ESTIMATION OF VOLATILE SULPHUR COMPOUNDS IN BEER

A method has been developed for the quantitative estimation of volatile sulphur compounds in beer at levels below 0.1 μg litre^?1. The method relies on the concentration of beer headspace volatiles onto a porous polymer followed by injection into a capillary gas chromatographic column using a thermal desorption cold trap injector. The volatile components are separated using temperature programming and detected by a flame photometric detector specific for sulphur compounds. The mean concentrations and the estimates of r₉₅ (μg litre^?1) for volatile sulphur compounds measured in a commercial lager were respectively: methanethiol (0.33, 0.30); dimethyl sulphide (19.3, 5.3); carbon disulphide (0.42, 0.32); ethylene sulphide (0.37, 0.05); 1-propanethiol (0.11, 0.02); methyl thioacetate (11.7; 2.7); methyl disulphide (0.34, 0.42). The method is a significant improvement over previous techniques for the quantification of sulphur-containing volatiles in beer .     

VICINAL DIKETONES IN BEER. PART II: INFLUENCE OF SULPHUR DIOXIDE IN THE DETERMINATION OF DIACETYL IN BEER

The accuracy of the determination of diacetyl in beer using o-phenylenediamine is impaired by the presence of sulphur dioxide. This effect may be overcome when the distillate from the beer is collected in strongly acidic solution¹.     

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.     

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.     

COMPARISON OF TWO DISTILLATION—TITRIMETRIC PROCEDURES FOR THE ESTIMATION OF SULPHUR DIOXIDE IN BEER

The modified Monier Williams procedure as recommended by the Institute of Brewing (IoB) and the European Brewery Convention (EBC) for estimating sulphur dioxide in beer has been compared with the Tanner method. For both beer and aqueous solutions of sulphur dioxide the Tanner method, favoured by the E.E.C., gave significantly lower results than those obtained by the IoB/EBC procedure. It was established that the discrepancy between the results was due to the design and mode of use of the Tanner apparatus and not to differences in the reagents employed.     

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%.     

DETERMINATION OF LOWER BOILING POINT VOLATILE COMPOUNDS IN BEER BY HEADSPACE GAS CHROMATOGRAPHY — COLLABORATIVE TRIAL

The Analysis Committee has collaboratively tested local routine headspace gas chromatographic methods for the determination of the lower boiling point volatile compounds in beer. The repeatability values (r₉₅) were dependent upon mean concentration (m) for acetaldehyde and alcohols but not for esters, whilst reproducibility values (R₉₅) were dependent upon concentration in all cases. The range of values of m and the estimates of r₉₅ and R₉₅ (mg/litre) for each compound were, respectively: acetaldehyde (5–8, 0.21m, 0.60m); propanol (9–23, 0.25m, 2.5+0.57m); isobutanol (5–22, 0.56+0.085m, 1.7+0.15m); methylbutanols (45–105, 0.14m, 0.22m); ethyl acetate (10–54, 3.1, 2.1+0.29m); isoamyl acetate (0.8–4.7, 0.36, 0.20+0.58m); and ethyl hexanoate (0.13–0.36, 0.073, 0.10+0.91m). No advantage was gained by diluting beer samples containing 9% V/V ethanol to 4% ethanol (used for the calibration mixtures) prior to analysis, but use of a standard method of sample preparation decreased most of the R₉₅ values. No recommendation is made in this interim report.     

TRACE VOLATILE CONSTITUENTS OF BEER DIMETHYL SULPHIDE FORMATION IN MALT

Dimethyl sulphide formation in malt occurs primarily during the kilning process, and is significantly dependent upon the maximum kilning temperature. During mashing a considerable fraction of the dimethyl sulphide of malt is transferred to the wort. Since this is not always completely removed by the wort boiling process, the remaining dimethyl sulphide, although decreased during fermentation, may be sufficient to impart a noticeable dimethyl sulphide aroma to the finished beer.     

THE FORMATION OF VOLATILE SULPHUR COMPOUNDS DURING PRESSURE COOKING OF GRAIN/WATER MIXTURES

Volatile sulphur compounds are formed during the pressure cooking of grain/water mixtures. The major components are carbonylsulphide, methanethiol and carbon disulphide, while hydrogen sulphide, dimethylsulphide, sulphur dioxide and dimethyldisulphide are formed in smaller quantities.     

DETERMINATION OF TOTAL SULPHUR DIOXIDE IN BEER BY FLOW INJECTION SPECTROPHOTOMETRY USING GAS-DIFFUSION AND THE MERGING ZONES TECHNIQUE

A flow injection system for the determination of total sulphur dioxide in beer is described. The methodology involves the same colorimetric reaction of the reference method between SO₂, prosaniline and formaldehyde. The sample is introduced into the manifold without previous pretreatment, the hydrolysis process, isolation of the SO₂ from the matrix and the colorimetric reaction, being carried out inside the flow tubes. To isolate the SO₂, a gas-diffusion unit was included in the system and to minimise p-rosaniline consumption, a merging zones technique was used. The results obtained were in good agreement with a reference method (relative deviations lower than 5%), the precision being better than 2.5% (relative standard deviation). A sampling-rate of 30 samples per hour was obtained and the consumption of p-rosaniline was reduced 10 fold when compared to the reference method.     

