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8.3.2.1 Hardware. Other than the replacement of glass by disposable plastic, relatively little has changed in the last 30 years in terms of the hardware required to perform microbiological testing in the brewery. Simpson (1996) has provided a useful summary of the fundamental items required for the microbiological testing of brewery samples.

Major developments however have occurred in the automation of microbiological 'routines'. Although at a cost, automatic plate readers and plate pourers have facilitated the removal of some of the drudgery of routine microbiology. Equally, the increasing sophistication of anaerobic incubators has provided an attractive alternative to the frustration and low capacity of gas jars. More mundane but equally liberating is the development of disposable sterile membrane filtration funnels that remove, at a stroke, one of the more frustrating elements of microbiological testing. This later development has also facilitated the important move away from pour and spread plating, enabling the wider use of membrane filtration in routine testing.

8.3.2.2 Media. One of the more emotive areas of brewing microbiology is the choice of growth media used to isolate, recover and quantify bacteria and yeast. Despite the efforts of many, there is no one microbiological medium that satisfies the needs of the brewery microbiologist (Smith et al., 1987). Consequently, over the years a raft of different media have been developed that reportedly are the 'best thing' for the selective recovery of wild yeasts, Gram-negative bacteria and so on. Invariably, the subsequent debate then hinges around the 'capability' of the various media to select for the group of interest. This leads to 'make it better' improvements and, consequently, tweaked variants of the original media appear that claim to offer superior performance. As succinctly noted by Casey and Ingledew (1981b):

'since 1974, there has been a new medium created almost every year for the lactic acid bacteria. In parallel with this has been a series of papers comparing the new media with the older media and, as different authors have reached different conclusions, it has created a great deal of confusion.'

It is beyond the scope of this chapter to do more than skim the surface of the 'media story'. To obtain a fuller insight into the evolution and composition of microbiological media the interested reader is referred to a series of reviews from Casey and Ingledew from the University of Saskatchewan in Canada. The articles cover general purpose media (Casey & Ingledew, 1981a), and media for lactic acid bacteria (Casey & Ingledew, 1981b), wild yeast and moulds (Ingledew & Casey, 1982b). The series was updated by Smith et al. (1987). More recently, Jesperson and co-workers have updated the story for brewery spoilage organisms in general (Jespersen & Jakobsen, 1998) and wild media in particular (van der Aa Kuhle & Jespersen, 1998). Without wishing to rehearse the various debates, some media formulations have found favour and are used by many brewing laboratories in their original form or, inevitably, with the occasional customisation! Where the composition of the media is reported (Tables 8.16-8.20) the details are invariably derived from the appropriate paper by Casey and Ingledew (or vice versa). With the exception of the lactic acid 'NBB' medium, the basal media are available commercially in formulations which, on occasion, differ marginally from those described here. The Oxoid Manual (Bridson, 1998) is a fund of

Table 8.16 Composition of WLN medium.

Component

Glucose 50 g

Monopotassium phosphate 0.55 g

Potassium chloride 0.425 g

Calcium chloride 0.125 g

Magnesium sulphate 0.125 g

Ferric chloride 2.5 mg

Manganese sulphate 2.5 mg

Casein hydolysate 5 g

Yeast extract 4 g

Agar 20 g

Bromocresol green 2.2 mg pH 5.5

Distilled water to 1000 ml

Table 8.17 Composition of MYGP + copper.

Component

Malt extract 3 g

Yeast extract 3 g

Glucose 10 g

Bacto peptone 5 g

Agar 20 g

Copper sulphate (5HzO) 312 mg

Distilled water to 1000 ml pH 6.2

Table 8.18 Composition of MRS medium.

Component

Peptone 10 g

'Lab-lemco' powder 10 g

Yeast extract 5 g

Glucose 20 g

Tween 80 1 ml

Dipotassium hydrogen phosphate 2 g

Sodium acetate 3HzO 5 g

Magnesium sulphate 7HzO 200 mg

Manganese sulphate 4H20 50 mg

Agar 20 g

Final pH 6-6.5

Distilled water to 1000 ml information on these and other microbiological media, and provides useful guidance on good practice for media preparation and storage.

• General purpose media - although a dangerous conclusion to draw, the Wallerstein Laboratories Nutrient (WLN) agar is used almost universally across the brewing industry. The history of the development of this medium is

Table 8.19 Composition of Raka Ray No. 3 medium.

