The influence of metal ions on concentrations of flavour-active sulphur compounds measured in beer using dynamic headspace sampling

Addition of copper(II) ions (≤1.6 mg litre^?1) to beer significantly reduced (P ≤ 0.05) concentrations of hydrogen sulphide and ethanethiol measured in beer, but had no effect on methyl sulphide, methyl thioacetate and methyl disulphide. Zinc, iron(III), manganese(II), nickel, lead(II) and tin(II) ions, at similar concentrations, had no effect on any of the sulphur volatiles. When metal ions were added at a concentration of 1 glitre^?1 zinc, iron(III) and lead(II) reversibly bound hydrogen sulphide and enthanethiol.     

The determination of dimethyl sulphoxide in aqueous solution

A method for the determination of dimethyl sulphoxide (DMSO) in aqueous extracts is described. DMSO is reduced using stannous chloride and hydrochloric acid to dimethyl sulphide which can be measured by gas-liquid chromatography. Other sulphur compounds, including S-methyl methionine, have no effect on the assay. Levels of DMSO in solution higher than 10 μg litre^?1 can be readily determined. In samples of brewery wort the levels of DMSO were within the range of 261-411 μg DMSO litre^?1 and repeated determinations on two samples of beer gave mean values of 417 and 627 μg DMSO litre^?1 with coefficients of variation of 13.7 and 11.8% respectively.     

THE ORIGIN AND OCCURRENCE OF VOLATILE SULPHUR COMPOUNDS IN BRITISH ALES AND LAGERS

A flame-photometric sulphur detector was used to identify, measure and determine the sources of the sulphur volatiles produced during the commercial processing of British ale and lager. Dimethyl sulphide was the main sulphur volatile present in malt but traces of hydrogen sulphide, diethyl sulphide, and dimethyl disulphide were also detected. Hops contained hydrogen sulphide, methyl mercaptan, dimethyl sulphide, diethyl sulphide, methional and dimethyl disulphide. Most of this material extracted into commercial worts was driven off during boiling. Brewing yeasts produced only traces of organo-sulphur volatiles both in laboratory fermentations of wort and during the processing of commercial ales and lagers. In contrast, brewery bacteria, particularly wort spoilage organisms, could generate dimethyl sulphide and sometimes traces of other sulphur volatiles in laboratory cultures. t-Butyl mercaptan was the only organo-sulphur volatile detected in significant concentrations during the primary fermentation and conditioning of commercial beers and this was attributable to the activity of wort-spoilage bacteria. Attempts to identify a volatile compound causing ‘sulphury’ smells in beer were unsuccessful but there was some evidence that it might not contain sulphur.     

Analysis of grapefruit sulphur volatiles using SPME and pulsed flame photometric detection

Sulphur volatiles are major factors in the perceived aroma of grapefruit juice, GFJ. The objective of this study was to develop a procedure to concentrate, separate, identify and quantify the major volatile sulphur compounds, VSC’s, in grapefruit juices. SPME parameters such as headspace atmosphere, fibre coating, extraction time and temperature were evaluated. High resolution capillary GC using ZB-5, DB-Wax and PLOT columns coupled with pulsed flame photometric detection, PFPD, were employed for separation and detection. Thirteen sulphur volatiles were identified including; hydrogen sulphide, sulphur dioxide, methanethiol, dimethyl sulphide, carbon disulphide, dimethyl disulphide, 2-methyl thiophene, 3-methyl thiophene, methional, dimethyl trisulphide, 3-mercaptohexylacetate, 2,8-epithio-cis-p-menthane and 1-p-menthene-8-thiol. Five additional VSC’s were tentatively identified. Canned reconstituted GFJ had more total sulphur volatiles and a greater number than fresh GFJ. Hydrogen sulphide comprised over 80% of total sulphur volatiles in fresh GFJ but only 5% in canned GFJ.     

A GAS CHROMATOGRAPHIC METHOD FOR THE ACCURATE DETERMINATION OF LOW CONCENTRATIONS OF VOLATILE SULPHUR COMPOUNDS IN ALCOHOLIC BEVERAGES

By using a new sensitive method the contents of dimethyl sulphide, dimethyl disulphide and dimethyl trisulphide have been determined in beers and wines, and the contents of these sulphides and diethyl sulphite measured in distilled beverages. The head-space volatiles are swept out from the beverage in a stream of nitrogen and adsorbed in a tube containing porous polymer. The volatiles are desorbed directly onto the column of a gas chromatograph by connecting the tube to the column and sliding the free end of the tube into the hot injection block from the oven side. The sulphur-sensitive flame photometric detector is used and the peak areas and identifications are collected on cassette and processed by computer to give the content of volatile sulphur compounds directly in μg/litre beverage. The detection limits are 2 μg/litre for dimethyl sulphide, 0.1 μg/litre for dimethyl disulphide and trisulphide, and 5 μg/litre for diethyl sulphite. The analysis time for each sample is 30 min and the coefficient of variation is 5%.     