DETERMINATION OF TOTAL CARBOHYDRATE IN BEER

A colorimetric procedure for the determination of total carbohydrate in beer has been evaluated in a collaborative trial. The method is recommended by the Analysis Committee.     

METHODS FOR THE DETERMINATION OF VOLATILE NITROSAMINES IN MALT,WORT AND BEER

Methods for the analysis of beer, wort and malt for NDMA at the one in 10⁹ level using the thermal energy analyzer have been evaluated. The beer and wort procedure using Preptube extraction has been shown to give excellent results. For malt, the preparation of an aqueous extract, followed by analysis by the beer procedure, also gives excellent results. Vacuum distillation from mineral oil has also been examined for the analysis of malt. Providing sufficient aqueous phase is initially present, this procedure yields results similar to those from aqueous extraction. The methods will also detect NDEA, although no NDEA has been observed in any of the beer, wort or malt samples examined. NDMA has been observed to be present in bound forms in malt and Preptubes and is released by the addition of water     

GAS CHROMATOGRAPHIC DETERMINATION OF TRIHYDROXYOCTADECENOIC ACIDS IN BEER

A rapid and quantitative method for measuring the content of trihydroxyoctadecenoic acids in beer is described. The acids are extracted from degassed beer with ethyl acetate and methylated with diazomethane. After washing, the methylated compounds are silylated and analysed by gas chromatography. The coefficient of variation of the method is 3·2%. The amounts of total trihydroxyoctadecenoic acids in commercial beer samples varied from 4 to 12 mg/litre. The effects of these acids on beer flavour and head retention are discussed.     

DETERMINATION OF THE FILTERABILITY OF BEER

A method for the determination of the filterability of beer has been evaluated by members of the European Brewery Convention Analysis Committee. The method is based on the filtration of a beer sample, with a standard amount of diatomaceous earth (DE), under standardised conditions. Repeatability of the test was quite good, however the predictive value of the test is still questionable since no direct correlation could be established between the different filtercake values and industrial filtration problems. No collaborative test was performed as the filterability has to be determined on unfiltered beer; the test is easy to run, and as for the other filterability tests, day to day practice in a brewery will reveal its usefulness.     

DETERMINATION OF CARBON DIOXIDE IN BEER

A rapid, simple and inexpensive method is described for the determination of the carbon dioxide content of beers.     

QUANTITATIVE DETERMINATION OF BITTER SUBSTANCES IN BEER,BY THIN LAYER CHROMATOGRAPHY

A method is described which allows the separation of hop bitter components and derivatives present in beer by combined thin-layer chromatography on a mixture of silica gel and cellulose. This method can be used to evaluate the purity of beer bitterness and to recognize the beer hopping.     

SIMULTANEOUS FLUOROMETRIC MICRODETERMINATION OF HYDROGEN SULPHIDE AND VOLATILE THIOLS IN BEER

A fluorometric method is presented for routine simultaneous determination of hydrogen sulphide and volatile thiols. Reproducibility, sensitivity and simplicity were achieved by the procedure described.     

FLUOROMETRIC DETERMINATION OF CYSTEINE IN BEER BY HIGH-PERFORMANCE LIQUID CHROMATOGRAPHY WITH PRECOLUMN DERIVATISATION

A high-performance liquid chromatographic procedure (HPLC) is described for the fluorometric determination of cysteine (CySH) in beer. Reduced CySH in beer is reacted with a fluorescent reagent, N-(9-acridinyl) maleimide (NAM), at pH 8·8 for 15 h to form a stable fluorescent derivative. The resulting reaction mixture is analysed by reversed-phase gradient elution HPLC. Under the optimised conditions for chromatography, ca 0·1 mg of CySH per litre of beer can be determined. Total CySH is determined by electrolytic reduction at a Hg pool electrode prior to the HPLC analysis. The levels of reduced and total CySH in 28 brands of commercial lager beers ranged from 0·2 to 0·9 mg/litre, and from 2·1 to 13·5 mg/litre, respectively.     

THE DETERMINATION OF IRON COPPER AND CALCIUM IN BEER

The atomic absorption methods for iron, copper and calcium in beer, published by the American Society of Brewing Chemists, are recommended.     

THE DETERMINATION OF DISSOLVED OXYGEN IN BEER

A survey is made of analytical techniques for oxygen determination and a recommended procedure for the determination of dissolved oxygen in beer using indigo carmine, is described. Dissolved oxygen meters and electrodes are reviewed.