The impact of different hop compounds on the growth of selected beer spoilage bacteria in beer

Beer spoiling lactic acid bacteria are a major reason for quality complaints in breweries around the world. Spoilage by a variety of these bacteria can result in haze, sediment, slime, off-flavours and acidity. As these bacteria occur frequently in the brewing environment, using certain hop products that inhibit the growth of these spoilers could be a solution to prevent problems. To investigate the impact of seven different hop compounds (α-acids, iso-α-acids, tetrahydro-iso-α-acids, rho-iso-α-acids, xanthohumol, iso-xanthohumol and humulinones) on the growth of six major beer spoilage bacteria (Lactobacillus brevis. L. backi, L. coryniformis, L. lindneri, L. buchneri, Pediococcus damnosous), two concentrations (10 and 25 mg/L) of each hop substance were added to unhopped beer. The potential growth of the spoilage bacteria was investigated over 56 consecutive days. A comparison of the results shows a strong inhibition of growth of all spoilage bacteria at 25 mg/L of tetrahydro-iso-α-acids closely followed by α-acids as the second most inhibitory substance. The results showed a high resistance of L. brevis to all hop compounds as well as an inhibition of L. coryniformis and L. buchneri at low concentrations of most hop components. In comparison with the control sample, L. lindneri showed increased growth in the presence of some hop compounds (rho-iso-α-acids, xanthohumol, iso-xanthohumol, humulinones). © 2020 The Authors. Journal of the Institute of Brewing published by John Wiley & Sons Ltd on behalf of The Institute of Brewing & Distilling     

Hop bitter acids inhibit carbohydrate metabolism,enhance biogenic amine metabolism and alter L-malic acid,glutamic acid and arginine metabolism of Lactobacillus brevis 49

Beer contains only limited amounts of readily fermentable carbohydrates and amino acids. Beer spoilage lactic acid bacteria have to come up with metabolic strategies in order to deal with selective nutrient content. The research was performed to investigate the influence of iso-α-acids on the metabolism of organic acids, biogenic amines (BAs), off-flavour compounds, carbohydrates and amino acids of Lactobacillus brevis 49. Only glyoxylic acid and ethyl formate in de man, rogosa, sharpe broth was consumed, and multiple organic acids, BAs and off-flavour compounds were produced. By supplementing a series of concentrations of hop iso-α-acids, consumption of L-malic acid, glutamic acid and arginine and generation of BAs were found benefit for bacteria to develop hop-resistance. In the metabolism of carbohydrates, glucose was preferred over maltose and maltriose, and hops significantly inhibited the utilisation of carbohydrates. The results provide comprehensive information of metabolites of L. brevis 49 under various concentrations of hop stress.     

Potential impact of medium characteristics on the isomerisation of hop α-acids in wort and buffer model systems

Many brewing variables, e.g., pH, boiling time, temperature, and the presence of metal catalysts, are essential in view of the isomerisation of hop α-acids into iso-α-acids. In this work, a comparative study on the impact of several selected factors on α-acids isomerisation was performed in both buffer model solutions and wort, in order to find out enhanced boiling conditions for higher isomerisation yields.     

Analysis of hop acids by capillary electrophoresis

Micellar electrokinetic chromatography (MEKC) was used to separate components of hop extracts. The separation of a sample of iso-α-acids by MEKC was better and faster than by an established HPLC method, giving <0.8% RSD on migration times and 5–10% RSD on peak areas. MEKC was also successfully used to separate the oxidation products of the α- and β-acids and thus to monitor the stability of hop products containing them. Furthermore, MEKC distinguished among samples of reduced iso-α-acids (rho-, tetrahydro- and hexahydro- derivatives).     

ANALYTICAL EVALUATION OF α-ACIDS IN HOP EXTRACTS AND IN HOPS*

The results of new investigations on hop extract analysis are presented. The main findings are:

• 1 The specific extinction (257) of α-acids at 276 nm in iso-octane used at present in CCD and paper strip analyses is 3·5% too high: analytical results have therefore been too low. The new extinction value to be used is 248.
• 2 The result of paper strip analysis depends very much on the amount of α-acids placed on the strip. The three-point paper strip method can only partly correct this. In general, figures with paper strip analysis have a tendency to be too high.
• 3 Conductometric titration of α-acids produces a figure about 2 to 10% too low. This can be partly corrected by adding 20% dimethylsulphoxide or dimethylformamide to the titration medium.
• 4 Conductometric analysis of hop extracts with removal of “iso-α-acids like material” before titration is possible by a very simple procedure (pH 7 buffer extraction).

