A cerveja é uma bebida alcoólica fermentada derivada do amido e aromatizada com lúpulo (Humulus lupulus L.). Os α-ácidos são extraídos do cone do lúpulo e durante o processo de cozimento do mosto são isomerizados em iso-α-ácidos (IAAs), na configuração cis- e trans-, conferindo qualidade de espuma e sabor amargo característico da cerveja. Neste trabalho, é reportado que IAAs sofrem degradação fotossensibilizada por flavinas (Φ = 4,8x10-3 mol einstein-1), mesmo na presença de compostos fenólicos (ácido ferúlico, Φ = 2,0x10-3 mol einstein-1) em excesso molar de 10 vezes, sugerindo que radicais formados pela desativação do estado tripleto excitado da riboflavina por compostos fenólicos possam também estar envolvidos na degradação dos IAAs. Foram identificados dímeros e trímeros derivados do ácido ferúlico e p-coumárico através de LC-ESI-IT-MS como principais fotoprodutos de degradação dos compostos fenólicos. Reportamos a reatividade dos diferentes diastereoisômeros de iso-α-ácidos frente ao radical 2,2-difenil-1-picrilhidrazila (DPPH•), como modelo de radical peroxila, k2 = 0,41 e 1,3 L mol-1 s-1 para a reação com cis-IAA e trans-IAA em meio de etanol acidificado com 1 % ácido fórmico a temperatura de 25 °C, respectivamente. Estas constantes de velocidade específica sugerem que a degradação dos ácidos amargos via reação térmica em processo radicalar é importante no armazenamento do produto já que as constantes de velocidade de reação dos IAAs com o radical DPPH• são competitivas com as observadas para as reações de antioxidantes naturalmente presentes na cerveja com o radical DPPH• ([ácido ferúlico] = 0,2mg/Lcerveja; k2 = 1,18.102 M-1s-1). A análise dos dados termodinâmicos (Mistura de IAAs, ΔH‡ = 25 kJ mol-1 e ΔS‡ = -155 J mol-1 K-1) sugere um mecanismo de oxidação dos IAAs pelo radical DPPH• via HAT/PCET. A diferença de reatividade observada para os diastereoisômeros (cis/trans) está aparentemente relacionada ao arranjo estereoquímico dos grupos laterais isohexonoil e prenil conectados aos carbonos C(4) e C(5), respectivamente. Desta forma, sugere-se que a proximidade espacial dos sítios de insaturação na espécie trans- ocasiona um aumento na densidade eletrônica ou um fator apenas estatístico já que os H-alílicos estão próximos espacialmente, favorecendo desta forma a oxidação via radicalóide. / Beer is a fermented alcoholic beverage based on starch and flavored by hops (Humulus lupulus L.). The α-acids are extracted from hop cones and isomerize into iso-α-acids (IAAs) during the wort boiling, in cis- and trans- configuration, providing foam quality and the characteristic bitter taste of beer. In this work, is reported that these compounds undergo degradation photosensitized by flavins (Φ = 4,8x10-3 mol einstein-1), even in the presence of phenolic compounds (ferulic acid, Φ = 2,0x10-3 mol einstein-1) in 10-fold molar excess, suggesting that radicals formed during the deactivation of triplet excited state of riboflavin by phenolic compounds may be involved in the degradation of IAAs. Dimers and trimers derived from ferulic and p-coumaric acids were identified by LC-ESI-IT-MS as the main photoproducts of the phenolic compounds. We report the reactivity of the different diastereoisomers of IAAs towards the 2,2-diphenyl-1-picrylhydrazyl (DPPH•) radical, as a model for peroxyl radical, k2 = 0,41 e 1,3 L mol-1 s-1 for the reaction with cis-IAA and trans-IAA in ethanol acidified with 1% of formic acid, at the temperature of 25 °C, respectively. These specifics rate constants suggest that the degradation of the bitter acids via thermal reactions in an radicaloid process is important during the storage of the product since the reaction rate constant for IAAs and the DPPH• radical are competitive with the reaction rate constants for naturally occurring antioxidants in beer with the DPPH• radical ([ferulic acid] = 0,2mg/Lbeer; k2 = 1,18.102 M-1s-1). The analysis of the thermodynamical data (IAAs mixture, ΔH‡ = 25 kJ mol-1 e ΔS‡ = -155 J mol-1 K-1) suggest a HAT/PCET oxidation mechanism of IAAs by DPPH• radical. The difference of reactivity observed for the diastereoisomers (cis-/trans-) is possibly related to the stereochemical arrangement of the isohexonoyl and prenyl side chains connected to C(4) and C(5) carbons, respectively. In this way, is suggested that the spatial proximity of the insaturation sites in the trans- species lead to a increase in electronic density or due to a statistical factor since the allylic-H are close spatially, which favors the oxidation via radicaloid.

