Milk pasteurisation

Pasteurisation is a relatively mild heat treatment designed to inactivate vegetative pathogenic microorganisms in milk. Pasteurisation, coupled with refrigerated storage of pasteurised product, makes milk safe for human consumption and also extends the shelf-life of the product. Pasteurised milk is not sterile, with refrigerated storage inhibiting / retarding the growth of thermophilic spore-forming bacteria which survive pasteurisation. Pasteurised milk typically contains low numbers of psychrotrophic bacteria, which eventually limit shelf-life. The process of pasteurisation is named after the French microbiologist Louis Pasteur, who discovered that wine could be preserved by inactivating bacteria by heating at a temperature below boiling. This approach was later applied to milk, with the first systems for commercial pasteurisation of milk being introduced in the last decade of the nineteenth century.

The early systems relied on heating of milk to approximately 63-65°C and holding for approximately 30 minutes in batch vessels, followed by rapid cooling to less than 12°C (i.e., low-temperature-long-time (LTLT) pasteurisation). While some plants (e.g., farmhouse dairy product manufacturers) may still employ this LTLT approach to pasteurisation, it has largely been superseded by highthroughput, continuous-flow plate heat exchanger (PHE)-based pasteurisers, in which milk is heated to 72-74°C and held for at least 15 seconds in a process called hightemperature- short-time (HTST) pasteurisation. HTST pasteurisation using PHE technology is the most common form of industrial pasteurisation today. Pasteurisation of milk is probably the largest-volume liquid processing operation in the modern food industry, and is consequently a well studied, highly controlled and optimised process.

The time-temperature parameters employed in the pasteurisation of milk were originally developed using knowledge of the thermal inactivation properties of pathogenic microorganisms and also with consideration of the negative impact of heat treatment on the flavour of milk. The time-temperature combination traditionally used for HTST (72-74°C for 15 seconds) of milk was based on the thermal inactivation kinetics of two bacteria (Mycobacterium tuberculosis and Coxiella burnettii), then considered to be the most heatresistant vegetative pathogenic bacteria likely to be present in raw milk. In the last 10 years or so, some evidence for the survival of emerging pathogens such as Mycobacterium avium subsp. paratuberculosis in pasteurised milk has renewed interest in this area which is the subject of ongoing research.

Technology and operation of HTST pasteuriser

Commercial-scale milk pasteurisation plants are almost entirely of plate heat exchanger (PHE) configuration. A typical PHE pasteurisation plant consists of a frame within which several vertical stacks of stainless steel plates (i.e., sections) are clamped. Different stages of the pasteurisation process (e.g., preheating, final heating, regeneration, cooling) take place in the different sections, each of which are precisely interconnected. The plate configuration facilitates indirect heat transfer between product and heating / cooling medium in all sections. The heating medium is typically hot water or steam, with cooling taking place by regeneration and chilled water. The capacity of a PHE pasteurisation plant is governed by the size and number of plates, with commercial installations having capacity as high as 100,000 litres per hour.

Stainless steel plates in a PHE pasteuriser have corrugated surfaces to increase the turbulence of the liquid flowing over them. Supporting points on the corrugations hold the plates apart during compression into stacks such that thin, rectangular channels are formed between plates. Gaskets, made from natural rubber or synthetic elastomers, around the edges of the plates and holes define the boundaries of the channels. Liquids flowing through the PHE enter and leave these channels by openings at the corner of the plates, with varying patterns of open and blind openings serving to route the liquid from one channel to the next. Milk is introduced through a corner opening into a channel between two plates and flows vertically through the channel, leaving through the opening at the opposite corner, bypassing and fully segregated from the next channel between the plates, with the latter channel being reserved for heating / cooling medium. Within a PHE, a single segment of parallel flow is known as a pass, with each section of a PHE containing several passes such that direction of flow of product or heating / cooling medium is changed several times.

For pasteurisation, cool (typically 4°C) raw milk is fed from a silo to a balance tank and pumped at a constant rate to the preheating (or regeneration, depending on exact plant configuration) section of the pasteuriser, where it is typically heated to 68-70°C. Heating of the incoming milk is achieved by it absorbing heat from outgoing pasteurised milk indirectly across the plates, with the latter being cooled simultaneously. This process is known as heat regeneration and contributes major energy savings as significantly less hot water / steam is required to bring the temperature of the incoming raw milk up to the pasteurisation temperature (typically 72-74°C). In a well designed plant, it is possible to recover about 94 per cent of the heat from the pasteurised milk.

