● BIOCHEMISTRY

Way through the wood biochemistry for brewers and distillers

By Ian Hornsey generally being used for ageing being towards the mid-range (18 gallons; kilderkin and 36 gallons; barrel). “Wooden have been arguably the most signifi cant Casks/barrels are characterised by shipping in history. They served Romans, being tapered at each end and consist of curved wooden slats (staves) along explorers, pilgrims, pirates, pioneers and samurai their long axis; ‘a bulbous cylinder through 2,000 years of civilisation.” Diana Twede, with fl at ends’. In this respect they have some structural relationship to packaging professor and author, 2005. the wooden bucket, except that these have a closure at one end only, and the ntil the advent of plastic, wooden solid foodstuffs, they were the result of component staves are not curved. Ucontainers, commonly referred to the skill of coopers who were masters The ability to curve wooden slats as ‘barrels’ in the US and called ‘casks’ of woodworking and metalworking. would have been learned from early in the UK (because there the barrel is The UK term ‘cask’ encompasses boat builders, as author Henry Work a measure), had been a feature of eve- a number of container sizes from related: “The evolution of wooden ryday life for nearly two millennia. Ca- 4½ gallons (pin) through 54 gallons barrels interweaves constantly with pable of holding a variety of liquid and (hogshead) to 216 gallons (tun). Those that of wooden boats – those built with

bung hole stave bilge

head hoop

rivet

cant

head stave joint

bilge hoop chime quarter hoop croze

Parts of a cask Ceramic dolia, used to ferment and store wine

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to casks filled with firewood pitch and tallow and were using them offensive- ly. Previously, in 238 BC, the emperor Maximinus, wanting to cross a swol- len river in order to secure Aquileia, a town in north-east Italy, ordered his engineers to fashion a ‘floating bridge’ out of redundant wine casks in order to achieve his goal. So casks had multiple uses! The celebrated Moving wine in Roman times find of casks found at Silchester, the Roman city of Calleva Atrabatum, Such wood is not ideal and would be indicated the extent of trade between avoided these days because it is dif- Britain and mainland Europe during ficult to bend and fashion. Because the 3rd century AD. Made from silver wooden artefacts do not usually sur- (Abies alba), they probably con- vive and therefore contribute little to tained Rhenish wine, and had latterly the archaeological record, it is difficult been used as well linings. to ascertain exactly when and where Kenneth Kilby notes “That by the ‘Tubs’ from tomb of Hesy-re; coopered above, casks evolved. The ‘favourites’ are the time of the Crusades (starting 1095) copper below (with lids); ca. 2690 BC Celtic tribes of central and western wooden casks were the standard timbers and planks”. But as Twede Europe who were skilled in both wood- means of transporting all manner says: “Wooden barrels are related to craft and metallurgy, but the Vikings of liquids and provisions ... and the basketry, which is one of the earliest and Romans also have claims. Dates cooper became one of the foremost of packaging technologies,” adding that for the original Celtic involvement with tradesmen.” By medieval times, the “The word ‘cooper’ is related to the casks seem to be around 900-800 BC. cask was Europe’s primary storage word ‘coop’, which originally referred Long before this, open wooden and transport container, and with the to a basket for chickens.” Others have buckets, clearly made by a cooper, huge variety of products contained proposed that ‘cooper’ comes from the are documented from the end of the therein, the nature of a cask’s con- Latin cupa, meaning ‘cask’. Old Kingdom in ancient Egypt. The tents would be known by tradesmen evidence comes from James Quibell’s by its size, style, and markings. By the Some early history drawings from the tomb of the ancient 16th century, casks were so important The robust wooden cask was an effec- Egyptian official Hesy-re (ca. 2690 BC), to trade of all kinds that ships were tive wine storage replacement for the which show wooden staved buckets as designed specifically to hold them. In ceramic , which was more part of an extensive row of measuring reality, casks were then a currency of fragile and suffered from an inherent . In Plate XIII he illustrates: sorts, their value being a combination instability, and the larger, ceramic, “Four tubs, two of wood two of cop- dolium which was used to ferment per... probably for measuring corn.” and store wine and was often semi- He remarks upon the graining of the buried for that purpose. Apart from wooden ‘tubs’, “which is best shown being more robust, casks could be in the largest tub but one, where five constructed to greater dimensions and planks are clearly distinguished...it will their very shape allowed close contact be noted that real cooper’s work is in- storage; in relation to amphorae, tended – barrels with bevelled staves. wooden casks could be made much The hoops [made of ebony] at top and larger without adding significantly to bottom are square in section, those in their weight and fragility. the middle are rounded.” Later, also Most authorities would accept that in ancient Egypt, a wall painting in the The Silchester casks in the Mediterranean region it was tomb of Beni Hassan (1900 BC) shows around AD 500 that wood replaced raisins (probably) being stored in open clay as the major construction mate- wooden-staved containers. rial for wine storage. The main use for wooden casks today is for ageing From the to the of alcoholic drinks, mainly wines and Crusades spirits although beers now figure more By the latter days of the Roman prominently. For many wines, distilled Empire, the use of casks was clearly beverages, and beers, wood ageing is widespread, for ’s one of the most important (and expen- Naturalis Historiae, in the section sive!) quality-defining factors. dealing with ‘Wine Vessels and Wine The 5th century BC Greek historian Cellars’, we find: “In the vicinity of the Herodotus of Halicarnassus (ca.484- Alps, they put their wines in wooden 425 BC), who travelled widely, related vessels hooped around.” He also that around 800-900 BC, wine was describes how Julius Caesar found exported by river from Armenia to himself facing an army of Gauls in Babylon in casks made of palm wood. south-west France who had set fire Cask identification marks (15th, 16th century) www.ibd.org.uk Brewer and Distiller International April 2016 z 49 l BIOCHEMISTRY

