Non-Conventional Yeast: Behavior under Pure Culture, Sequential and Aeration Conditions in Beer Fermentation

The use of wild yeasts, isolated from different environments, is becoming the most interesting option for the production of new beers. The objective of this study is to evaluate the potential of seven non-conventional yeast strains from five different species (Saccharomyces cerevisiae, Hanseniaspora guilliermondii, Metschnikowia pulcherrima, Torulaspora delbrueckii, and Zygosaccharomyces bailii) isolated from Madrid agriculture to produce type ale beer. Wild yeast strains were evaluated at laboratory and pilot plant scales under different fermentation conditions (pure, aerated, and sequential culture). Strain S. cerevisiae SafAle S-04 was used as a reference. Throughout the fermentation of beer, volatile compounds were determined by GC and residual sugars by HPLC, among other parameters. The yeast strains used for the fermentation in pure culture conditions were unable to ferment maltose and maltotriose (0.73-1.18% v/v of ethanol). The results of the study under aerated conditions showed varying levels of higher alcohol and ester concentrations. It should be noted that the strain CLI 1057 (S. cerevisiae) fermented maltose in the presence of oxygen (Kluyver effect). This strain also showed a high production of 4-vinyl guaiacol, making it suitable for producing beers with a phenolic profile. Finally, three strains (H. guilliermondii, Z. bailii, and T. delbrueckii) were evaluated in sequential culture together with commercial strain and found to improve the organoleptic characteristics of the brewed beer. These approaches offer the opportunity to add new product characteristics to the beers.

Elucidating the Odor-Active Aroma Compounds in Alcohol-Free Beer and Their Contribution to the Worty Flavor

Alcohol-free beers (AFBs) brewed by cold-contact fermentation exhibit a flavor reminiscent of wort which affects consumer acceptability. The aims of this study were to identify the odor-active compounds in AFB and elucidate the contribution of these to the overall aroma and worty character of the beer. Using a sensomics approach, 27 odor-active aroma compounds were identified and quantitated using gas chromatography-mass spectrometry. The most odor-active compound was methional (boiled potato-like aroma), followed by 3-methylbutanal (cocoa-like), (E)-β-damascenone (apple, jam-like), 5-ethyl-3-hydroxy-4-methyl-2(5H)-furanone (curry, spicy-like), and phenylacetaldehyde (floral, honey-like). The important contribution of these flavor compounds to the worty and honey aroma of AFB was determined by sensory assessment of the recombinate in a beer-like matrix with omission tests. The role of 5-ethyl-3-hydroxy-4-methyl-2(5H)-furanone in AFB aroma was reported for the first time. The outcomes from this study are of relevance for the brewing industry to design strategies for the reduction of the wortiness of AFB.

Simultaneous Optimization of Acetaldehyde and DMS Concentrations for Better Sensory Quality of Beer Fermented on an Industrial Scale

The levels of selected volatile components that affected the sensory properties of a lager beer were optimized under high-gravity brewing conditions (15.5 °P) in an industrial plant. The influence of different pitching rates (6–10 million cells/mL), aeration levels (8–12 mg/L), times (4.5–13.5 h) of filling CCTs (cylindroconical tanks, 3850 hl), and fermentation temperatures (8.5–11.5 °C) on the contents of acetaldehyde, diacetyl, acetone, 2,3-pentanedion, dimethyl sulfide (DMS), and on the sensory properties of beer were investigated. Response surface methodology (RSM, Box–Behnken design) was used to research the possibilities for optimizing the concentration of selected volatile components and sensory properties of bottom-fermented lager beers. Statistical analyses of the results showed that the experimental factors had a significant influence (R-squared for the original model with no significant lack-of-fit) on some of the volatile components. Based on the Multiple Response Optimization analysis, the values of independent factors that ensured the highest beer sensory quality were the following: a pitching rate of 10 million cells per mL; a fermentation temperature of 11.5 °C; an aeration level of 12 mg/L; and a CCT filling time of 4.5 h. These results proved that RSM modelling can be successfully applied to optimize fermentation and lagering processes in an industrial plant to manufacture lagers of enhanced sensory quality.

Determination of sulphur compounds in beer using headspace solid-phase microextraction and gas chromatographic analysis with pulsed flame photometric detection

A simple and sensitive method for the analysis of volatile and semi-volatile sulphur compounds in beer at trace levels was developed using headspace solid-phase microextraction (SPME) and gas chromatography with pulsed flame photometric detection. Different SPME fibres were tested and a Carboxen-polydimethylsiloxane coated fibre was found to be the most appropriate. The adsorption and desorption conditions were optimised. The effect of ethanol concentration in the sample on the extraction of analytes was examined. A 60 m non-polar capillary column preceded by a 10 m length of a polar column was found to be capable of separating a wide range of C1-C6 sulphur compounds. The pulsed flame photometric detector enabled increased sensitivity to be obtained over previous methods, such as dynamic headspace followed by conventional flame photometric detection or sulphur chemiluminescent detection, with high sulphur selectivity. Two sulphur compounds, 2-methyl-1-butanethiol and 3-methylthiophene, were identified in beer for the first time.

Single- and multiple-dose pharmacokinetics of nufenoxole in healthy human subjects

1. In 12 healthy subjects, after single doses of 20, 40 and 80 mg of nufenoxole, mean peak plasma drug concentrations of 400, 815 and 1463 ng/ml were reached at 2.2, 2.5 and 2.5 h respectively. 2. Nufenoxole was absorbed with an apparent half-life of less than one hour at all three doses. Nufenoxole concentrations declined biphasically after the peak, with an initial and terminal half-life of four to five hours and about 27 h respectively. These half-lives were independent of the administered dose. 3. AUC and Cmax increased with increasing dose, but AUC did not increase proportionately to dose, due to a lower value for 80 mg than expected, possibly reflecting reduced absorption. 4. Observed nufenoxole concentrations, in another 12 healthy subjects receiving single, daily 80 mg oral doses of nufenoxole for eight days, were in excellent agreement with those predicted from single-dose pharmacokinetics.

The chemical nature of the urinary odour produced by man after asparagus ingestion

1. The pungent urinary odour produced by certain individuals within a few hours of eating asparagus has been shown to be due to a combination of up to six sulphur-containing alkyl compounds identified as methanethiol, dimethyl sulphide, dimethyl disulphide, bis-(methylthio)methane, dimethyl sulphoxide and dimethyl sulphone. 2. The possible roles of S-methylmethionine and asparagusic acid as precursors of these odorous substances are discussed in relation to the known chemistry of the vegetable.

[Biosynthesis of aroma compounds by microorganisms II. Formation of sulphur containing flavour substances from methionine by Saccharomyces cerevisiae (author's transl)]

Using methionine as single nitrogen source 3-(methylthio)-1-propanol, 3-(methylthio)-propanal, 3-(methylthio)-propylacetate and 2-methylthiophan-3-one were formed in anaerobic model fermentations by Saccharomyces cerebisiae. The sulphur compounds were enriched by liquid-liquid extraction and detected by a selective flame photometric detector (FPD). The identities of the components were confirmed by mass spectrometry. It could be shown by quantitative determiniations that methionine was metabolized with 56--61% to 3-(methylthio)=1-propanol, when it was given to the medium in concentrations from 10 to 100 mg/1. 3-(methylthio)-propylacetuate, 3-(methylthio)-propanal, and 2-methylthiophan-3-one only could be detected in traces.