In the production of market milk, many other unit processing operations may be incorporated with pasteurisation into an overall integrated process designed to optimise energy efficiency, processing time and product quality. Examples of such unit operations include (1) clarification for removal of bacteria, somatic cells and debris, (2) centrifugal separation for removal of fat, (3) microfiltration for removal of spore forming bacteria, (4) standardisation of fat content and (5) homogenisation of fat to prevent creaming during storage. The optimum temperature for conducting these unit operations on milk is approximately 50-55°C. For this reason, the regeneration section used to preheat incoming raw milk is typically divided into a number of subsections to facilitate routing of milk at 50-55°C from the PHE to complete clarification, separation, standardisation and / or homogenisation, before being returned to the PHE to complete the pasteurisation process.

An off-line hot water generator supplies heating medium, which is circulated through the PHE using a pump. The temperature of the hot water heating medium is maintained at a temperature differential of 2-7°C (specific for product type and required holding time) above that of the product pasteurisation temperature by injecting steam via a steam regulating valve, which is controlled by an automatic temperature controller.

Pasteurisation requires that the milk is held for a specified time (e.g., 15 seconds) at the pasteurisation temperature. This is normally achieved using a specific section of plates within the PHE or in an external holding tube; the holding tube configuration permits greater control and is therefore more common. The length of the holding tube is determined from the flow-rate of the milk and the required holding time. No forced heating or cooling takes place in the holding tube. A thermometer (thermistor) is normally located at the end of the holding tube (may occasionally be located at the beginning of the holding tube) to measure temperature of the pasteurised milk product. The potential for a drop in temperature of the milk from start to end of the holding tube is often minimised by insulating the holding tube. Alternatively, the milk may be heated to a slightly higher temperature in the PHE to compensate for projected reduction in temperature in the holding tube – although this is not preferred as it implies overheating of the milk which may have a deleterious impact on product flavour.

A flow diversion valve (FDV) is located downstream of the thermistor – typically at the end of the holding tube (although may be located after the cooling section). The role of the FDV is to prevent unpasteurised milk from progressing to the packaging stage of the process, by returning such milk to the pasteuriser balance tank if the temperature as measured by the thermistor drops below the target (set point) pasteurisation temperature. As a default, the FDV is in the closed (i.e., divert) position and the valve opens only when the product temperature exceeds the set point. A programmable logic controller (PLC), or equivalent, normally controls the interfacing of the thermistor and the FDV, and also records such deviations and sounds an alarm for pasteurisation plant operators.

A booster pump is often located in the pasteurised product line after the holding section, which increases the pressure in the milk line slightly. This ensures that, if a leak / pinhole were to develop in one of the plates of the PHE, then the direction of leakage would be from the pasteurised milk side to the raw milk side, preventing post pasteurisation con tami na tion. This preventative effect may also be achieved by increasing the number of passes in the regeneration section of the PHE, which has the effect of increasing the back pressure within the pasteurised milk section of the pasteuriser.

Maintenance, testing and safety of pasteurisers

All pasteurisation plants should have a schedule in place for routine inspection to identify any signs of wear or damage which may cause issues with product quality or plant efficiency. The principal components of the plant requiring regular inspection include:

• Temperature gauges must be positioned correctly and calibrated regularly

• The FDV must be checked regularly for signs of wear or damage and must be shown to be operating correctly. Diversion temp – erature should be checked on a daily basis
• Heating plates should be inspected regularly for integrity (e.g., cracks and pinholes). Differential electrolyte analysis, helium leak detection systems, differential pressure analysis and endoscopy analysis may be used to check PHE plants for plate integrity. Such checks of plate integrity conducted in situ should be complemented by regular full strip down of the plate packs for visual assessment, to include detection of scaling or bio-film and checks on gasket integrity
• Exterior of plant should be inspected daily for general cleanliness and signs of leaks

Such pasteuriser checks should be recorded and records retained as part of the overall quality management system, in accordance with relevant legislation and be readily available for inspection as part of troubleshooting or product quality issues.