New photometric and conductometric procedures based on these results give figures which are in agreement for something which must be practically true α-acids.     

LACTIC ACID PRODUCTION IN SWEET WORTS

The production of D-(-)-lactic acid in sweet wort was due to growth of Lactobacillus delbrueckii whereas L-(+)-lactic acid was produced by Bacillus coagulans. Incubation of sweet worts at 45°C favoured the production of D-(-)-lactic acid whereas L-(+)-lactic acid was produced in greatest amounts at 55°C. Low concentrations of hop iso-α-acids (4–12 ppm) inhibited the growth of both B. coagulans and L. delbrueckii at 55 °C.     

Detection of Beer‐spoilage Lactobacillus brevis strains by Reduction of Resazurin

Lactobacillus brevis is the most common beer‐spoilage bacteria found in breweries. Due to its high prevalence and biodiversity, it is necessary to differentiate between the strains based on their hop tolerance. Forcing tests are often conducted for different types of beer, which can vary in cereal base, fermentation type, ethanol‐ and hop content. These conventional tests are considered to be the safest way to determine the ability of a given strain to cause beer‐spoilage, but they are very time consuming and costly. Since NADH₂ is used as cofactor by many cellular dehydrogenases, this study used the reduced form of intracellular nicotinamide adenine dinucleotide (NADH₂) as an indicator for microbial metabolic activity and thus ability to spoil beer, in order to reduce the time required to conduct the forcing tests for beer production. Beer‐spoiling L. brevis strains were detected among other strains in beer samples within two days (of sampling).     

An approximate shelf life prediction of elaborated lager beer in terms of degradation of its iso-α-acids

A 3-l laboratory-scale beer production protocol was designed and developed to produce beer. Concentrations of cis-/trans-iso-α-acids were quantified by HPLC. The trans-/cis-ratio varied from 0.62 to 0.49 after accelerated aging from 4 °C to 50 °C. The degradation of trans-iso-α-acids in elaborated pale lager beers followed first order reaction and this degradation conforms to the Arrhenius equation. The activation energy (E_a) and frequency factor (A) for trans-iso-α-acids in elaborated pale lager beer ranged from 69.2 kJ mol⁻¹ and 1.88 × 10¹⁰ days⁻¹ for trans-isocohumulone to 74.4 kJ mol⁻¹ and 1.3642 × 10¹¹ days⁻¹ for trans-isoadhumulone. The higher value of E_a demonstrated greater temperature sensitivity of trans-iso-α-acids during accelerated storage. The average half life of trans-iso-α-acids in elaborated pale lager beer was found to decrease from 471 days to 12 days when temperature increased from 4 °C to 40 °C. Using the activation energy of trans-α-acid degradation and the temperature profile of the accelerated aging, a mathematical model was employed to predict the loss of iso-α-acids, when the initial concentration of iso-α-acids in the product is known. The results obtained in the investigation can be of great importance to the industry in predicting the alteration of beer bitterness during warm periods and in tropical countries where summer temperature can reach 40 °C.     

Amino acids improve acid tolerance and internal pH maintenance in Bacillus cereus ATCC14579 strain

