This section is to briefly explain the Pasteuriser and the Pasteurisation process for Beer and Beverage Cans.
Principle of the Pasteurisation Process:
The purpose of the pasteuriser is to maximise the shelf life of drinks contained in Beverage Cans. This is achieved by a heat treatment process whereby the harmful microbiological organisms are killed by the heat. The treatment is controlled in such a way that there will be a minimum effect on the physical stability and flavour of the Can contents, together with a maximum extension of biological stability. When this is achieved, there will be an increased shelf life of the product contained in the Cans.
The pasteurisation process involves three stages:
Gradual heating of the beverage in the filled Cans from the filling temperature to the pasteurisation temperature.
The actual pasteurisation, where the Cans are kept at the pasteurisation temperature for a certain time (holding time), giving the required pasteurisation effect.
Gradual cooling of the beverage Cans to the desired discharge temperature.
In modern pasteurisers, the heat treatment is performed by spraying hot water (at different regulated temperatures in the various water tanks/zones) on to the filled Cans as they pass through the pasteuriser tunnels.
Description of the Pasteuriser:
As the Cans are usually pasteurised upside down, the domes become filled with pasteuriser water.
It is extremely important that the air knives remove this water as the Cans exit the pasteuriser. Failure to do this, will result in problems with the printing of date codes on the bases of the Cans, as well as potentially damp packs.
The pasteuriser consists of a tunnel, water tank system and temperature control.
There can be one, two or multi tunnels on a pasteuriser, depending on the design; a single tunnel for a single deck, or a double tunnel for a double deck. For the purpose of this section, a one/two tunnel pasteuriser will be referred to.
A pasteuriser is normally constructed of stainless steel and has a series of water tanks beneath the lower tunnel.
Depending on the make/model/design of the pasteuriser will determine the number of water tanks required. Usually, they have eight, nine, or ten tanks. For the purpose of this section, a ten tank pasteuriser will be referred to. The ten water tanks also relate to the ten zones of the pasteuriser.
The pH of the pasteuriser water is very important, as alkalinity can cause staining of the Cans and Ends, also softening and peeling of the Can decoration could occur. Ideally the pH should be maintained between 6.5 and 7.5.
At the top of each tunnel, there are a series of stainless steel spray bars, which spray circulated water of variable temperatures and pressures, depending on which zones they are spraying. The water is pumped from the individual water tanks through spray bars on to the filled Cans.
Beneath the spray bars, there are normally perforated stainless steel plates which have raised holes that minimise clogging and facilitate the cascade – like spraying of the Cans with large amounts of water in such a way that all of the Cans that are passing beneath them receive an equal amount of treatment.
Cans of filled product are transferred through the processing area of the pasteurising tunnel either on flat bed plastic belts or steel belt systems, or on earlier models, a walking beam conveyor system.
The whole conveying system is placed above the water tanks and below the water spray bars.
The perforated plates below the spray bars are located above the conveyor system.
On modern pasteurisers, there is a control chart or graph showing a continuous display of running temperatures in the various zones. The functions of these zones are as follows.
Heating zones 1 – 3
Temperatures are increased in stages through these zones as Cans pass through the pasteuriser, until they reach the superheat zone.
Superheat, zone 4
The Cans in the superheat zone are exposed to a temperature slightly higher than the pasteurisation zones (5 & 6), to enable the centre of the beverage in the Cans to reach the pasteurisation temperature, within a reasonable amount of time.
Pasteurisation zone, 5 and 6
These are the actual pasteurisation zones. The temperature of the beverage contained in the Cans is maintained at a defined level for a specified period of time. Both of these parameters determine the number of pasteurisation units (PUs) to ensure optimum product quality and preservation.
Re-cooling zones 7 – 10
The temperature of the Cans decrease as they pass through each of the re-cooling zones; The Cans should leave the pasteuriser at the ambient temperature or slightly above.
The average time it takes for Cans to pass through the pasteuriser without stopping is 45 minutes.
