Yeast and Beer UNIT

1 Dipl. Brew. Module 2: Unit 2.7 – The Properties of Beer – Section 2.7.1

MODULE 2: Yeast and Beer

UNIT 2.7: The Properties of Beer

SECTION 2.7.1: Beer Hazes

ABSTRACT: Appearance and taste are the two sensory attributes on which beer consumers judge the acceptability of the product and these parameters may be used to evaluate critically every glass of beer drunk. The flavour attributes are considered in detail in section 2.8, but in this unit we will address the key features of appearance, viz. clarity (haze stability), head formation and retention (foam stability), colour and lack of gushing! These parameters can be physically measured and, thereby, controlled.

LEARNING OUTCOMES: On completion and comprehension of this unit you will be able to:

1. Describe the nature and typical composition of hazes. 2. Know the scientific principles behind non-biological haze formation. 3. Explain the measurement of non-biological hazes. 4. Understand how accelerated haze formation can predict shelf life.

PREREQUISITE UNDERSTANDING: To have studied Section 2.5

2.7 The Properties of Beer______3 2.7.1 Beer Hazes ______3 2.7.1.1 Introduction ______3 2.7.1.2 Mechanism of Haze Formation______3 2.7.1.3 Methods of Haze Prevention ______6 2.7.1.4 Haze Stabilisation ______8 (a) Processing Aids to Reduce Sensitive Proteins ______8 (b) Processing Aids to Reduce Polyphenols______9 (c) Combination Treatments ______9 (d) Summary ______10 2.7.1.5 Measurement Methods for Non-Biological Haze ____ 10 (a) Zero Time Tests ______11 (b) Haze Stability Prediction Tests ______11

2.7 THE PROPERTIES OF BEER

2.7.1 BEER HAZES

2.7.1.1 INTRODUCTION When beer from the primary fermenter is chilled to 0°C, it usually becomes hazy due to the precipitation of material which typically is mainly a complex of protein, carbohydrate and polyphenol plus a small amount of inorganic substances. When the beer is warmed, the haze tends to disappear. It is therefore called chill haze.

The beer may be subject to a succession of alternating periods of chilling and warming, with the beer becoming hazy and then clearing again. Gradually, however, the haze formation ceases to be reversible. That which is stable at 20°C is called permanent haze.

2.7.1.2 MECHANISM OF HAZE FORMATION Not all hazes are associated with protein and tannins. Some may arise from calcium oxalate crystals although these are unlikely if there is excess calcium present during wort production. Other hazes comprise carbohydrates, especially β-glucan material.

The most common however are derived from protein fractions, chiefly degradation products of hordein. These protein precursors are acidic (isoelectric points below 5.5), are initially of rather low molecular weight (10,000-60,000) and are therefore better described as polypeptides. Associated with the polypeptides are various carbohydrates and polyphenols; probably a proportion is covalently linked, another part hydrogen-bonded and the rest free.

An essential role in the polymerisation of the polypeptide material is played by the polyphenols or tannins, particularly the dimers as shown in Fig. 1. Such polyphenols easily oxidise to become highly reactive so that the level of dissolved oxygen in the beer has importance in this context. Heavy metals also seem important as they link oxidised polyphenols and polypeptides. The metals which are remarkably effective are titanium, tin and lead, with copper and iron moderately effective at concentrations over 1mg/litre.

Figure 1. Dimeric polyphenol present in beer and believed to be important in haze formation.

Thus the building up of visible haze particles is particularly rapid in the presence of dissolved oxygen and heavy metals. The oxidised polyphenol units become linked with polypeptide units firstly by the hydrogen-bonding. Chill haze represents such reversible association, the material coming out of solution because of the decreased solubility at low temperature. Permanent haze is characterised by the more durable covalent linkages.

In summary, chill haze forms when beer is cooled to 0 ºC and redissolves on warming to 20 ºC, whereas permanent haze forms on cooling to 0ºC and does not redissolve on warming.

The mechanism of formation of chill haze and permanent haze is illustrated diagrammatically in Fig. 2.

Figure 2. Mechanism for the formation of Chill haze and Permanent haze .