Component

Yeast extract 5 g

Trypticase 20 g

Liver concentrate 202-3 1 g

Maltose 10 g

Fructose 10 g

Betaine hydrochloride 2 g

Diammonium citrate 2 g

Potassium aspartate 2.5 g

Potassium glutamate 2.5 g

Magnesium sulphate (7HzO) 2 g

Manganese sulphate (H20) 0.5 g

Dipotassium hydrogen phosphate 2 g

N-acetyl-glucosamine 0.5 g

Tween 80 10 ml

Agar 20 g

Final pH 5.4

Distilled water to 1000 ml

Table 8.20 Composition of modified NBB medium.

Component

Casein peptone 5 g

Yeast extract 5 g

Meat extract 2 g

Tween 80 0.5 ml

Potassium acetate 6 g

Sodium phosphate, dibasic 2 g

L-cysteine monohydrochloride 0.2 g

Chlorophenol red 70 mg

Glucose 15 g

Maltose 15 g

L-malic acid 0.5 g

Agar 15 g

Final pH 5.8

Beer/distilled water (1:1) to 1000 ml detailed in Casey and Ingledew (1981a). As with all successful media, WLN is purchased commercially from one of the many suppliers of microbiological media. Its composition (see Table 8.16) is notable for containing only glucose (5% w/v) as the sole carbon and for the inclusion of the pH indicator, bromocresol green. In passing, modifying WLN by doubling the concentration of bromocresol green has found application in the simple differentiation (colony colour and size) of closely related brewing yeasts (see Section 4.2.5.1). However, WLN is not a 'selective' medium as it supports the growth of both brewery bacteria and yeast. The media is made selective for bacteria by the inclusion of 15mgl 1 cycloheximide (see Section 4.2.5.1) to inhibit the growth of brewing and some (but not all) 'wild' yeasts and is then known as WLD (Wallerstein

Laboratories Differential) agar. The inclusion of isomerised hop extract (e.g. isohopC02n at 400 mgl *) in WLN and WLD is useful in suppressing the growth of spore-forming bacillus.

Despite the claims of the original authors (Casey & Ingledew, 1981a), WLD is comparatively poor in terms of supporting the growth of the more fastidious lactic acid bacteria. Despite this, WLD has found application as a 'routine' medium for the detection of aerobic bacteria (e.g. acetic acid bacteria) and some cycloheximide resistant wild yeast in brewing (Quain, 1995; Hammond, 1996; Simpson, 1996; Anonymous, 1999).

• Wild yeasts - at the time of writing it seems unlikely that a single method for the unified detection of wild yeasts will be forthcoming in the near future. This is not surprising given the sheer diversity of the Saccharomyces and non-Saccharomyces wild yeasts (see Section 8.1.3). For a wider review of the pros and cons of many wild yeast media, the interested reader should consult Ingledew and Casey (1982a) together with the update of Smith et al. (1987). Ingledew and Casey (1982b) should be consulted for a consideration of the more specialist media for moulds.

Arguably, the nearest thing to a single medium for wild yeasts (both Saccharomyces and non-Saccharomyces) is the MYGP + copper medium of Taylor and Marsh (1984). Although described in detail in Table 8.17, this medium is normally prepared from commercial MYGP media (YM or 'yeast mould agar') with the addition of sterile filtered copper sulphate (200mgl *) to tempered medium. To assure consistent results, this medium should be prepared freshly to ensure the 'availability' of the copper. Although Smith et al. (1987) were ambivalent about the application of this method, van der Aa Kuhle and Jespersen (1998) reported that this medium detected wild yeast (Saccharomyces, Candida and Pichia) in brewery samples with a success rate of 80%. In this study, other older approaches such as lysine, crystal violet etc. (see Ingledew & Casey, 1982a) were shown to be less successful in detecting wild yeasts in 4656% of the contaminated samples. Others who have reported the use of copper supplemented media to detect wild yeast include Quain (1995), Hammond (1996), Simpson (1996) and Anonymous (1999).

Inevitably, this medium is open to 'tweaking' the concentration of copper in an attempt to improve its capability. Rather than replacing the existing media, there is an opportunity for an additional medium with a lower concentration of copper to catch the more copper-sensitive strains. For example MYGP + Cu (100mgl *) may have some utility although it is necessary to test the resistance of primary brewing yeasts as a handful have been found to be capable of growth. Presumably, these strains have acquired enhanced resistance to the toxic effects of copper, through inadvertent exposure to the metal during processing in copper vessels. Resistance is down to sequestration via a meta-lothionein coded by the gene CUP1 which, in resistant strains, is tandem repeated up to 15 times (Macreadie et al., 1994).

