THE BIOCHEMISTRY OF MATURATION

Negative and positive aspects of maturation are respectively related to aroma and taste modifications. Vicinal diketones, hydrogen sulphide, acetyldehyde being primarily responsible for ‘green’ beer flavours an important feature of maturation is the adjustment of their concentration during the lagering period. The role of secondary fermentation in the removal of these undesirable by-products and the importance of sulphury compounds in determining the typical character of lager beer are reviewed. Particular emphasis is placed upon new enzymatic and genetic approaches to overcome vicinal diketone problems in accelerated fermentation systems using free and immobilized cells. The presence in beer of amino acids, peptides, nucleotides and organic as well as inorganic phosphates is, in part, due to the secretion of these materials by the yeast during lagering. Most of these compounds are contained in internal pools and their actual participation in flavour maturation depends upon intracellular breakdown and accumulation, changes in cell permeability and subsequent exchange possibilities between the yeast cell and the surrounding beer. Participation and practical implications of medium chain length fatty acids in the development of autolytic and yeasty flavours are discussed.     

Analytical‐Sensory Determination of Potential Flavour Deficiencies of Light Beers

Although currently light beers have a high market share in the US, this beer style is still not accepted on the European continent because light beers are considered watery, mainly on account of lack of mouthfeel. In this paper, the flavour quality, including flavour stability, of five commercial light beers was evaluated in comparison with their corresponding pilsner beers derived from the same breweries. Through detailed physico‐chemical and sensory evaluation, followed by multivariate data analysis, the fresh beers could be differentiated according to their origin (US or European) and beer style (light or pilsner). Potential flavour deficiencies of fresh light beers were determined as too much sweetness, lack of bitterness, and especially reduced fullness. As a result, the fresh light beers were less preferred than their corresponding pilsner. Upon ageing, the light beers were generally less flavour stable than their pilsner counterparts. This is mainly ascribed to increased sensory perception of cardboard and ribes staling flavours in most aged light beers. Nevertheless, based on detailed analytical/sensory investigation, it was clearly demonstrated that one of the light beers involved in this study showed enhanced flavour stability with respect to all other beers, including the pilsner beers.     

DIFFERENTIATION OF ALE AND LAGER FLAVOURS BY PRINCIPAL COMPONENTS ANALYSIS OF FLAVOUR CHARACTERIZATION DATA

The application of principal components analysis to flavour characterization data has been examined by comparing and contrasting 32 beer samples. These comprised two samples of each of 16 brands of beer selected to represent 4 different types, contrasting ales and lager beers of two different strengths. Two-dimensional plots of results using the first two principal components as axes showed resolution of the four groups of beers and the close proximity of the majority of the duplicate samples. Differences between samples thus revealed are in accordance with known differences between the beer flavours.     

PRODUCTION-SCALE APPARATUS FOR THE INJECTION OF HOP EXTRACTS INTO BEER USING LIQUID CARBON DIOXIDE

Extracts of hops made with liquid carbon dioxide can be substituted for dry hops to introduce the flavour of hop oils into beer. Apparatus has been devised so that such extracts can be redissolved in liquid carbon dioxide and then metered into beer. Successful trials have been carried out on the commercial scale.     

LAGER-TYPE BEER MADE FROM EXTRUDED MALT; SENSORY AND CHEMICAL ANALYSES

A lager-type beer was brewed from a mixture of partially dried green malt and extruded green malt (9:1). Small flavour differences were observed in the beer made from the extruded malt when compared with two other types of beer, namely standard lager and beer made from 100% green malt which was partially dried. More than 90 flavour components were described upon sniffing of the GC eluate. Most of them were present in trace amounts which did not allow their identity to be established. However, in all beer samples several unidentified components were described as giving a “roasted”, “burnt”, “bread crust” or “biscuit” odour. These odour characteristics probably derive from the alkylpyrazines present. Positively identified pyrazines were only found in the “extruded” beer, namely methylpyrazine and 2,5 (and/or-2,6)-dimethylpyrazine. However, when the beer was tasted, no characteristics associated with a roasted or burnt flavour were registered. Among the identified flavour components esters and alcohols were abundantly represented.     

