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Pyruvate

Sulphur dioxide

Hydrogen sulphide

Methyl mercaptan

Acetyl-CoA -

Methyl thioacetate

Fig. 2.31 The origins of other sulphur-containing volatiles in beer (see also Fig. 1.17 in Chapter 1).

higher than that specified for the beer. There is another complication, insofar as some of the SMM is converted into a third substance, DMSO, during kilning. This is not heat-labile but is water-soluble. It gets into wort at quite high levels and some yeast strains are quite adept at converting it to DMS.

Hydrogen sulphide (H2S) can also be produced by more than one pathway in yeast. It may be formed by the breakdown of amino acids such as cysteine or peptides like glutathione, or by the reduction of inorganic sources such as sulphate and sulphite (Fig. 2.31). Once again, yeast strain has a major effect on the levels of H2S that are produced during fermentation. For all strains, more H2S will be present in green beer if the yeast is in poor condition, because a vigorous fermentation is needed to purge H2S. Any other factor that hinders fermentation (e.g. a lack of zinc or vitamins) will also lead to an exaggeration of H2S levels in beer. Furthermore, H2S is a product of yeast autolysis, which will be more prevalent in unhealthy yeast.

When the bitter iso-a-acids are exposed to light, they break down, react with sulphur sources in the beer and form a substance called 3-methyl-2-butene-1-thiol (MBT), which has an intense skunky character and is detectable at extremely low concentrations. There are two ways of protecting beer from this: do not expose beer to light or else bitter using chemically modified bitter extracts, the reduced iso-a-acids.

The addition of hops during beer production not only contributes much of the resulting bitterness, but also imparts a unique so-called 'hoppy' aroma. This attribute comes from the complex volatile oil fraction of hops. Most of the component substances do not survive the brewing process intact and are chemically transformed into as yet poorly defined compounds. Certainly, there does not appear to be one compound solely responsible for hop aroma in beer, although several groups (e.g. sesquiterpene epoxides, cyclic ethers and furanones) have been strongly implicated.

The point at which hops are added during beer production determines the resulting flavour that they impart. The practice of adding aroma hops close to the end of boiling (late hopping) still results in the substantial evaporation of volatile material, but of the little that remains, much is transformed into other species (e.g. the hop oil component humulene can be converted to the more flavour-active humulene epoxide). Further changes then occur during fermentation, such as the transesterification of methyl esters to their ethyl counterparts. The resultant late hop flavour is rather floral in character and is generally an attribute more associated with lager beers.

In a generally distinct practice, hops may be added to the beer right at the end of production. This process of dry hopping gives certain ales their characteristic aroma. The hop oil components contributed to beer by this process are very different to those from late hopping, with mono- and sesquiterpenes surviving generally unchanged in the beer.

Malty character in beer is due in part at least to isovaleraldehyde, which is formed by a reaction between one of the amino acids (leucine) and reductones in the malt. The toffee and caramel flavours in crystal malts and the roasted, coffee-like notes found in darker malts are due to various complex components generated from amino acids and sugars that cross-react during kilning - the Maillard reaction (see Chapter 1).

Acetaldehyde, which is the immediate precursor of ethanol in yeast, has a flavour threshold of between 5 and 50 mg L-1 and imparts a 'green apples' flavour to beer. High levels should not survive into beer in successful fermentations, because yeast will efficiently convert the acetaldehyde into ethanol. If levels are persistently high, then this is an indication of premature yeast separation, poor yeast quality or a Zymomonas infection.

The short-chain fatty acids (Table 2.9) are made by yeast as intermediates in the synthesis of the lipid membrane components. For this reason, the control of these acids is exactly analogous to that of the esters (discussed earlier): if

Table 2.9 Some short-chain fatty acids in beer.

Fatty acid

Flavour threshold (mg L 1)

Perceived character

Acetic

175

Vinegar

Propionic

150

Acidic, milky

Butyric

2.2

Cheesy

3-Methyl butyric

1.5

Sweaty

Hexanoic

8

Vegetable oil

Octanoic

15

Goaty

Phenyl acetic

2.5

Honey

yeast needs to make fewer lipids (under conditions where it needs to grow less), then short-chain fatty acids will accumulate.

Some beers (e.g. some wheat beers) feature a phenolic or clove-like character. This is due to molecules such as 4-vinylguaiacol (4-VG), which is produced by certain Saccharomyces species, including Saccharomyces diastaticus. Its unwanted presence in a beer is an indication of a wild yeast infection. 4-VG is produced by the decarboxylation of ferulic acid by an enzyme that is present in S. diastaticus and other wild yeasts, but not in brewing strains other than a few specific strains of S. cerevisiae, namely the ones prized in Bavaria for their use in wheat beer manufacture.

