There are analogies between the raw commodity of hops and the product beer. The hop components react just like the beer:

In the chapters on the hop varieties and the hop products we have seen that the brewer has a wide range of choices available. Hop products are homogenous, comparatively durable and are controllable in their application. The respective use is dependent on the brewer's goal:

Hops in the brew-house

Traditionally the hopping takes place In the wort copper. The time for the dosage varies from the first wort hopping, via the additions directly before casting, up to direct aroma hopping in the whirlpool. After being dosed the hop components pass through the following stages:

  1. mechanical distribution
  2. emulgation i.e. dissolving the components
  3. components are transformed by heat
  4. evaporating volatile (partly undesired) hop oils
  5. partially combining with proteins and precipitation

At the same time the following processes occur during the wort boil:


Wort boiling

Hop bitter substances dissolve poorly in a watery solution, as they are mainly non-polar. The solvency of the alpha-acids with a pH of 5.0 is around 40 mg/l. The solvency of the beta-acids is even less with wort pH. It is around 1.5 mg/l. Only traces of beta-acids are found in the beer but their oxidation products such as the hulupones are stable and soluble and remain in the solution up until the end product. 

The formation of the smallest drops of resin with a large surface is prerequisite for the resins dissolving well in the wort.

Due to thermal reactions the humulones isomerise into isohumulones. The isohumulones are then present in the solution as cis- and trans-stereo-isomers.


In the wort the ratio between cis- and trans- isomers is about 7:3. A small part of the isohumulones can be found in the wort as allo-isohumulones, abeo-isohumulones and spiro-isohumulones. The spatial structure of the molecule influences its behaviour in wort and in the beer.

There are solubility, isomerisation and aging differences between the various iso-humulone fractions and their cis- and trans-stereo-isomers. Only the dissolved iso-alpha-acid imparts the bitterness typical for beer.

Cohumulone can enter the solution more easily due to its slightly higher polar character as opposed to the n- and adhumulone. A faster isomerisation could not be determined. Iso n- and iso-adhumulones are less polar, are enriched in the froth and are probably responsible for a creamy foam.

The iso-alpha-acids are sensitive to oxygen and form a unspecific amount of oxidation products while the wort is boiling. These may also be relevant for the aroma.

Parameters which influence the isomerisation:

Yield of bitter substances

In the brewery the yield results from the ratio between the remaining bitter substances in the finished beer and the bitter substances dosed with the hops according to the following rule:

Yield = (bittersubstances in beer / bitterhop in doses) x 100

and is calculated as the isomerising rate as follows:

Isomerisierungsrate = ( Iso - ? - Säuren in Würze oder Bier / ? - Säuren dosiert ) x 100

The HPLC analysis is the best way to determine the alpha and iso-alpha acids actually dissolved.
The method for analysing the bitter substances in wort and beer can be seen in the chapter "The Plant" - under the subtitle: Hop analytics.
In the brewery there are various ways and means of influencing the yield of bitter substances and quality of the bitter substances: ways and means massnahmen.pdf

What happens with the alpha-acids dosed?


What is the ratio between IBU and content of alpha and iso-alpha-acids?


Hop oils

Here we can only address this interesting subject superficially within this framework. The hop aroma is very complex and cannot be described by one or a few substances as in the case of many plants and fruits (e.g. vanilla, berries).

Aroma substances from the malt and the yeast metabolism prevail in the beer. The hop aroma characteristics in the beer differ from those of the hops before the dosage. We are very familiar with the hop oils which are present in the leaf hops, but know little about the changes which occur throughout the entire brewing process. Epoxy and conversion products of the hitherto more than 300 oils form a mixture of substances relevant for the aroma, which can individually only be detected with difficulty but which as a mixture have a considerable influence on the aroma of the beer.

A problem in the analysis are the very low concentrations of these oxidation and conversion products in the hop oils which are in the area of ppb and ppt. Despite state-of-the-art technology the usual analysis equipment meets it's accuracy limits here. There are also hardly any pure substances to calibrate the euipment with.

The hop flavour in the beer bouquet are often described as citrussy, flowery, estery, fruity, spicy or sharp. Hop oil fractions with the characteristics mentioned are sold commercially. It Is assume that increased sulphurous ester compounds of some hop varieties could give the beer an onion or garlic tang. Also aroma substances attached to polyphenols and glycosides are also responsible for the hop bouquet.

There is a substance which is nowadays used as a reference for the aroma hopping: linalool. This terpene alcohol can be relatively easily proved in the beer and can be regarded as a guiding substance for the hopping. Late hop dosages in the brewhouse quantitatively bring linalool and other aroma substances into the beer. Linalool is not a substance specifically found In hops. It can not only be found in hops but also in e.g. coriander or marjoram.

The hop character Is linked with oxidized terpenes. Sesquiterpenes are susceptible to auto-oxidation. At the same time epoxides such as e.g. caryophyllene-4.5-epoxide are formed. Humulene di-epoxides can be found not only in hops but also in the beer.

