The chemical compounds in beer give it a distinctive taste, smell and appearance. The majority of compounds in beer come from the metabolic activities of plants and yeast and so are covered by the fields of biochemistry and organic chemistry. The main exception is that beer contains over 90% water and the mineral ions in the water (hardness) can have a significant effect upon the taste.
Four main ingredients are used to make beer in the process of brewing.
The carbohydrate source is an essential part of the beer because unicellular yeast organisms convert carbohydrates into energy to live. Yeast metabolize the carbohydrate source to form a number of compounds including ethanol. The process of brewing beer starts with malting and mashing, which breaks down the long carbohydrates in the barley grain into more simple sugars. This is important because yeast can only metabolize very short chains of sugars. Long-carbohydrates are polymers, large branching linkages of the same molecule over and over. In the case of barley, we mostly see polymers called amylopectin and amylose which are made of repeating linkages of glucose. On very large time-scales (thermodynamically) these polymers would break down on their own, and there would be no need for the malting process. The process is normally sped up by heating up the barley grain. This heating process activates protein enzymes called amylases. The shape of these enzymes, their active site, gives them the unique and powerful ability to speed these degradation reactions to over 100,000 times faster. The reaction that takes place at the active site is called a hydrolysis reaction, which is a cleavage the linkages between the sugars. Repeated hydrolysis breaks the long amylopectin polymers into simpler sugars that can be digested by the yeast.
Hops are the flowers of the hops plant Humulus lupulus. These flowers contain over 250 essential oils, which contribute to the aroma and non-bitter flavors of beer. However, the distinct bitterness especially characteristic of pale ales comes from a family of compounds called alpha-acids (also called humulones) and beta-acids (also called lupulones). Generally, brewers believe that α-acids give the beer a pleasant bitterness whereas β-acids are considered less pleasant. α-acids isomerize during the boiling beer boiling process in the reaction pictured. The six-member ring in the humulone isomerizes to a five-member ring, but it is not commonly discussed how this affects perceived bitterness.
In beer, the metabolic waste of yeast is a significant factor. In aerobic conditions, the yeast will use the simple sugars from the malting process in glycolysis, and send the major organic product of glycolysis (pyruvate) into carbon dioxide and water via cellular respiration, many homebrewers use this aspect of yeast metabolism to carbonate their beers. However, under anaerobic conditions yeast cannot use the end products of glycolysis to generate energy in cellular respiration. Instead, they rely on a process called fermentation. Fermentation converts pyruvate into ethanol through the intermediate acetaldehyde.
Water can often play a very important role in the way a beer tastes, as beer is around 90% water and different brands of water can taste different. Further, the ion species present in the water can affect the metabolic pathways of yeast. For example, calcium and iron are essential in small amounts for yeast to survive in water, because these metals are usually required cofactors for yeast enzymes.
In aerobic conditions, yeast turns sugars into pyruvate then converts pyruvate into water and carbon dioxide. This process can carbonate beers. In commercial production, the yeast works in anaerobic conditions to convert pyruvate into ethanol, and does not carbonate beer. Beer is carbonated with pressurized CO2. When beer is poured, carbon dioxide that is dissolved in the beer forms bubbles. These bubbles grow and accelerate as they rise by feeding off of nearby smaller bubbles, a phenomenon known as Ostwald ripening. These larger bubbles lead to “coarser” foam on top of poured beer.
Beers can be carbonated with pressurized Nitrogen gas or other gases instead of carbon dioxide gas. These gases are not as soluble in water as carbon dioxide, so they form bubbles that do not grow through Ostwald ripening. This means that the beer has smaller bubbles and a more creamy and stable head. This less soluble gas gives the beer a different and flatter texture. In beer terms, the mouthfeel is smooth, not bubbly like beers with normal carbonation. Nitro beer could taste less acidic than normal beer.
A particular problem with beer is that, unlike wine, its quality tends to deteriorate as it ages. A cat urine smell and flavor called ribes, named for the genus of the black currant, tends to develop and peak. A cardboard smell then dominates which is due to the release of 2-Nonenal. In general, chemists believe that the "off-flavors" that come from old beers are due to reactive oxygen species. These may come in the form of oxygen free radicals, for example, which can change the chemical structures of compounds in beer that give them their taste. Oxygen radicals can cause increased concentrations of aldehydes from the Strecker degradation reactions of amino acids in beer.
Amino acid and carbohydrate degradation has been linked to presence of air in bottled beers, through Maillard reactions.