User:MattAldred/PUMonitor

A P.U. Monitor is a special purpose electronic data logger for measuring the performance of a tunnel pasteuriser. Tunnel Pasteurisation is one of the most common methods for increasing the shelf life of pre-packaged food and drink. It is important to ensure that the product is exposed to the correct temperatures for the correct amount of time to ensure microbiological stability without degradation in the taste of the product. The measurement of P.Us and therefore the usage of a P.U. Monitor is mainly during the pasteurisation of beer, cider, fruit juices, mineral water and soft drinks.

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.

PU Units

During the 1950s Del Vecchio(??) carried out scientific tests with a range of harmful microorganisms typically found in beer. By subjecting the beer to the temperatures of the pasteurisation process he found that an increase in temperature of 7’C increased the destruction of microorganisms 10 fold. From his work it is possible to create a graph showing how which combinations of time and temperature are lethal to the microorganisms undertest.

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: PU = t x 10^((T-60)/7) 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 eco-friendly, 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.

History Microbiological Paper Chart