The third and final part of our in-depth look at brewing terminology.


Contrary to common preconceptions traditional ale is not a vegetarian product. It is true that beer’s ingredients are vegetal – barley and hops fermented with yeast. However, once the beer is fermented brewers use Isinglass a finings agent that is derived from fish. Finings agents remove residual yeast and protein from the beer and ensure a clear pint.

Rather surprisingly Isinglass is taken from the swim bladder of the sturgeon or perch. Perhaps not what a drinker would expect to be mixed with their pint of cask ale.

Isinglass is positively charged and so attracts negatively charged yeast into a complex which settles at the bottom of the cask. Only a small amount is required – half a pint or so per 9-gallon cask.

Isinglass is by far the most effective fining agent for clearing yeast in beer. Clearing a full cask within 24 – 48 hours ensuring a publican’s beer is ready to serve soon after delivery.

Alternatives are less effective. Carrageenan seaweed extract is often added as well as Isinglass and will work on its own but not as well. Plant extracts are also an option but so far these have not proved reliable. It is possible to produce clear beer for bottles, kegs and cans without using Isinglass by using centrifugation in the brewing process, for example. Clear cask ale, however, requires fining. The rise of craft brewing means more and more beers are now being served with a yeast haziness. Some years ago this would have been viewed negatively by drinkers with many wrongly assuming there was something wrong with the beer. The fact is a hazy bee is just unfined and as such is more natural and less processed.


Juniper is normally associated with gin production providing the familiar strong spicy taste we mix with a tonic. But Juniper is also bee used in brewing. Its distinctive taste can be found in some traditional Scandinavian beers such as Sahti. It is also used as an ingredient in beverages and medicinal treatments in North America where it known for its health benefits and antiseptic properties.

When brewing beer juniper can contribute to the fermentable extract and also be used to separate the wort at the end of mashing. Where a layer of juniper branches, sometimes mixed with straw, provide a dense filter allowing the wort to separate from the residual malt grains – as contributing flavour as the beer seeps through.

Juniper berries (or cones) can also provide fermentable materials to use in a brew but require careful extraction. Berries and branches can both provide flavour when added to the boil and, being antimicrobial, help limit bacterial contamination. The climate in Northern Scandanavia does not lend itself to malt and hop production so it is easy to see how the Juniper tree became the basis for local beers.

Sahti production is still practised in Finland today. Where brewers produce a beer of 6 – 8% alcohol by volume from a grain mix of barley, rye malts and separated by Juniper twigs. The beer is spontaneously fermented and has a very distinctive resinous and spicy flavour. Residual grains are often incorporated into local bread too providing the perfect accompaniment for a glass of beer.


Ketone flavours such as diacetyl may not be a common topic for discussion between drinkers in our bars and taprooms. But they’re often front of mind in the brewery – especially when judging the quality of a beer. Ketones are produced during yeast growth with their levels increase during fermentation. They are noticeable in fresh beer as honey, butterscotch or caramel flavour.

In a beer with bitter and hop character these flavours are viewed as contaminants with brewers of pale ales and lagers becoming paranoid over their presence. Real ales prove more resilient as Ketone levels decrease when they are metabolised by yeast. And in darker beers, they are hidden in a raft of other flavours and even contribute positively to a rounded mouthfeel.

Ketones can also be produced in high levels by bacteria so giving you advance notice of a bad beer before you take a sip. Yeast strains differ in the levels they produce with classic British yeasts generating higher levels than yeasts for other beer styles. There is even an argument that international brewers target low levels to distinguish their beers as clean – or perhaps part of their unflavoured character.

Ketones are regarded as unpleasant by many drinkers. Even though a quarter of the UK population are not sensitive to diacetyl and may even enjoy its presence. Look out for butterscotch flavours when you next taste a real ale a few yeast strains will have left a trace for your palette to detect.