Cooperages of different ages

Wooden hoops; Northern Europe, 16th century of their size and content and because of this, casks transcended language 18th century French cooperage and cultural differences. Traders, pi- rates and customs officials in the 16th century would have learned the worth of a cask size in much the same way people today learn the worth of a £50 or $50 note! Because of their value, casks con- taining wine or spirits were susceptible to tampering. “Sucking the monkey” is a term that was applied to the act of Cooperage at T. & R. Theakston, Masham, ca. drilling two small holes in a cask hold- 16th century German cooperage 1990 ing alcohol and the offender would then blow through one hole which would (or ‘dry’). The former is for storing North America or in much of conti- induce the liquid to flow from the other. and transporting liquids, the latter for nental Europe, for example, the climax After the theft, the holes would be other goods. Some dry cooperage was vegetation could be mixed deciduous plugged with small pieces of wood. for carrying dry cargo that must not forest and there were plentiful sup- get wet (e.g. gunpowder) and this is plies of species. This meant that The wood of choice defined as ‘dry-tight’ wines and spirits produced in these ar- The metal hoops used to hold staves eas could be stored in new casks, but together were not in common use until Wood and drink in most of Scotland, where the climax the late 18th century, prior to this time Experience tells us that fresh distilled vegetation is mixed pine/birch forest, staves were bound by strands of a spirit can be undrinkable because of there was a paucity of mature oak and flexible wood such as willow, hazel, or its harsh taste and pungency, as well so a constant supply of new wood was chestnut. With plentiful supplies, oak as the fact that its consumption can not forthcoming, meaning that second- became the wood of choice in Europe produce harmful side effects. Pro- hand containers became de rigueur. and North America for wine storage. longed storage in oak casks imparts These had mostly been used for In addition, the porosity of oak wood colour, a complex aroma, and harmony the maturation of bourbon or for the permitted evaporation and oxygenation in the mouth. Not only are complex fermentation and shipment of sherry. of the liquid contents. In the days when wood phenolics extracted into the In Scotland, ex-bourbon (from US) and beer was fermented and then shipped spirit, but structural molecules, such ex-sherry casks (from Spain) are used in wooden casks, internal pressure as cellulose and hemicellulose are de- repeatedly (‘re-filling’) until they can was such that leakage was a major polymerised and the products thereof no longer fulfil their function (i.e. ‘ex- problem and to counteract this, an in- are also extracted. In all, around 200 hausted’) whence they may be regen- ner layer of pitch was added to casks. substances can be extracted or trans- erated in the cooperage. Sherry casks Overall, there are two basic types of formed from oak by ethanol which will have a longer ‘maturation capability’ cooperage; ‘tight’ (or ‘wet’) and ‘slack’ directly participate in the formation of than ex-American bourbon casks, flavour, aroma, and mouth feel of the and would normally be preferred but final product. demand for these exceeds supply. 1st stave cut Since the production of spirits In the case of Scotch whisky, the 3rd stave cut and wine would have originally been maturation period became so im- heartwood seasonal activities, it is assumed that portant that it became incorporated the practice of maturing these drinks into the legal definition of the drink! sapwood 2nd would have evolved from the need to To conform to this, one finds that a stave cut store the final product. Manufactur- considerable proportion of the produc- ers would soon have found that oak tion costs of Scotch whisky distillers casks were usually the most suitable are attributable to cooperage oak containers for doing so. Even so, John investment. Storage in oak casks is an Conner believes that: “Historically the expensive process in any stored/aged quarter sawing origins of maturation are obscure, with alcoholic drink. medullary the majority of whisky in the 18th and rays 19th centuries being probably drunk Tree type and basic wood un-matured.” Bio-geographical crite- chemistry ria would determine the wherewithal An oak is a tree or shrub in the genus Stave cuts in timber processing (from Russell, 2003 – by for storage which would vary with tree Quercus. There are around 600 spe- kind permission) species availability. In many parts of cies of Quercus worldwide, of which