DERIVATION OF A PROFILE METHOD FOR SENSORY ANALYSIS OF BEER FLAVOUR

Flavour assessment ultimately depends on sensory methods of analysis. Profile methods are considered to offer the best current solution to the problem of describing, and as far as possible quantifying, beer flavour. The sensory characteristics of flavour must be identified and described systematically and objectively, without prejudice due to preferences. A flavour ‘vocabulary’ is needed which is applicable to all types of beer. This involves selection of significant terms which have the same meaning for different people. Any system universally applicable to all types of beer must necessarily be complex, but such a “universal” system is essential for research. It also provides a “dictionary” from which terms can be selected for simpler profiles for quality control of particular products. The steps in the development of a profile system are described. Development has depended and will continue to depend on the collaboration of a large number of individuals and groups of people within the industry. Profile systems cannot be static but must continuously evolve.     

DIMETHYL TRISULPHIDE,ITS MECHANISM OF FORMATION IN HOP OIL AND EFFECT ON BEER FLAVOUR

The precursor of dimethyl trisulphide (DMTS) in hops is destroyed by sulphur dioxide during kilning, but then re-forms slowly during storage. A redox mechanism is suggested and S-methyl-cysteine sulphoxide is postulated as the DMTS-precursor. The effect of DMTS on beer flavour is described. The flavour threshold value for DMTS in a commercial beer has been found to be 0·10μg/litre.     

PROGRESS TOWARDS AN INTERNATIONAL SYSTEM OF BEER FLAVOUR TERMINOLOGY

In an attempt to bring order into the existing chaos of beer flavour terminology, the joint working parties formed by the three above-mentioned organizations began by compiling a list of flavour terms in current use in the major languages. Terms were grouped according to related flavour characteristics. Synonyms and ill-defined terms were then eliminated and the most appropriate name was selected for each separately identifiable flavour note. This paper summarises the resultant flavour terminology, which attempts to arrange, in a logical way, terms describing sensory components of beer flavour which can be objectively characterized. It is recommended that the proposed terminology be used in the brewing industry for a trial period of a year or more, and then reconsidered in the light of experience, so that an agreed definitive list of flavour terms, with detailed definition and reference standards, may subsequently be prepared. Ongoing activities of the joint working parties on reference standards and on flavour thresholds are briefly discussed.     

THE STALING OF BEER

It has previously been shown that unsaturated fatty acids must be considered as one class of precursors responsible for the staleness of beer. It is now demonstrated that oxidation products of carotenoids may also contribute to the stale flavour.     

THE OCCURRENCE OF TWO GEOMETRICAL ISOMERS OF 2,4,5-TRIMETHYL-1,3-DIOXOLANE IN BEER

Two geometrical isomers of 2,4,5-trimethyl-1,3-dioxolane have been detected in a range of commercial and experimental beers at levels up to ca 0.1 ppm. These compounds, plus a number of other dioxolanes, are present in unboiled wort but are lost by evaporation during wort boiling. The trimethyldioxolanes are then reformed during subsequent fermentation. The flavour threshold of a mixture of the two trimethyldioxolanes was found to be ca 0.9 ppm, at which concentration it produced in beer ‘phenolic’ and ‘astringent/drying’ flavour notes. However, these compounds are not present in sufficiently high concentrations to make a significant contribution to the flavour of beer.     