Late hopping

Any serious drinker will be aware of the impact hops have on the flavour of their beer. Both in providing bitterness and aroma. In general, we respond positively to both of these enjoying a bitter taste and a complex hop aroma. Today greater interest in beer flavours has encouraged brewers to state their hop varieties on pump clips, bottles and web sites. Where before we could judge the degree of hop character now we can characterise and document its varieties, its times of addition and how it is processed.

One of these means is the addition of hops towards or after the end of the boil either in the copper or even later in cask as dry hopping. Traditional late hopping would refer to throwing in a charge of hops five or ten minutes before the end of the boil with the reasoning that because many flavours are volatile a late addition will provide some fresh aroma to the beer. This is true but it also alters the flavour balance.

Hop flavours are as diverse as the styles of beers and differ greatly in their volatility – some are so volatile they dissipate up the brewery chimney within minutes. Myrcene is a good example of a predominant hop aroma with a range of flavours from spicy to fruity depending on its concentration but giving a very distinctive signature of fresh hops. However, with a boiling point of 167ºC its level rapidly declines during boil allowing other, less volatile aromas to become apparent. Some such as geraniol have higher boiling points – 230ºC in geraniol’s case – and are more persistent at the end of the boil.

Adding hops later during or after the boil will restore the proportion of myrcene giving a more intense and recognisable hop character. It will, however, subdue the flavour of possibly more interesting flavours – geraniol, for example, has the floral aroma you smell in geranium and roses which is unusual in a beer but which may make it stand out in a crowd.

The timing and varieties used in late hopping may dictate a beer’s hop character while early hopping provides its bitterness. Trying to identify which variety of hops has been added and when is not easy but always a good challenge especially when trying something new.


Malt is known as ‘the body of beer’. It provides the sugars which ferment and turn to alcohol. Different styles of beer use different malts to generate the varying flavours and colours which are immediately apparent on viewing and tasting your beer.

A major feature of malt is its complex biochemistry. Not only are we drinking the alcohol from its sugars but also its residual, non-fermented sugars, its proteins and vitamins, minerals and tannins. Malt can produce a range of complex flavours such as biscuit, flour, grainy, sweet and astringent.

Malt is derived from its parent cereal. This is normally barley but could be wheat, rye or oats in the UK and Sorghum or other grains overseas. Cereal grains are packed full of starch so must be malted before they can be used in brewing. Without this process, little starch would be digested and little sugar or alcohol produced. Enzymes present in the cereal grains digest the starch during the malting process releasing the sugars we brewers need.

Malt protein also helps generate and stabilize the head of beer. As well as contributing to a pint’s appearance foam also controls which flavours leave the beer. Poor malt or a recipe with too little malt will give a flat beer with a different aroma. Some malts, particularly wheat malt, have high levels of protein and thus produce a large head but also a haze in the beer. Haze can also happen if a draught ale is chilled too much as this makes the malt proteins precipitate. Normally these dissolve as the beer warms so if you are pulled a beer with a light unwanted haze it may be worth waiting a while before complaining.

While different cereals and different barley varieties provide different flavours to beers it is the roasting of malt that’s the basis of different beer styles. Crystal, black and chocolate are the major roasted varieties each enable different recipes from mild to stout and everything in between.


A beers chemistry is critical to its quality. Nonenal is a particular characteristic rarely discussed outside a brewer’s laboratory. Sometimes known as staling nonenal is an unsaturated aldehyde that leaves a musty taste reminiscent of cardboard, papery or wet clothes. Far from the fresh hoppy and fruitiness, we would expect from a freshly brewed beer.

Nonenal arises when beers age. Or to be more precise (and technical) from the oxidation of lipids present in malt a reaction which is accelerated by the presence of oxygen. That’s why keeping beer in sealed vessels ensure it has a longer shelf life when compared to open fermenters for example. Small scale production and home brewing are inevitably more prone to nonenal due to their greater exposure to air during handling and bottling. So a rule of thumb is ‘the bigger your brew the better’.