Testing for pasteurisation

It is essential that regulatory agencies and process / quality control groups have a rapid and robust method for measuring the efficiency of pasteurisation. The most widely used and universally recognised method is based on thermal inactivation kinetics of the indigenous milk enzyme alkaline phosphatase (ALP). ALP is marginally more resistant to thermal inactivation than the target bacterial pathogens, thus, if ALP is inactivated by heating then one can conclude that the target bacterial pathogens were also inactivated and that the legal requirements for pasteurisation were met. The assays used for measurement of ALP activity in milk may be colorimetric, fluorometric, chemiluminescent or immunochemical in principle. In most modern dairy testing laboratories, fluorimetry-based testing equipment (e.g., the FluorophosTM system) is used for simple, rapid, automated testing of residual ALP activity in pasteurised milk and dairy products. Using this approach, a small amount of milk is mixed with a substrate (nonfluorescent) for ALP, with any residual ALP activity hydrolysing the substrate to yield a fluorescent product which can be quantified. For some dairy products requiring higher pasteurisation temperatures (e.g., 78°C for cream), residual ALP activity is not a suitable indicator. Another indigenous milk enzyme, lactoperoxidase, is often used for such applications. Indeed, lactoperoxidase may also be used as an indicator of excessive heating during pasteurisation of milk, as HTSTpasteurised milk should be ALP negative, but lactoperoxidase positive.

Developments in pasteurisation technology

Increasing consumer demand for nutritious and fresh beverage products that are micro – biologically safe, have good sensory properties and have long shelf life has resulted in significant research on novel (some non thermal) pasteurisation technologies in recent years. Examples of technologies which are currently being evaluated and / or developed include, but are not limited to, microfiltration, superpasteurisation, high hydrostatic pressure, high pressure homogenisation, pulsed electric fields, supercritical gas pressurisation, ohmic heating and sonication. A segment of the dairy business growing strongly is that of extended shelf life (ESL) milk products; such products have a shelf life of 20-40 days under refrigerated storage conditions. Approaches to the manufacture of ESL milk involve either physical removal or thermal inactivation of spore forming bacteria. Super-pasteurisation for ESL milk production involves high temperature (typically greater than 100°C) processing of milk with very short holding times (typically one to four seconds), to significantly reduce numbers of spore forming bacteria. Super-pasteurisation of ESL milk may be achieved using retrofitted / upgraded conventional PHE pasteurisation technology or specifically designed steam injection / infusion technology. Microfiltration and centrifugal separation (e.g., bactofugation) are also used commercially for the removal of bacterial spores from milk prior to pasteurisation.

About the Author

Dr. Seamus O’Mahony graduated from University College Cork (UCC) with a BSc in Food Science and a PhD in Food Science and Technology in 2001 and 2005, respectively. He was awarded the National University of Ireland travel bursary in Food Science and Technology in 2002, which allowed him to conduct part of his PhD studies at the University of Wisconsin-Madison, USA under the joint supervision of Professor John Lucey (UW) and Professor Paul McSweeney (UCC). On completion of his PhD studies in 2005, Dr. O’Mahony was awarded an Embark Postdoctoral Fellowship by the Irish Research Council for Science, Engineering and Technology to progress his academic studies at the Teagasc, Moorepark Food Research Centre under the supervision of Dr. Tim Guinee. In 2006, Dr. O’Mahony took up an industrial research and development position with a multinational pharmaceutical / food company specialising in the development of infant nutritional products. Over a four year period, he held a number of different roles with increasing responsibility working at the interface between Research & Development, Product Development and Operations on multidisciplinary ingredient, formula, process and product development projects. In September 2010, Dr. O’Mahony joined the academic staff of the School of Food and Nutritional Sciences at UCC as a lecturer in Food Science. He lectures on topics including dairy product technology, physical chemistry, food product development and innovation, formu – lation science and technology and advanced analytical methods. His current research programme focuses on (1) food ingredients, structure and functionality, (2) isolation, enrichment and purification of food constitu – ents, (3) nutritional product formulation, processing and functionality and (4) proteincarbohydrate interactions in food systems.