1999_hoppolyphenols

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Technical Publications

Hop Polyphenols - do more than just cause turbidity in beerHopfenrundschau International, 68 74, 1999

A. Forster, B. Beck and R. Schmidt

Introduction

Even today polyphenols originating from barley-malt or hops are considered to be a nuisance by manyfarmers, as certain groups of polyphenols promote the formation of irreversible turbidity in the bottled

beer. In order to attain a high physical stability they prefer to reduce the dosage of polyphenols or

partially remove them. This can be done e.g. by using specially cultivated varieties of barley free of

proanthocyanidine such as Caminant or by using pure-resin hop extracts free of polyphenol. Alter-

natively turbid-active polyphenols can be reduced by adsorption on polyvinylpolypyrrolidon (PVPP)

during ltration.

However this is contrary to an interesting development: In food and plant research increased efforts

are being made to nd natural antioxidants e.g. in herbs, tea or seeds. The effects of these anti-

oxidants, which are often of polyphenolic character, are seen in two different ways. They can protect

food such as fats from being spoiled by oxygen as far as the taste is concerned. On top of this it is

thought that they are used to catch radicals in the human body thus exerting an anticarcinogenic ef-

fect. Examples of this are rosemary, tea and especially green tea, or red-wine.

Now the hop is a plant with comparatively high contents of polyphenols (4-6 weight -%) which raises

the question whether and how this group of substances can specically be used.

Analytics

Until a few years ago, for the most part there were only unspecic colouring methods available for the

analytical description of polyphenols. In the meantime considerably more informative methods existusing high-pressure liquid chromatography (HPLC) linked with a diode array detector (DAD). By this

means it is possible to separate a polyphenol compound into its individual components.

In this article analytical details such as the preparation of samples, chromatographical conditions,

identication of peaks and their calibration will deliberately be avoided. Meanwhile about 100 single

polyphenolic hop components can be separated. Chart 1shows the main groups of substances with

reference values on their respective contents. By way of the permanent spectral recording a DAD

makes it possible to secure the identity of peaks and by comparing the spectren of known substances

with those of test peaks enables them to be classied into substance groups. Furthermore, simultane-

ous representation is possible in several wave-lengths (e.g. with 265, 280, 310 and 360 nm). With a

logically devised method the polyphenols can be analyzed analogue in worts and beer.

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Inuences of hop varieties and production area

The analyses of several years relating to the inuence of important hop varieties on the composition

of the polyphenols will soon be completed. A detailed description of the results will not be prejudged

here. In Diag. 1 just to illustrate the HPLC chromatograms (wave-length 360 nm) two hop varieties

produced in the Hallertau i.e. Perle and Nugget are compared as nger-prints. This very comparison

already clearly shows that the hop variety exerts a denite inuence on the composition of polyphe-

nols. Aroma hops contain larger amounts of low-molecule polyphenols.

As far as fruit is concerned, e.g. paprika or oranges, it is already known that the production area with

its climatic conditions exerts an inuence on the composition of avonoids. Here too analyses on hops

are nearly nished and will soon be made public. Diag. 2 shows the HPLC chromatogram of the vari-

ety Nugget from the Hallertau and Yakima production areas. The peak distributions differ signicantly.

According to this climate and production area play a role in the composition of polyphenols in hops. A

moderate climate such as in the Hallertau has a positive effect.

Brewing trials

Many brewers are prejudiced against polyphenols as they promote the formation of turbidity in beer

and therefore impair its physical stability. Allegedly they also have a negative effect on colour, foam

and taste of the beer. In order to clarify these matters the following brewing trials were carried out in

the 120 l pilot brewery of the experimental station of the Swiss breweries in Zrich (5):

Hop polyphenols in the form of a bracteole fraction, as produced by the mechanical concentration of

hop powder type 45, was added to the brew hopped with pure-resin extract free of polyphenols. The

comparison or neutral brew did not contain any of this hop polyphenol fraction (HPF). In the 1st series

the dosage, except in the case of the comparison brew, was with 300 and 600 g/hl of a HPF from Saaz

hops not only at the beginning of the boil but also at the end. In the 2nd series amounts between 50

and 300 g/hl HPF were used , besides the comparison brew. The factors hop variety and boiling time

of the HPF in the wort were varied. The features foam, colour, physical stability, reduction power, poly-

phenol values and particularly taste were observed.

