fermentation Review The Impact of Simple Phenolic Compounds on Beer Aroma and Flavor Michael Lentz Department of Biology, University of North Florida, Jacksonville, FL 32224, USA; [email protected]; Tel.: +1-904-620-1064 Received: 2 February 2018; Accepted: 13 March 2018; Published: 19 March 2018 Abstract: Beer is a complex beverage containing a myriad of flavor- and aroma-active compounds. Brewers strive to achieve an appropriate balance of desired characters, while avoiding off-aromas and flavors. Phenolic compounds are always present in finished beer, as they are extracted from grains and hops during the mashing and brewing process. Some of these compounds have little impact on finished beer, while others may contribute either desirable or undesirable aromas, flavors, and mouthfeel characteristics. They may also contribute to beer stability. The role of simple phenolic compounds on the attributes of wort and beer are discussed. Keywords: hydroxycinnamic acid; yeast metabolism; fermentation; beer; volatile phenols 1. Introduction There are approximately 1000–2000 chemical compounds in beer [1]. Many of these are present at levels well below their flavor, aroma, and other perception thresholds, and therefore, do not contribute to beer perceptions. The wide array of flavors, aromas, and mouthfeel attributes across world beer styles result from the presence, to varying degrees, of many organic and inorganic compounds. For many compounds, we have little understanding of the potential contribution to beer aroma and flavor, while others have been investigated extensively. A wide range of phenolic compounds are present in beer, extracted from raw materials during the malting, mashing, and brewing process. These include complex polyphenols that contribute to mouthfeel, beer haze, antioxidant properties and stability, as well as foam retention [2,3]. Simple phenolic compounds can have a major impact on beer aroma and flavor, both positive and negative. In this paper, the role of these simple phenolic compounds in raw ingredients, their transformation during brewing and fermentation, and their impact on beer organoleptic properties is reviewed. 2. The Role of Phenolic Compounds in Plants Simple and volatile phenols in beer all originate from plant material, primarily malted grains and hops. The vast majority are extracted from raw materials during the brewing process (see Section3 below). Other sources are less direct, either through extraction of phenols from wood during barrel aging, or from burning of wood in smoked malt beers. These indirect sources are still plant-derived. Phenolic compounds are an integral and abundant component of the plant cell wall. Ferulic acid, p-coumaric acid, and sinapic acid are hydroxycinnamic acids (HCAs) that are converted to their hydroxyl-cinnamyl alcohol derivatives. These compounds are exported to the plant cell wall in a poorly understood process. In the cell wall, these “monolignols” react to form the complex polymer lignin, a major cell wall component that protects cell wall fibers and contributes to cell rigidity [4]. These same hydroxycinnamic acids also provide crucial cross-links between lignin and non-lignin fiber components of the cell wall, most commonly hemicellulose. Ester linkages between HCAs and Fermentation 2018, 4, 20; doi:10.3390/fermentation4010020 www.mdpi.com/journal/fermentation Fermentation 2018, 4, 20 2 of 13 Fermentation 2018, 4, x 2 of 13 celland wall cell polysaccharides, wall polysaccharides, particularly particularly arabinoxylans arabinoxylans are well-documented. are well-documented. Dehydrodiferulic Dehydrodiferulic acid also participatesacid also participates in polysaccharide-polysaccharide in polysaccharide-polysaccharide and polysaccharide-lignin and polysaccharide-lignin bridges [5,6 bridges]. [5,6]. InIn addition addition to theirto their structural structural role, HCAsrole, HCAs contribute contribute to plant to defense plant againstdefense pathogenic against pathogenic microbes. Theymicrobes. reduce They the biodegradabilityreduce the biodegradability of cell wall of polymers, cell wall servingpolymers, as aserving structural as a defensestructural to defense cell wall to breakdowncell wall breakdown [6]. Free HCAs [6]. Free as well HCAs as other as well phenolic as other cell phenolic wall components cell wall arecomponents directly toxic are todirectly microbial toxic cellto membranes,microbial cell leading membranes, to increased leading ion to permeability,increased ion thereby permeability, reducing thereby cell viability reducing [7 ].cell Common viability representative[7]. Common HCArepresentative structures HCA are shown structures in Figure are shown1. in Figure 1. FigureFigure 1. 1.Hydroxycinnamic Hydroxycinnamic acid acid structures. structures. 