Although protein – polyphenol complexes are the most common cause of beer haze, other materials can cause hazes in beer. The protein-polyphenol complex may also have carbohydrate material (e.g. β-glucan) and/or metal ions (e.g iron, copper) associated with the complex, but undegraded α-glucan (starch) or β-glucan can form

© The Institute of Brewing and Distilling (Dipl. Brew. 2 Revision Notes Version 1 2008) 6 Dipl. Brew. Module 2: Unit 2.7 – The Properties of Beer – Section 2.7.1 gelatinous precipates in beer. Further, calcium oxalate crystals (if present in beer at greater than 20 mg/l) can precipate at low temperatures and collapsed beer foam (bubble “skins”) can also cause haze. Finally, carried over filter powder may leave a particulate residue in finished beer.

2.7.1.3 METHODS OF HAZE PREVENTION There are several possibilities open to brewers to slow down the formation of haze in packaged beer. Various combinations are normally selected from list below:

Avoid contamination of water and raw materials by heavy metals and avoid their introduction from materials of construction of equipment.

Select grist materials so that the total nitrogen of the wort is not excessive (say <800 mg/litre for SG 1040) and particularly the high molecular weight (non- dialysable) nitrogen.

Select grist materials so that the level of polyphenols is relatively low. (Analytical problems make it difficult to provide exact figures.)

Ensure adequate boiling of mash (where directions apply) and of wort to precipitate protein – polyphenol material.

Promote proteolytic action during mashing where necessary, such as stands in the range 45-55°C.

Remove all hot trub from the wort and most of the cold trub.

Ensure a strong production of hot trub be getting a good balance of high molecular protein and polyphenols. This involves the choice and amount of hop material added. Thus some hop extracts contain little or no polyphenol.

Avoid last runnings from the mashing unless they are to be used for mashing-in ( as in high gravity brewing).

Remove any brown scum which appears during fermentation.

Ensure strong yeast growth so that new cells will absorb onto their surfaces protein – polyphenol material.

Treat the beer with proteolytic enzyme – purified papain is particularly effective.

Use a protein adsorbent such as silica gel. (Bentonite is less selective and is difficult to separate from the beer).

Use a polyphenol adsorbent such as PVPP (polymerised polyvinyl pyrrolidone)

Reduce the dissolved oxygen content of the beer by care in processing and the use of reducing agents ( e.g. SO2 plus ascorbic acid ) or the enzyme glucose oxidase. Attempt to balance polypeptide and polyphenol levels by adding tannic acid or protein ( e. g . isinglass finings).

Chill the beer to as low a temperature as possible before very fine filtration.

Keep beer free of dissolved oxygen and heavy metals in packaging.

Hold packaged beer in cold store (or at least avoid warm storage conditions.

It must be emphasized that some of these measures will be rejected by many brewers on the grounds of cost. If only a short shelf life is required, only elementary measures are adopted. Other measures above are rejected on the grounds of violating food regulations. Thus in Germany, only water, malt, hops and yeast may be used; even enzyme additions are precluded. But protein and polyphenol adsorbents add nothing to the beer and do not move on in the processing. They are therefore permitted.

At this juncture, it should also be emphasized that certain measures listed above will, in addition, slow down the deterioration of flavour and aroma of beer held in package. This particulary applies to the reduction in dissolved oxygen content, to as low as possible and the storage of packaged beer in cold conditions. Indeed, the present state of the art of brewing is that there is confidence in achieving colloidal stability ( i. e, haze prevention) and attention has switched to improving flavour stability.

One of the most intriguing points about colloidal stability is the effectiveness of papain. This enzyme has a pH optimum for activity of about 5.5 - 6.0. Yet it is effective in beers with pH values in the range 3.8 - 4.4. Furthermore it can be used at temperatures in the range 0-15°C and give results in 2-7 days. It is an endo-enzyme and therefore cleaves the polypeptides at a point several amino units from the end of a chain. The enzyme’s shape is known to be like that of Paramecium , the ciliate protozoan with an oral groove. The polypeptide is taken party into a pouch-like region and is cleaved at

© The Institute of Brewing and Distilling (Dipl. Brew. 2 Revision Notes Version 1 2008) 8 Dipl. Brew. Module 2: Unit 2.7 – The Properties of Beer – Section 2.7.1 the point where the “mouth” is. One concern when using papain is whether it will have action on foam-forming as well as haze-forming polypeptides.

2.7.1.4 HAZE STABILISATION

Stabilisation ensures that beer is protected from subsequent haze formation and avoids protein – polyphenol complexes forming, as described in detail in section 2.7.1.3

The protein or polypeptide molecules involved in haze formation are rich in proline residues.

