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The Chemistry Behind Beer Flavor
Beer is one of the most widespread and largely consumed alcoholic drinks in the world. The total world’s beer production amounts to about 1.7 billion liters. It is a complex alcoholic beverage, containing numerous flavor-active compounds over a wide range of concentrations. Beer flavor is a delicate balance of all these compounds, and for the brewers it is a challenge to produce their products consistent in flavor, and to maintain the flavor balance for as long as possible in the market place.
Brewing is a multistage process. It starts with the mixing of barley malt and brewing water (so-called mashing) and heating of the slurry. Enzymes in the malt degrade starch and proteins and a mixture of sugars, peptides, and amino acids are formed.
Malt contains a range of carbohydrates, composed of insoluble cellulose and soluble hemicellulose, dextrin, starch, and sugars. Starch, which accounts for about 50–60% of the weight of malt, is composed of amylose, which decomposes during mashing into maltose and maltotriose and amylopectins which decomposes into glucose molecules (1).
The most important reaction during mashing is the conversion of starch into low-molecular weight fermentable sugars and unfermentable higher molecular weight dextrin. Maltose (2), the most common carbohydrate associated with brewing consists of two glucose units and maltotriose (3) of three glucose units (Figure 1). Maltotriose is still fermentable by most brewing yeast strains while higher dextrins are not.2 Sucrose, another disaccharide, is also present in malt though in low concentration. The cellulose components in the malt do not give fermentable extract or flavor.
Time, temperature, and pH are important factors influencing the enzymatic breakdown of the starch molecules. The principal enzymes, alpha- and beta-amylase, have a different temperature and pH operating range. Alpha-amylase is more temperature resistant and has an optimum between 72 and 75 °C, but is destroyed at 80 °C. It has an optimum pH between 5.6 and 5.8. For beta-amylase, the optimum temperature is between 60 and 65 °C and the pH between 5.4 and 5.5. The difference in temperature optimum is used by the brewer to control the composition of the mash and the ratio of fermentable and nonfermentable sugars. The higher the temperature used for the mashing process, the greater the proportion of unfermentable dextrins in the liquor. The latter contribute to the body and the mouthfeel of the final beer. Mashing at lower temperatures results in more fermentable sugars and subsequently a higher alcohol production during fermentation.
Malted barley contains polyunsaturated fatty acids, such as linoleic and linolenic acid, which readily form oxidation products, which can be the precursors for aging compounds formed in the final beer.3., 4., 5. and 6. During mashing enzymatic and nonenzymatic oxidation of the unsaturated fatty acids takes place. Reduction of oxygen contact during mashing has a positive effect on the flavor stability of the final beer.7 Brewing with barley-malt lacking the enzyme lipoxygenase-1 also results in better flavor stability of the final beer.8. and 9.
After the mashing is completed, filtration is carried out to obtain a solution containing about 12–14% (w/w) sugar, which is called sweet wort. With the filtration of the mash (called lautering or mash filtration) solid materials such as spent grains are removed. Together with the solids and the turbidity much of the unwanted fatty acid materials are also removed. The effects of the clarity of the wort after lautering on the fermentation performance and later on the flavor stability of the final beer has been a subject of many studies.10. and 11.
After the lautering, the sweet wort is boiled for at least 1 h together with hops, the flowers (so-called cones) of the female hop plant which provide flavor to beer. The boiling serves several purposes: sterilization, deactivation of enzymes, protein precipitation, color formation, removal of unwanted volatile components and, very important, the conversion (isomerization) of the main constituents of the hops, the α-acids, into the iso-α-acids, the main bittering compounds found in beer. During boiling of the wort the following changes occur.
1) Proteins and phenolic compounds from the malt form insoluble complexes and precipitate. This is important to increase the colloidal stability of the final product.
2) The wort becomes darker because of the formation of melanoidins, as a result of reactions of sugars with amino acids, oxidation of polyphenols, and caramelization of sugars.
3) Many volatile compounds, which are present in the malt and hops, such as volatile sulfur components, aldehydes, and hydrocarbons, are evaporated. This is important for the quality of the final beer, as many of these volatile compounds are considered negative for beer flavor.
Dimethyl sulfide (DMS) is a particularly important malt component, which is rapidly lost during the boiling of the wort. To decompose its precursor, S-methylmethionine (SMM), adequate boiling time is required. If the boiling is stopped too soon the remaining SMM can still decompose during the cooling of the wort, but without evaporation of the DMS formed. Consequently, a very high concentration of DMS can carry through in the final beer where it is considered an off-flavor.
Boiling concentrates the wort to its desired strength for fermentation. On average, the volume decreases by 8–10% per hour of boiling. Finally, boiling also sterilizes the wort, which is important to avoid microbiological spoilage during the next steps in the process, fermentation and maturation. After the boiling, the wort is cooled and solid materials, precipitated proteins, spent grain, and spent hops, are removed and the clear liquid (hopped wort) is ready for fermentation. Yeast is added and the solution is aerated to facilitate the yeast growth. During the main fermentation phase, yeast converts the fermentable carbohydrates in the wort into ethanol and carbon dioxide. During fermentation numerous other flavor-active volatile components, such as esters, aldehydes, and higher alcohols, are being formed as by-products, which have an important contribution to the flavor of the final beer. The composition of these flavors depends on the yeast strain and the fermentation conditions, enabling the brewers to create unique flavors in different beer types.
After the main fermentation the liquid, called green beer or young beer, is not yet ready for consumption. It contains too many undesirable flavor components, also formed during the main fermentation. It requires a period of maturation or conditioning of several weeks at low temperature during which off-flavor compounds are either transformed (reduced) into less flavor-active compounds by the remaining yeast cells or are purged by the carbon dioxide which is still formed in this phase of the process.
The most dominant compounds, which need monitoring during the maturation phase, are diacetyl and 2,3-pentanedione. These compounds are particularly unwanted in lager-type beers because of their very low flavor threshold value. Only when the content of these flavor-active compounds has decreased to below their critical concentration the beer is ready for filtration and can eventually be packaged in kegs, bottles, or cans.
In order to avoid problems with microbiological contamination in the packaged beer, the bottled or canned beer may be pasteurized. Alternatively, cold sterile filtration can be used before bottling of the beer. A simplified scheme with the steps in the brewing process is depicted in Figure 2.
Malting and brewing technology have remained very traditional over the years, but the efficiency of the process has increased through understanding of the technology and the underpinning science. Innovation in the brewing industry is driven by cost reduction, for example, by more efficient use of the raw materials and lower energy consumption, and the need for improved quality, safety, and wholesomeness of the final product.12
Extensive state-of-the-art knowledge of brewing science and practice is described in a standard work by Briggs et al. 13 Research and innovation in brewing process and technology and their effects on beer flavor have been reviewed by Bamforth 14 and by Meilgaard. 15
This excerpt was taken from the article, Beer Flavor by Leen C. Verhagen. The article examines the origin and formation of the dominant flavors and off-flavors in beer, with emphasis on the hop, which is a minor ingredient in beer brewing, but with a huge impact on the sensory and physical quality of the products. Flavor changes occurring during the storage of beer, and the possible precursor of some of them are highlighted. Read more here.
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