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about 20 are economically important. species, and its wood is denser and grain and few knots. Slow-growing Just to confuse taxonomy, many more coarse-grained than European specimens produce wood with tighter Quercus species readily form hybrids. . Other American oak species that grain. Ideally, trees should be around Historically, the species most used have been used include: Q.bicolor, Q. 100 years old, with a straight unblem- in cooperage are Q. alba (American macrocarpa, Q. stellata, and Q. lyrata, ished trunk, and around five feet in white oak), and two European spe- while Q.pyrenaica is an alternative circumference. Only the wood from cies, Q. robur (pedunculate oak) and Q. European species. The important ground level to the appearance of petraea (sessile oak). In practice, they Mediterranean species, Q. suber, is the first lateral branches is used and can often serve the same purposes, only used for cork manufacture. this should be able to yield up to four but there are differences in wood The best oak wood for casks is casks, depending on size. extractives; most notably the fact that from slow-growing trees grown in Once staves are cut from the trunk, American oak wood contains a lower dense groves, which produces an they are left to dry for 3-5 years (sea- ellagitannin content than the European upright trunk growth with a straight soning), a process that will reduce the Wood polymers

oak heartwood

major cell-wall components extractives

lignin hemicellulose cellulose phenolics fatty acids other extractibles

guaiacyl & hexoses, hexose: polyphenols simple phenols lactones syringyl propanes pentoses D-glucose (ellagitannins) alcohols hydrocarbons norisoprenoids inorganic substances

