Question 6 (AME 1 2001 – Question 7)

Discuss the constituents of hops that are relevant to brewing and give a detailed explanation of why? [15]

What changes in quality can occur during storage of whole hops? [5]

USE OF HOPS IN BREWINGAlthough hops have had a traditional minor role in medicine and baking, hops are now almost exclusively used to provide aroma and bitterness in beer, with the following properties:

Hops provide the bitter taste in beer (with alpha acid the principal precursor)

The oils provide aroma. Hops modify yeast performance during fermentation. Hops contribute to beer texture (mouthfeel) Hops have bacteroistatic properties which protects beer against some

biological spoilage organisms. Hops reduce over foaming during wort boiling. Hops aid in protein coagulation during the boil. Hops act as a filter medium when a hopback is used. Hops are a foam active agent in beer. Cone Hops contribute tannins which may increase the reducing power

of a beer, and hence its resistance to oxidative staling. Tannins may also contribute to a tendency to produce chill haze.

HOP CHEMISTRYThe hop cone contains lupulin glands consisting of a mixture of hard and soft resin and hop essential oils, otherwise the bulk of the hop cone is made up of vegetative matter containing cellulose, proteins, tannins etc. It is the soft resins (soluble in hexane) that are most important in brewing, and these consist of alpha acids, beta acids and uncharacterised soft resins.

Extraction of hop resins

The alpha acid and the beta acids provide the bittering principals and exist as a mixture of different compounds. These acids are largely insoluble in cold water and, while they are more soluble in boiling water, any material dissolved while hot will precipitate out on cooling.During wort boiling, however, the alpha acids are

isomerised into iso-alpha acids that are much more soluble ( see Figure 1.5.9.2.). The beta acids are less soluble and are largely unchanged during wort boiling, and hence contribute little to the final bitterness of the beer, unless they are oxidised to hupulones in which case they will also contribute bitterness (see section 1.5.6.).

Isomerisation of alpha acids into iso-alpha acids.

Hops also contain beta acids which are sparingly soluble and in fresh hops make little contribution to beer bitterness. The beta acids are principally responsible for the antiseptic properties of the hops. In older hops after the beta acids have been oxidised they contribute more to the bitter character of the beer (see figure 1.5.7.1).

Figure 1.5.9.3 Structure of beta acids.

Hops contain a wide range of polyphenolic compounds of low molecular weight, which generally survive wort boiling to go through to the final beer, where particularly in the presence of oxygen, they convert slowly into products that contribute to chill haze and chemical and flavour instability. However the tannins contribution from hops are generally considered to be less problematical than those contributed from the malt. Between 2-4% of the hop material consists of higher molecular weight tannins/polyphenols which together with a larger (by weight) contribution from the malt tannins, precipitate with proteins during wort boiling and help to improve the kettle break.

EXTRACTING BITTERNESS FROM HOPS: HOP UTILISATION.The bittering principle forms only a small proportion of the total hop cone:

Table 1.5.10.1. Typical composition of Commercial Hops.

Composition %Water 10Total resin 15Essential oils 0.9Tannins/polyphenols 4Monosaccharides 2Pectin 2Amino acids 0.1Proteins (nitrogen x 6.25) 15Lipids and wax 3Ash 8Cellulose, lignin etc. 40TOTAL 100

The majority of the bittering from hops comes when the alpha acids are isomerised during wort boiling.. With bitterness playing such a vital role in beer flavour, it is important to achieve a consistent level of bitterness in the final beer. Table 1.5.10.3. shows the main factors that influence hop utilisation.

Table 1.5.10.3 Factors affecting hop utilisation in brewing.

Factor Influence UtilisationWort pH Increased pH ImprovedLength of boil Longer boil Initial improvement

then decrease over 90 mins.

Hop addition rate Higher addition LowerWort original gravity Increased gravity LowerFoaming in kettle Greater foam LowerFoam in fermenter Greater foam LowerAge of hops Older hops LowerAlpha acid content (%) Higher % alpha LowerTrub volume More trub LowerCold break volume More cold break LowerYeast strain Yeast dependentPressure boiling (higher temperature)

Higher boiling temp Quicker isomerisation

Not all beers have the same degree of bitterness, and the bitterness levels of beers vary substantially according to the style of beer being brewed and the quality of bitterness required in different geographical areas.