Volatile compounds in Bacillus‐fermented soybeans

Thua nao, a traditional, proteolytic, fermented soybean condiment of northern Thailand, was prepared from cooked whole soybeans by natural flora fermentation. The microbial flora during the fermentation was dominated by Bacillus species. The formation of volatile compounds during the fermentation was studied. In addition, the volatile compounds of two samples of commercial dried thua nao and two samples of commercial Japanese natto were analysed. Fermentation led to a large increase in the concentration of total volatile compounds, from 35 µg kg^?1 wet weight in cooked soybeans to 3500 µg kg^?1 wet weight in 72 h fermented material. The major volatile compounds in fermented beans were 3‐hydroxybutanone (acetoin), 2‐methlybutanoic acid, pyrazines, dimethyl disulphide and 2‐pentylfuran. Sun drying of 72 h fermented material resulted in the loss of 65% of total volatiles, including important aroma compounds. The commercial dried thua nao samples had low concentrations of total volatile compounds (380 µg kg^?1 wet weight). It is suggested that improved drying/preservation methods are needed to retain aroma compounds in the traditional products. The natto samples were devoid of aldehydes, aliphatic acids and esters, and sulphur compounds, whereas the thua nao samples contained a diversity of these compounds. Previous investigators have reported these compounds in natto and it is not possible to suggest the existence of systematic differences between the volatile compounds in traditional thua nao prepared with an undefined, mixed microbial flora and those in natto fermented with Bacillus subtilis. © 2001 Society of Chemical Industry     

Volatile sulphur compounds in UHT milk

Several volatile sulphur compounds have been detected in raw and processed milk. These are hydrogen sulphide, methanethiol, carbonyl sulphide, dimethyl sulphide, carbon disulphide, dimethyl disulphide, dimethyl trisulphide, dimethyl sulphoxide, and dimethyl sulphone. Many of these increase in milk during heat processing and are associated with the cooked flavor of heat-treated milks, particularly UHT and sterilized milk. Several researchers have attempted to explain the origin of these volatiles in both raw and processed milk, and how to reduce the associated cooked flavor that has a negative impact on consumer acceptability of processed milk. These compounds are difficult to detect and analyze due to their high volatility, sensitivity to oxidation and heat, and in some cases, their very low concentrations. However, methods of detection and quantification have improved in recent years. Pre-concentration methods such as solid phase microextraction (SPME) together with gas chromatography equipped with sulphur-selective detectors now enable low concentrations of these compounds to be analyzed. In this review, methods of extraction and analyzes of these volatile sulphur compounds are compared, and their occurrence in milk is reviewed.     

Studies on the effect of protectants on Lactobacillus acidophilus strain dehydrated under controlled low-temperature vacuum dehydration and freeze-drying by using response surface methodology

Ca²⁺ (0–60 g litre^?1), glycerol (0–100 g litre^?1), and non-fat dry milk solids (NFDMS, 0–140 g litre^?1) were combined to study their effects on the survival of Lactobacillus acidophilus in controlled low-temperature vacuum dehydration and freeze-drying. A three-variable (each at three-levels) design method was used and analysed by fitting a quadratic response surface. These factors were found to be significantly effective on survival. Fitted quadratic regression equations were plotted using a graphing software package. Conditions for the best survival of freeze-drying was found to be Ca²⁺ 5.34 g litre^?1. NFDMS 100.8 g litre^?1, and glycerol 41.5 g litre^?1 and that of controlled low-temperature vacuum dehydration was Ca²⁺ 5.67 g litre^?1, NFDMS 107.1 g litre^?1, and glycerol 41.0 g litre^?1. Models obtained were tested using t-tests. Survival of freezing under different conditions was found to be not significantly different.     

DETERMINATION OF THE VOLATILE COMPONENTS OF BEER

A method relying on headspace sampling and gas chromatography for the determination of the lower boiling point volatile compounds (fusel oils) in beer, has been approved by the Analysis Committee of the Institute of Brewing (IOB), for inclusion in Recommended Methods of Analysis. In view of the large values obtained for precision in terms of repeatability (r₉₅) and reproducibility (R₉₅) and the differences in gas chromatographic equipment used by individual brewing laboratories, the method is considered not to be ideal, but is given as a guideline only. The IOB Analysis Committee has recommended a method relying on gas chromatography and flame photometric detection for the determination of dimethyl sulphide in beer. Repeatability (r₉₅) and reproducibility (R₉₅) values of 3.3 μg/litre and 3.66+0.168 m μg/litre (where m is concentration), respectively, were obtained over the range 20–50 μg/litre. Methods currently used in brewing laboratories, for the measurement of vicinal diketones, are being surveyed with a view to obtaining a suitable method for collaborative testing by the Analysis Committee.     