• Lactic acid bacteria - in their review, Casey and Ingledew (1981b) detailed the composition of no less than 18 different microbiological media for the detection of lactic acid bacteria. This is a reflection of the importance of lactic acid bacteria in brewing microbiology and difficulty in cultivation as 'different strains require different growth factors, use different sugars and have different requirements for oxygen' (Smith et al., 1987). Overlain on these considerations is the complex relationship that lactic acid bacteria have with hop acids (see Section 8.1.2.2).

The market leaders for the detection of lactic acid bacteria are MRS (deMan et al., 1960 - Table 8.18), Raka Ray (Saha et al., 1974 - Table 8.19) and the modified NBB medium (Kindraka, 1987; Takemura et al., 1992 - Table 8.20). All three media differ significantly in composition and all have their adherents. NBB ('Nachweismedium fuer Bierschadliche Bacteriem') was first developed as a general purpose medium by Back (1980) before being modified and rebadged by Kindraka (1987) for anaerobic lactic acid bacteria. NBB includes beer and a pH indicator (chlorophenol red). Both MRS and Raka Ray have long been available as dehydrated media whereas NBB has yet to be available commercially, a factor which has doubtless hampered its usage.

The above selection is by no means definitive and many microbiologists may disagree with this choice and, indeed, strongly advocate alternatives such as UBA, VLB-S7 and KOT. However as 'no single medium appears to be capable of detecting all strains of lactic acid bacteria' (Jesperson & Jakobsen, 1996), the debate will doubtless rumble on. Further developments can be anticipated as the 'optimal medium for detection of lactic acid bacteria has yet to be identified' (Jesperson & Jakobsen, 1996).

Whatever the choice of media, lactic acid bacteria are routinely detected after 5-7 days of anaerobic incubation. This is for two reasons: first to eliminate the growth of aerobes and second because the growth rate of Lactobacillus and Pediococcus species is enhanced anaerobically. Although not requiring oxygen, lactic acid bacteria tolerate oxygen. Recent work (Marty-Teysset et al., 2000) has shown aerobically grown L. delbrueckii subsp. bulgaricus to reduce oxygen and to accumulate hydrogen peroxide, an oxidative stress which triggers early entry into stationary phase. This results in a concomitant reduction in biomass yield and, from the perspective of growth on agar plates, smaller colonies. In addition, the media are made more selective by the inclusion of inhibitors. These include cycloheximide (to inhibit yeast), 2-phenylethanol (to inhibit Gram-negative bacteria - Casey & Ingledew, 1981b) and vancomycin (to inhibit non-beer spoilage Gram-positive bacteria - Simpson & Hammond, 1987; Simpson et al., 1988b). As noted in Section 8.1.2.2, Simpson and Hammond (1991) have advocated the inclusion of the 20 (iM trans-isohumulone in MRS media to better select for potential beer spoilage Lactobacillus.

Compared to the developments in the detection of wild yeast and lactic acid bacteria, the Gram-negative bacteria (see Section 8.1.2.1) have, rightly or wrongly, received relatively little attention! However, Casey and Ingledew (1981c) were still able to contribute a review on the subject which was updated by Smith et al. (1987) and Jespersen and Jakobsen (1996).

MacConkey's agar (Table 8.21) remains the favourite medium in brewing microbiology to detect specialist Gram-negative bacteria, particularly coliforms. WLD continues to find favour for the enumeration of acetic bacteria and O. proteus

Table 8.21 Composition of MacConkey's agar.

Component

Bactopeptone 17 g

Proteose peptone 3 g

Bacto-lactose 10 g

Bacto-bile salts No. 3 1.5 g

Sodium chloride 5 g

Bacto-agar 13.5 g

Bacto-neutral red 30 mg

Bacto-crystal violet 1 mg

Distilled water to 1000 ml pH 7.1

(Fernandez et al., 1993). The more fashionable, strictly anaerobic bacteria Mega-sphaera and Pectinatus are detected in a variety of pre-reduced media such as NBB or MRS (Smith et al., 1987; Jesperson & Jakobsen, 1996). Detection of these microorganisms is complicated by the need to achieve anaerobiosis during sample processing and incubation.