HOP AROMAThe essential oils in hops are the source of aroma compounds. These oils are volatile and will be almost entirely vaporised from the kettle if they are present from the start of a 60 to 90 minute boil, although some will be converted by heat or chemical reaction. To compensate for this, many brewers who want beer with a "hoppy" character add selected aroma varieties into the kettle between 5 and 20 minutes before the end of the boil; this gives sufficient time to extract the hop aroma but ensures that all the oil is not lost in the vapour.Late hop character is often described as "floral" or "citrus", but it can be unpleasant if present in too high a concentration. The variety of hop, the timing of the addition, as well as the kettle shape and the material of construction all have a major influence on the subtlety of the final beer aroma.Hops can also be added to beer after fermentation - to the maturation vessel or to the cask for ales - to give beer a dry hop flavour often described as "resinous" "spicy" and "citrus". Because the alpha acids are only slightly soluble in cold beer, there is hardly any increase in the bitterness of beer with dry hopping.Hops produce up to 3% of essential oils during the later stages of ripening after the bulk of the resin synthesis is complete. The composition of the oil reflects not only the variety but also the degree of ripeness. Over 300 hop oil compounds have been isolated and they are usually separated and identified using gas chromatography with mass spectroscopy (GCMS). The essential oils can be divided into three classes:HYDROCARBONS (comprising 50 - 80% of the total)The principal hydrocarbons are the monoterpene (C10) myrcene, and the sesquiterpenes (C15) caryophyllene and humulene. Farnesene and selinene occur in some varieties, but not all, and can be used as "markers" for hop variety identification. All are very volatile and few, if any, hydrocarbons survive the wort boiling process in to beer, even when added as a late hop addition to the kettle. Traces of these compounds will dissolve in dilute alcohol, and both myrcene and humulene have been detected in beers which have been dry hopped.OXYGENATED COMPOUNDS (comprising 20 - 50% of the total)Some of the oxygenated compounds are produced by the esters of the corresponding hydrocarbons which produce a more polar and chemically complicated group of compounds. These are better able to survive wort boiling, particularly from a late kettle hop addition, and find their way into beer. However it is not known precisely which, if any, of these compounds are responsible for the desirable hop aroma since many of the compounds will be volatilized during the last minutes of boiling, and those which survive in to the wort may be lost with CO2 purging or be chemically modified during fermentation.SULPHUR COMPOUNDS (comprising < 1% of the total)Although the sulphur compounds are present in only very low concentrations in hop oil they have potent aromas. Few sulphur compounds survive wort boiling, but late addition of hops can introduce trace amounts of sulphur compounds as can the addition of hop oils. Some of the sulphur compounds give cooked vegetable, onion-like, rubbery sulphur aromas with very low perception thresholds.

It appears that hop flavours in the finished beer are a result of a synergistic mixture of compounds. These may be modified by:

variety, type of hop length and nature of boil oxidation and degradation products during the brewing process yeast metabolism time and nature of hop storage.

Column chromatography of an oil rich carbon dioxide extract of hops gave five fractions which were then added to beer:Giving a hop aroma characacter to a beer not only requires selecting an appropriate hop variety to provide the essential oils but also to add the hops so that at an appropriate stage in the process.KETTLE HOPS: when hops are added at the beginning of the kettle boil, little if any aromatic oils persist into the finished beer. There will be some chemical modification or combination of flavour compounds.LATE HOP ADDITION: hops may be added at any stage from 20 to 5 minutes before the end of the boil, or fresh hops added to a hop back to allow sufficient time for the extraction of some of the hop oils without them all being lost due to vaporisation. Not only will some of the more volatile hop oils be lost in the last few minutes of boiling or while waiting for wort cooling, but the remaining oils may be modified or lost during fermentation. It is usually the heavier esters and ketones which are retained to give the fruit citrus characters found in many late hop lagers. The extent to which these characters persist depends on the kettle design as well as the hop variety.DRY HOP: appropriate hop varieties may be added during maturation or to cask to impart a hop aroma and taste particularly to traditional ales. A wider range of hop oils is extracted than during late hopping, which imparts a floral fragrant note to beer often with spicy characters which can be astringent if overdone. When whole hops are added to a cask the extraction and chemical reaction of the oils will continue throughout the drinking life of the cask which produces a constantly evolving change in palate over time.