The tannins in beer are derived from malt and hops and include polyphenols and anthocyanogens and it is the oxidised and more complex polyphenols that bind with protein/ ploypeptide molecules to form chill haze (by hydrogen bonding) and permanent haze (via covalent bonding).

Consequently there are two approaches to preventing haze formation:

Reducing the level of “sensitive” protein

Reducing the level of polyphenols.

(a) Processing Aids to Reduce Sensitive Proteins

Adsorbents – Silica Hydrogels and Xerogels

Silica gels (acidified sodium silicate) are very effective at removing proteins by adsorbing into pores within the gel structure. The size of the pores can be used selectively to remove proteins in terms of their molecular size (molecular weight). Hydrogels contain up to 60% w/w moisture and are easily handled, quick dispersing and fast settling, whereas Xerogels (with less than 10% w/w moisture) have higher adsorbency, but are more expensive and more difficult to handle due to the very low bulk density. Silica gels are readily removed as tank sediments or at filtration. Hydrogels can also be used as filter body feed.

Bentonite is now rarely used for protein stabilisation in beer.

Precipitants – Tannic Acid

Tannic acid is a very effective protein precipitant added to maturation tank, but tends to produce large volumes of loose solid deposits, which can lead to relatively high beer losses. Tannic acid is a polyphenol extracted from gall nuts.

Proteolytic Enzymes – Papain

This theoretically acts by reducing protein molecular size (see Section 2.5.1.5) and so increase polypeptide solubility. Most enzyme activity will occur during pasteurisation and the enzyme will be retained in the final product and also might adversely affect head forming proteins.

(b) Processing Aids to Reduce Polyphenols

The most common material is PVPP (polyvinyl poly pyrrolidone) which is a polyamide (derivative from nylon, which itself used to be used as a polyphenol removing agent).

PVPP acts as a synthetic protein to which polyphenols bind and are removed from solution as a tank deposit or at filtration.

PVPP is actually recoverable and re-useable if added after primary filtration and removed on a separate filter (candle or horizontal leaf design). The powder is recovered in slurry form and regenerated by heating at 80ºC in 2% caustic soda solution. The cost effectiveness of use of PVPP in this way has to be balanced with the capital investment required for a separate and additional, recovery filter after the primary filter.

In addition, an alternative material composed of a silica hydrogel basis to which PVP (polyvinyl pyrrolidone) is also available for addition to maturation tank.

For long shelf life products, it may be necessary to synergistically treat beer to remove both protein (with silica gels) dosed into maturation tank and/or at filtration (as body feed), followed by post filter addition of PVPP (for polyphenol removal).

(d) Summary

Figure 1 (below) provides a summary of Stabilisation Techniques

2.7.1.5 MEASUREMENT METHODS FOR NON -BIOLOGICAL HAZE

The measurement of haze is based on light scattering techniques. Light is reflected from particles in solution and the amount of light scattered depends on the size of the particles. There are visual methods (involving merely viewing beer in a glass in front of a light source) and instrumental methods that measure the quantity of light scattered by the presence of suspended particles, either at 90º or 13º to the incident light beam; 90º measurement detects particles <0.4µm (that is, invisible to human eye), with 13º measurement detecting particles >0.4 µm (ie. visible). Modern instruments can measure turbidity at both 90º and 13º, but most beer specifications refer to EBC or ASBC haze units measured at 90º. The instrument light source is usually green light, but since results can be affected by beer colour, instruments can use near infra red light sources for dark beers.

The following table indicates correlation between visual standards of turbidity and instrument hazes measured against Formazin standards (prepared from mixtures of hydrazin and tetramethylene tetramine).

Visual Description EBC Haze Units ASBC Haze Units

Brilliant 0 - 0.5 0 - 34.5 Bright 0.5 - 1.0 34.5 - 69 Slight Opalescence 1.0 - 2.0 69 - 138 Opalescent 2.0 - 4.0 138 - 276 Hazy 4.0 - 8.0 276 - 552 Very Hazy > 8.0 > 552

1 EBC unit = 69 ASBC units

(a) Zero Time Tests

Initial hazes are usually quoted as: - Room Haze - measured at 20ºC - Chill Haze - measured after storage at 0ºC for 24 hours.