At the Technical University of Munich-Weihenstephan linalool, alpha-terpienol and humulenol II were definitely identified by GC-MS as aroma components in the beer originating from hops. Their threshold values should be at about 10 µg / l, 500 µg / l, 2,400 µg / l liegen. Other substances such as myrcenol, 5.5 dimethyl-2(5H) furanon, alpha- and beta-eudesmol and selinenol could be assigned with a high degree of probability to other peaks of the chromatogram.

This brief excursion into the hop chemistry is only intended to demonstrate how complex the subject is. We shall not go into it deeper here as even a summary of the present literature would take up a lot of pages and anyway would not even produce statements of universal validity on the hop aroma.

Many scientists have tried to find out what contribution hops make to the beer aroma and its taste. Nowadays in the USA, England, New Zealand and Germany they are trying to decipher the complexity of the aroma by applying the latest methods. In this field especially Prof. Peter Schieberle has shed some light on the deep mystery of the hop aroma.

At present we do not know exactly which hop oil/which hop oil fraction or which synergies enfolding in flavour between them, are actually responsible for an elegant hop bouquet. Here too it is unlikely that hops will reveal their secret. The practice up to now shows that …


1.… a late hopping before casting or in the whirlpool imparts a stronger sensorial "hop bouquet".
2.… with pellets the aroma substances dissolve faster in the wort than with extracts. After a while this effect evens out. This phenomenon is of interest as far as the last hop dosage is concerned.
3.… the manner and intensity of boiling influences how the aroma components react.
4.… to a certain extent there are distinct differences in aroma and taste between the various hop varieties.
5.… a low ester content in the hop oil can be advantageous to the taste.
6.… also substances from the leaf fraction are relevant for aroma and taste.
7.… the hop aroma in the beer decreases due to the beer aging.
8.… a perceptible hop character can mask the aging taste.
9.… the same hop doses can convey other flavours in different beers.
10.… over 1000 years of experience with the hopping practice has shown in an almost "Darwinist" way what has to be done to obtain a fine hop bouquet and a first-class taste in the beer. In this respect you only have to study the old brewing recipes.



Depending on the kind of hopping up to 30% of the polyphenols in the wort can come from the hops.

Nowadays polyphenols from barley malt or hops are more likely to be regarded as intrusive by the brewer. Because certain groups of polyphenols facilitate the formation of irreversible turbidity in the bottled beer. Therefore, in order to obtain a high physical stability they prefer to reduce the dosage of polyphenols or remove them. This is done e.g. by using especially bred proanthocyanidin-free barley varieties such as Caminant or by the use of polyphenol-free hop pure resin extracts. Alternatively polyphenols actively causing turbidity can be reduced by absorbing PVPP during the course of filtration.

However, this is in the face of an interesting development: Efforts have been increased in food and plant research to find natural antioxidants e.g. in herbs, tea or seeds. The effect of these antioxidants can be seen in two different directions: They can protect food - such as fats - from being affected in taste by oxygen or they act as radical catchers in the human body thus exerting an anti-carcinogenic effect. Examples of this are rosemary, tea - especially green tea - or red wine.

The dissolving behaviour of polyphenols in wort and beer is not identical for all components. Hydrophilic groups of substances such as hydroxybenzoles - or hydroxy-cinnamic acids, flavanols or proanthocyanidins - dissolve more easily than the more lipophilic prenyl-flavonoids; flavonoids are between these. The yield of the low molecular HPLC polyphenols with on an average 66 % is higher than that of the EBC polyphenols with 40 %, which is due to the better solubility of the hydrophilic, low molecular structure. Losses occur due to thermal changes and excretions with hot or cold break respectively, yeast and dregs as well as in the filtration.

Hitherto works on the polyphenol contribution hops make to the beer have shown the following:

• Froth and colour of the beers are not influenced negatively.
• With shorter boiling times no hard bitterness occurs - more likely the opposite occurs.
• Definite flavours can be achieved.
• The reducing power of the beers increases, the flavour stability increases. This also applies at least for the pilot extractions with relatively high oxygen content.
• Nevertheless the turbidity tendency increases particularly with longer boiling times (e.g. 80 minutes).
• The nitrate content increases according to the amounts dosed.

Therefore positive flavour effects can be achieved with hop polyphenols, especially when using aroma hops with short boiling times. An increase in the reducing power can be analytically accounted for and in the form of an improved flavour stability. The polyphenol dosage has a negative effect on the amounts of nitrate and - especially where there are longer boiling times - on the physical stability.

Other hopping methods

Like the normal hopping all the other hopping methods also depend on what you want to achieve. 

"Dry Hopping"
With this method hops are dosed as cones, powder or pellets to the already fermented beer. This either takes place in the storage tank or - as with some English beers - in the barrel. Dry hopped beers have a distinct hop bouquet, the aroma is reminiscent of fresh hops. From a microbiological aspect this process does involve risks as the sterile beer comes into contact with a fair amount of non-sterile hops.

Old hops 
Certain types of beers - like Belgian Lambics - use traditionally aged hops in their formula. The oxidised resins and oils convey an altered bitter taste, the aroma differs from when fresh hops are used. This hopping is typical of this type of beer and Is an integral part of the formulas. 