HOP PRODUCTSBeside whole hops (hop cones) and pelletised hops, other hop products and extracts are used either as alpha acids added to the kettle or as isomerised iso alpha acids added to the kettle or after fermentation. Also hop oils and hop bittering products can be added after fermentation to give a dry hop character, or to modify the character of the final beer.Processed hop products represent a convenient and controlled system for adding both bitterness and aroma character to beer, and can be very useful for adjusting beers produced outside specification.

The benefits include: Reduced bulk Reduced transport costs Reduced storage costs Potential for automating hop addition in the brewery Uniformity - (can be standardised to % a alpha value) Greater stability (long term storage)

Improved utilisation (pellets up to 10%: Post fermentation bitterness up to 70% improvement)

Reduced beer and wort losses Improvements in beer quality and consistency Decreased level of polyphenols/tannins Decrease (or absence) of nitrates Decrease (or absence) of pesticide residues

These potential benefits have to be balanced against:X Increased production/processing costsX Possible inclusions of solvents and toxic residues (not with CO2

extract)X Purist/public attiudes to the use of processing and solventsX Possible adverse effects on beer quality and taste.

All of the products are derived from whole hops in the first instance, and are generally used to help the Brewer overcome the effects of ageing, storage and consistency problems.

Hop storageThe common practice is to purchase hops, particularly bitter varieties, on the weight of alpha acid (which may vary between 5% and 14% depending on variety) rather than the weight of hops themselves. Specialist aroma varieties which have lower % alpha acid (2.5% - 6% approximately) are generally purchased by weight of hop and command a premium for their aroma characters.There are four principal methods used for measuring the alpha acid content in the laboratory: lead conductance value LCV (lead salt formation with lead acetate solution) - principally measures the alpha acid contentultraviolet light absorption - spectrophotometry - measures alpha acids and beta acids.optical rotation of polarised light - polarimetry - measures only the alpha acidshigh performance liquid chromatography - measure both alpha and beta acids as well as many of the modified hop compounds which are increasingly finding favour for hop bittering.The lead conductance value method is still widely used in Europe to purchase hops, although spectrophotometry is the usual method in the USA. Hops, and alpha acid in particular, undergo oxidation and degradation during storage. Some varieties show more rapid loss of alpha acid than others, for example Target, although the rate is affected by the form and conditions of storage. Since hops are only harvested in the autumn, unless some are imported from the other hemisphere, they need to be kept in good condition for the Brewer for at least 18 months. To minimise deterioration, whole hops are stored in a cold store, and many hops are processed into hop pellets or hop extracts to improve storage and usage characteristics. As hops oxidise, the percentage of soft resin decreases and the percentage hard resin increases, with a reduction in alpha and beta acids. The oxidative cleavage of the acyl side chains of the hop resin give rise to volatile fatty acids which produce the cheesy flavours in old hops. Many of the hop oils will deteriorate and change either through evaporation, oxidation or internal

reactions - changing the overall balance of the aroma and flavour characteristics of the hop, unless the hops are stored cold or processed to reduce the exposure to oxidation.Although there is a decrease in alpha acid as measured by lead conductance, the oxidation of the beta acid into hulupones gives a new soluble bittering agent which reduces the loss of perceived bitterness as shown in figure 1.5.6.1. When using old hops, the perceived bittering value will be greater than that expected from the alpha acid determination.

Figure 1.5.7.1. Changes in alpha acid analysis values and percieved bittering value during storage

Bittering value can be measured, but the character and aroma quality of aroma hops can only be satisfactorily determined through brewing trials.

@ Ambient

Time

alph

a ac

id

cont

ent

Alpha acid by HPLC

Bittering value

leadconductancevalue

Deterioration of hops with storage

Oxidation of alpha acids decreases the bittering valueOxidation of beta acids to hulupones enhances the bittering valuechanges in alpha acid analysis values and percieved bittering value during storage

Tempratures

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