Oak heartwood components

Wood tissue is composed of discreet cells and an intercellular zation of three phenypropanoid monomers; coniferyl, sinapyl and material that separates the cells, called the ‘middle lamella’, p-coumaryl alcohols.” The latter is only founds in grasses. and three insoluble polymers comprise the main part of (oak) In oak, lignin is formed by the polymerisation of coniferyl al- wood: cellulose, hemicellulose and lignin. cohol and sinapyl alcohol, yielding ‘guiaiacyl lignin’ and ‘syringyl Cellulose, which represents about half of total wood content, alcohol’ respectively. Cell walls contain all three polymers, while constitutes the wood ‘framework’. It is a linear polymer with a the middle lamella, which cements cells together, consists of a uniform chain structure, individual glucose-based units being pectic base mainly impregnated with lignin (around 30% of all bound by β(1-4)-glycosidic linkages. Internal H-bonding between lignin). Oak woods vary in composition, and the heartwood of Q. chains yields a cell wall framework onto which other molecules alba, for example, is composed of cellulose (49-52%) lignin (31- can adhere. 33%), hemicellulose (22%), and a fraction which can be extracted A matrix for the cellulose superstructure is provided by in hot water or ethyl ether (7-11%). The latter contains volatile hemicelluloses which are branched heteropolymers containing oils, volatile and non-volatile acids, sugars, steroids, tannic com- a variety of sugars, such as hexoses and pentoses. In oak wood, pounds, pigments, and inorganic compounds. hemicelluloses comprise some 15-30% cell wall dry weight, and In their native form, the three major wood polymers contrib- they are largely xylose based (i.e. xylans). ute nothing to beverage flavour, but via a series of reactions, The third major component, lignin, normally around 15- such as hydrolyses, oxidations, esterifications, acetalisations, 30% in woods, is a highly-branched three-dimensional polymer and polymerisations, their structures are modified to provide the structure, and, after, cellulose, is the second most abundant required compounds for the visual, taste and flavour characters polymer in the plant kingdom. Lignin confers mechanical for the stored beverage. Pyrolysis of cellulose and hemicel- strength to a tree by helping to bond cellulose and hemicellulose lulose during coopering leads to the formation of substituted together, as well as providing protection through its antioxidant furans and pyrans, the most significant entities being furfural capabilities. and 5-hydroxymethyl furfural (5-HMF). Phenolics resulting from HO A descrip- the breakdown of lignin and which are often present in seasoned tion of lignin and toasted wood include hydroxybenzoic acids (e.g. gallic, OH coniferyl alcohol would be: “An syringic, and vanillic) and hydroxycinnamic acids (e.g. p-cou- H3CO amorphous, maric and ferulic). Related compounds include hydroxybenzoic polyphenolic OCH3 aldehydes, such as vanillin, syringaldehyde, coniferaldehyde, material aris- and sinapaldehyde. There are several other volatile phenols and HO ing from an en- most of these compounds can be categorised according to their zyme-mediated biogenic or chemical origin; for example those based upon the OH dehydrogena- sinapyl alcohol guaiacyl (4-hydroxy-3-methoxyphenyl) or syringyl (4-hydroxy-3, H CO tive polymeri- 3 5-dimethoxyphenyl) nucleus emanate from lignin degradation. www.ibd.org.uk Brewer and Distiller International April 2016 z 51 l BIOCHEMISTRY

ellagitannins have been characterised in oak wood; castalagin; Tannins vescalagin; grandinin, and roburins A-E, and most can be extracted by aqueous alcoholic solutions (and other organic solvents). The Classification two most abundant are the stereoisomers vescalagin and casta- Non-hydrolysable tannins were called ‘condensed tannins’, and lagin, which comprise 40-60% of all ellagitannins. and the others consisted of oligomeric and polymeric proanthocyanidins. To com- are either dimers of these two or differ by presence of a pentose plicate matters, some ellagitannins are not totally hydrolysable substituent. because they possess additional bonds (due to the C-C coupling In the European oaks, Quercus robur and Q. petraea, ellagitan- of their catechin units to glycosidic moieties). Such tannins were nins can comprise up to 10% dry weight of heartwood. In addition, originally called ‘non-classified tannins’, but this changed to ‘com- flavanols, including dihydroflavonols, often occur in association with plex tannins’. Over the years there have always been nomenclatu- ellagitannins to give flavano-ellagitannin derivatives such as acutis- ral problems over the disparity in ‘tannin’ molecule size, with both simin A and acutissimin B. These compounds can occur in whisky, large and small molecules exhibiting ‘tannin properties’. as can the related gallagyl-glucosides peduncalagin and punica- lagin. The level of these latter compounds in a product depends tannins upon oak species and wood seasoning and toasting.