3.3. Extraction Extraction of of Simple Simple Phenolics Phenolics into into Beer Beer Wort Wort 3.1.3.1. Raw Raw Materials Materials 3.1.1.3.1.1. Grains Grains and and Malt Malt CerealCereal grains grains and and malt malt account account for for the the majority majority of theof the total total phenolic phenolic acid acid content content of beer of beer wort wort [8]. In[8]. barley In barley malt, ferulicmalt, ferulic acid and acidp-coumaric and p-coumaric acid predominate. acid predominate. HCA levels HCA vary levels among vary barley among varieties, barley andvarieties, may be and influenced may be influenced by malting by process, malting growing process, environment growing environment and barley and handling barley handling post-harvest. post- Publishedharvest. Published values vary values widely, vary ranging widely, from ranging 10 to from 15 µ g/g10 to dry 15 weightµg/g dry combined weight combined [9,10] to highs [9,10] of to ~100–300highs of µ~100–300g/g dry weightµg/g dry for weightp-coumaric for p-coumaric acid, and acid, ~400–600 and ~400–600µg/g dry µg/g weight dry for weight ferulic for acid ferulic [11, 12acid]. These[11,12]. wide These variances wide variances may be due may to be extraction due to extracti method,on method, analytical analytical methods, methods, and whether and whether free, bound, free, orbound, all forms or all of HCAsforms of are HCAs analyzed. are analyzed. In all varieties In all investigated,varieties investigated, ferulic acid ferulic predominates acid predominates by a factor by ofa 2–5factor [3 ].of 2–5 HCAs [3]. inHCAs malt in are malt available are available for extraction for extraction into into wort wort in twoin two forms, forms, free free and and esterified. esterified. EsterifiedEsterified HCAs HCAs areare primarilyprimarily bound to to arabinoxylans. arabinoxylans. Greater Greater than than 90% 90% of the of theHCA HCA in malt in malt is ester- is ester-bound,bound, and and its availability its availability for for malt malt extraction extraction an andd later later conversion conversion to to free HCAHCA isis dependentdependent on on arabinoxylan-degradingarabinoxylan-degrading enzymes enzymes in in the the barley, barley, mainly mainly active active during during the the mashing mashing process process [11 [11–15].–15]. MostMost analyses analyses of of barley barley and and barley barley malt malt have have been been carried carried out out on on pilsner pilsner or or other other base base malt malt varieties.varieties. Vanbeneden et et al. al. [13] [13 ]ha haveve shown shown that that some some specialty specialty malts malts with withhigher higher degrees degrees of kilning of kilninghave altered have altered potential potential extraction extraction of HCAs. of HCAs. Worts Worts produced produced with withlighter lighter kilned kilned specialty specialty malts malts such suchas Munich as Munich and andVienna Vienna did didnot notdiffer differ in HCA in HCA extrac extractiontion from from base base malt malt worts. worts. Higher Higher kilned kilned malts malts(caramunich (caramunich and carafa) and carafa) yielded yielded lower lower ferulic ferulic acid acidto the to wort, the wort, with with a corresponding a corresponding increase increase in 4- invinylguaiacol, 4-vinylguaiacol, the thedecarboxylation decarboxylation product product of ferulic of ferulic acid acid[13,16]. [13 It,16 is]. proposed It is proposed that higher that higher kilning kilningtemperatures temperatures lead to lead some to degree some degree of thermal of thermal decarboxylation decarboxylation during during the kilning the kilning process. process. These Theseanalyses analyses were were carried carried out outon onlaboratory laboratory worts worts prepared prepared from from 50% 50% pilsner pilsner malt andand 50%50% specialty specialty malt.malt. It It is is expected expected that that at at the the lower lower percent percent of of specialty specialty malts malts often often used used in in brewing brewing that that these these malts malts willwill not not significantly significantly impact impact the the HCA HCA composition composition of of the the wort wort [13 [13].]. ReleaseRelease of of ester-bound ester-bound HCAs HCAs into into wort wort occurs occurs primarily primarily during during mashing. mashing. Barley Barley enzymes enzymes includeinclude arabinofuranosidases arabinofuranosidases andandβ β-xylan-xylan endohydrolases,endohydrolases, whichwhich degradedegrade andand solubilizesolubilize the the arabinoxylanarabinoxylan chains chains and and allow allow extraction extraction into wortinto [wort15]. Perhaps [15]. Perhaps more important more important is cinnamoyl is cinnamoyl esterase, whichesterase, releases which HCAs releases from HCAs xylan subunits,from xylan