Pasteurisation
The theory of PU is easily found on the Internet as well as, for example, in Andrew's et
al. book Fermented Beverage Production, page 379.
It is defined by an equation : PU = (time in minutes) x 1.393 exp(T-60), where T is the
temperature in Celsius.
From this equation, you get
0.2 PU from 1 minute at 55C,
1 PU from 1 minute at 60C,
5 PU from 1 minute at 65C
27 PU from 1 minute at 70C
Hence if you want 50 PU, you would need less than 2 minutes at 70C, or 10 minutes at
65C, or 50 minutes at 60C, or 250 minutes at 55C, or some combination that would
yield the total desired.
Louis Pasteur©
Pasteurisation was developed and named after its inventor Louis Pasteur who in 1862 first
successfully tested his method of heating wine to prevent further fermentation and spoiling. This
method has been refined to extend the shelf life of wine, beer, milk and numerous other
beverages and food stuffs. The pasteurisation process does not sterilise the product as the heat
required for this would normally impair the flavour, however enough heat is used to allow a shelf
life of up to several years.
The first pasteurisers used commercially were crude vats of water kept roughly at temperature.
Bottles were loaded into baskets that were lowered into the vat for a given period of time. This
labour intensive method was superseded by tunnel pasteurisation.
Bottles in a pasteuriser
Tunnel Pasteurisation
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; water is preheated by
spraying onto hot bottles within the cooling zones , then the water is heated externally and
sprayed onto the bottles within the pasteurising zones, before being cooled by spraying onto the
cold bottles in pre heating zones. A run through the pasteuriser takes between 20-50 minutes at
temperatures from 60°C to 90°C. Due to the length of time the bottles must be heated, tunnel
pasteurisers can be extremely large. However, solutions are available with double deck designs
to optimise the use of space.
•
Advantages
Simple to install and operate
◦
Easy to guarantee quality
◦
•
Disadvantages
Take up a lot of space
◦
Can use a lot of energy if improperly controlled
◦
Flash Pasteurisation
Although proposed during the 1800s, commercial flash pasteurisation only took off seriously in
the 20th Century. This was when the extreme standards of hygiene required for sterile fill
technology became practical. With flash pasteurisation, product is forced through a stack of
corrugated plates; the plates act as heat exchangers to quickly bring the product up to
temperature, hold it for 25-30 seconds, then cool the product back down to near ambient. The
flash pasteuriser itself is relatively compact and takes up much less room that a tunnel
pasteuriser.
Flash pasteurisation must be used in conjunction with sterile fill technology and therefore has the
risk of post-pasteurisation contamination if hygiene standards are not rigorously enforced. Flash
pasteurisation is often used for the pasteurisation of bulk products such as keg beer and milk.
Often when flash pasteurisation is used for bottled or canned beer, tunnel pasteurisation is used
as a secondary process for export beer in order to prolong the shelf life.
•
Advantages
Can offer improved flavour
◦
Space and energy saving
◦
•
Disadvantages
Expensive to set up and maintain a sterile environment
◦
Difficult to retrofit into an existing bottling/canning line
◦
Risk of downstream contamination
◦
No possibility to independently monitor the PUs achieved
◦
Cannot be used for pulpy products such as Orange juice
◦
Cannot process more than one product without downtime
◦
Sterile Filtration
Mentioned only for completeness this is an alternative to pasteurisation that uses advanced filters
to remove sufficient microbiological contamination to extend the shelf life of the product. This
method has the advantage of having minimal impact on flavour. However, as with flash
pasteurisation there is a risk of downstream contamination.
Redpost
Redpost produces PU Monitoring equipment for tunnel pasteurisers and therefore the rest of
these science pages focus exclusively on the tunnel pasteurisation of bottled and canned
beverages.