hydrolysable tannins complex tannins condensed tannins

gallotannins

ellagitannins

Recent advances in tannin chemistry have seen a reassess- ment of classification and this has meant that tannins and ‘related polyphenols’ can now be placed in two categories: Type A, with “constant structures” and Type B, “of variable composition”. This distinction can be used alongside the previously mentioned stand- ard classification of ‘hydrolysable’ and ‘condensed’ tannins. From the point of view of this article, it is hydrolysable tannins that provide the major compounds in oak heartwood, which con- tains high concentrations of these phenolic extractives, principally the ellagitannins. With more than 500 natural products identified so far from the plant kingdom, these are by far the largest group of known tannins. Gallotannins are the simplest hydrolysable tannins, containing a polyphenolic and a polyol residue, mostly derived from D-glucose. The hydroxy functions of the polyol residues may be partly, or fully, substituted with galloyl units. Gallotannins are eas- ily degraded by many microbes, while ellagitannins are relatively The eight oak heartwood ellagitannins resistant because of the more complex structure conferred by the The amount of vescalagin and castalagin that is extractable by further C-C coupling. The presence of gallotannins in oak heart- organic solvents declines as the heartwood ages, due to a decrease wood has not been unequivocally determined. in solubility caused by tannin polymerisation. Hydrolysis of casta- Ellagitannins are derived from HO lagin gives ellagic acid and castalin and that of vescalagin gives biosynthetic stepwise oxidation ellagic acid and vescalin. Despite the high solubility of hydrolys- of gallotannins and subsequent O able ellagitannins in ethanol-water solutions, they are not found in oligomerisation processes, and HO OH spirits or wines that have been matured in oak casks. Admittedly, can be classified according to their OH ellagitannins are rapidly denatured when wood is heated during biogenic oxidation stages. They are HO toasting but since they are not present in spirits aged in non- HO esters of hexahydroxydiphenic acid OH toasted casks, this cannot totally explain ellagitannin absence in the (HHDP) and a polyol, usually glu- final product. It has long been known that the relationship between cose or quinic acid. HHDP is the O known wood-derived compounds and distilled alcohol is not always product of the first-stage biogenic OH straightforward, for some, such as vanillin, are often found in new- oxidation of galloyl groups. Ellagi- Hexahydroxydiphenic acid (HHDP) make spirit, albeit at low levels. In malt whisky production, this is tannins have enormous structural likely to be attributable to barley lignins emanating from the wash. variability because of the different linkages of HHDP residues with Using the ratio of vescalagin to castalagin, it has been shown the glucose moiety and their strong tendency to form dimeric and that the composition of ellagitannins varies with heartwood and that oligomeric derivatives. this is independent of the decline in the concentration of extract- able ellagitannins with increasing heartwood age. Wood density Ellagitannins in oak wood is another important factor, as is flexibility which is governed by When exposed to acids or bases, ester bonds are hydrolysed and way in which the long wood (‘medullary’) cells are arranged. The HHDP spontaneously rearranges into water insoluble ellagic acid conversion of sapwood into heartwood is critical since it determines (EA). Ellagitannins are thought to derive from a common gallotan- the number of blockages (‘tyloses’) formed in medullary cells and nin biosynthetic precursor, penta-O-galloyl-β-D-glucose. Eight these reduce the likelihood of leakage.

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likelihood of cask leakage and extract more tannin from the wood. Cutting the trunk for stave production is a real skill and follows a set pattern. Sea- soning results in wood dehydration, until it becomes aligned with ambient humidity. During the process, there is a reduction of hydrolysable polyphe- nols, such as ellagitannins, which are polymerised and precipitated – thus decreasing astringency. Most of this occurs on the wood surface, but is also known in inner wood. A combination of the effects of rainfall, temperature variation, and UV radiation contribute to these chemical changes.