The results of both series can be summarized as follows:

Colour and foam were not inuenced by the dosage of a HPF.

The physical stability suffered especially when the HPF was boiled for a longer time.

The reduction power only ascertained in the 2nd series in accordance with MEBAK method 2.20.1.

improved from 45 (comparison brew) via 50 (approx. 100 g/hl HPF) up to 55 (150-300 g/hl HPF).

The polyphenol ratios in the beers are shown in Chart 2.

The beers were tested in a three-glass test with the comparison beer and with 11-15 tasters.

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In the 1st series 4 beers were available:

Comparison or neutral beer without hop polyphenols = 0

600 g/hl HPF hopped at beginning of the boil (BB) = 600 BB

300 g/hl HPF hopped at end of the boil (EB) = 300 EB

600 g/hl HPF hopped at end of the boil (EB) = 600 EB

Chart 3 shows the results which can be summarized as follows:

There were signicant ndings with regard to differentiation and preference, by which the beers

containing spent hops surprised the tasters with their pleasant, hoppy, slightly fruity aroma and

taste. It is still not clear whether these impressions of taste can also be linked with the observa-

tions on glycosidic compounds in hops (1).

Only where the 600 BB beer was concerned could a somewhat broader bittering be ascertained.

The results were unchanged when the beer-tasting was repeated with the same samples. In the

second round again the same tasters preferred neither the neutral beer nor the beers with HPF.

According to this, denite sensory effects can be attained with the addition of a HPF which the

majority of tasters judged positively but some negatively.

This basic judgement did not change after 4 weeks storage at 27C.

The beers had aged badly over 15 months at 22C. The beer-tasting again produced clear results.

The comparison beer without HPF was totally aged and was undrinkable. This was followed by the

beer with 600 g HPF, hopped at the beginning of the boil (600 BB). The beers with 300 and 600 g

HPF - hopped at EB - had aged but were perfectly drinkable. However it must be taken into consi-

deration that the freshly bottled beer from the pilot brewery had a total oxygen content of about 0.8

to 1.0 mg/l.

Unfortunately negative effects offset these positive results (see Chart 4). The nitrate values of the

beers correspond to the supplementary amounts hopped with the HPF and increased considerab-

ly. The physical stability also deteriorated correspondingly.

The beer-tasting results of the 2nd series with 50 up to 300 g HPF can be summarized as follows:

The addition of a HPF can be seen with 50-100 g/hl and one-star signicance, with 150 g/hl and

two-star signicance and from 300 g/hl upwards with three-star signicance.The signicance of the classication decreased with a longer boiling time and with bitter hops.

Having tasted the beer several times a minority of the tasters preferred the comparison beer, a

majority the beers with HPF. This preference was clearer when the boiling times were short and

when aroma hops were added.

The beers with HPF were designated typically hoppy, fruity, grassy.

A so-called tannin bitter was only found in beers from 300 g/hl HPF upwards and with a long boi-

ling time.

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From this the following conclusions for the dosage of hop polyphenols can be drawn:

There is no negative inuence on foam and colour of the beers.

No hard bitters occur with shorter boiling times. The opposite tends to be more likely.

Denite tastes can be achieved.

The reducing power of the beers increases, the stability of taste improves. This applies at least for

the pilot draughts with relatively high oxygen content.

However the tendency to become turbid increases especially where there are longer boiling times.

The nitrate content increases corresponding to the size of the dosage.

In practice this means a deliberate dosage of hop pellets in fact amounting to about 50 g/hl upwards.

The actual expectations relating to the nitrate content in beer and the physical stability act as a brake

for high dosages.

