses involving high-efficient chromatographic variants (e.g. reversed phase HPLC)12 and electro-driven separation methods (e.g. CZE)13 have superseded classical unspecific protocols. Validated quantification of various hop-derived compounds must support beer quality control from a modern brewing perspective (www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422000000100019).