8.3.2.3 Validation. The use of controls to validate analytical measurements has long been accepted laboratory practice. However, it is only relatively recently that controls and concepts such as proficiency testing have found application in brewing microbiology. Although by no means 'rocket science', both disciplines add value to routine microbiological testing by adding certainty to the results be they good or bad!

Although far from new, the use of media controls was reported by Quain (1995) in support of the microbiological analyses supporting 'yeast supply'. Here (Table 7.2), the different media are challenged with micro-organisms that should and should not grow on the various microbiological media. Such controls are powerful in confirming that the media 'work'. This approach is especially useful to validate the performance of batches of media that need to be uniquely identified to allow traceability.

Proficiency testing has long been used to assess capability and competency in brewing analysis. Its use in brewing microbiology is relatively recent, owing much to the development of proficiency testing in the water industry. BAPS Microbiology, a popular scheme in the UK, is operated by the Laboratory of the Government Chemist together with BRi. In essence, blind samples are supplied to participating laboratories for qualitative and quantitative analysis using routine microbiological methods. This approach enables the measurement of the capability of the laboratory to recover, quantify and identify what should be familiar brewery micro-organisms. In addition, proficiency testing is useful in validating the competence of test media and the microbiologist. Used appropriately, proficiency testing is a valuable tool for improvement and for assuring performance via an independent route.

In passing, it is worth noting some of the common pitfalls in microbiological testing that undermine performance. Perhaps one of the most common is the failure post-preparation to adjust the pH of the medium. Kindraka (1987) is fulsome in the importance of this adjustment when considering the efficacy of NBB medium. Temperature abuse features frequently in the inconsistent performance of micro biological media. This theme includes the addition of heat sensitive inhibitors (cycloheximide, vancomycin, phenylethanol) prior to autoclaving or directly to very hot media ex-autoclave and 'storage' of media prior to pouring plates at too high a temperature and for too long. The guidelines - which are all too easily ignored -require microbiological media to be stored or 'tempered' at 46+ 1°C for no longer than three hours. It is good laboratory practice to add heat sensitive materials to tempered media. Similarly, tempered media should be used where larger sample volumes (e.g. 5 ml) are being processed as 'pour plates'. However, in terms of avoiding heat stress and achieving consistency, it is preferable to avoid pour plates and process such volumes via filtration and incubation of the membrane on the top of the plate.

Arguably the nirvana for brewing microbiology is that QA systems and in-line monitoring is so sophisticated and interactive that there is no requirement for conventional sampling and testing! Today's reality is such that the focus has been on accelerating testing methodologies, with the ultimate objective of achieving results in 'real time' on a par with the likes of beer colour, C02, and ABV. Similarly, much effort has gone into exploiting new technologies for the quicker and less equivocal identification of brewing micro-organisms.

In the last 20 years or so, brewing microbiology has sought to exploit a variety of new technologies that offer improvements over traditional approaches. With the honourable exception of ATP bioluminescence testing in CiP, the 'real-time' objective has been revised down to being quicker or more 'rapid' over and above traditional methods. Although not as dramatic, these developments should not be decried as they frequently reduce testing times by many days. In a world where 'time is money', these developments have value, and frequently potential. For a fuller review than can be given here the interested reader is directed toward Dowhanick (1995), Russell and Dowhanick (1996) in Brewing Microbiology and Storgards et al. (1997).

8.3.3.1 ATP bioluminescence. There is no doubt that ATP (adenosine triphosphate) bioluminescence is the great success story of brewing microbiology in the late twentieth century. Indeed, the implementation of this technology has truly achieved a step change in our approach to hygiene testing in particular and microbiology in general. It is now accepted practice to validate CiP performance in real time and, where required, to reclean. With the technology evolving by leaps and bounds, testing is now easily performed in process areas with all-in-one dipstick tests and small portable testing units. Accordingly, hygiene testing is no longer the preserve of the QA laboratory but is performed and responded to by the appropriate people, the process owners. In addition to these tangible changes, ATP bioluminescence has played its part in raising the profile of hygiene and microbiology in the brewing industry. For many, this technology has made microbiology 'real'!