N-nitrosamine content of New Zealand beer and malted barley

The nitrosamine content of 16 New Zealand beers and one Australian beer, all manufactured in 1980, was determined by chemiluminescence. The levels, calculated as N-nitrosodimethylamine (NDMA) ranged from 0.2–1.8 μg dm^?3 with a mean of 0.76 μg dm^?3. Over a period of 2 years, the NDMA levels in New Zealand malt have been reduced as the result of changes in kilning technology, introduced to control the level of nitrogen oxides in kiln air. For example, the mean value for directly-fired malt was 5.4 μg kg^?1, that for malt dried in air heated by burners to which sulphur had been added was 2.4 μg kg^?1 and that for malt dried in air to which SO₂ had been released was 0.3 μg kg^?1. For the purposes of quality control, screening beer or malt for nitrosamines can be accomplished adequately by the injection of CH₂Cl₂ extracts directly into a thermal energy analyser.     

The effects of defaunation on the metabolism of lactic acid in the rumen

Two experiments examined the effects of defaunation on the ruminal metabolism of lactate. Three rumen-cannulated sheep given a diet of molassed sugar beet pulp and barley (80:20) were used to study the effects of defaunation (with manoxol-OT) on the metabolism of lactic acid produced endogenously from rumen fermentation. Defaunation increased mean ruminal concentrations of lactate from 3.4 mmol litre^?1 to 8.9 mmol litre^?1 but other rumen measurements remained virtually unchanged: pH, 6.3 and 6.3; molar proportions of acetic acid, 645 and 645 mmol mol^?1; propionic acid, 189 and 197 mmol mol^?1 and butyric acid 142 and 115 mmol mol^?1 for the faunated and defaunated states respectively. In a second experiment, two groups each of four rumen-cannulated sheep were used to study the effect of defaunation on the ruminal metabolism of added Na-D, L lactate. One group of four was defaunated using a rumen-washing technique whilst the other group of four remained faunated. The animals were given a diet of rolled barley and hay (60:40) and were given a series of intraruminal doses of Na-D, L lactate from 0 to 100 g day^?1, increasing by 20 g day^?1 every third day. Disappearance of added lactate was much more rapid in faunated animals: at the 100 g day^?1 dose rate, L-lactate concentrations had fallen from a peak of 6 g litre^?1 to >1 g litre^?1 after 3 h whereas in defaunated sheep the peak of 6.5 g litre^?1 was reduced to >1 g litre^?1 only after 7 h. In faunated animals lactate addition caused an increase in the ruminal concentration of total volatile fatty acids (VFA) from 147 to 217 mmol litre^?1 between the zero and 100 g day^?1 dose rates, accompanied by an increase in the molar proportion of propionic acid from 190 to 320 mmol mol^?1. However, in defaunated animals there was only a small increase in total VFA concentrations from 94 to 106 mmol ml^?1 with no change in the molar proportion of propionic acid and a small increase in butryric acid from 140 to 180 mmol mol^?1.     

CONTROL OF SULPHURY IMPURITIES IN BEER AROMA

Volatile sulphur compounds, particularly hydrogen sulphide and mercaptans, are normal products of yeast metabolism. Although they tend to be purged out of beer during fermentation and maturation, it is not uncommon for finished beer to retain enough of them—up wards of 0·02 ppm as sulphur—to impair its aroma. The addition of a small amount of copper sulphate to sulphury beer cleanses its aroma by transforming volatile sulphur into non-volatile copper sulphide and mercaptides, but since a considerable proportion of this added copper is liable to be inactivated by complexing with nitrogenous constituents of the beer, the removal of sulphur may necessitate the addition of so much copper as to threaten the shelf life of the beer. It has been found that not only may traces of copper be dosed electrolytically into beer with great precision but that the copper so dosed reacts nearly quantitatively with the volatile sulphur present, thus abolishing the latter with no appreciable increase in the net copper content of the beer. The process has given satisfactory results on an industrial scale, improving the flavour of beers without prejudice to their other characteristics.     