Chemical Dosing of Pasteurisers:
There are usually two types of chemicals added to the pasteuriser water; These are as follows:
This is a disinfectant chemical and is used in the warm up and cooling tank zones(1,2,3 & 7,8,9,10), The main purpose of this chemical is to eliminate microbiological growth and diseases such as Legionella. Due to the high temperatures reached in the superheat and pasteurising zones(4,5 & 6), there is no need to dose them.
Overdosing of this chemical could lead to corrosion of the Cans, tab staining on the Ends and possibly accelerate stress corrosion cracking of the Can Ends. Certain parts of the pasteuriser would also suffer with corrosion.
Under dosing could lead to bug growth, slime build up and blocked spray bar nozzles, resulting in poor pasteurisation performance on the product.
All chemicals used in customers pasteurisers must be compatible with the coatings, inks and lacquers that are used on the finished Can and End.
For clarification of this, please seek the advice of your local Rexam CTS Engineer.
The main purpose of this chemical is to prevent corrosion of the Cans and Ends as well as the pasteuriser itself. This chemical is normally fed into the main water supply to the pasteuriser, therefore the inhibitor is present in all ten tanks/zones.
Overdosing of this chemical has no real adverse effect other than the cost of the process.
Under dosing of inhibitor can lead to dome staining of the Cans, tab staining of the Ends and also corrosion of some pasteuriser machine parts.
Both these chemicals are normally added using an automatic dosing system and have to be closely monitored by the customer. These systems are generally excellent, but it is not uncommon for the feeder tank of chemical to run out, or the dosing pump to fail. If these conditions remain undetected it can lead to Can issues such as dome staining/rusting or tab staining on the End. It is important that procedures/controls are implemented to ensure that this does not occur.
The customer must strictly control the pasteurisation temperature. Failure to do so will result in out of specification PU’s (Pasteuriser Units) and therefore potentially poor product quality. Issues with the Cans and Ends could also occur.
During the pasteurisation of Cans, the factors creating internal pressures have to be considered in order not to exceed the dome reversal strength of the Cans or reach the peaking pressure of the Ends.
The major factors contributing to pressure inside the Cans are:
Air is the headspace.
Level of carbonation.
Excessive exposure to temperature.
See also Liquid Nitrogen Dosing
Special charts apply for the calculation of expected internal pressure and help can be obtained from our CTS Representatives. The charts are made up of ordinary C02 holding products. If liquid nitrogen or other gasses are used, there are other means of calculation.
It is also worth noting that all the four problems with Cans and Ends that are mentioned in this section could occur if the fill volume/Nitrogen dosing/carbonation of the Cans are too high.
Typical problems that can occur with the Cans and Ends if the pasteuriser temperature is too high are:-
Dome Growth (Cans)
Dome Reversal (Cans)
Tab Over Seam (Ends)
If there are problems resulting in Cans being delayed in the pasteuriser for several minutes or hours, it is possible that the above problems will occur if the process is not controlled.
Pasteurisers are normally equipped with a "Quench" system, so that when Cans are delayed in the pasteuriser, cool water is administered to the super heat and pasteurisation zones to automatically control the temperature within specified limits.
Some pasteuriser conveyors stop when the filler stops, therefore filled Cans are held up for extended periods in the pasteuriser. This could also result in the above problems occurring.
Rexam has a Pasteurisation control logger which measures the pasteurisation process and gives the user valuable information regarding the customer’s filling/pasteurisation conditions.
The logger transducer fits inside the Can, which is then filled and seamed, placed in a special container and then passed through the pasteuriser. On exit from the pasteuriser, the logger is removed from the Can and the Data is downloaded giving accurate recordings achieved on the internal pressure, temperature and pasteuriser units (PU’s).
General Maintenance and Sanitation
The water in the Can warmer/pasteuriser needs to be kept clean and changed at regular intervals, to avoid build up of corrosive salts and sludges. This should be incorporated along with a pasteuriser water treatment programme, incorporating biocides and inhibitors.
Due to the development of water efficient pasteurisers with advanced heat exchange systems, the above is even more important, or there could be a relatively sharp increase in chemical instability, leading to problems with the overall appearance of the filled Cans.
If the above is incorporated as part of an overall pasteuriser maintenance programme, it will also minimise the risk of spray bar nozzle blockages, thus giving more consistent product quality and maintain the attractive appearance
of the Can.