A cerveja é uma bebida alcoólica fermentada derivada do amido e aromatizada com lúpulo (Humulus lupulus L.). Os α-ácidos são extraídos do cone do lúpulo e durante o processo de cozimento do mosto são isomerizados em iso-α-ácidos (IAAs), na configuração cis- e trans-, conferindo qualidade de espuma e sabor amargo característico da cerveja. Neste trabalho, é reportado que IAAs sofrem degradação fotossensibilizada por flavinas (Φ = 4,8x10-3 mol einstein-1), mesmo na presença de compostos fenólicos (ácido ferúlico, Φ = 2,0x10-3 mol einstein-1) em excesso molar de 10 vezes, sugerindo que radicais formados pela desativação do estado tripleto excitado da riboflavina por compostos fenólicos possam também estar envolvidos na degradação dos IAAs. Foram identificados dímeros e trímeros derivados do ácido ferúlico e p-coumárico através de LC-ESI-IT-MS como principais fotoprodutos de degradação dos compostos fenólicos. Reportamos a reatividade dos diferentes diastereoisômeros de iso-α-ácidos frente ao radical 2,2-difenil-1-picrilhidrazila (DPPH•), como modelo de radical peroxila, k2 = 0,41 e 1,3 L mol-1 s-1 para a reação com cis-IAA e trans-IAA em meio de etanol acidificado com 1 % ácido fórmico a temperatura de 25 °C, respectivamente. Estas constantes de velocidade específica sugerem que a degradação dos ácidos amargos via reação térmica em processo radicalar é importante no armazenamento do produto já que as constantes de velocidade de reação dos IAAs com o radical DPPH• são competitivas com as observadas para as reações de antioxidantes naturalmente presentes na cerveja com o radical DPPH• ([ácido ferúlico] = 0,2mg/Lcerveja; k2 = 1,18.102 M-1s-1). A análise dos dados termodinâmicos (Mistura de IAAs, ΔH‡ = 25 kJ mol-1 e ΔS‡ = -155 J mol-1 K-1) sugere um mecanismo de oxidação dos IAAs pelo radical DPPH• via HAT/PCET. A diferença de reatividade observada para os diastereoisômeros (cis/trans) está aparentemente relacionada ao arranjo estereoquímico dos grupos laterais isohexonoil e prenil conectados aos carbonos C(4) e C(5), respectivamente. Desta forma, sugere-se que a proximidade espacial dos sítios de insaturação na espécie trans- ocasiona um aumento na densidade eletrônica ou um fator apenas estatístico já que os H-alílicos estão próximos espacialmente, favorecendo desta forma a oxidação via radicalóide. / Beer is a fermented alcoholic beverage based on starch and flavored by hops (Humulus lupulus L.). The α-acids are extracted from hop cones and isomerize into iso-α-acids (IAAs) during the wort boiling, in cis- and trans- configuration, providing foam quality and the characteristic bitter taste of beer. In this work, is reported that these compounds undergo degradation photosensitized by flavins (Φ = 4,8x10-3 mol einstein-1), even in the presence of phenolic compounds (ferulic acid, Φ = 2,0x10-3 mol einstein-1) in 10-fold molar excess, suggesting that radicals formed during the deactivation of triplet excited state of riboflavin by phenolic compounds may be involved in the degradation of IAAs. Dimers and trimers derived from ferulic and p-coumaric acids were identified by LC-ESI-IT-MS as the main photoproducts of the phenolic compounds. We report the reactivity of the different diastereoisomers of IAAs towards the 2,2-diphenyl-1-picrylhydrazyl (DPPH•) radical, as a model for peroxyl radical, k2 = 0,41 e 1,3 L mol-1 s-1 for the reaction with cis-IAA and trans-IAA in ethanol acidified with 1% of formic acid, at the temperature of 25 °C, respectively. These specifics rate constants suggest that the degradation of the bitter acids via thermal reactions in an radicaloid process is important during the storage of the product since the reaction rate constant for IAAs and the DPPH• radical are competitive with the reaction rate constants for naturally occurring antioxidants in beer with the DPPH• radical ([ferulic acid] = 0,2mg/Lbeer; k2 = 1,18.102 M-1s-1). The analysis of the thermodynamical data (IAAs mixture, ΔH‡ = 25 kJ mol-1 e ΔS‡ = -155 J mol-1 K-1) suggest a HAT/PCET oxidation mechanism of IAAs by DPPH• radical. The difference of reactivity observed for the diastereoisomers (cis-/trans-) is possibly related to the stereochemical arrangement of the isohexonoyl and prenyl side chains connected to C(4) and C(5) carbons, respectively. In this way, is suggested that the spatial proximity of the insaturation sites in the trans- species lead to a increase in electronic density or due to a statistical factor since the allylic-H are close spatially, which favors the oxidation via radicaloid.