Bioluminescence is the phenomenon in which visible light is emitted by an organism. The history and evolution of ATP bioluminescence (Simpson, 199 lb) dates back to 1884 when DuBois demonstrated that a crude extract of fireflies (Photinus pyralis) emitted light ('bioluminescence') that eventually faded and disappeared. He also coined the terms 'luciferin' and 'luciferase' for, respectively, the reaction's substrate (a 6-hydroxybenzothiazole) and enzyme. Intriguingly, in the wild, bioluminescence acts as a signal to arrange sexual encounters between consenting fireflies! In 1947, McElroy identified the missing piece of the jigsaw with the demonstration that ATP triggered light formation from the firefly extract. The reaction involves the oxidation of luciferin by luciferase with the oxidant being molecular oxygen:

ATP + Luciferin + Oj^^^AMP + Oxyluciferin + C02 + PPi + Light (Amax = 532 nm)

These early observations have spawned a technology built around the release of 'living light' in proportion to very small amounts of ATP - the 'universal' energy molecule found in all living cells. The quantum yield from the luciferin-luciferase reaction with ATP approaches unity, with a photon of light being emitted for every molecule of ATP utilised. In passing, bioluminescence in the jellyfish (Aequorea aequorea) is triggered by calcium ions. This has been exploited in the laboratory for tracking the movements of Ca2+ in biological systems.

Inevitably, today's familiar technology has taken time to deliver with numerous innovations along the way. Initially, in the 1960s, ATP bioluminescence was considered for glamorous applications such as the detection of extraterrestrial life forms in NASA's 'Life on Mars' programme (Simpson, 1991). Since then, although the ambition remained the same, the technology has been brought down to earth for the detection of micro-organisms. However, up and until the late 1980s progress in developing ATP technology was 'exasperatingly slow' (Stanley et al., 1997). Since then, fuelled by innovation, change has been dramatic with the development of simple, user-friendly analyses and portable, robust 'bioluminometers'. The scope of bioluminescence is impressive and can be gauged by presentations at an international conference on bioluminescence (ATP 96) (Stanley et al., 1997). These included personal care products, milk hygiene, animal and poultry carcasses, effluent, pharmaceutical products and brewing.

As described in Section 8.2.1.1, bioluminescence is used in brewing to validate CiP operations in real time, which today is within seconds of sampling! At its simplest, a dirty tank has a high bioluminescence. The source of the ATP can be microbial cells or 'soil'. For this reason it is foolhardy to seek to correlate ATP (RLU) against conventional plate counts. Indeed, failure to achieve a correlation should be expected but it should be appreciated that this does not imply a failure in the capability of either approach! In a landmark paper (see below), Hysert et al. (1976) estimated yeast cells to have about 100 times more ATP than bacterial cells. They found 0.24 fmol ATP (or 0.24 x 10 15 moles) in an 'average' yeast cell compared to 0.0025 fmol ATP in an 'average' bacterial cell). In terms of 'soil', Simpson et al. (1989) reported the concentration of ATP in beer to range widely from a low of 0.01 nM to a high of 100 nM, with a mean of 5 nM. In this context, the report of Boyum and Guidotti (1997) is of particular interest as they showed the glucose dependent efflux of ATP from S. cerevisiae. There is no apparent reason why these laboratory observations cannot extrapolate to brewery fermentations. Further work in this intriguing area would be most welcome!

Although differentiation is possible ('total' v. 'free' ATP), the source of the ATP is generally immaterial. From the perspective of poor CiP, it is usually sufficient to recognise that the vessel is not clean and to be unconcerned whether the ATP originates from predominately micro-organisms or beer residues. What is more important is real-time action in response to a real-time result. This enables appropriate corrective action to be taken at the time - an opportunity that was hitherto unavailable through traditional microbiological sampling and testing.

The first reported application of ATP bioluminescence in brewing (Hysert et al., 1976) anticipates much of what has subsequently happened and makes fascinating reading. The technology however is very different with DMSO extraction of ATP and light measurement using a liquid scintillation counter! Tellingly in the final paragraph of this paper, Hysert et al. (1976) note the applications as 'monitoring biocide effectiveness in recirculating cleaning systems, evaluating the general microbiological state of brewing facilities and basic biochemical brewing research'. This predication has proved to be remarkably accurate with ATP bioluminescence now being used routinely for CiP validation, but also finding increasing application in reducing incubation times for product testing.