Pasteurisation Units (PUs)
Lethal Rate Graph
The Pasteurisation Unit (PU) is defined as relating to the sterilising effect observed when the
product is held for one minute at a temperature termed the Base value. At this temperature
therefore, 1 PU per minute is achieved.
Experiments on various mixtures of the common brewery biological contaminants showed that at
temperatures over about 50°C there is an approximately ten-fold increase in sterilising effect for
every 7°C increase in temperature. For example, if the time required to kill a population of
micro-organisms at 60°C is found to be 5 minutes then if the temperature were to be increased to
67°C the time required would be only 0.5 minutes. The increase in temperature required to
produce a ten-fold increase in kill rate is termed the Z value. The Z value in this case is therefore
7°C.
The formula for PUs is normally shown like this:
Note this assumes a Base value of 60°C and a Z value of 7°C.
The same definition is often used for soft drinks and other beverages. Where there is any
possibility of spore-forming bacterial contamination being present (for example in tomato juice)
very much higher temperatures are required for pasteurisation. In these situations a Base value of
80°C and a Z value of 10°C is often used.
PU Control
There are three reasons why tight control and monitoring of PU accumulation is important.
Shelf Life
First and foremost, PUs must be controlled to ensure no package on the line receives less than
the minimum PU value required to reach the desired shelf life. Failure to achieve this results in
costly product wastage, repasteurisation, product recall or at worst spoiled product on
supermarket shelves. Most beer requires only around 5 PU to maintain product stability, however
it is normal for brewers to aim for between 15 and 30 PU to be certain of stability. Redpost
equipment gives an instant, empirical and accurate measure of the PUs achieved, allowing longer
shelf life without over-pasteurising.
Taste
The flavour of a beverage can be severely influenced by temperature, especially when the oxygen
content of the beverage is high. There has been a lot of work over many years to investigate this
oxidation or staling and much improvement has been made to the filling process to reduce the
dissolved oxygen content and oxygen within the headspace. The staling effect can be held back
with the use of additives in the product, however consumer preference and in some cases
national regulations can prevent their use. The best way to maintain flavour is to keep the number
of PUs to a minimum and pasteurise at as high a temperature and for as short a time as possible.
However, this of course runs contrary to the above goal of guaranteed shelf life and can lead to
problems with package vulnerability.
There has been some work to quantify the effect that pasteurisation has on flavour, Redpost
equipment is able to calculate this Thermal Degradation effect in addition to the PU result.
Energy Efficiency
With rising energy costs around the globe and consumer pressure on producers to be more ecofriendly, energy efficiency is extremely important. Until the 1950s pasteurisation was really only
based on guesswork with most beverages being extremely over-pasteurised and therefore wasting
a vast amount of energy. Since the '70s, regenerative pasteurisers have been available to improve
efficiency, and better control since the '80s and '90s has improved this yet again. Redpost
monitoring equipment is able to ensure product stability without wasting energy.
PU Monitoring
Typical growth plate©
Microbiological
The precursor to PU monitoring, the culturing of microorganisms from pasteurised beer, is still
widely used today. Although, whereas previously used as the sole measure of product stability, it
is now used mainly as an infrequent quality check or for research purposes. This method of
monitoring was extremely slow and was often unreliable, it was these shortcomings that led to
the development of the electronic monitors available today.
Paper chart recorder
Paper Chart
The first PU monitors were simple repurposed chart-based temperature loggers, travelling
through the pasteuriser in a waterproof and heat proof enclosure. They were bulky and
unreliable. The logger would also not calculate PUs directly, this had to be done by hand back in
the laboratory. This was not much of an improvement on the microbiological methods and it was
this inadequacy that led Redpost to develop the first solid-state electronic PU monitor.
