Sterilization of Beer
Absolute sterility of the bottled beer is essential given the fact that bottles are shipped over long distances, subject to varying temperature regimes, and often remain on the shelf for fairly long periods. Traditionally sterility of beer was accomplished by pasteurizing the beer in the bottle after filling and crowning by a process known as tunnel pasteurization. Alternatively, the beer can be sterilized prior to packaging either by flash pasteurization or sterile filtration. Craft brewers almost universally do not use tunnel pasteurization or flash pasteurization for microbiological assurance but rather sterile filtration.
Flash pasteurization is a method of heat pasteurizing beer prior to filling into containers for the purposes of killing spoilage microorganisms. This approach can be applied to virtually any packaging vessel: bottles, cans, or kegs. for conversion of adjunct starches (those that lack enzymes) during mashing.
Theory and Principles
Flash pasteurizers utilize a HTST (high temp short time) concept, while tunnel pasteurizers run more of a LTLT (low temp long time) concept. The amount of heat imparted to the product during the elevated temperature and time period is expressed in Pasteurization Units (PU’s). A Pasteurization Unit (PU) is defined as a one-minute exposure to a temperature of 60 degrees C (140°F). A PU is a measure of the lethal effect on microorganisms of the heat treatment.
A flash pasteurization system generally includes (Figure 17.1): a high-head centrifugal beer pump, a triple-stage plate heat exchanger with regeneration, and heating and cooling sections.
The number of rows of kernels makes for easy identification of two- and six-row varieties. In six-row varieties, two-thirds of the kernels are twisted in appearance because of insufficient space for symmetrical development.
Flash Pasteurization Water Treatment
When water is to be heated during use, as in cooling water or in the pasteurizer, it needs to have been softened, demineralized or otherwise treated to prevent the deposition of sludges and scale, which can cover surfaces and interfere with heat exchange and may even block pipework.
Advantages and Disadvantages
The advantages of flash pasteurization compared to conventional tunnel pasteurizer are: (1) kill-off is assured independent of the type of microbe, (2) less space is required, (3) lower capital and operating costs, (4) the PU input is finite and under closer control than it is in tunnel pasteurization, and (5) less of an impact on flavor than tunnel pasteurization.
An alternative to flash pasteurization and sterile filtration is tunnel pasteurization. Tunnel pasteurization is performed on bottled or canned beer only. Tunnel pasteurization isn’t typically used with kegs because of their compact size. The bottles are loaded at one end of the tunnel pasteurizer and passed under sprays of water as they move along the conveyor (Figure 17.2).
As mentioned, the bottles and cans pass through several zones and are sprayed with water heated to increasing and then decreasing temperatures. In the case of bottles, zone temperatures are carefully graduated to avoid thermal shock and subsequent bottle breakage. During heating, gradients in excess of 22 degrees C (40°F) are common on both bottles and cans. However, because bottles are more sensitive during cooling, brewers should adhere to an upper limit of 22 degrees C (40°F) of the gradient between cooling zones (Alarcon et al., 2014).
Tunnel Pasteurization Water Treatment
Tunnel pasteurizers can experience microbiological growth, bio-fouling, corrosion and inorganic scale deposition. If not addressed, these issues can have a negative impact on safety, heat transfer efficiency, utility usage, process reliability, maintenance, and system integrity. Package and beverage quality, productivity, operational efficiency and total cost of ownership can also be affected. Proliferation of Pseudomonas and other microorganisms can lead to the development of biofouling.
Advantages and Disadvantages
Tunnel pasteurization has proven to be a reliable and effective method under an almost infinite variety of brewery conditions. Because it takes place after the filling and closing operation, it is effective against microorganisms introduced during these operations. Also, it is a variable process and the degree of treatment can be optimized depending on a particular brewery’s needs.
Sterile filtration has the advantage over pasteurization of giving very gentle treatment to the beer; with no heating and cooling there is no potential for flavor changes. The additional filtration can also improve non-biological stability and clarity of the beer. Sterile filtration physically removes beer spoilage organisms from the beer as it passes through a cartridge membrane filter. It is essential the beer first undergo primary diatomaceous earth filtration or sheet pad filtration before it undergoes sterile cartridge membrane filtration.
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