Sorption of flavour compounds in aqueous solution into polyethylene film

Sorption of flavour compounds by the polyethylene (PE) liner of laminated pouches (PET/Al/PE) containing a flavour solution has been studied. The solution contained 43 volatile compounds (each 2 μl litre^?1) in sugar ester solution (3 g litre^?1). The distribution ratios of the flavour compounds for sorption into various PE liners increased with carbon chain length of the flavour compounds from 0.01 to 0.1 for alcohols, from 0.03 to 1.23 for aldehydes, from 0.02 to 5.77 for aliphatic esters and from 0.11 to 11.6 for benzoates. In each series (all straight-chain compounds) the distribution ratios increased about threefold for each methylene group, but in the compounds composed of 11 or more carbon atoms the increment was less or negative. The distribution ratio for a given PE film was proportional to the film thickness. The distribution ratios increased with decreasing crystallinity of the PE film and the results for the films of different crystallinities suggested the existence of an impermeable boundary region surrounding the crystalline material amounting to about 20% of its volume.     

Inexpensive,rapid screening method for aflatoxins in peanuts and peanut products

Surveys of major Brazilian foodstuffs demonstrated that peanuts and peanut products continue to be very susceptible to aflatoxin contamination. To prevent, or at least minimise, the problem the aflatoxins need to be monitored by a rapid and inexpensive screening method. The AOAC Romer method has been used and found highly reliable. However, the clean-up step utilises anhydrous FeCl₃ and basic CuCO₃ which are expensive and not readily available in Brazil. Thus, the extraction (with a mixture of 270 ml methanol plus 30ml 40 g litre^?1 aqueous KCl) and clean-up (150 ml 100 g litre^?1 aqueous CuSO₄) steps of the method of Soares and Rodriguez-Amaya (1985) were combined with the AOAC minicolumn to provide a rapid, inexpensive screening technique. Fifty-two sample lots of peanuts and peanut products were screened by this and Romer's method, and the results were in complete agreement: 28 samples were negative, four < 20 μg kg^?1, 12 in the range 20–50 μg kg^?1, three in the range 50–100 μg kg^?1 and 5 >100 μg kg^?1. The results also agreed well when the extracts obtained by the two methods were submitted to quantitative TLC.     

The yeast flora of ensiled whole-crop maize

The yeast flora in samples of 13 different whole-crop maize silages was shown to be predominated by Candida krusei, C. lambica, Saccharomyces dairensis, S. exiguus, C. holmii or C. milleri. All these strains fermented glucose but, except for the latter three species, not sucrose, α-trehalose or raffinose. Under conditions resembling those in silage, i.e. at pH 4.0 in the presence of a complex nitrogen source, all strains assimilated lactic and acetic acids. This result suggests that the predominant yeasts in silage should not be distinguished into lactate utilisers and lactate non-utilisers. All strains, except those of Saccharomyces dairensis, tolerated acetic acid (5 g litre^?1) and grew at pH 4.0 in a mineral salts medium containing lactic acid (10 g litre^?1), acetic acid (5 g litre^?1) and yeast extract (1 g litre^?1). Growth in this medium was completely inhibited by the cationic detergent Arquad C33W (20 mg litre^?1), propionic acid (5 g litre^?1), benzoic acid (5 g litre^?1) or salicylic acid (10 g litre^?1).     

Chemical studies on the herring (Clupea harengus). XI.—Quantitative estimation of volatile sulphur compounds produced during the cooking of herring

The volatile sulphur compounds in fresh and canned herring were estimated by a colorimetric method. It was found that there was an overall decrease in volatile sulphur compounds after canning and also that nobbed herring was richer than filleted herring in hydrogen sulphide. The skin produced relatively larger amounts of methyl mercaptan. The reasons for these findings are discussed.     

Determination of Cu(II) in Beer by Derivative Potentiometric Stripping Analysis

Derivative Potentiometric‐Stripping Analysis (dPSA) is described as an inexpensive and rapid method for the determination of Cu(II) in beer. Beer samples were analysed directly after degassing and addition of the analytical reagents (hydrochloric acid, mercury(II) chloride, and potassium metabisulfite). It was not necessary to digest the sample. During dPSA the metal ions are deposited on a glassy carbon‐working electrode and then stripped by a suitable oxidant. Quantitative analysis was carried out by the method of standard additions. The recovery of the method was tested by adding 50 μg L^?1, 75 μg L^?1 and 100 μg L^?1 of Cu(II) (as Cu(II) chloride solution) to the beer. The mean recoveries of Cu(II) ranged from 95 ± 5% to 98 ± 2%. The reproducibility was evaluated by three repetitive analyses for each sample and the standard deviation ranged from 0.6 μg L^?1 to 5.5 μg L^?1. The detection limit was 0.8 μg L^?1. The Cu(II) concentration determined in seven beer samples ranged from 28 μg L^?1 to 48 μg L^?1 and the results obtained were not significantly different from those obtained by atomic absorption spectrophotometry (AAS).     