This study investigated the involvement of glutamate-, arginine- and lysine-dependent systems in the Acid Tolerance Response (ATR) of Bacillus cereus ATCC14579 strain. Cells were grown in a chemostat at external pH (pH_e) 7.0 and 5.5. Population reduction after acid shock at pH 4.0 was strongly limited in cells grown at pH 5.5 (acid-adapted) compared with cells grown at pH 7.0 (unadapted), indicating that B. cereus cells grown at low pH_e were able to induce a marked ATR. Glutamate, arginine and lysine enhanced the resistance of unadapted cells to pH 4.0 acid shock of 1-log or 2-log populations, respectively. Amino acids had no detectable effect on acid resistance in acid-adapted cells. An acid shock at pH 4.0 resulted in a marked drop in internal pH (pH_i) in unadapted cells compared with acid-adapted cells. When acid shock was achieved in the presence of glutamate, arginine or lysine, pH_i was maintained at higher values (6.31, 6.69 or 6.99, respectively) compared with pH_i in the absence of amino acids (4.88). Acid-adapted cells maintained their pH_i at around 6.4 whatever the condition. Agmatine (a competitive inhibitor of arginine decarboxylase) had a negative effect on the ability of B. cereus cells to survive and maintain their pH_i during acid shock. Our data demonstrate that B. cereus is able to induce an ATR during growth at low pH. This adaptation depends on pH_i homeostasis and is enhanced in the presence of glutamate, arginine and lysine. Hence evaluations of the pathogenicity of B. cereus must take into account its ability to adapt to acid stress.     

啤酒花苦味酸对血糖调节作用的研究

观察啤酒花超临界CO2 浸膏、β- 酸和六氢β- 酸对糖尿病小鼠血糖的调节作用。对四氧嘧啶复制的糖尿病小鼠的动物模型进行为期1w 的啤酒花超临界CO2 浸膏、β- 酸和六氢β- 酸的高、低剂量(30、15mg/kg)组和阳性药物格列本脲片灌胃治疗,观察小鼠的糖尿病“三多一少”的症状和治疗前后血糖和糖耐量的变化。与模型对照组相比,治疗组糖尿病小鼠的“三多一少”症状得到明显地改善,治疗前后血糖和糖耐量的变化结果表明,超临界CO2 浸膏和β- 酸的高剂量组治疗后的血糖值与正常组(p ＞ 0.05)无明显差异。对治疗后小鼠的血糖值采用单一自由度正交比较的方差分析表明,正常组小鼠的血糖值与模型组有极显著的差异,模型对照组与治疗组之间也具有非常显著的差异(p ＜ 0.01);在各治疗组之间,灌胃优降糖片的小鼠的治疗效果与灌胃不同剂量的β- 酸、六氢β- 酸和超临界CO2 浸膏样品的小鼠的总体治疗效果相当(p ＞ 0.05)。啤酒花超临界CO2 浸膏、β- 酸和六氢β- 酸对小鼠糖尿病症状均有一定的治疗作用,该作用与灌胃样品的剂量呈正相关关系。     

Effect of structurally different microbial homoexopolysaccharides on the quality of gluten-free bread

The effect of structurally different bacterial homoexopolysaccharides on gluten-free bread quality and their properties to act as hydrocolloids was investigated. Furthermore, exopolysaccharides (EPS) were analyzed structurally by asymmetrical flow field flow fractionation and methylation analysis. Breads were made of buckwheat and rice flour with EPS of Lactobacillus (L.) curvatus TMW 1.624, L. reuteri TMW 1.106, L. animalis TMW 1.971, and L. sanfranciscensis TMW 1.392 or hydroxypropylmethylcellulose (HPMC) at 1?% w/w flour base. Water-holding capacity, specific volume, crumb analysis, baking loss, moisture content, and crumb hardness were determined. Only HPMC and the glucan of L. curvatus TMW 1.624 retained water, and all supplements increased the specific volume. Furthermore, crumb hardness was decreased by additives to different extents. The moisture content, baking loss, and crumb firmness were improved most by dextran of L. curvatus TMW 1.624. Structure analysis of EPS revealed that L. animalis TMW 1.971 produces a fructan and a glucan and that the dextran of L. curvatus TMW 1.624 had highest molecular weight of analyzed EPS, ranging from 118 to 242 MDa. A methylation analysis demonstrated differences in branching. Dextran of L. reuteri TMW 1.106 is branched in position 4 (18?C19?%), whereas dextran of L. curvatus TMW 1.624 is branched in position 3 (8?C9?%). Overall, this study gives insight into structure function relations of different EPS. A structure function relation is suggested in which high weight average molar mass (Mw) and branching at position 3 of the glucose monomer foster a compact conformation of the molecule, which enables an increased water-binding capacity and promotes superior (structural) effects in gluten-free breads. The dextran of L. curvatus TMW 1.624 was the most promising candidate for applications in gluten-free bread quality improvements as it retains its size distribution and root mean square even with increasing Mw and forms an increasingly compact molecule.     