They possibly limit them being used to a greater extent for qualitative or physiological reasons. Att-

empts can be made to make deliberate use of hop polyphenols up to these limits which apply for every

brewery or for the individual beer type. Here the following recommendations can be given:

Suitable hop varieties should be selected with a higher proportion of low-molecular polyphenols

(e.g. aroma varieties from moderate climatic zones).

Fresh hops or pellets should preferably be used instead of older ones, in order to maintain the

low-molecular character of the polyphenols.

The boiling time for the respective hopping in the wort-kettle should be limited.

If e.g. the nitrate determines the possible amounts of pellets, pure-resin extract can be added at

the beginning of the boil and pellets only after the boiling time is half-way.

In order to keep the doses of polyphenols constant over the years, the annual uctuations of the

(-acids can be balanced out by means of a exible mechanical concentration (variable type).

Therefore the ratio (-acids to polyphenols) remains constant. A lupulin concentration makes sense

as the hop spindle with a relatively low polyphenol content is discharged and therefore the result is

a more favourable ratio of polyphenols to nitrate in the pellet.

Polyphenols during the brewing process

The dissolving behaviour of polyphenols in wort and beer is not the same for all components. Hydro-

philic groups of substances such as hydroxybenzoin - or hydroxy cinnamic acids, avanols or proan-

thocyanidins dissolve more easily than the more lipophil prenylavonoids; avonoids are in between.

Supplementary to Chart 2, reference values can be calculated for the polyphenol yields from the 1st

series of brewing trials. In this respect the unspecic EBC-colouring method was compared with the

specic HPLC method. Chart 5shows the results. The initial values of the HPF used were:

5.2 weight -% polyphenol content according to EBC

1.2 weight -% calibrated polyphenols with HPLC (without xanthohumols)

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The average yield of the low-molecular HPLC polyphenols is with 66% higher than that of the EBCpolyphenols with 40%, which is due to the better solubility of the hydrophilic, low-molecular structure.

Losses occur due to thermal conversions and eliminations with hot or cool sludge, yeast and dregs as

well as during ltration.

As far as the prenylavonoids are concerned (main representative: xanthohumol) the ratios are diffe-

rent. Xanthohumol is not as soluble as all the other polyphenols and must rst be isomerized similar to

the (-acids. Chart 6shows the development resulting from one trial.

In this trial 5.0 mg xanthohumol were added per liter wort with an average boiling time of 35 min. For

the most part xanthohumol is converted into isoxanthohumol. The high losses can be explained by

poor solubility and adsorption. Even with long boiling times and reduced discharge due to turbidity and

ltration only about 30% yield in the form of isoxanthohumol can be expected in relation to the xantho-

humol used.

Polyphenols causing turbidity are extracted from beer that is treated with PVPP to make it keep longer

or improve its physical stability. The amount g/hl of PVPP added is primarily regarded as variable, but

possibly the type of PVPP used is too. As an example Chart 7shows the reduction of some polyphe-

nol groups through a PVPP ltration. There were differences in a moderate and in a strong treatment.

In fact there is a stronger adsorption of the proanthocyanidins and avonols causing turbidity, but other

polyphenols are reduced. The PVPP treatment is therefore not a strictly selective process of totally

removing the polyphenols which cause turbidity.

Summary

The dosage of hop polyphenols does in fact go along with an increase in nitrate and an increased

tendency to turbidity. On the other hand when a technologically specic procedure is used, their appli -

cation is linked with interesting sensory effects. Therefore there is no reason to refrain from using hop

polyphenols under the doctrine of low nitrate and high turbid stability.

Recently modern selective HPLC methods facilitate the choice of suitable raw commodities, e.g. fresh

aroma hops from temperate production zones. Preferably medium and short boiling times instead of

long boiling times. Therefore neither the colour nor the foam on the beer suffers, the reduction power

increases, taste and taste stability can be inuenced positively.

For brewing technologists, who want to develop a polyphenol philosophy there are sufcient ndings

and possibilities to produce one.

Literature: see previous article by M. Biendl

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