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Fig. 8.21 Comparative microbiological risks of tunnel pasteurisation (scheme 1), flash/plate pasteurisation immediately before prior to packaging (scheme 2), flash/plate pasteurisation into sterile tank prior to packaging (scheme 3) and sterile filtration (scheme 4) (redrawn from Beer Pasteurisation, Manual of Good Practice, EBC Technology and Engineering Forum. 1995).

Fig. 8.21 Comparative microbiological risks of tunnel pasteurisation (scheme 1), flash/plate pasteurisation immediately before prior to packaging (scheme 2), flash/plate pasteurisation into sterile tank prior to packaging (scheme 3) and sterile filtration (scheme 4) (redrawn from Beer Pasteurisation, Manual of Good Practice, EBC Technology and Engineering Forum. 1995).

the most effective route to achieving 'commercial sterility' as reinfection under normal process conditions is difficult to conceive. The risk is markedly greater with flash pasteurisation as heat-treated beer is transferred to a 'sterile' buffer tank and then to a packaging line and, typically, keg. In passing, it is noteworthy that Dymond (1992) has described the economic benefits of coupling flash pasteurisation with aseptic packaging into bottle. Whatever the final package, care must be taken to guard against the threat of plate cracking or seal degradation. These events can lead to contamination of pasteurised beer with unpasteurised beer or, more alarmingly from a public relations perspective, secondary refrigerant from the cooling section of the pasteuriser. However, both tunnel and flash pasteurisation are significantly more microbiologically robust than the alternative non-thermal process, sterile filtration. To be successful, this process (see Section 8.2.1.3) requires operating to extremely high hygienic standards to prevent the contamination by micro-organisms post-sterile filtration.

Measurement ofpasteurisation. Despite its universal use in the brewing industry, the pasteurisation unit (PU) - one minute of heating at 60°C - was proposed but never published by H.A. Benjamin of the American Can Company. Indeed, the paper in which this relationship was first reported (Del Vecchio et al., 1951) remains, despite subsequent critical comment, one of the seminal papers of brewing microbiology. Accordingly, it is appropriate to consider the paper of Del Vecchio et al. (1951) and its findings in some detail. Essentially, Del Vecchio et al. (1951) reported a series of 'thermal death curves' for an 'abnormal' yeast, 'torula' (presumably Candida species) and a selection of brewing bacteria. These were inoculated into an 'end fermented' beer containing wort (5% v/v) and survival time noted at various temperatures between 48.8 and 65.5°C. A semi-logarithmic plot of maximum survival time against temperature (see Fig. 8.22) provided a 'thermal death curve' for each organism. From this, a Z' value could be calculated (in Fig. 8.22 Z' = 7.2°C) which provided a comparative measure of thermal sensitivity for a micro-organism. This was defined by Del Vecchio et al. (1951) as the 'number of degrees Fahrenheit (or centigrade) traversed by the curve in passing through one log cycle'. Alternatively, the Z-value can be defined as the increase in temperature required to achieve a 'tenfold increase in the rate of thermal inactivation' (European Brewery Convention, 1995). In other words, in the case of Fig. 8.22, for every increment of 7.2°C there is a corresponding ten-fold decrease in time required to kill the organism. Consequently, the steeper the slope (i.e. smaller Z-value), the greater the kill rate for a given temperature step. Del Vecchio et al. (1951) also note that such an analysis enables the thermal resistance at 60°C to be determined. In the case of Fig. 8.15, this is 5.6 minutes or 5.6 PU. This figure has become recognised as the minimum number of PU required to achieve 'commercial sterility'.

Fig. 8.22 Example of a thermal death curve for a micro-organism at various temperatures (°C).

The continuing importance of the observations of Del Vecchio et al. (1951) is that the familiar pasteurisation tables are derived from the thermal death curve reported for the 'abnormal yeast'. These tables enable the calculation of PUs (or lethal rates) for other temperatures relative to 60°C. Presented graphically (Fig. 8.23), it can be seen that this a logarithmic relationship and, consequently, relatively small increases in temperature result in large increases in PUs. As noted in Beer Pasteurisation Manual of Good Practice (European Brewery Convention, 1995), an increase of 2 and 7°C increase the PUs by factors of approximately 2 and 10. These relationships (Fricker, 1984) are explored further in Fig. 8.24. PUs (or lethal rate) can be calculated

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