(b) Haze Stability Prediction Tests

A number of tests can be carried out to check that haze stabilisation treatment has been effective and also to predict shelf life in terms of haze stability (forcing or accelerated ageing tests).

(i) Forced (accelerated) Ageing

Many companies have their own methods of haze prediction based on forcing or accelerated ageing, all being variations on a theme of observing the increase in haze determined after holding for a set period of time (several days) at an elevated temperature (say, 50 - 60ºC), followed by 24 hours at 0ºC and reading the chill haze value. Several cycles of heating and cooling may also be used. Shelf lives can then be based on these predictive results.

(ii) Alcohol Chill Test

This is a rapid method of haze prediction based on measuring the potential to form chill haze, determined as the time taken for the observed haze to increase by cooling the beer to sub zero temperatures after the addition of alcohol to suppress the freezing point.

(iii) Sensitive Protein

This test is used to check on the effectiveness of silica gel treatment by measuring the level of residual sensitive proteins, that is the fraction that is tannic acid precipitable. Determined as the amount of haze formed after addition of 10 mg/l tannic acid.

(iv) SASPL Protein

This measures the amount of saturated ammonium sulphate precipitable proteins (sensitive proteins) left in beer. Determined as the amount of saturated ammonium sulphate solution required to produce haze.

(v) Tannoid Polyphenols

This is a measure of the level of polyphenols remaining that are precipitable with PVP. Determined as the amount of PVP (mg/l) required to cause maximum precipitation.

NB Tests (ii) to (v) can all be carried out by a single instrument called a Tannometer.

MODULE 2: Yeast and Beer

UNIT 2.7: The Properties of Beer

SECTION 2.7.2: Beer Foam

LEARNING OUTCOMES: On completion and comprehension of this unit you will be able to:

1. Understand the principles of foam formation, stabilisation and collapse. 2. Describe the factors affecting stability of bubbles and foam and the causes of por head retention. 3. Describe the methods used for measuring foam formation.

PREREQUISITE UNDERSTANDING: To have studied Section 2.5

2.7 The Properties of Beer______3 2.7.2 Beer Foam ______3 2.7.2.1 Introduction and Summary ______3 2.7.2.2 Foam Formation ______4 2.7.2.3 Factors Affecting Foam Stability ______5 (a) Foam Positives ______5 (b) Foam Negatives______5 2.7.2.4 Processing Factors Affecting Foam Stability ______6 2.7.2.5 Measurement of Beer Foam ______6 (a) NIBEM ______7 (b) Rudin______7

2.7 THE PROPERTIES OF BEER

2.7.2 BEER FOAM

2.7.2.1 INTRODUCTION AND SUMMARY

The beer drinker usually demands that when his drink is poured it forms a white dense foam which persists while beer is in the glass. He or she may also wish to see an attractive lacing of the glass as it is emptied. Foam formation, foam retention and lacing are three separate but interrelated phenomena. Foam formation depends on the presence of gas - usually carbon dioxide, but sometimes mixtures of CO2 and air or CO2 and nitrogen. It also depends on a favourable balance of solutes which promote foam against those that inhibit it. The most effective foam promoters are thought to be glycoproteins – proteins conjugated to carbohydrate. Those derived from malt tend to be polypeptides with many times their weight of carbohydrate conjugated to them. The molecular weight of these materials is controversial but it is likely that the polypeptide component is 10,000- 20,000 and the total molecular weight is 60,000-300,000. These materials are characterised by isolelectric points in excess of pH 5.5.

The polypeptides are important in foam formation because they are hydrophobic and accumulate around the gas/liquid interfaces of the bubbles. If they are to be effective in foam formation and retention, they must have a long hydrophilic tail. Small polypeptides without such a tail are inhibitory to foam because they compete for space at the interfaces with the effective molecule. Under some circumstances, these short polypeptides may be converted into effective molecules by associating with beer carbohydrates or melanoidins through a joining molecule such as isohumulone. Alternatively, certain gums, alginates or cellulose esters may be added to the beer so that they couple within the short polypeptides. These are the so-called foaming or “heading agents”.

Fatty materials and certain hop materials are known to be foam inhibitors. For this reason, care is taken to keep beer lipids to a minimum. However, unsaturated fatty acids are desirable for yeast growth.

