Pasteurization in beer production
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Pasteurisation is a process of heating beer, and many other consumable products, to a temperature that will kill any living microbes without changing the chemistry or flavour. It is used by some brewers to sterilize and stabilize their product. In the brewery packaging environment, two methods of pasteurisation have evolved for beer – flash and tunnel pasteurisation.
One pasteurization unit (PU) is the microorganism death that occurs in a product held at 60°C (140°F) 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.
The number of PU's required for a particular beverage depends on several factors, such as the microorganisms 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, with a range of 2.4 to 45.6 PU's.
The success of pasteurization (that is, what percentage of the microorganisms 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 trade-off: 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 process for beer can be estimated from :
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.
Tunnel pasteurisation is usually employed for beer which is to be packed in cans or bottles. It is a longer process than flash pasteurisation but the temperature the beer is exposed to is lower, around 60°C (140°F). During the process the beer passes through a tunnel which is broken up into a number of chambers that are at different temperatures.
In tunnel pasteurisation bottles or cans are filled and closed in the normal way, then funnelled into the pasteuriser 'tunnel' before any labelling is added. The tunnel has a low ceiling with spray heads at regular intervals. Temperature controlled water is sprayed down on to the packages. The bottles or cans move through the pasteuriser slowly on either a walking beam or conveyor belt. The tunnel is divided into many temperature zones to slowly bring the product up to temperature, keep them at a specified holding temperature and then bring them back down to room temperature.
Modern tunnel pasteurisers contain sophisticated control systems to manage the temperatures, deal with line hold-ups and slow-downs in a way to prevent over or under pasteurisation of the product. Water is normally recirculated to improve energy efficiency.
As the beer moves through the tunnel it is gradually heated to the pasteurisation temperature of 60°C(140°F) and held at that temperature for a defined period of time before being cooled back down to 20°C (68°F). The amount of time that the beer is held at 60°C (140°F) is determined by the brewer and is measured in pasteurisation units (PU) where one PU is the lethal effect on micro-organisms obtained by holding beer for one minute at 60°C(140°F) (see above for more detailed explanation). Therefore a beer that has received 30 PUs has been held at 60°C (140°F) for 30 minutes.
Flash pasteurisation is generally used when beer is to be filled into kegs and exposes beer to higher temperatures but for a much shorter period of time. This is possible because the higher the temperature the more rapidly micro-organisms are destroyed.In this process the product is handled in a controlled, continuous flow and subjected to a temperature, normally in the range of 71.5°C (160°F) to 74°C (165°F), for a time period of 15 to 30 seconds. Because a higher temperature is employed the time for which the beer is exposed to heat is so short any impact on flavour is greatly reduced.
The flash pasteurisation temperature and hold-time are dependent upon individual beers, the live cell count and a brewers specification to his system supplier.Typical parameter for American Lagers is 160.7°F (71.5° C) for 20 seconds (≅15PU) and for European Lagers and Ales 161.6°F (72C) for 30 seconds (≅26PU).
When used in combination with a "Sankey" style keg and suitably adapted keg, bottle or can filling line to maintain an aseptic fill, a flash pasteurisation system will accomplish an economical and microbially stable fill without impacting the beer or beverage colour and flavour profiles.
Tunnel vs Flash Pasteurisation
Major factors differentiating flash pasteurisation from tunnel pasteurisation in the container are:
Flash Pasteurisation is done prior to filling and does not have any effect on organisms introduced during filling. Therefore, a well-controlled, sterile filling operation to prevent re-introduction of organisms is essential.
Flash pasteurisation of beer typically uses a two or three stage plate heat exchanger with hot water as the heat exchange media. This affords the use of controlled beer flow and thin film heat transfer which assures that the beer is evenly heated by hot water temperatures 2-4°C (3.5-7°F) higher than the desired pasteurisation temperature. This thin film, (or sheeting) of beer between the heat exchanger plates allows for rapid heating to higher temperatures and a short holding time at temperature, sometimes referred to as High Temperature Short Time (H.T.S.T.), then rapid cooling, all of which ensures that all of the beer receives the same P.U. input with a limited time at elevated temperatures.
Due to the length of time bottles/cans must be heated, tunnel pasteurisers can be extremely large. However, solutions are available with double deck designs to optimise the use of space.
- Pasteurization in breweries
Literature: Case study: Murauer Brewery (Joints)
- Typical parameters of the process
|Beer Products||Temperature [°C]||Pressure (bar)||Heat ransfer medium||Residence Time||Details||Literature|
|Beer||~70||water||1min||Case study: Murauer Brewery(JOINTS)|