Alpha acids (α acids) are a class of chemical compounds primarily of importance to the production of

beer Beer is an alcoholic beverage produced by the brewing and fermentation of starches from cereal grain—most commonly malted barley, although wheat, maize (corn), rice, and oats are also used. The grain is mashed to convert starch in the ...

. They are found in the resin glands of the flowers of the hop plant and are the source of hop bitterness. Alpha acids may be isomerized to form iso-alpha acids by the application of heat in

solution Solution may refer to: * Solution (chemistry), a mixture where one substance is dissolved in another * Solution (equation), in mathematics ** Numerical solution, in numerical analysis, approximate solutions within specified error bounds * Solu ...

. Iso-alpha acids (iso-α-acids) are typically produced in beer from the addition of hops to the boiling

wort Wort () is the liquid extracted from the mashing process during the brewing of beer or whisky. Wort contains the sugars, the most important being maltose and maltotriose, that will be Ethanol fermentation, fermented by the brewing yeast to prod ...

. The degree of isomerization and the amount of bitter flavor produced by the addition of hops is highly dependent on the length of time the hops are boiled. Longer boil times will result in isomerization of more alpha acids and thus increased bitterness. Common alpha acids include

humulone Humulone (α-lupulic acid), a Vinylogy, vinylogous type of organic acid, is a Bitterness (taste), bitter-tasting chemical compound found in the resin of mature hops (''Humulus lupulus''). Humulone is a prevalent member of the class of compounds ...

, adhumulone, cohumulone, posthumulone, and prehumulone. The most common iso-α-acids are ''cis''- and ''trans''- isohumulone.

Bittering

The alpha acid "rating" on hops indicates the amount of alpha acid as a percentage of total weight of the hop. Hops with a higher alpha acid content will contribute more bitterness than a lower alpha acid hop when using the same amount of hops. High alpha acid varieties of hops are more efficient for producing highly bitter beers. Alpha acid percentages vary within specific varieties depending on growing conditions, drying methods, age of the hop, and other factors. For example, this list shows the typical range of alpha acids found in some common varieties (percentages are based on total dried weight). The choice of a hop variety used in beer brewing depends on the beer style. For instance,

lager Lager (; ) is a Type of beer, style of beer brewed and Brewing#Conditioning, conditioned at low temperature. Lagers can be Pale lager, pale, Amber lager, amber, or Dark lager, dark. Pale lager is the most widely consumed and commercially availab ...

styles use hop varieties with a low alpha acid content (such as Saaz and Hallertauer) while IPA styles use hop varieties with a high alpha acid content (such as Cascade, Centennial and Chinook).

Anti-bacterial properties

Iso-α-acids have a

bacteriostatic A bacteriostatic agent or bacteriostat, abbreviated Bstatic, is a biological or chemical agent that stops bacteria from reproducing, while not necessarily killing them otherwise. Depending on their application, bacteriostatic antibiotics, disinfec ...

effect on many common

Gram-positive In bacteriology, gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall. The Gram stain is ...

bacteria found in beer. While the iso-α-acids are very effective at preventing serious contamination from Gram-positive bacteria such as the

lactic acid bacteria Lactobacillales are an order of gram-positive, low-GC, acid-tolerant, generally nonsporulating, nonrespiring, either rod-shaped (bacilli) or spherical ( cocci) bacteria that share common metabolic and physiological characteristics. These bact ...

, there are some strains that are quite resistant to the effects of the iso-α-acids. The iso-α-acids have no effect on

Gram-negative Gram-negative bacteria are bacteria that, unlike gram-positive bacteria, do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. Their defining characteristic is that their cell envelope consists ...

bacteria, and therefore the brewer must rely on maintaining proper sanitiation and anaerobic conditions of the finished beer to ensure shelf stability.

References

External links

{{Commons category-inline, Alpha acids, chemical structures of alpha acids Organic acids Brewing Bitter compounds Humulus