Tunnel pasteurisation

Tunnel pasteurisers are divided into zones where water is sprayed at different temperatures, with the total transit time and temperature profile calculated to provide the required degree of pasteurisation. It is common in practice to have a superheat zone before the holding zone to ensure the required tanperature has been reached. Heat recovery is important to ensure maximum energy efficiency and minimise costs. Each zone of the tunnel pasteuriser will contain a water tank, a pump and a water distribution system. Heat recovery is achieved by water being moved to different zones where its temperature is appropriate. 

Heat transfer to beer inside bottles or cans takes place through the walls of the beer container which causes a lag in the heating process, bottles having a longer lag than cans. Convection currents are also generated in the beer being heated. Because of this there will be a point near the base of the container, known as the cold spot, where the lowest rate of heat transfer occurs. It is this point that calculations of PU applied to the beer must be made. The temperature rise cannot be too rapid or there is a risk of 

The first heating stage will gently warm the container approximately 10 °C and subsequent stages will steadily raise the temperature to at least 60 °C. The most important stage is the superheat zone which must be accurately controlled at 61-65 °C to ensure the container has reached 60 °C as it enters the holding zone. 

To prevent over pasteurisation, and associated deterioration in product flavour and risk of producing haze, it is important that controls are in place to adjust the heat delivered in the event of a stoppage. Modem tunnel pasteurisers can calculate the total PUs delivered to containers as they travel though the pasteuriser. 

Travelling recorders are also routinely used, consisting of a dummy bottle or can on a base plate with sensors and recorders. This can be passed through the pasteuriser to measure the temperature profile that containers are exposed to both as they pass through different zones of the pasteuriser and in different positions that containers may occupy. This means cold spots due to blocked spray jets or other problems can be detected. 

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In-pack pasteurisation normally demands very large and expensive tunnel pasteurisers which have several stages. Bottles are introduced into a pre-heating stage, typically around 40°C, to reduce thermal shock, and then into the pasteurisation zone, which will normally be at 70°C, for some 20 min. Following this are two hydro-cooling zones. The first of these reduces product temperature to around 40°C the second, to ambient temperature. Recovery of heat is essential to an economically viable operation. 

Two main methods are used to pasteurise beer tunnel pasteurisation, where bottles or cans are passed through a series of water jets applying heat, and flash pasteurisation, where the beer is heated rapidly in a plate heat exchanger and holding mbe before packaging. 

Products passed through a tunnel pasteuriser can have a shelf life of up to a year (Boulton Quain, 2006). 

The warm and wet conditions inside a tunnel pasteuriser make it a good environment for slime to grow and corrosion problems to develop. The water used for spraying may be treated with biocides, softened to prevent scaling, and treated to minimise corrosion. Care must be taken with addition rates, however, to ensure compliance with local regulations, the effects on the equipment and personnel and even possible effects on cans. 

A key design parameter for tunnel pasteurisers is ease of cleaning. Water tanks, spray jets and strainers must have easy access so scale, broken glass and other foreign bodies can be easily removed. Spray nozzles must have large bores and be easily removed and replaced. 

As flash pasteurised beer is not pasteurised inside a sealed container a higher degree of hygiene and sterility is required than for tunnel pasteurisation. It is particularly critical once the beer has left the pasteuriser, and the buffer tank, filling equipment and all... 

Verification of the process will need to be based on likely errors that may occur and the organisms that are most likely to cause problems. The different processes employed in tunnel pasteurisation and flash pasteurisation mean they have different potential problems and monitoring must take this into account. For example, blocked jets or gas breakout are specific to particular methods though poor maintenance or calibration can apply to both. 

For tunnel pasteurisers time temperature indicators that travel through the tunnel recording the temperatures that containers are subjected to, and for what time, can be used to monitor the process. 

Figure 123 Graph to show teimperature during travel through a tunnel pasteuriser.

As breweries strive to reduce their water and energy usage the trend has been to move away from mnnel pasteurisation to flash pasteurisation. It has been estimated that the costs of flash pasteurisation are only 15% of that of tunnel pasteurisation... 

Hyde, 2001). The capital expenditure required for replacement and the durability of tunnel pasteurisers means their replacement will proceed slowly but it has been predicted that they will be gone by 2030 (Nelson, 2009). There has also been an increase in using sterile filtration as an alternative to pasteurisation. 

S Reduced pasteurisation. At present, pasteurisation is the preferred means for destroying spoilage bacteria, in particular, lactobacilli. Tunnel pasteurisation is employed for cans and glass and flash pasteurisation for beers destined for kegs or polyethylene terephthalate (PET) bottles. It is essential that both excessive and insufficient filtration is... 

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