DISCRIMINANT ANALYSIS OF SENSORY AND INSTRUMENTAL DATA ON BEER

Stepwise discriminant analysis has been used to examine sensory and instrumental data on forty six different brands of ale from five brewing companies. The beers were classified also according to four evenly spaced bands of original gravity within the range 1·030 to 1·050. Two samples of each beer from brews several months apart were analysed. Eighty of the beers were correctly assigned to brewing companies from data on four sensory and five instrumental parameters. The most important of these parameters were iso-amyl alcohol content and caprylic flavour, both of which relate to products of fermentation. Eighty six of the beers were correctly classified into gravity bands using data on thirteen sensory parameters relating to the beer flavours. In this case the three most important parameters were body (palate-fullness), viscous (thick) mouthfeel and aldehydic/estery character.     

STABILITY OF ESSENTIAL OIL OF HOPS

The stability of hop essential oil in beers, in hops and in aqueous emulsions has been investigated. Hop character of beers treated with hop oil emulsion has stability on pasteurization and storage similar to that of dry hopped beers. When bottled with high levels of headspace air, beers lose hop character. Beers dry-hopped with stored hops or with hops damaged during pelleting tend to develop sulphury flavours. However, hop oil emulsions prepared from such hops give rise to a sound hop character in beer. Hop oil emulsion produced by the new process shows good stability physically, chemically and biologically, particularly when mechanically homogenized and stored under an inert atmosphere. The extent of any chemical alteration due to contact with air may be estimated spectrophotometrically.     

INFLUENCE OF pH ON FLAVOUR STALING DURING BEER STORAGE

A decreasing pH accelerated an increase in the chemiluminescence production and degradation of isohumulones and procyanidins during the storage of beer and using a model system. The sensory test showed that the addition of HCI to fresh beer accelerated the flavour staling during beer storage but that the addition of HCI to stored beer did not significantly accelerate the flavour staling. Therefore, it was thought that the acceleration of beer flavour staling is not dependent on a decrease in pH such that the decreasing pH isolates stale flavour aldehydes by a dissociation from staling-flavour aldehyde adducts but based on the fact that the decreasing pH accelerates the flavour staling reactions, free radical reactions, during beer storage.     

FLAVOUR OF FATTY ACIDS IN RELATION TO THEIR PHYSICAL STATE IN SOLUTION

The flavour of a fatty acid in beer is dependent on its physical state in solution. Thus, how and when a fatty acid is added to beer governs the apparent flavour threshold value. This could apply also to other flavour components which take part in intermolecular associations.     

A RAPID METHOD FOR THE EXTRACTION AND ANAYLSIS OF BEER FLAVOUR COMPONENTS

Dispersal of beer on a commercially available tube of kieselguhr and elution with dichloromethane was found to be a much faster and more efficient method of obtaining a beer flavour extract than liquid-liquid extraction. The constitution of each type of extract was qualitatively similar. The use of an internal standard allowed major flavour components to be quantitatively analysed.     

FLAVOUR STABILITY STUDIES: THE USE OF FRACTIONAL FACTORIAL DESIGNS

The number of taste tests that can be undertaken by the average flavour profile panel places a severe constraint on the study of flavour stability. The use of fractional factorial designs has been shown to be an effective approach to this problem. In this work, the effect of eight process variables on both beer flavour and flavour stability have been assessed using only eight experimental beers plus one control. The efficacy of the method is demonstrated by the ease with which the flavour profile panel results can be explained by accepted theories of flavour stability. Thus the variables examined could be divided into three groups. The first group includes headspace air, pasteurization and copper content: an increase in any of these variables leads to a marked increase in oxidized flavours during storage. The second group, which includes sulphur dioxide, PVPP treatment and lupulin (dry hop) addition, reduces oxidation. The remaining variables, proteolytic enzymes and iron content had no obvious effect on oxidation.     

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.     

EFFECT OF WORT-EVAPORATION ON FLAVOUR AND OTHER BEER QUALITIES

Pilot scale trials have been carried out to determine the importance of evaporation during wort-boiling at atmospheric pressure. Using normal brewing materials It has been established that an evaporation of only 2% of the total volume of a wort is sufficient to produce beers of sound flavour. Further evaporation had no significant effect on the flavour or other qualities of the finished beer.