Hygiene testing. Since the early work and proposals of Kilgour and Day (1983) and Simpson et al. (1989), the application of ATP bioluminescence in hygiene testing has been slowly threatening to come of age. The seminal paper, 'Practical experiences of hygiene control using ATP-bioluminescence' (Ogden, 1993) took the technology out of the laboratory into routine application in the brewery. This two-year study applied bioluminescence in the real time testing of hygiene swabs. 'Go/no go' specifications were established which had to be passed before the plant could be used. As is shown in the paper, this initiative clearly demonstrated a 'significant improvement in the hygiene and cleanliness of the plant, which could only have a beneficial effect on the quality of the product' (Ogden, 1993). Despite this positive endorsement, take-up of the technology immediately thereafter remained patchy.

With hindsight, the most likely explanation for the sluggish response was that the technology was still focused on 'swabs' and was still in the domain of the 'specialist', who was comfortable with user-hostile laboratory-based testing. Indeed, it is instructive to revisit the assay as used at the time by Ogden (1993). Here, swabs were wetted with a nutrient medium and, after use, agitated for 10 seconds in a cuvette containing 200 (il of a 'nucleotide releasing reagent'. After adding the enzyme (100 (il) and mixing, the cuvette was 'loaded' into the bioluminometer and read. Up until the late 1990s developments were primarily embellishments of this basic approach with the inclusion of a rinse method, replacement of pipetting with dropper bottles and general improvements in detection sensitivity.

The step change that did more for the transfer of technology was the introduction of all-in-one 'single shot' testing. This together with the genuinely small, robust bioluminometers with increasingly sophisticated data capture and manipulation, has provided the framework to implement real-time testing in breweries by, as noted above, the right people, the process owners. Popular single-shot systems include the Celsis SwabMate and, from Biotrace, the Aqua-Trace™ and Clean-Trace™. The design of these single shot devices is quite different and accordingly there is no universal bioluminometer. Accordingly, Celsis use the SystemSURE™ and Biotrace the

Uni-Lite® and Uni-Lite® Xcel. The principle behind the single-shot systems can be appreciated from the simplicity of the Biotrace approach. Here dipsticks are available in two formats, with a swab (Clean-Trace™) or with a series of small plastic rings that capture by capillary action a small volume of a final rinse (Aqua-Trace™). The bioluminescence reagents are stored in a separate compartment at the bottom of the dipstick holder. Although robust enough to prevent leakage, the partition is breached by plunging the dipstick through into contact with the reagents. This activates the reaction and, after briefly mixing, the ensuing bioluminescence is quantified in a bioluminometer. Promotional images that capture 'single shot' testing are presented in Fig. 8.30.

Bizarrely, despite the obviousness of'ATP', there is no universal standard unit for expressing ATP concentration. The output from bioluminometers is invariably RLU or relative light units. Unfortunately, for a known amount of ATP the RLU from one manufacturer's bioluminometer will be different to RLU from another manufacturer's machine. For the consumer's point of view this is unsatisfactory. Consequently, it would be a welcome development and would aid comparison, if the bioluminescence industry could agree to standardise their results in terms of ATP concentration.

Within Bass Brewers Limited, bioluminescence has been implemented with great gusto! Using Biotrace technology, CiP validation is captured within the sampling plan (Section 8.3.1.2) with global specifications for a pass (< 150 RLU), caution (150-299 RLU) and fail (>300 RLU). As described in Section 8.2.1.1, should a rinse fail, the vessel or main is retested, CiP set visually checked and, if necessary recleaned (where commercial pressures allow). Ironically, because of its simplicity and timeliness, there is a 'that's all I need to do' attitude to the use of bioluminescence in the validation of cleaning. This is not the fault of the method but of the user! There remains the need (as described in Section 8.3.1.2) to validate the inputs (causticity, carbonate etc.), the outputs (visual assessment) of a CiP cycle and periodic microbiological checks to characterise the microflora found in the system.

Perhaps the most popular criticism of bioluminescence is that of sensitivity. The typical argument is that traditional microbiology has the capability to detect a single micro-organism in up to 11 of sample. Conversely, bioluminescence systems are far less sensitive. Comparison of the different manufacturers' packages has shown varying detection sensitivities for ATP (Table 8.22). Although obviously an important criterion, detection sensitivity has to be considered alongside factors such as cost, sample type, application, ease of use and, where required, data handling. So is

Table 8.22 Comparison of commercial bioluminometers (unpublished results of Andrew Price, Brewing Research International).

Bioluminescence 'package'

Minimum detection of ATP (fmol)

Capable of detecting x yeast cells per assay

Hughes Whitlock Bioprobe

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