A further undesirable note is a metallic character which, if present in beer, is most likely to be due to the presence of high levels (>0.3 ppm) of iron. One known cause is the leaching of the metal from filter aid.

The flavour of beer changes with time. There is a decrease in bitterness (due to the progressive loss of the iso-a-acids), an increase in perceived sweetness and toffee character and a development of a cardboard note. It is the cardboard note that most brewers worry about in connection with the shelf life of their products. Cardboard is due to a range of carbonyl compounds, which may originate in various precursors, including unsaturated fatty acids, higher alcohols, amino acids and the bitter substances. Most importantly, their formation is a result of oxidation, hence the importance of minimising oxygen levels in beer and, perhaps, further upstream.

Any drinker who has ordered a beer containing nitrogen gas will appreciate that one can talk of the mouthfeel and texture of beer. N2 not only imparts a tight, creamy head to a beer, but it also gives rise to a creamy texture. More specifically, the partial replacement of carbon dioxide with nitrogen gas suppresses several beer flavour attributes, such as astringency, bitterness, hop aroma as well as the reduction in the carbon dioxide 'tingle'. Other components of beer, such as the astringent polyphenols, may also play a part. Physical properties, such as viscosity, are influenced by residual carbohydrate in the beer and might also contribute to the overall mouthfeel of a product. It is thought that turbulent flow of liquids between the tongue and the roof of the mouth results in increased perceived viscosity and therefore enhanced mouthfeel.

Foam

A point of difference between beer and other alcoholic beverages is its possession of stable foam. This is due to the presence of hydrophobic (amphipathic) polypeptides, derived from cereal, that cross-link with the bitter iso-a-acids in the bubble walls to counter the forces of surface tension that tend to lead to foam collapse.

Gushing

Foaming can be taken to excess, in which case the problem which manifests itself in small pack is 'gushing', that is, the spontaneous generation of foam on opening a package of beer. This is due to the presence of nucleation sites in beer that cause the dramatic discharging of carbon dioxide from solution. These nucleation sites may be particles of materials like oxalate or filter aid, but most commonly gushing is caused by intensely hydrophobic peptides that are produced from Fusarium that can infect barley unless precautions are taken.

Spoilage of beer

Compared with most other foods and beverages beer is relatively resistant to infection. There are several reasons for this, namely the presence of ethanol, a low pH, the relative shortage of nutrients (sugars, amino acids), the anaerobic conditions and the presence of antimicrobial agents, notably the iso-a-acids.

The most problematic Gram-positive bacteria are lactic acid bacteria belonging to the genera Lactobacillus and Pediococcus. At least ten species of lactobacillus spoil beer. They tolerate the acidic conditions. Some species (e.g. Lactobacillus brevis and Lactobacillus plantarum) grow quickly during fermentation, conditioning and storage, while others (e.g. Lactobacillus lindner) grow relatively slowly. Spoilage with lactobacilli is especially problematic during the conditioning of beer and after packaging, resulting in a silky turbidity and off-flavours. Pediococci are homofermentative. Six species have been identified, the most important being Pediococcus damnosus. Such infection generates lactic acid and diacetyl. The production of polysaccharide capsules can cause ropiness in beer.

Many Gram-positive bacteria are killed by iso-a-acids. These agents probably disrupt nutrient transport across the membrane of the bacteria, but only when they are present in their protonated forms (i.e. at low pH). This is one of the reasons why a beer at pH 4.0 will be more resistant to infection than one at pH 4.5. Some Gram positives are resistant to iso-a-acids and most Gram negatives are.

Important Gram-negative bacteria include the acetic acid bacteria (Acetobacter, Gluconobacter); Enterobacteriaceae (Escherichia, Aerobacter,

Klebsiella, Citrobacter, Obesumbacterium); Zymomonas, Pectinatus and Megasphaera. Acetic acid bacteria produce a vinegary flavour in beer and a ropy slime. It is most often found in draft beer, where there is a relatively aerobic environment close to the beer, for example, in partly emptied containers. Enterobacteriaceae are aerobic and cannot grow in the presence of ethanol. They are a threat in wort and early in fermentation and they produce cabbagy/vegetable/eggy aromas. Zymomonas is a problem with primed beers (it uses invert sugar or glucose, but cannot use maltose). Although it has a metabolism reminiscent of Saccharomyces (it's actually used to produce alcoholic beverages in some countries), it does tend to produce large amounts of acetaldehyde.