<p>Hop (<i>Humulus lupulus</i> L) belongs to the Cannabaceae family and is well-known to be a dioecious, perennial climbing plant. The common hop is native to temperate climates, but due to its widespread commercial use in the brewing industry, hop plants are grown worldwide. The bittering components in hops, especially the a-acids, provide a pleasant bitterness and characteristic flavors to beer. On their own, a-acids do not contribute much to beer bitterness but do so after they are converted to iso-a-acids during the kettle boil step in brewing. b-acids in hops are not as important as a-acids, since they only supply about a third of the bittering power as a-acids but are mostly responsible for hops’ antimicrobial properties. The goal of this study was to investigate the effects of storage conditions on the degradation rate of hop constituents. Pelletized hops from two varieties grown in Knightstown, IN were studied: Cascade (typically used as aroma hops) and Chinook (mostly used as bittering hops). The impact of storage conditions on hop chemical constituents, hop storage index (HSI), and color parameters was evaluated at three storage temperatures (4℃, 25℃, 35^oC) with hop pellets exposed to two different gases in headspace (nitrogen or air) for a duration of up to 168 days (4℃ and 25℃) and 70 days (35℃). Hop acids content was determined by UV/Vis spectrophotometry and HPLC-DAD. Results showed that increased storage temperature decreased the a-acid concentration in hops. At 4°C and 25°C the loss in a-acids was (14.5 – 23.4%) whereas at 35°C there was a greater loss in a-acids. The a-acid loss between nitrogen flushed and air exposed pellets was not significantly different (p>0.05) except for Chinook pellets stored at 35°C. b-acids remained mostly stable for the duration of the study; however, at 35℃ the loss of b-acids over time became significant for both varieties. HIS values increased over time for both varieties at 4 and 25^oC, however, those values remained below the recommended 0.40 value. At 35^oC, the HSI values indicated unsuitable hops for brewing at the end of the study for both varieties. The color parameters L* and b* remained constant after 168 days at 4 and 25^oC, while the a* value and the hue angle showed a significant decrease with time and increasing temperature. At the conclusion of the study it was demonstrated that the loss in a-acids from Cascade pellets at 35℃ was lower than the loss in Chinook, suggesting that at high temperature the a-acids in cascade were more stable than those in Chinook.</p>

Os iso-&alpha;-ácidos são os principais constituintes responsáveis pelo sabor amargo da cerveja, sendo estes facilmente degradados durante o seu processo de envelhecimento ou exposição a radiação luminosa, em particular o diastereoisômero trans-. O radical 1-hidroxietila é o radical majoritário formado na cerveja durante o processo de envelhecimento. O presente trabalho descreve a reatividade dos iso-&alpha;-ácidos frente ao radical 1-hidroxetila através do uso da técnica de spin-trapping com detecção por espectroscopia de ressonância paramagnética de elétrons (RPE) e espectrometria de massas (ESI-(+)-MS/MS). Observou-se que ambos os diastereoisômeros cis- e trans-iso-&alpha;-ácidos são degradados na presença do radical 1-hidroxietila com constantes de velocidade aparentes de 1,8 108 e 9,2 109 L mol-1 s-1, respectivamente. A reatividade dos dihidro-iso-&alpha;-ácidos com o radical estudado foi similar à reatividade da mistura diastereoisomérica dos iso-&alpha;-ácidos, apresentando constante de velocidade aparente de 1,5 109 L mol-1 s-1. Os análogos tetrahidro-iso-&alpha;-ácidos não apresentaram reatividade para com o radical 1-hidroxietila, sugerindo os hidrogênios alílicos como sítio reacional. Adicionalmente, os cálculos ab initio por DFT demonstraram que os valores de BDE para os hidrogênios alílicos das cadeias laterais prenila e isohexenoila são equivalentes e, desta maneira, sugerindo a maior reatividade do diastereoisômero trans- a ser creditada a um fator entrópico, já que ambos os grupos estão no mesmo plano espacial. Os produtos de oxidação foram determinados por LC-ESI-MSn e verificou-se a formação dos hidroxi-allo-iso-&alpha;-ácidos, recentemente descritos na literatura. O conjunto de resultados obtidos possibilitou a proposta de mecanismo para processo de oxidação e perda dos ácidos amargos da cerveja, observado no envelhecimento da bebida. / The iso-&alpha;-acids are the main responsible constituents for the bitter taste of beer, they are easily degraded during the aging and light exposed process, specially the trans- diastereoisomer. The 1-hydroxyethyl radical is the major radical produced during the beer aging process. The present work describes the reactivity of iso-&alpha;-acids towards the 1-hydroxyethyl radical as probed by the spin-trapping technique and detected by electron paramagnetic resonance (EPR) and mass spectrometry (ESI-(+)-MS/MS). It was observed that both diastereoisomers cis- and trans-iso-&alpha;-acids are degraded in the presence of 1-hydroxyethyl radical with apparent rate constant of 1.8 108 e 9.2 109 L mol-1 s-1, respectively. The reactivity of dihydro-iso-&alpha;-acids towards the studied radical was similar to the reactivity of the iso-&alpha;-acids diastereoisomeric mixture, showing apparent rate Constant of 1.5 109 L mol-1 s-1. The tetrahydro-iso-&alpha;-acids analogues did not observed reactivity towards the 1-hydroxyethyl radical suggesting the allilic hydrogens as the reaction sites. In addition, the ab initio DFT calculations demonstrated that the BDE values for the allilic hydrogens of the prenyl and isohexenoyl side chains are equivalents and according to that suggesting the higher reactivity of the trans- diastereoisomer to be accounted to an entropic factor since both goups are in the same plane of the space. The oxidation products were determined by LC-ESI-MSn and its was verified the formation of hydroxyl-allo-iso-&alpha;-acids. The data colected allows a mechanism to be proposed for the oxidation process and loss of bitter acids of beer during the beverage aging.