When foam forms, the gas is preferably in tiny bubbles of even size. If there are large bubbles also present they will capture the small ones and the beer head tears to pieces. Some breweries use a mixture of 60% nitrogen and 40% carbon dioxide for dispensing draught beer; others introduce air into their draught beer during

© The Institute of Brewing and Distilling (Dipl. Brew. 2 Revision Notes Version 1 2008) 4 Dipl. Brew. Module 2: Unit 2.7 – The Properties of Beer – Section 2.7.2 dispense. Gas mixtures such as these tend to give a more stable foam, partly because of the high partial pressures of nitrogen and oxygen above the foam.

Wort has finite concentration of foam-positive substances. Any foaming of the wort, or the beer derived from it, will deplete this pool of substances. Even the production of a yeast head during top fermentation in a shallow open vessel reduces the pool. ( A yeast head is a foam with entrained yeast cells.) Indeed, some breweries use antifoams of lipid or silicone to inhibit head formation during fermentation. After removing the antifoam from the beer by filtration, the beer has a greater foaming potential than beer produced normally (the main reason for use of antifoam is to fill fermentors to a greater depth without fear of overfoaming). It should be explained that, when foam forms, some at least of the polypeptides involved become denatured. They may in fact not completely redissolve, leaving submicro-scopic “skins” of the bubbles in suspension. These particles act as foci for premature release of gas; in other words they cause beer to gush.

Foam formation and retention are helped be relatively high viscosity; the liquid between the bubbles is then unable to drain quickly. This is one of the reasons why carbohydrates (especially gums, alginates and cellulose) assist foam retention. Low surface tension is also important and in the context hop resins, especially isocompounds and reduced isocompounds, are of great consequence. If beer foam is analysed it is richer than the beer below it in isocompounds and certain metals, such as nickel, which form salts with the resins.

Lacing of foam on the glass appears to be associated in part with glass surfaces that are not completely clean but have a very thin (possibly monomolecular) film of lipid. The hydrophobic molecules become trapped on this surface. Excess lipid would completely collapse the foam.

2.7.2.2 FOAM FORMATION

Foam forms as CO 2 bubbles are released by reduction in pressure as the beer container is opened and these bubbles collect surface active materials as they rise. The surface active materials form an active skin around the bubbles and are principally proteins with hydrophobic side chains that project into the bubble interior (away from water), but also with hydrophilic groups that project away from CO 2 into water. Foam collapses due to bubbles bursting and coalescing and drainage of the “skin” materials. The rate of drainage from the foam depends also on viscosity.

The stability of foam is dependent on the presence and nature of the surface active materials. A number of factors influence foam stability by being either foam positive or foam negative.

2.7.2.3 FACTORS AFFECTING FOAM STABILITY

(a) Foam Positives

The protein and polypeptides involved are relatively high molecular weight (up to 60000) and act as surfactants by virtue of their amphipathic nature (possessing both hydrophobic and hydrophilic groups in the same molecule). They are glycoproteins and the polysaccharide side chains are hydrophilic and encourage interaction between protein molecules, with chains forming hydrogen bonds, allowing concentration around the bubbles, increasing viscosity and resisting drainage.

Iso-α-acids (especially reduced iso-α-acids) are also foam positive, with the hydrocarbon side chains forming hydrogen bonds with hydrophobic groups on proteins and so forming a thicker and more stable hydrophobic layer.

Bivalent metal ions, such as magnesium, form ionic bonds in the hydrophobic areas and help to cross link iso-α-acid molecules with proteins.

The gas content of beer will affect foam stability in that the higher the gas solubility the less stable is the foam. CO 2 is very soluble, but more foam is formed at higher carbonation levels. Nitrogen is much less soluble and so produces a more stable foam, with smaller bubble size.

The use of Foam Stabilisers, such as propylene glycol alginate (PGA) will also enhance foam. PGA is an ester of alginate (which is an acidic polysaccharide, composed of manuronic and galacturonic acids, extracted from seaweed) and propylene glycol. It enhances foam by protection against foam negatives, by electrostatic interaction between the carbonyl groups on the alginate with amino acid residues in the polypeptides within the bubble wall.

(b) Foam Negatives

Factors that are negative towards beer foam formation include:

Lipids Detergents Antifoam compounds

These factors act by being more hydrophobic than proteins and so replace protein in the bubble layer, but because they have no hydrophilic groups to dissolve in the beer or to react to strengthen the bubble, then bubble collapse occurs.