Table 2.10 Major beer styles.

Style

Origin

Notes

(a) Ales and stouts

Bitter (pale) ale England

India Pale Ale England

Alt (n.b. Alt means Germany

Mild (brown) ale England

Porter England

Stout Ireland

Sweet stout England

Imperial stout England

Barley wine England

Kölsch Germany

Weizenbier (wheat Germany beer)

Lambic Belgium

Saison Belgium

(b) Lagers

Pilsener Czech

Bock Germany

Helles Germany

Märzen (meaning 'March' for Germany when traditionally brewed)

Vienna Austro-

Hungaria

Dunkel Germany

Schwarzbier Germany

Rauchbier Germany

Malt liquor USA

Dry hop, bitter, estery, malty, low carbonation

(on draught), copper colour Similar, but substantially more bitter Estery, bitter, copper colour

Darker than pale ale, malty, slightly sweeter, lower in alcohol

Dark brown/black, less 'roast' character than stout, malty Black, roast, coffee-like, bitter Caramel-like, brown, full-bodied Brown/black, malty, alcoholic Tawny/brown, malty, alcoholic, warming Straw/golden colour, caramel-like, medium bitterness, low hop aroma Hefeweissens retain yeast (i.e. turbid). Kristalweissens are filtered. Very fruity, clove-like, high carbonation Estery, sour, 'wet horse-blanket', turbid. Lambic may be mixed with cherry (kriek), peach (peche), raspberry (framboise), etc. Old lambic blended with freshly fermenting lambic = gueuze Golden, fruity, phenolic, mildly hoppy

Golden/amber, malty, late hop aroma Golden/brown, malty, moderately bitter Straw/golden, low bitterness, malty, sulphury Diverse colours, sweet malt flavour, crisp bitterness

Red-brown, malty, toasty, crisply bitter

Brown, malty, roast-chocolate Brown/black, roast malt, bitter Smokey

Pale colour, alcoholic, slightly sweet, low bitterness

A wild yeast is any yeast other than the culture yeast used for a given beer. As well as Saccharomyces, wild yeast may be Brettanomyces, Candida, Debaromyces, Hansenula, Kloeckera, Pichia, Rhodotorula, Torulaspora or Zygosaccharomyces. If the contaminating yeast is another brewing yeast, then the risk is a shift in performance to that associated with the 'foreign' yeast (i.e. you will not get the expected beer). If the contaminant is another type of yeast, the risk is off-flavour production (e.g. clove-like flavours produced by decarboxylation of ferulic acid) or a problem like over-attenuation as might be caused by a diastatic organism such as S. diastaticus.

Beer styles

An indication of the complexity of beer styles available worldwide will be gleaned from Table 2.10. In relation to the immediately foregoing discussion, we might note the lambic and gueuze products of Belgium, whose production depends not only on Saccharomyces species, but also inter alia Pediococcus, Lactobacillus, Brettanomyces, Candida, Hansenula and Pichia.

Bibliography

Bamforth, C.W. (2003) Beer: Tap into the Art and Science of Brewing, 2nd edn.

New York: Oxford University Press. Baxter, E.D. & Hughes, P.S. (2001) Beer: Quality, Safety and Nutritional Aspects.

London: Royal Society of Chemistry. Boulton, C. & Quain, D. (2001) Brewing Yeast and Fermentation. Oxford: Blackwell Publishing.

Briggs, D.E. (1998) Malts and Malting. London: Blackie.

Briggs, D.E., Boulton, C.A., Brookes, P.A. & Stevens, R. (2004) Brewing: Science and

Practice. Cambridge: Woodhead. MacDonald, J., Reeve, P.T.V., Ruddlesden, J.D. & White, F.H. (1984) Current approaches to brewery fermentations. In Progress in Industrial Microbiology, vol. 19 (ed. M.E. Bushell), pp. 47-198. Amsterdam: Elsevier. MacGregor, A.W. & Bhatti, R.S., eds (1993) Barley: Chemistry and Technology.

St Paul, MN: American Association of Cereal Chemists. Neve, R.A. (1991) Hops. London: Chapman & Hall.

Food, Fermentation and Micro-organisms Charles W. Bamforth Copyright © 2005 by Blackwell Publishing Ltd

Chapter 3

The Miracle Of Vinegar

The Miracle Of Vinegar

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