Production of nonenal is also accelerated by the presence of an enzyme in malt – lipoxygenases. Since malt varieties have different levels of this enzyme beer styles will have different potential for staling. Light beers are generally more susceptible than darker styles. Dark beers will also have stronger flavours to mask nonenal but also benefit from compounds which absorb oxygen preferentially so limiting the impact.

Other approaches to managing stale flavours include control of the malting process whereby low germination and high kilning temperatures reduce the levels of lipoxygenase. Other options include using barley varieties specifically bred to have low or no enzyme. The most well known of these is PolarStar developed for Sapporo brewery and although not widely available it does suggest that, in future, malts may have less potential for developing off-flavours.

Malts are not entirely to blame for staling. Hops also contribute when their bitterness components are oxidised – again to aldehydes but also to isovaleric acid which has more of a ‘cheesy’ flavour. Highly hopped beers have more potential for this with their greater levels of bitterness. Light coloured, very bitter beers are thus doubly disadvantaged. They are maybe beautiful beers when fresh can be a real problem when aged.


Oak can provide a subtle extra dimension to a beer’s flavour. Barrel-aged beers are increasingly popular both with drinkers and breweries looking to add to their brewing portfolio. Barrel ageing is as old as brewing itself and only fell out of use as metal vessels and casks became widespread within the industry. While stainless steel has many hygiene advantages and helps brewers produce more consistent beers. Traditional wood casks and barrels are still widely recognised as a great way to add flavour and microbes to beer.

Flavours imparted directly from wood include coconut from lactone lipids, vanilla from vanillin and aromatic phenols providing clove and cinnamon cahracteristics. Tannins contribute slowly over time to provide astringency which can make beer too dry if aged for extended periods.

Wood is not inert and often contains microorganisms within its porous structure, particularly the increasingly acclaimed yeast Brettanomyces and lactic acid bacteria.

Sourness and a variety of metabolic flavours including diacetyl may arise from these and, in time, produce a sour beer.

Further sourness may develop from gradual ingress of air to the cask allowing acetic acid bacteria to grow and provide a slight ‘vinegar’ taint. Different woods have different porosity with French oak more so than American so providing different oxygenation. Similarly, large casks will have less surface area and be less effective than smaller casks.

Finally, it is common to reuse spirit casks for beer ageing and so pick up some of the residual characteristics of the spirit. Whisky, sherry and wine casks are popular but brandy and port also provide interesting alternatives.


When is a beer an off beer?  Certainly when it is oxidized.  A stale character is a good indication of poor brewing, packaging or cellaring whereby air has had contact with the beer and its oxygen has reacted with beer components particularly lipids.

As beer has so many components there is a wealth of results from such reactions.  Principle staling flavours in beer are those with a papery or cardboard character.  As most of us refrain from tasting such materials these flavours are often recognised by association and smell but think dull and damp for a general impression.

As well as their own character staling flavours greatly distract from the desirable flavours of beer particularly hop and fruit so accentuating the displeasure.  More specific flavours may arise such as leathery for highly oxidised or blackcurrant depending of the beer style and ingredients.  In a few occasions a slow oxidation may produce specific flavours of almond which may be positive in a few styles as in mature wine but certainly not in standard ales.

A good brewer will take steps to minimise oxygen uptake by keeping the wort and beer away from air.  Not stirring the mash and flushing vessels and bottles with carbon dioxide are essential precautions as are checking valves and connections for leaks.  In some cases oxidation can occur in the pub cellar particularly when cask ale is dispensed.  Leaving a cask open for too long and not hard spiling between sessions will allow air extensive contact with the beer.  Such treatment will also lead to loss of carbonation and a flat pint.  Coupled with oxidation we certainly have the worst combination – off flavours and a flat pint.  Time for some feedback to the bar staff.


Written by Dr Keith Thomas
Brewlab Director

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