Toasting the wood Later, during coopering, it is toasting

(or charring, at higher temperatures) Brown-Forman.com where the most significant changes take place, the most important being Tannins are polyphenolic secondary species differences as well as those the decrease in ellagitannins and the metabolites of higher plants, some- due to geography, etc. American oak concomitant increase in ellagic acid. times called ‘plant polyphenols’ and, has a lower ellagitannin content than In addition, the three main polymers because of their enormous structural its French (European) counterparts, pyrolyse to yield volatile extractables, diversity, have enjoyed several dispa- and higher levels of β-methyl-γ- such as guaiacol, 4-methylguaiacol, rate ‘definitions’ over the years. For octalactone particularly the cis isomer. trans-oak lactone, cis-oak lactone, and example, one (not so ancient!) organic These cis- and trans-lactones are of vanillin which are formed mainly from chemistry text defines tannins as: “C- great importance in Scotch whisky the degradation of lignin. Over 100 and O-glycosidic derivatives of gallic ageing and are known as the ‘whisky volatile phenolics have been identified acid (3,4,5-trihydroxybenzoic acid)”, lactones’. They are formed during from oak wood, with the oak lactones which is far too specific because not all charring of bourbon casks or toasting being most important. Heat will also tannins necessarily contain a galloyl of wine casks alongside coloured com- destroy undesirable resinous com- unit or derivative (i.e. condensed tan- pounds, which give the finished whisky pounds in wood (e.g. ‘sawdust’ aroma, nins which are built up from flavanoid a ‘mature colour’. mainly due to trans-2-nonenal). precursors. The name ‘tannin’ was Certain types of molecule, exhibit When considering seasoning and originally given to any plant extract great variation between these two oak toasting oak, it is important to realise that exhibited astringency, without any types and can be used as markers for that many of the numerous flavour regard for its chemical structure. oak wood origin. Most notable in this compounds are produced at different Features that distinguish tannins respect are norisoprenoids, several of temperatures, and that oak species from other types of plant polyphenol which have been found in significant toast differently. Seasoning and toast- are the ability of the former to bind amounts in American oak, but are ing ensure the structural integrity of to proteins, basic compounds, large absent/present in trace amounts in the finished cask. The former prevents molecular compounds, pigments, and European oak. Conversely, 4-oxo-7,8- shrinkage, while toasting is aimed at metallic ions. Many tannins also have dihydro-β-ionol was a major noriso- stabilising the curve of the wood. It is important biological characteristics, prenoid in European oak, but absent the wood ultra-structure of certain oak including anti-oxidant, antimicrobial, in Q. alba. Over thirty norisoprenoids species that marks them out as ideal and anti-tumour activity. The proper- have been identified from oak, the first for tight cooperage. For details of cask ties of tannins, which are moderately being β-ionone. manufacture see Conner et al., (2003) size molecules, are based on the fact In bourbon manufacture, casks and Work, (2014). that they have two or three phenolic tend to be re-used many times, re- hydroxyl groups on a phenyl ring. Ac- sulting in a diminution of the overall From sapwood to heartwood cording to the polyphenol groups in quality of substances leached from Large quantities of tannins and related their molecules, tannins were once the wood and the depth at which polyphenols are laid down in oak dur- classified into two groups: pyrogallol- leaching occurs increases. In some ing the transformation of sapwood type and catechol (catechin)-type. experiments with cask staves pre- into heartwood. These compounds Subsequently, these two groups were pared from Q. alba, it was found that provide protection for the plant against renamed to ‘hydrolysable tannins’ and repeated exposure to whisky led to invasive micro-organisms; such resist- ‘condensed tannins’ respectively. less leaching. New charred (unused) ance being conferred by the ability of bourbon staves had more extractable tannins to complex with proteins and Whisky lignin hydrolysis products, whisky polysaccharides. In addition to species Regarding the chemical nature of lactones and coloured substances differences, the content of oak extrac- oak woods, most attention has been than either used Bourbon or first fill tives varies with factors such as geo- paid to trees grown in France and Scotch staves, and ‘exhausted’ Scotch graphical origin and forestry practice. North America, where there are clear staves had the least. It has been www.ibd.org.uk Brewer and Distiller International April 2016 z 53 l BIOCHEMISTRY