2.7.2.4 PROCESSING FACTORS AFFECTING FOAM STABILITY

Several factors during beer processing can influence foaming ability and stability:

Grist composition: Sufficient quantity of malt (not over modified); +ve Wheat protein has high foam potential; +ve Maize and Rice have high oil content; -ve

Mashing Low temperature encourages proteolysis; -ve Low pH encourages debranching enzymes to hydrolyse carbohydrate chains of glycoproteins; -ve

Iso-α-acids (especially reduced compounds) are foam +ve

Cloudy wort contains lipids: avoid trub carry over; -ve

Excess fobbing during fermentation causes loss of foam +ves

Antifoam will reduce fobbing, but carryover into beer is –ve

Yeast autolysis releases proteolytic enzymes; -ve

Avoid fob formation during beer transfers and carbonation; -ve

Many negative factors affecting foam occur in trade: Dirty glasses Rinse aid residues from glass washers Poorly controlled and maintained dispense equipment.

2.7.2.5 MEASUREMENT OF BEER FOAM

The methods used to measure foaming ability or stability (retention) are often criticised for not representing foam stability as assessed by the consumer. The key factor is establishing a uniform pouring or foam generation procedure.

Analytical methods either measure the time taken for a set level of foam to collapse (e.g. visual systems, either by eye or by camera, and NIBEM) or measure drainage of beer from a foam formed artificially from de-gassed beer (e.g. Rudin).

(a) NIBEM

The principle of the NIBEM equipment is to measure the time taken for the surface of beer foam to collapse by 10, 20 and 30mm, followed by a descending conductivity probe. The beer is foamed into a glass by a standardised procedure in a flashing apparatus. A plate with moveable electrodes is lowered on to the top of the foam. As the foam collapses, the signal received by the electrodes reduces and the plate moves down to maintain contact with the foam. The more rapidly the probes move down to maintain contact, the less stable is the foam. The results are recorded as the time taken for 10, 20, or 30mm of foam to collapse. A good result is usually > 250 seconds for 30mm. The key advantage of this method is that the foam assessed is generated form the intrinsic gas content of the sample and tends to more representative of beer in glass under real dispense conditions.

(b) Rudin

The principle of the Rudin apparatus is previously de-gassed beer is sparged with CO 2 via a scinter in a narrow glass tube to form foam. The drainage of liquid form the foam is followed by timing the rise in liquid level between two marks on the tube 2.5 cm apart. A god result is usually > 95 seconds. The key advantage of this method is that the intrinsic ability of the beer to generate foam is measured, but the major disadvantage is that the foam is formed in a narrow glass tube and is not necessarily representative of foam in a beer glass. Further, since the sample is de-gassed prior to analysis, there is no assessment of the influence of the actual gas content (CO 2 and/or N 2).

MODULE 2: Yeast and Beer

UNIT 2.7: The Properties of Beer

SECTION 2.7.3: Colour in Beer

LEARNING OUTCOMES: On completion and comprehension of this unit you will be able to:

1. Describe the constituents of beer responsible for colour and their origins (melanoidins, oxidised polyphenols, etc.) 2. Understand how processing factors affect colour. 3. Explain how beer colour can be measured and the advantages and disadvantages of the methods used.

PREREQUISITE UNDERSTANDING: To have studied Section 2.5.

2.7 The Properties of Beer______3 2.7.3 Colour in Beer ______3 2.7.3.1 Introduction ______3 2.7.3.2 Melandoidin Formation ______3 2.7.3.3 Formation of Oxidised Polyphenols______4 2.7.3.4 Processing Factors Affecting Beer Colour______5 2.7.3.5 Caramel ______5 2.7.3.6 Methods of Colour Measurement ______6 (a) Visual Comparison ______6 (b) Spectrophotometric Analysis ______6 (c) Tristimulus______6

2.7 THE PROPERTIES OF BEER

2.7.3 COLOUR IN BEER

2.7.3.1 INTRODUCTION

Colour is a major determinant of beer quality and type and is derived entirely from raw materials and processing or from the addition of materials specifically for colour adjustment, such as caramel.

The colours of beer are complex and absorb light over a range of wavelengths. In fact, there is no obvious peak in the visible region and the balance between yellow and red varies from pale to dark samples. Consequently, beer colour is difficult to measure.