Fertilized hops 
According to the EU regulation only those hops can be designated as "hops without seeds" which contain less than a 2% proportionate weight of seeds. Some English hop varieties are fertilized and pollinated which is characteristic for these varieties. The potential negative effect on the flavour due to the fats and lipids contained in the seeds is controversial. However it is generally accepted that lipids - whether from the malt or the hops - influence the formation of aging compounds in the beer and have a negative effect on the beer froth.

Hop oils 

In order to obtain a stronger hop bouquet, oil fractions can be added e.g. at the end of the boil. This can take place in the form of a fractionated CO2 extract or through distilled hop oils. The aim is to produce a distinct hop bouquet which corresponds to a marked aroma hopping at the end of the boiling time. There are also &auot;post-fermentation" hop oil preparations which are used to fine-tune the flavour.

Beta-acid products  
Beta- acids are gained in the fractionation of the resins. These products are dosed into the copper as emulsions (partly together with the oil fraction) in the brewhouse. This process can be used In making "light stable" beers In order to avoid the copper frothing over, to produce a certain microbiological protection and to counteract a too malty wort-flavour in the beer. Some unspecified oxidation products of the beta-acids enter the solution in low concentrations. 

Isomerised products 
The use of pre-isomerised extracts and pellets is not intended to obtain a special character in the beer but is generally made for reasons of cost. Isomerized products should contribute a basic bitterness in the form of iso-alpha-acids. The hop oils and polyphenols of these products are considerably changed or reduced compared with the fresh hops. At the same time the original characteristics of a hop variety are lost.
The aim is to save money In the use of raw commodities.
As pre-isomerized downstream extracts iso-alpha acids can be added prior to filtration to correct the bitter units. In this way a product with stronger bitterness can be produced e.g. also from a mildly hopped beer in the filtration.

Reduced products  
Also used as downstream products these hop products are usually added prior to filtration.
Consequently two main problems are dealt with:
Froth problems and light struck flavour with the beer in light glass bottles.
Increased contents of reduced iso-alpha-acids can cause an untypical froth and an unusual, changed bitter taste. Therefore special attention must be paid when dosing them.

Xanthohumol preparations
They are a new product in the beverage industry. New preparations are being developd for dosage In the brewhouse as well as for the post-fermentation (following the fermentation). In the beer xanthohumol in higher concentrations produces a lasting turbidity which is similar to the yeast turbidity in white beer

Light struck flavour

As far as light struck flavour is concerned, the chemist means 3-methyl-2-buten-1-thiol (MBT).

In English-speaking countries this thiol, which has a very active odour, is also known as "skunky thiol", and is already perceptible in concentrations of a few nanograms per litre.


The iso-alpha-acids are sensitive to light and disintegrate at wave lengths between 350 nm and 500 nm. This phenomenon, which has been recorded in literature since 1875, occurs if beer is exposed to normal daylight. In the presence of sulphurous amino acids such as cystein the dissolved iso-alpha-acids react to the known thiol. Presumably riboflavin also plays a role in the photolysis of the iso-alpha-acids.

The formation of MBT can be checked by two methods:

1. by protecting the beer in dark glass bottles
2. by using reduced iso-alpha-acids

In the paper by Prof. D. De Keukeleire, M. D. E. Forbes et al. "Mechanism for Formation of the Lightstruck Flavor in Beer" the chemical reactions were investigated during the formation of MBT. With regard to the reduced iso-alpha-acids the scientists came to the following result:

"Apart from storing beer in light-proof containers, such as dark glass or cans, or immediate consumption, the photosensitivity can be circumvented by reduction of the iso-a-acids so that the deleterious photochemical process is prohibited.

This has been conclusively shown in the present work for the dihydroiso-a-acids in which the photoreactive a-hydroxyketone group is reduced to a photoinactive 1,2-diol entity, resulting in complete light resistance.

Conversely, the tetrahydroiso-a-acids are as photoreactive as the iso-a-acids;
however, 3-methylbut-2-ene-1-thiol (";skunky" thiol) cannot be formed from these compounds subsequent to photocleavage. As a consequence, the lightstruck flavor derived from tetrahydroiso-a-acids must be distinctly different from the "natural" lightstruck flavor, perhaps having less obnoxious organoleptic features."

As investigations by the German Research Institute for Food Chemistry show, the intensities of methional and phenylacetaldehyd are also increased through the exposure - compared with the unexposed beer. 

To sum up it can be said:
Although it is possible to counteract the volatile mercaptan odour by reduced iso-alpha-acids, it is essential that beer is not regarded as altogether light-protected: On the one hand e.g. tetra-hydro-iso-alpha-acids can allow hitherto not analysed light struck compounds to arise, secondly light causes a faster, stronger formation of aging components, which considerably changes the original taste. The photo-oxidation of the lipids shown by Prof. Karl Wackerbauer in transparent bottles and the research carried out by Dr. Carsten Zufall on beer aging "light stable beers", which were presented during the EBC Congress 2005 in Prague, should be studied by interested brewers.