Kristinae Brewery

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%

Fig. 8.3 Electron micrograph of Acetobacter species (kindly provided by Bill Simpson of Cara Technology).
Fig. 8.4 Electron micrograph of Zymomonas mobilis (kindly provided by Bill Simpson of Cara Technology).
Fig. 8.5 Electron micrograph of Obesumbacterium proteus (kindly provided by Bill Simpson of Cara Technology).

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 8.1.5.3). 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).

8.1.2.2 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.

Family

Genus

Major species

Characteristics

Beer spoilage

Lactic acid bacteria Lactobacillus

Lactic acid bacteria Pediococcus

Bacillus

Micrococcus

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

B. coagulans

M. kristinae M. varians

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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Responses

  • glen grant
    Are facultative anaerobes beer spoilers?
    10 months ago

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