Colour in beer (and wort) is associated with a complex array of compounds, such as melanoidins, oxidised polyphenols and charred products. These are formed:

in the raw materials in the grist

during mashing, wort boiling

added deliberately to adjust colour

formed during ageing.

2.7.3.2 MELANOIDIN FORMATION

Melanoidins are soluble pigments with yellow, amber, reddish brown colours and are formed by complex reactions involving the interaction of reducing sugars with amino acids and the amino groups on peptides, via the Maillard reaction (Figure 1.)

Maillard reactions occur during germination and kilning of malt, roasting (of barley and malt), mashing and wort boiling and cooling. Reducing sugars form melanoidins more readily than non-reducing sugars and glycine and alanine react very readily to form the deepest colours.

For beers produced with pale malts, it is estimated that some 66% of the beer colour is produced by malt kilning and 33% by wort boiling.

For the colour effects of coloured malts, see module 1, Section 1.4.3

MELANOIDINS R re-arrangements Amadori O (pigments) Furans -H2O

Reducing sugars. +NH REDUCTONES 3 + Amino + R (α-Diketones) acid Amino N acids. Pyrroles

ααα -Ami no ketones +H 2S

O 2 R S N Thiophenes R

N Pyrazines

Valine + Fructose Malt flavour

Glycine + Glucose Dark colour

Glycine + Maltose Malty aroma & very dark colour

Figure 1. The Maillard reaction.

2.7.3.3 FORMATION OF OXIDISED POLYPHENOLS

Polyphenols (such as catechin) are derived from malt and hops and react with oxygen during wort boiling to form complex polymeric structures by hydrogen bonding, such as Anthocyanogen,

Proanthocyanidin and Tannins. Under normal wort boiling conditions, some oxidised polyphenols survive into clarified cold wort and hence contribute to colour of beer; polymerised polyphenols are reddish brown colour.

Last runnings during wort separation, especially at high pH, can have a high content of polyphenols and iron and copper can catalyse the further polymerisation to tannins.

2.7.3.4 PROCESSING FACTORS AFFECTING BEER COLOUR

The major determining factors of beer colour are the selection of grist materials and the addition of colourants, such as caramel. However several processing conditions will influence colour development:

Temperature; prolonged stands at high temperature during boiling can risk sugar caramelisation or even “charring” or burning on.

pH; high pH during sparging risks increased extraction of polyphenols.

Oxidation; wort oxidation and excessive aeration or oxygenation prior to yeast pitch risks higher degree of polyphenol polymerisation.

Time; prolonged mash conversion and sparging during wort separation risks increased extraction of polyphenols; long boil times risk higher extraction of polyphenols from hops; delays in trub separation (e.g. long Whirlpool stand times) riks enhanced polyphenol oxidation.

2.7.3.5 CARAMEL

Brewing caramel is prepared from high glucose syrup by heating with ammonia (Maillard reaction). It is electro-positive (rather than electro- negative) to avoid adverse reaction with beer polypeptides and haze formation. The colour produced is very high (up to 40000 EBC), but caramel is usually diluted for addition, either to wort during kettle boil or post-fermentation for “fine-tuning” of beer colour.

2.7.3.6 METHODS OF COLOUR MEASUREMENT

Beer colour is expressed in EBC units or, by ASBC method, in SRM (Standard Research Method); the relationship between the two methods is approximately 1 ºSRM = 2 ºEBC.

There are two main methods of analysis:

Comparator - visual method

Spectrophotometric absorbance at 430 nm.

However, because of differences in hue and other factors, determination of colour by Tristimulus has been proposed.

(a) Visual Comparison

The principle of this simple method is direct visual comparison of the sample colour against Lovibond coloured glass discs in a Tintometer. The colour scale comparators are developed to grade beer (and wort colour) into visual units from 2 to 27, using fixed pathlength glass cells (usually 25 mm). Dark beers require dilution to fit on to the scale and the method is somewhat subjective and operator dependent.

(b) Spectrophotometric Analysis

The principle is the measurement of light absorbance at 430 nm in a 10mm cell. Then colour (º EBC) = absorbance (@ 430nm) x 25.

430 nm is used because all beers absorb to some degree at this wavelength, but dark beers again need dilution (to achieve absorbance less than 0.8)

All hazy samples must be clarified.

