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pasteurization unit

One pasteurization unit is the microorganism death that occurs in a product held at 60°C for 1 minute. It was defined in 1951 by Del Vecchio et al,¹ and is employed in rating the effectiveness of pasteurization processes for beer and other products. Symbol, PU.

The number of PU's required for a particular beverage depends on several factors, such as the microrganisms it contains and even on the type of packaging. “Wild” yeasts, for example, such as occur in some types of Belgian ales, are more resistant to heat than domestic yeasts. A survey of large American breweries in 1955 found their processes averaged 14.8 PU's, wih a range of 2.4 to 45.6 PU's.

The success of pasteurization (that is, what percentage of the microrganisms are killed) is affected by both temperature and by the length of time for which the product is held at that temperature. It is a tradeoff: high temperatures for short times or lower temperatures for longer times. Unfortunately, higher temperatures tend to affect the taste of the beverage.

The total number of PU's for a particular pasteurization processs for beer can be estimated from²:

PU = t × 1.393^{(T − 60)}

where T is the temperature in degrees Celsius, and

t is the time in minutes at which the beer is held at that temperature.

In reality, of course, the beverage does not instantaneously go to the desired temperature, nor does it cool down instantaneously.

1. H. W. Del Vecchio, C. A. Dayharsh, and F. C. Baselt.
Thermal death time studies on beer spoilage organisms.
Proceedings of the American Society of Brewing Chemists, 1951, page 45.

2. Andrew Geoffrey Howard Lea and John R. Piggott, editors.
Fermented Beverage Production. 2nd edition.
Springer, 2003.

Page 379.

for further reading

Institute of Brewing. Method 13.37, Measurement of Pasteurization Units (PU's). Last amended Jan. 1997. Now archived.

Carsten Zufall and Karl Wackerbauer.
The biological impact of flash pasteurization over a wide temperature interval.
Journal of the Institute of Brewing, vol. 106, no. 3, pages 163-168 (2000).

“It is now clear that the pasteurization unit is certainly not a good descriptor of product damage by thermal stress. The object of this work was to elucidate whether the biological impact of different flash pasteurization procedures can be compared by PU calculations over the temperature interval from 50 to 90°C.”