Os iso-&alpha;-ácidos são os principais constituintes responsáveis pelo sabor amargo da cerveja, sendo estes facilmente degradados durante o seu processo de envelhecimento ou exposição a radiação luminosa, em particular o diastereoisômero trans-. O radical 1-hidroxietila é o radical majoritário formado na cerveja durante o processo de envelhecimento. O presente trabalho descreve a reatividade dos iso-&alpha;-ácidos frente ao radical 1-hidroxetila através do uso da técnica de spin-trapping com detecção por espectroscopia de ressonância paramagnética de elétrons (RPE) e espectrometria de massas (ESI-(+)-MS/MS). Observou-se que ambos os diastereoisômeros cis- e trans-iso-&alpha;-ácidos são degradados na presença do radical 1-hidroxietila com constantes de velocidade aparentes de 1,8 108 e 9,2 109 L mol-1 s-1, respectivamente. A reatividade dos dihidro-iso-&alpha;-ácidos com o radical estudado foi similar à reatividade da mistura diastereoisomérica dos iso-&alpha;-ácidos, apresentando constante de velocidade aparente de 1,5 109 L mol-1 s-1. Os análogos tetrahidro-iso-&alpha;-ácidos não apresentaram reatividade para com o radical 1-hidroxietila, sugerindo os hidrogênios alílicos como sítio reacional. Adicionalmente, os cálculos ab initio por DFT demonstraram que os valores de BDE para os hidrogênios alílicos das cadeias laterais prenila e isohexenoila são equivalentes e, desta maneira, sugerindo a maior reatividade do diastereoisômero trans- a ser creditada a um fator entrópico, já que ambos os grupos estão no mesmo plano espacial. Os produtos de oxidação foram determinados por LC-ESI-MSn e verificou-se a formação dos hidroxi-allo-iso-&alpha;-ácidos, recentemente descritos na literatura. O conjunto de resultados obtidos possibilitou a proposta de mecanismo para processo de oxidação e perda dos ácidos amargos da cerveja, observado no envelhecimento da bebida. / The iso-&alpha;-acids are the main responsible constituents for the bitter taste of beer, they are easily degraded during the aging and light exposed process, specially the trans- diastereoisomer. The 1-hydroxyethyl radical is the major radical produced during the beer aging process. The present work describes the reactivity of iso-&alpha;-acids towards the 1-hydroxyethyl radical as probed by the spin-trapping technique and detected by electron paramagnetic resonance (EPR) and mass spectrometry (ESI-(+)-MS/MS). It was observed that both diastereoisomers cis- and trans-iso-&alpha;-acids are degraded in the presence of 1-hydroxyethyl radical with apparent rate constant of 1.8 108 e 9.2 109 L mol-1 s-1, respectively. The reactivity of dihydro-iso-&alpha;-acids towards the studied radical was similar to the reactivity of the iso-&alpha;-acids diastereoisomeric mixture, showing apparent rate Constant of 1.5 109 L mol-1 s-1. The tetrahydro-iso-&alpha;-acids analogues did not observed reactivity towards the 1-hydroxyethyl radical suggesting the allilic hydrogens as the reaction sites. In addition, the ab initio DFT calculations demonstrated that the BDE values for the allilic hydrogens of the prenyl and isohexenoyl side chains are equivalents and according to that suggesting the higher reactivity of the trans- diastereoisomer to be accounted to an entropic factor since both goups are in the same plane of the space. The oxidation products were determined by LC-ESI-MSn and its was verified the formation of hydroxyl-allo-iso-&alpha;-acids. The data colected allows a mechanism to be proposed for the oxidation process and loss of bitter acids of beer during the beverage aging.