The modern PU Monitor
A modern PU monitor is a special purpose electronic temperature recorder, it is battery powered
and housed in a rugged watertight enclosure. The PU monitor has a means of supporting the
package under test and a means of ensuring that the temperature probe stays constantly at the PU
pick-up point. The PU monitor travels through the pasteuriser tunnel with the product under test
and has to operate correctly under the high temperature, high humidity, high vibration
environment inside. The enclosure must not only survive the harsh environment within the
pasteuriser but also the rough handling it may receive in the packaging hall. Above all the PU
monitor should be accurate and not affect the package under test to the extent that it becomes
non-representative of the other packages on the line.
Redpost has over 30 years' experience producing dedicated PU monitors for the brewing and
beverage industry. As well as temperature and PUs, Redpost monitors are available that monitor
many other variables including Spray temperature, Pressure, Flavour Degradation and Lethal PU.
The difference a Redpost PU monitor can make can be dramatic; one new customer believed
they were accumulating 20 PU, though when demonstrating their new Redpost PU Monitor it
was shown they were actually accumulating over 200PU. Redpost equipment is regarded so
highly that it is used by Pasteuriser manufacturers to calibrate the management software when
commissioning their new installations.
PU Pick-up point
Convection currents
This refers to the location within the package where the temperature used in calculating PUs
should be measured. Redpost recommends the use of the Cold-spot as the PU pick-up point,
however, some prefer to use the centre of mass of the package, or 1/3 of the way up from the
bottom of the package.
Cold-Spot
The liquid within a package cannot be considered to be a uniform mass of equal temperature.
The liquid is heated by the spray water indirectly through the walls of the package and this
causes convection currents to form. Computer analysis has shown that a thin layer of heated
liquid rises quickly up the walls of the package, slowly forcing a wide column of cooler liquid
down the centre of the package. At the end of the package that is in contact with the pasteuriser
bed there is a thin layer of turbulence. Above this, the wide column of falling liquid is at its
coolest, this is the cold-spot.
As any microbiological component of the product is swept along with the convection currents,
normal practice is to use the cold-spot as the PU pickup point in order to get the 'worst-case'
measurement for PUs. If you measure PU at the cold-spot you can be sure that all of the product
has been pasteurised to this value or higher and so can be sure you have attained your required
product stability.
Accuracy
The calculation of PUs is exponential (See above). This means that small errors can affect the
resultant PU values significantly. An error of only 0.1°C in the recorded temperature can cause
an error of 3.3% in the calculated PU. The accuracy should be maintained over the entire
temperature range of the pasteurising run i.e. Cutoff temperature to Maximum temperature.
It is therefore important to have a modern accurate PU monitor, all current Redpost PU Monitors
offer a worst case PU accuracy of better than 8% over the entire temperature range, incorporating
all sources of error. It is common to see the errors of some PU monitors not stated, or separated
into different types and it is not made clear that these errors should be added together.
PU Cutoffs
At product temperatures below about 50°C it is doubtful that any significant sterilising effect is
achieved even though from a strictly mathematical point of view the correct PU total could still
be obtained, given enough time in the pasteuriser. For this reason it is desirable to set a
temperature level below which no PUs are added to the total calculated by a PU monitor. This is
termed the PU cutoff temperature.
Set the PU cutoff temperature at least 5°C lower than the planned product temperature in the
holding zone of the pasteuriser. Higher values will give a very conservative figure for PU
achieved and will reduce the repeatability of the result because the product temperature will be
changing only slowly as it crosses the cutoff temperature threshold. For beer the cutoff is not
normally set below 50°C.
A packaging specification will normally set the PU definition and a target PU value for each
product to ensure sufficient pasteurisation. This packaging specification will have been
determined by microbiological analysis.
Z-Values
The Z-Value is the parameter of the PU calculation that controls the increase in lethality with
temperature. As stated above, for beer it is normally stated that for every 7°C rise there is a 10
fold increase in lethality. This means that the Z-Value is 7°C. In some fruit or vegetable juices
higher temperatures are required for pasteurisation and a Z value 10°C is often used. Another
alternative Z-Value for brewing is 6.94°C, this is equivalent to a value of 12.5°F and is often
used in American text books.
