Alpha acids are a class of compounds primarily of importance to the production of beer

Beer

Beer is the world's most widely consumed andprobably oldest alcoholic beverage; it is the third most popular drink overall, after water and tea. It is produced by the brewing and fermentation of sugars, mainly derived from malted cereal grains, most commonly malted barley and malted wheat...

. They are found in the resin glands of the flowers of the hop

Hop (plant)

Humulus, Hop, is a small genus of flowering plants native to temperate regions of the Northern Hemisphere. The female flowers of H. lupulus are known as hops, and are used as a culinary flavoring and stabilizer, especially in the brewing of beer...

 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

In chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...

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

Wort

Wort may refer to:* Wort, the liquid created by the mashing of malted barley to use in brewing beer* Worting, Hampshire, a large district and suburb of the town of Basingstoke, in Hampshire, England....

. 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 of the available alpha acids.

Common alpha acids include humulone

Humulone

Humulone is a bitter-tasting chemical compound found in the resin of mature hops . Humulone is a prevalent member of the class of compounds known as alpha acids, which collectively give beer its characteristic bitter flavor.-Chemistry:In terms of structure, humulone is a phloroglucinol derivative...

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

Isohumulone

Isohumulones are compounds that contribute to the bitter taste of beer. They are in the class of compounds known as iso-alpha acids. The bitterness of beer is measured according to the International Bitterness Units scale, with one IBU corresponding to 1 part per million of 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).

• Cascade 4.5-8%
• Centennial 9-11.5%
• Chinook 12-14%
• East Kent Goldings 4.5-7%
• Hallertauer Hersbrucker 2.5-5%
• Mt. Hood 3.5-8%
• Saaz 2-5%
• Styrian Goldings 4.5-7%
• Willamette 4-7%

Anti-bacterial Properties

Iso-α-acids have a bacteriostatic effect on many common Gram-positive

Gram-positive

Gram-positive bacteria are those that are stained dark blue or violet by Gram staining. This is in contrast to Gram-negative bacteria, which cannot retain the crystal violet stain, instead taking up the counterstain and appearing red or pink...

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

Lactic acid bacteria

The lactic acid bacteria comprise a clade of Gram-positive, low-GC, acid-tolerant, generally non-sporulating, non-respiring rod or cocci that are associated by their common metabolic and physiological characteristics. These bacteria, usually found in decomposing plants and lactic products, produce...

 and Pediococcus

Pediococcus

Pediococcus is a genus of Gram-positive lactic acid bacteria, placed within the family of Lactobacillaceae. They usually occur in pairs or tetrads, and divide along two planes of symmetry, as do the other lactic acid cocci genera Aerococcus and Tetragenococcus. They are purely homofermentative...

, 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

Gram-negative bacteria are bacteria that do not retain crystal violet dye in the Gram staining protocol. In a Gram stain test, a counterstain is added after the crystal violet, coloring all Gram-negative bacteria with a red or pink color...

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