Enterobacteriaceae Klebsiella ('coliforms')
K. terrigena K. aerogenes K. pneumoniae
Zymomonas Z. mobilis
Pectinatus P. cerevisiiphilus
Megasphaera M. cerevisiae
Forms phenolic off-flavours (4-vinylguaiacol) like some wild yeasts and DMS
Rods (1-1.4 x 2-6 anaerobic but tolerate oxygen, catalase positive, ferment glucose efficiently to ethanol, do not ferment maltose, very ethanol tolerant, grows best at 25-30°C
Obligately anaerobic (consequently difficult to recover using conventional microbiological methods), curved rods (0.8 x 2-30 - elongated in old cultures)
Obligately anaerobic, cocci (1.0-1.2 nm in diameter), catalase negative, difficult to detect
Specific to ale fermentations/breweries (temperature) and glucose primed b eers, off-flavours include hydrogen sulphide and acetaldehyde
Oxygen sensitivity restricts risk to low oxygen conditions in process or in package, off-flavours include hydrogen sulphide (and other sulphur compounds), acetaldehyde, propionic and other weak acids, grows best at elevated pH (4.5-6) with growth being weaker at pH 3.7-4 _
Oxygen sensitivity generally restricts risk o to low oxygen conditions in process or § in package, off-flavours (described as S 'foul', 'putrid' with a 'faecal' aroma) include hydrogen sulphide, butyric and ^ other short-chain fatty acids, cannot grow at pH <4.1 or ethanol >3.5-5.5%
Section 8.2.2) of pitching yeast and scrupulous attention to process hygiene. Mega-sphaera and Pectinatus (Fig. 8.6) present a more contemporary threat to product stability. Both are relatively 'new', having been first described as brewing contaminants in the 1970s. However, the threat is growing with contamination by these organisms having been reported in Germany, USA, Scandinavia, Japan and France. Megasphaera (Haikara & Lounatmaa, 1987) and Pectinatus (Haikara et al., 1981) are both 'strict' anaerobes and the by-products of their metabolism are notably noxious. The threat from these organisms is increasing with the worldwide drive to reduce in-process and in-package oxygen concentrations to improve the flavour stability of beer. There is also evidence that Pectinatus cerevisiiphilus can survive the early aerobic stages of fermentation and that yeast provides protection to oxygen (Chowdhury et al., 1995). This is exacerbated by the difficulties in recovering these organisms via conventional microbiological methods such as membrane filtration. Consequently, less than adequate methods, such as protracted 'forcing' tests, have been used for the routine but retrospective detection of these organisms (Haikara, 1985). There is currently no evidence that Escherichia coli and other Gram-negative pathogens can grow in beer (see Section 22.214.171.124). However, it is prudent to maintain a 'watching brief on developments in the wider food industry, particularly the emergence of acid tolerant strains (Jordan et al., 1999).
Fig. 8.6 Electron micrograph of Pectinatus cerevisiphilus (kindly provided by Bill Simpson of Cara Technology).
126.96.36.199 Gram-positive bacteria. The major Gram-positive bacteria found in breweries are summarised in Table 8.2. For an authoritative review, see Priest (1996) in Brewing Microbiology. For a wider appreciation of Gram-positive bacteria see Bergey's Manual of Determinative Bacteriology (Holt et al., 1994).
Of the microflora found in the brewery, the Gram-positive lactic acid bacteria are the most feared. In addition to being potent beer spoilers, the lactic acid bacteria have a reputation for being 'difficult' in terms of detection, recovery from spoilt product and typing. These concerns reflect the nutritional fastidiousness of these bacteria and their variable response to the anti-microbial effects of hop iso-a-acids. The complex chemistry of the myriad of these hop compounds is outside the scope of this chapter (for a review see Stevens, 1987). The major bittering (and antimicrobial) substances in beer include isohumulone, isocohumulone and isoadhumulone and their cis and trans
Table 8.2 Gram-positive bacteria.
Lactic acid bacteria Lactobacillus
Lactic acid bacteria Pediococcus
L. brevis L. damnosus L. casei L. fermentum L. buchneri L. delbrueckii L. lindneri
P. damnosus (syn. P. cerevisiae) (almost exclusively in breweries) P. inopinatus
Aerotolerant anaerobes, catalase negative (but novel protection to reactive oxygen species), 0.6-1.2 x 1-15 ^m, metabolism is either homofermentative (main product is lactic acid) or heterofermentative (products include lactic acid, acetic acid, ethanol, and carbon dioxide
Aerotolerant anaerobes catalase negative (but occasional novel protection to reactive oxygen species), cocci, 0.7 ^m in diameter, occur as tetrads, homofermentative
Aerobic, catalase positive rods that produce resistant spores, thermophillic
Catalase positive, typically strict aerobes although M. kristinae is a facultative anaerobe, cocci
Potent beer spoiler growing optimally at pH 4-5, forms 'silky' turbidity, acidity, occasionally viscous 'ropiness' and -notably - diacetyl, impact on beer in process and in package
Variable sensitivity to hop iso-a-acids -beer spoilers 10 x more resistant than sensitive strains
Resistant to hop iso-a-acids and ethanol (< 10%, v/v), impact on beer in process and in package
Spoilage through diacetyl production ('sarcina sickness') acidity and haze
Sensitive to hop iso-a-acids, not beer spoilers, found in hot brewing liquor and sweet wort, form lactic acid in wort at 55-70°C, implicated in the formation of ATNCs in sweet wort
Widely distributed in breweries, can survive in beer but rarely cause spoilage, M. kristinae reported to spoil products ('fruity' aroma) with high pH, low bitterness tú JO
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