Thermal Degradation Units (TDUs)
Sometimes called Flavour Degradation Units or FDUs.
Sometimes called Taste Degradation Units.
Thermal Degradation Units give a measure of the amount of flavour change that has been caused
by the pasteurisation process. Flavour degradation is produced as a result of chemical reactions
within the product. It is generally accepted that chemical reactions occur twice as fast for each
increase of 10°C. From the flavour degradation point of view it is better to achieve the PU target
by using a higher temperature for a shorter time.
The formula for TDUs is normally shown like this:
The calculated value is divided by 10 and shown as TDU/10 or dTDU. The definition therefore
becomes 0.1 TDU/10 per minute at 20°C. This gives better alignment with standard PU
calculation. (It gives 1.6 TDU/10 per minute at 60°C, TDU = PU at approx. 62°C). No TDUs are
accumulated below 20°C. The definition does not change when working in Fahrenheit:- 0.1
TDU/10 per minute at 68°F.
A packaging specification may set a target TDU/10 value to ensure that even though pasteurising
conditions may be correct the product flavour is not harmed.
TDU calculation is built into Redpost Redlink software.
Spray Monitoring
Monitoring the spray temperature is used to view the temperature of the individual temperature
zones and to identify the location of any bent or clogged spray heads. As the bed is in constant
motion a sample time of at least 10 seconds is required to determine an accurate location for a
damaged head.
Delta T (ΔT) can also be measured i.e. the difference between the temperature inside the package
and outside. This is of interest when pasteurising bottles, as too great a Delta T during the
heating or cooling cycles would likely lead to breakages.
The Redpost RPU-352 and RPU-353 have two temperature channels to allow measurement of
product and spray temperatures.
Pressure Monitoring
Peaked Can
During pasteurisation the pressure in the container increases. This is partly due to the rise in
temperature acting on the CO2 gas already in the headspace and partly due to the vibration,
which encourages CO2 to be released from supersaturated solution in the product. The practice of
using nitrogen for purging containers and for fobbing produces a mixture of gases in the
headspace that may increase the pressure rise.
Over the years there has been a trend towards ever thinner-walled can packages, as well as
reduced head space volumes. Both these factors make problems of over-pressure more
significant, with peaking or breakages the result. When pasteurising cans with internal pressure
reservoirs or 'widgets' the internal pressure also needs to be monitored closely.
A PU Monitor with a pressure measurement channel can assist with this; it is attached to the
bottle or can and is designed to make as little change to the headspace of the package as possible
so as not to affect the results. The graph of pressure can then be examined to observe maximum
pressure along with any obvious spikes.
The Redpost RPU-353 has a pressure measurement channel as well as two channels for product
and spray temperature.
Lethal PUs (LPUs)
Lethal PUs are used by some brewers and bottlers as an extension to the PU system. Rather than
having just one PU Cutoff value, under this system there are Total-PU values and Lethal-PU
values. The Total-PU cutoff would be set at say 55°C and the Lethal-PU cutoff at 65°C the PU
Monitor would then give the two PU results.
The common reason for using this methodology is as a backup to ensure that the product has
completed the expected temperature curve. With some resilient microorganisms the temperature
curve is important and the product must be above the Lethal Cutoff for a certain time to ensure
stability.
Lethal PU calculation is available in Redpost Redlink software.
Exit Temperature
The temperature that the bottles or cans leave the tunnel pasteuriser can be very important. If the
product leaves at too high a temperature the flavour of the product may be affected. Because of
this some package specifications state a required product exit temperature. As the PU Monitor
recording run may not be stopped immediately the product leaves the tunnel, it may be difficult
to determine the exit temperature by looking at the temperature graph alone.
To solve this problem all Redpost PU Monitors are fitted with a sensor that determines when the
PU monitor leaves the pasteuriser and the temperature of the product is recorded at this point.
More Information