The effect of interaction between genotype CAST/RsaI (calpastatin) and MYOG/MspI (myogenin) on carcass and meat quality in pigs free of RYR1T allele

The purpose of the studies was to demonstrate to what degree genotypes of calpastatin (CAST/RsaI) and myogenin (MYOG) genes as well as the interaction between them may affect the carcass and meat quality of pigs. The investigations were conducted on 397 stress resistant pigs (free of RYR1^T allele). It was demonstrated that the favourable effect of the variants of CAST and MYOG genes on carcass quality traits depends on the cut. The gene variant favourably affecting the weight of ham simultaneously had a negative effect on the weight of the loin. It was also shown that the interaction between CAST and MYOG genotypes has a significant effect on backfat thickness. The effect of a given combination of CAST and MYOG genotypes on carcass traits is related to the weight of a substantial cut (ham, loin).Genotypes at loci CAST/RsaI and MYOG have a significant effect on the value of certain traits and parameters of meat quality and its technological value (genotype CAST on pH at 35 min and 2, 3, 24, 48, 96, 144 h post-mortem (pH₃₅, pH₂, pH₃, pH₂₄, pH₄₈, pH₉₆, pH₁₄₄, respectively), R₁ (IMP/ATP), electrical conductivity at 3 and 4 h post-mortem (EC₃, EC₄), technological yield of meat in curing and thermal processing (TY) and protein content in the muscle tissue, while genotype MYOG on pH₄₈, EC₃₅, EC₃, EC₂₄ and dry matter content).     

Ability of Lactobacillus brevis strains to degrade food phenolic acids

The potential to degrade 15 food phenolic acids was investigated for several Lactobacillus brevis strains isolated from different sources. All the strains analysed in this study showed a similar metabolism on phenolic acids. Among the cinnamic acids assayed, only p-coumaric, ferulic and caffeic acids were metabolized by the L. brevis strains. These acids were decarboxylated to produce their corresponding vinyl derivatives. Contrarily to the results previously reported on Lactobacillus plantarum, the L. brevis strains analysed in this study were unable to subsequently reduce or metabolize these vinyl derivatives. In L. brevis, vinyl phenol, vinyl catechol, and vinyl guaiacol were the final metabolic products from p-coumaric, caffeic or ferulic acids, respectively. From the benzoic acids analysed, and similarly to L. plantarum strains, only gallic and protocatechuic acids were modified by L. brevis strains. Both acids were decarboxylated to pyrogallol and catechol, respectively. Currently, the enzymes involved in the metabolism of phenolic acids in L. brevis remain uncharacterized.     

Purification and biochemical characterisation of a novel glutamate decarboxylase from rice bran

BACKGROUND: Glutamate decarboxylase (GAD) is a useful enzyme whose main function is to catalyse the irreversible α‐decarboxylation of L ‐glutamate to produce γ‐aminobutyric acid. The cheap and abundant rice‐processing by‐product rice bran contains a high amount of GAD, the purification and characterisation of which have not yet been reported. In this study, research on rice bran GAD was initiated. RESULTS: Rice bran GAD was purified to homogeneity via a combined purification protocol of ammonium sulfate fractionation, ion exchange chromatography and two gel filtrations, with a purification fold of 128.6 and an activity recovery of 21.3%. The enzyme was active at pH 5.5 and 40 °C and retained 80% of its original activity in the pH range 5–9 and the temperature range 30–50 °C. GAD activity was significantly enhanced in the presence of Ca²⁺ but strongly inhibited by Ag⁺, Hg²⁺, sodium dodecyl sulfate and CH₃COOH. Kinetic determination of the apparent K_m for L ‐glutamate and pyridoxal 5′‐phosphate gave values of 27.4 mmol L^?1 and 1.16 µmol L^?1 respectively. CONCLUSION: Considering that rice bran is cheap and commercially available and that rice bran GAD is relatively stable, the development of cost‐effective rice bran GAD‐related functional foods would seem to be feasible. Copyright © 2010 Society of Chemical Industry     

Effects of stress by unfamiliar sounds on carcass and meat traits in bulls from three continental beef cattle breeds at different ageing times