and heavy), an increase in concentra- tion of nine monophenols (vanillin, acetovallinone, syringaldehyde, aceto- syringone, guaiacol, 4-ethylguaiacol, eugenol, thymol, and salicylalde- Wood-ageing beer at Epic Brewing, Denver, CO hyde) was observed. Wood origin and suggested that the concentrations of beers because of the decreasing level degree of toasting influenced both lignin-derived guaiacyl and syringyl of ethanol contained in these drinks. monophenol concentrations and their compounds could be used as an indi- For convenience, most experi- sensory effects. Medium toasted cator of cask exhaustion. ments on beer seem to have been chips promoted eugenol, thymol, and carried out by using oak chips. During salicylaldehyde levels in beer, while Beer brewing, malt and hops are the pri- the other six monophenols were most Most research into wood ageing marily responsible for imparting aro- associated with heavy wood toasting. relates to spirits and wines, with matics to beer. Phenolic compounds Beer flavour was greatly influenced relatively little published on beer. such as ferulic acid, gallic acid, vanillic by wood, with notes such as ‘va- With the never-ending search for new acid, quercetin and vanillin are known nilla’, ‘woody’, ‘smoky’, ‘creamy’, and products, especially by craft brew- beer phenolics and contribute to fla- ‘burned’ being to the fore. ers, beer wood-ageing is now a fertile vour, colour and body as well as haze Further work with model solu- field of study and there is an increased and astringency. tions showed that process parameters consumer interest in such beer. In a Brazilian study, a young influenced the availability of the nine Superficially, one might propose that, Pilsner-style beer was subjected to a monophenols mentioned above. They using the same wood and conditions, 3-month maturation period (at 0°C) all increased quasilinearly in intensity a different rate and type of in the presence of French oak cubes with increasing amounts of oak chips uptake in wood extrac- with different toasting regimes. and their extraction was enhanced tives might occur Oak barrels were also used. Beer by low pH (3.5) and higher ethanol in spirits, sample strength varied between content (→8-10%v/v). Elevated storage wines and 4.34 - 5.19% v/v and samples were temperature (20°C) also enhanced analysed monthly. Results indicated extractability, but, conversely, pres- that there were some interactions ence of yeast and oxygen decreased between wood and beer but they extraction of some phenolics. Vanillin, were “smaller than expected,” and acetovanillone, syringaldehyde, and this was attributed to the low level of acetosyringone are particularly unsta- alcohol in the beer, low maturation ble when oxygen is present. temperature and short maturation period. The lager stored with the Other wood for cooperage most heavily toasted cubes had the In a comparison of East European (Ro- highest concentration of low-molec- mania, Ukraine, Moldova) Q. robur and ular weight phenolics, followed by Q. petraea wood with that from French- beer matured in an oak barrel. It was grown samples, and with American agreed that neither oak cubes nor Q. alba, discrete chemical differences wooden casks affected the quality of in their extractives were revealed. As the beer. judged by their ellagitannin and whisky In experiments carried out at KU lactone levels, the greatest disparity Leuven over a period of 60 days with was between American- and French- an 8.3% ABV ‘neutral blonde beer’ grown wood, whereas East European suffused with oak chips (identical samples were somewhat intermedi- dimensions) of Q. robur and Q. alba, ate between these extremes. Eastern Oak wood toasted to different degrees (medium European wood could be characterised