The major problem with the above methods is that beers with similar absorptions at 430nm (or compared to the narrow range of coloured discs in a Tintometer) may have very different light absorptions or transmissions at many other wavelengths in the visible spectrum. In

© The Institute of Brewing and Distilling (Dipl. Brew. 2 Revision Notes Version 1 2008) 7 Dipl. Brew. Module 2: Unit 2.7 – The Properties of Beer – Section 2.7.3 other words, beers of different “hue” may achieve the same colour measurement, but are clearly identifiable by eye. It has been propsed to measure colour as a combination of red, green and blue transmitted or reflected light, at separate wavelengths: - red 400nm, designated X - green 530nm, designated Y - blue 700nm, designated Z

These are measured on a scale of 0 (black) to 100 (saturated colour); equal intensities of X,Y and Z represent a neutral grey colour. X, Y and Z represent “Tristimulus values”.

In reality, colour comprises three variables:

Hue; the actual colour in terms of frequency of vibration of light waves and indicates the combination of red/green/ blue transmitted light

Luminoscity; the lightness or darkness of a colour

Chroma; the saturation or purity of a colour (how “dull” or “vivid”).

Tristimulus instruments integrate XYZ data into a 3-dimensional plot of hue and luminoscity by quantification of red, green and blue light transmitted or reflected by a sample, taking account of lightness/ darkness and purity.

The advantages of this approach are that errors associated with subjective comparisons and single wavelength measurements are overcome in that a complete colour assessment in the whole visible spectrum is achieved. Also the instruments are full automated, accurate and reproducible. The key disadvantages are the instruments are expensive ans highly sophisticated and tend to be regarded as over-complicateing a relatively straight forward analysis. Further, correlation with EBC units is not directly comparable. All-in-all, tristimulus has found very limited appeal for beer colour assessment, but is used frequently by packaging material manufacturers to achieve good reproducibility of colours for labels and packaging decorations, etc.

MODULE 2: Yeast and Beer

UNIT 2.7: The Properties of Beer

SECTION 2.7.4: Gushing in Beer

LEARNING OUTCOMES: On completion and comprehension of this unit you will be able to:

1. Explain the phenomenon of gushing in beer. 2. Understand the relevance to gushing of grain infection, beer particulates and oxidised hop compounds.

PREREQUISITE UNDERSTANDING: To have studied Section 2.5.

2.7 The Properties of Beer______3 2.7.4 Gushing in Beer______3 2.7.4.1 Summary ______3 2.7.4.2 Prediction of Gushing Potential ______4

2.7 THE PROPERTIES OF BEER

2.7.4 GUSHING IN BEER

2.7.4.1 SUMMARY

Gushing is the rapid and uncontrolled loss from beer of carbon dioxide in supersaturated state.

The main causes of gushing are:

the presence of calcium oxalate crystals in beer providing foci for gas evolution;

the use of “weathered” barley or badly stored barley for malting. This may lead to the growth of various moulds such as Aspergillus, Fusarium, Rhizopus and Stemphylium. These produce polypeptides, of molecular weight less than 15,000, which induce gushing;

the occurrence of certain materials in some isomerised hop extracts;

the presence of certain metallic ions, especially nickel, iron, tin, cobalt and molybdenum, all of which complex with isocompounds;

the presence of less soluble gases in beer, for instance hydrogen.

Remedies that have been suggested to each of these are:

have excess calcium ions present at mashing and boiling so that calcium oxalate is precipitated early in the processing;

ensure that unweathered or badly stored barley is rejected;

check out isomerised hop extracts to make certain they do not initiate gushing;

keep heavy metal contents low;

do not use aluminium spots in crown closures.

There are, in beer gushing, inhibitors as well as promoters. Thus dehydrated humulinic acid promotes gushing, while α – acids and cohuluphone inhibit it.

2.7.4.2 PREDICTION OF GUSHING POTENTIAL

There are currently no reliable tests to predict Gushing potential.

Further it is not possible to measure the “gushing factor” in infected barley. However, it is possible to detect Fusarium infection in barley by determining the presence of a mycotoxin produced by Fusarium . The mycotoxin is Deoxynivalenol (DON) and the presence of the mycotoxin is indicative that the fungus responsible has been growing on the barley for some time. The concentration of DON should be < 0.5 ppb in barley, as determined by gas-liquid chromatography.