One-hundred-and-twenty-four young bulls of three Continental beef cattle breeds were used to study the effect of pre-slaughter stress by unfamiliar sounds on carcass traits and quality characteristics of beef aged for three different periods. Stress due to unfamiliar noises had a moderately negative effect on carcass and meat quality. Carcasses from group Stressed had higher (P < 0.05) values of pH₀, pH₂₄, a*, and Warner-Bratzler shear force, and lower (P < 0.05) values of L*, b* and pressure losses than from Unstressed animals. There were significant differences (P < 0.05) among breeds in L*, a* and pressure losses. The ageing time had a significant effect on chromatic variables, WBSF and pressure losses. The cooking losses were not significantly affected by any of the three factors discussed. These results emphasize the importance of implementing appropriate management practices during pre-slaughter handling of cattle in order to reduce any possible risk factor for stress, as well as the different sensitivity of the breeds to similar stimuli.     

ENZYMIC DEGRADATION OF α-ACIDS IN HOPS

Evidence is presented for the biological oxidation of α-acids in hop cones by an enzyme having molecular weight 17,000 daltons, K_m 6–0 and Mn as the active metal ion. The enzyme is located in bracts and bracteoles and not in lupulin glands. The presence of an endogenous inhibitor was demonstrated. The specific activity of protein in hop cones increased with maturity and this was probably associated with release of α-acids from lupulin glands.     

Effect of ethanol and low pH on citrulline and ornithine excretion and arc gene expression by strains of Lactobacillus brevis and Pediococcus pentosaceus

The accumulation of citrulline and ornithine in wine or beer as a result of the arginine catabolism of some lactic acid bacteria (LAB) species increases the risk of ethyl carbamate and putrescine formation, respectively. Several LAB species, which are found as spoilage bacteria in alcoholic beverages, have been reported to be arginine degrading. This study evaluates the effect of ethanol content and low pH on the excretion of citrulline and ornithine by two strains belonging to the potential contaminant species Lactobacillus brevis and Pediococcus pentosaceus. In the conditions that most affected cell viability, arginine consumption per cell increased noticeably, indicating that arginine utilization may be a stress responsive mechanism. L. brevis showed a higher accumulation of ornithine in the media than P. pentosaceus. In the presence of ethanol, a higher expression of the arcC gene was found in P. pentosaceus, which resulted in a lower excretion of citrulline and ornithine than in L. brevis. This suggests that L. brevis is more likely to produce these amino acids, which are precursors of ethyl carbamate and putrescine.     

ESTIMATION OF α- AND β-ACIDS IN HOP EXTRACTS BY HPLC

A procedure relying on high performance liquid chromatography for the estimation of α-acids and β-acids in hop extracts has been collaboratively tested by members of a Sub-Committee of the Institute of Brewing Analysis Committee and is recommended for use. No significant differences were found between precision values obtained using peak height and peak area measurements. For α-acids, the mean repeatability (r₉₅) and reproducibility (R₉₅) values were 1-3 and 2-4% m/m respectively over the range 41–62% m/m. For β-acids they were 0-9 and 2-0% m/m respectively over the range 11 to 35% m/m. The precision values were judged to be independent of concentration.     

Antioxidant activity and γ-aminobutyric acid (GABA) content in sea tangle fermented by Lactobacillus brevis BJ20 isolated from traditional fermented foods

GABA-producing lactic acid bacteria were isolated from kimchi and salt-fermented Jot-gal, which are traditional Korean fermented foods. The strain, BJ-20, isolated from salt-fermented Jot-gal (cod gut), possessed the highest GABA-producing ability in MRS broth with 1% monosodium glutamate (MSG), as determined by thin layer chromatography. The BJ-20 strain was identified as Lactobacillus brevis and designated as L. brevis BJ20. A sea tangle solution was fermented over 5 days to produce GABA using L. brevis BJ20. During fermentation, the GABA concentration dramatically increased, while the glutamic acid concentration decreased. This result indicates that the glutamic acid was converted to GABA by L. brevis BJ20 in the fermented sea tangle solution. Furthermore, the fermented solution exhibited strong antioxidant activities, such as DPPH scavenging, superoxide scavenging, and xanthine oxidase inhibition, which were higher than those of BHA as a positive control.