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by high levels of eugenol, 2-phenyle- acacia, cherry and mulberry (in thanol and aromatic aldehydes. that order). Over 50 different Oak wood is still the main mate- volatiles were identified from rial used in cooperage, certainly these woods, with each spe- for products such as vinegar, cies showing a characteristic cider, and some spirits such as profile. Oak proved to be brandy, although only oak and the richest in volatiles, with chestnut (Castanea sativa) chestnut running second; are at present approved by mulberry had the lowest the International Organisa- levels of volatiles. tion of Vine and Wine (OIV) for winemaking. Some produc- Accelerated ageing ers prefer their drink to be For drinks that must be ma- non-oak matured, and this is tured for extended periods, such not always on financial grounds. as brandy, where some 200 phe- A compromise would be to use oak nolics may be extracted from wood, casks with non-oak staves incorpo- any means of hastening the process rated. has to be considered. Whatever the When assessing other woods for method, there should be no deleteri- ageing suitability, it is the phenolic ous effect on those phenolics neces- fraction that is the most important sin- Chestnut sary for taste, aroma, etc. Several gle parameter. Among woods that have physical methods have been used to been studied extensively for possible tion of castalagin or vesacalagin with a accelerate wine ageing processes, cooperage use are chestnut (Castanea gallic acid residue. including γ-irradiation, ultrasonics, sativa), , cherry (Prunus avium) acacia Chestnut and oak are both in the UV visible light, as well as electric (Robinia pseudoacacia), ash (Fraxinus same flowering plant family, Fagace- fields (EFs), including pulsed AC excelsior; F. americana), and to a lesser ae, as is (Fagus),and both are electric field (PEF) and static electric extent, mulberry (Morus alba; M. nigra). characterised by having high levels of field (SEF). Before toasting, each wood exhibits ellagitannins after seasoning, espe- In general, these methods are a different and specific polyphenolic cially vescalagin and castalagin. After efficient, non-thermal, and inexpen- profile, with both qualitative and quan- light toasting of seasoned chestnut, sive. For example, it has been shown titative differences. Toasting changes concentrations decreased by 70%, that treatment with 20 kHz ultrasonic these profiles according to the level and this was increased to a 95% loss waves aged rice wine much more of heat applied and wood thus treated after an intense toasting. Levels of quickly than standard methods, and will assume its own set of phenolic ellagitannins in seasoned and lightly that γ-irradiation rapidly produced markers. toasted oak followed the same pat- very high quality rice wine. Treatment In toasted cherry wood, for exam- tern, but detected levels were lower of a young red wine with PEF gave an ple, methyl syringate, benzoic acid, than chestnut; even an elimination of increase in the concentration of most methyl vanillate, p-hydroxybenzoic ellagitannins from heartwood surface phenolics. acid, 3,4,5-trimethylphenol, and p- layers being observed after heavy When EF treatments were ap- coumaric acid, the flavonoids narin- toasting. Since it is only stave wood plied to brandy ageing, promising genin, aromadendrin, isosakuranetin that has to be toasted, higher levels results were obtained. Most beneficial and taxifolin can be used as markers. of ellagitannins will be available for substances, such as tannins, total Cherry wood also contains many con- extraction from cask head wood. phenols, volatile phenols, and, esters densed tannins of the procyanidin type Gallotannins are present in chestnut were increased, while some harmful which can be useful markers. In acacia wood and were found at their highest ones (acetaldehyde, acetal, and higher heartwood, where there are relatively levels in seasoned wood. They also alcohols) were decreased. Most of this low levels of condensed tannins and showed a 70% decrease after a light sort of work is carried out on small no hydrolysable tannins, one finds high heat treatment and a 95% reduction casks (i.e.2L or 5L), but scientists levels of the flavonoids robinetin and at higher temperature. Such facts believe that similar effects should dihydrorobinetin, which are found in no allow us to distinguish chemically exist when EF treatments are applied other cooperage woods. The latter is a between these two woods. to production size casks (i.e. 225L or marker for ‘upmarket’ vinegars aged When oak, chestnut, acacia, above). in acacia. cherry and mulberry woods were exposed to a 50:50 ethanol-water References Chestnut wood solution (‘model spirit’) and a 12% Kilby, K. The Cooper and His Trade. J. Baker, Chestnut wood, which is especially ABV ‘model wine’ solution, there was London. 1971. rich in gallic acid and ellagitannins, considerable variation in polyphe- Russell, I. (ed.) Whisky, Technology, Pro- has been used enologically around nol extractability. The model spirit duction and Marketing, Academic Press, London. 2003. the Mediterranean for years. It is now extracts of chestnut and mulberry ex- Twede, D. The cask age: the technology and known that chestnut heartwood has a hibited the highest total polyphenols, history of wooden barrels. Packaging and polyphenolic profile very similar to that followed by cherry, acacia and, finally, Technology Science. 18(5): 253-264. 2005. of oak. A difference, however, revolves oak, whereas the model wine solution Work, H.H., Wood, Whiskey and Wine: A around 1-O-galloyl castalagin, which extracted most polyphenolic mate- History of Barrels, Reaktion Books, London. seems to originate from the esterifica- rial from oak – followed by chestnut 2014. www.ibd.org.uk Brewer and Distiller International April 2016 z 55