Effect of Carbonyl Sulphide (COS), Ethyl Formate (EF) and Carbon Disulphide (CS2) on the Malting Quality of Barley and the Flavour Profile of Beer

New fumigants are needed to control grain insects because methyl bromide is being phased out as an ozone‐depleting substance under the Montreal Protocol and because resistance to phosphine is increasing. Alternative fumigants also provide one strategy to manage resistance. To assess the effect of candidate fumigants on barley quality, malting and brewing trials were conducted with malting barley (Schooner), 9.4% moisture content, fumigated with carbonyl sulphide (COS), ethyl formate (EF) and carbon disulphide (CS₂) at concentrations of 20, 90 and 36 mg/L respectively for 7 days at 20°C in riveted steel silos containing 33 t of barley. The appropriate industry body, the Malting and Brewing Industry Barley Technical Committee (MBIBTC) set the quality parameters to be evaluated and allocated work to the appropriate quality laboratories. Germination of fumigated barley was not affected by the fumigants. Residues of COS, EF and CS₂ in outloading barley samples were 0.085 ± 0.0052, 0.4 ± 0.1 and 2.22 ± 0.07 mg/kg respectively, which were below the experimental permits of maximum residue levels (MRL). Residues of COS, EF and CS₂ in malt, wort and beer were indistinguishable from those in unfumigated (or non‐fumigated) barley. Ethyl formate and CS₂ affected wort in increasing the apparent attenuation limit (AAL) and CS₂ affected barley colour. Sulphur volatiles in the trial beer made from EF and CS₂ (unlike COS) fumigated barley decreased significantly. Beer from COS, EF and CS₂ fumigated barley had a somewhat higher 24‐hour chill haze level (0.62–0.84) than beer from the untreated barley (0.53). The total alcohols and isoamyl acetate increased in beer made from EF fumigated barley. Carbonyl sulphide, EF and CS₂, however, had no effect on beer flavour or quality.     

Improvement of wine aromatic quality using mixtures of lysozyme and dimethyl dicarbonate,with low SO2 concentration

The use of sulphur dioxide (SO₂) in the treatment of foodstuffs presents some problems as it could lead to pseudo-allergies in some people. The aim of this research work was to study the addition of different preservative mixtures and their influence on the concentration of volatile compounds and sensorial quality in wine. To do so, vinifications were carried out using Garnacha must to which lysozyme, dimethyl dicarbonate (DMDC) and mixtures of these with SO₂ were added at different doses (25 and 50 mg l^?1). The results were compared with a control sample to which only SO₂ had been added (50 mg l^?1). In general, mixtures of SO₂ with lysozyme and DMDC favoured the formation of volatile compounds in the wines. Wines obtained from the mixtures of lysozyme and DMDC with 25 mg l^?1 of SO₂ had better sensorial quality than the wines obtained with 50 mg l^?1 as the only preservative used.     

APC values and volatile compounds formed in commercially processed,raw chicken parts during storage at 4 and 13 °C and under simulated temperature abuse conditions

Skinless chicken breast fillets, thighs, wings and boneless breast with skin were selected from the production line of a commercial processor. Samples were evaluated over time in storage at 4 and 13 °C and under temperature abuse conditions for microbial proliferation and production of volatile compounds (VCs). Aerobic plate counts (APCs) were enumerated and VCs quantitated at 24 h storage intervals; however, APC increases were not reflected by significant correlations with headspace VCs. Compounds isolated from sample headspaces by solid phase microextraction (SPME) and direct headspace sampling and then analysed by gas chromatography (GC), gas chromatography/mass spectrometry (GC/MS) and gas chromatography/chemiluminescence detection (GC/SCD) were hydrogen sulphide, methanethiol, ethanol, acetone, C5, C6 and C7 hydrocarbons, dimethyl sulphide, methyl ethyl ketone, carbon disulphide, 1‐propanol, ethyl acetate, 1‐butanol, S‐methyl thioacetate, 3‐methyl‐1‐butanol, dimethyl disulphide and dimethyl trisulphide. Compounds appearing in the samples with any degree of consistency and therefore considered pertinent for these analyses were ethanol, acetone, methyl ethyl ketone, ethyl acetate, hydrogen sulfide, dimethyl sulphide and carbon disulphide. Elevated APCs and VC diversity occurred more prevalently in those samples from higher‐temperature storage. Published in 2000 for SCI by John Wiley & Sons, Ltd.