Liquid milk can be delivered to the consumer after various heat treatments: none (raw milk), pasteurized or sterilized, and either packaged or not (although sterilized milk is, of course, always packaged). The properties of liquid milk that require the most attention are safety to the consumer, shelf life, and flavor.
Pasteurization is a mild heat treatment that is used on a wide range of different types of food products. The pasteurization process was developed by Louis Pasteur. It has been described as the process of heating milk to such temperature and for such periods of time as are required to destroy any pathogens which may be present, whilst causing minimum changes in the composition, flavour and nutritive value (Ahmad, 2012). The two primary aims of pasteurization are to remove pathogenic bacteria from foods, thereby preventing disease, and to remove spoilage (souring) bacteria to improve its keeping quality.
Pasteurization ensures the safety and greatly enhances the shelf life of the product. This heat treatment, kills all pathogens that may be present (especially Mycobacterium tuberculosis, Salmonella spp., enteropathogenic E. coli, Campylobacter jejuni, and Listeria monocytogenes) to such an extent that no health hazard is left. Such pasteurization inactivates alkaline phosphatase to the extent as to be no longer detectable. Most of the spoilage microorganisms in raw milk, such as coliforms, mesophilic lactic acid bacteria, and psychrotrophs, are also killed by pasteurization.
The term “Pasteurization”, “Pasteurised” and similar terms shall be taken to refer to the process of heating every particle of milk of different classes to at least 630C (sixty three degree centigrade) and holding at such temperature continuously for at least 30 minutes or heating it to at least 71.50C (seventy one point five degree centigrade) and holding at such temperature continuously for at least 15 seconds or an approved temperature time combination that will serve to give a negative Phosphatase test.
All pasteurised milk of different classes shall be cooled immediately to a temperature of 100C, or less. (less than 10 degree centigrade)
International Dairy Federation (IDF) definition
International Dairy Federation (IDF) definition of pasteurization is as follows: “pasteurization is a process applied to a product with the objective of minimizing possible health hazards arising from pathogenic microorganisms associated with the product (milk) which is consistent with minimal chemical, physical, and organoleptic changes in the product.” This definition is also applicable to products other than milk, including, creams, ice cream mix, eggs, fruit juices, fermented products, soups, and other beverages.
Pasteurization does not inactivate all microorganisms; those that survive pasteurization are termed thermodurics and those that survive a harsher treatment at 80-1000C for 30 min are termed spore formers. Traditionally, pasteurization was carried out in a batch process- the Holder process- at 630C for 30 min, but this was replaced by the introduction and acceptance of continuous HTST processes.
Batch/ Holding Pasteurization
This process is also called the Low –Temperature-Long-Time (LTLT) method. This process is not popular in the dairy industry any more after HTST system was introduced. The milk and milk products is heated or cooled in batches in one, two, or three tanks. The process involves heating the milk to a temperature between 62.8 and 65.60C, holding it at that temperature for 30 min, and rapidly cooling it to below 100C. Batch processing involves filling the vessel, heating, holding, cooling, emptying the vessel and filling into containers, and cleaning the vessels (Lewis and Jun, 2012).
The heating and cooling of the product is done through a metal wall. The milk is heated in open vats by using steam or hot water. Then the product is heated or cooled, gentle agitation is done for rapid heat transfer. The pasteurizers may be of three types:
Water-jacketed vat: This is double walled around the sides and bottom in which hot water or steam under partial vacuum circulates for heating, and cold water for cooling. The outer wall (lining) is usually insulated to reduce heat loss. The heat-exchange takes place through the wall of the inner lining. The difference between the temperature of the heating water and the milk is kept to a minimum. The milk is agitated by slowly moving (revolving) propellers. When heating, the vat cover is left open for escape of off flavours; and when holding, the cover is closed. During the holding period, an air space/foam heater (steam or electrically heated) prevents surface cooling of milk.
Water-spray type: A film of water is sprayed from a perforated pipe over the surface of the tank holding the product. The product is agitated as above. A rapidly moving continuous film of water provides rapid heat transfer.
Coil-vat type: The heating/cooling medium is pumped through a coil placed in either a horizontal or vertical position, while the coil is turned through the product. The turning coil agitates the product (but additional agitation may be necessary).
The batch pasteurizers are relatively cheap, simple and well suited for small plants and for low volume products (De, 2001).
High temperature short-time (HTST) pasteurization
This was first developed by A. P. V. Co. in the United Kingdom in 1922. The HTST pasteurizer gives a continuous flow of milk. The HTST process involves heating milk to 72-750C with a 15 second holding time before it is cooled.
One of the main advantages of continuous systems over batch systems is that energy can be recovered in terms of regeneration. Regeneration efficiencies up to 95% can be obtained, which means that a pasteurized product requiring heating to 720C would be heated up to more than 680C by regeneration when initial temperature of milk is 4 deg C. Although high regeneration efficiencies result in considerable saving in energy, they necessitate the use of larger surface areas because of the lower temperature driving force and there is a slightly higher capital cost for the heat exchanger.
The following parts and functions are involved as milk passes through the HTST pasteurization systems:
1. Float-Controlled Balance Tank (FCBT): Maintains a constant head of milk for feeding the raw milk pump; also receives any sub-temperature milk diverted by flow diversion valve (FDV).
2. Pump: Either a rotary positive pump between the regenerator and heater, or a centrifugal pump with a flow control device to ensure constant output, after FCBT is used.
3. Plates: The Plate Heat Exchanger is commonly used in the HTST system, especially for heating to temperatures which are below the boiling point of milk. The plate heat exchanger is a compact, simple, easily cleaned and inspected unit. Its plates may be used for heating, cooling, regeneration and holding. A space of approximately 3 mm is maintained between the plates by a non-absorbent rubber gasket or seal.
4. Regeneration (Heating): The raw cold incoming milk is partially and indirectly heated by the hot outgoing milk (milk-to-milk regeneration). This adds to economy of the HTST process, as the incoming milk requires less heating by hot water to raise its temperature for holding.
5. Filter: Variously shaped filter units to connect directly to the HTST system are placed after the pre-heater or regenerative (heating) section. These units, using 40-90 mesh cloth, are usually cylindrical in shape. Usually two filters are attached but they are used at a time.
6. Holding: The holding tube or plates ensures that milk is held for a specified time, not less than 15 sec, at the pasteurization temperature of 720C or more
7. Flow diversion valve (FDV): This routes the milk after heat treatment. If the milk has been properly pasteurized, it flows forward through the unit; that which is unpasteurized, it flows forward through the unit; that which is unpasteurized is automatically diverted back to FCBT for reprocessing. It is usually operated by air pressure working against a strong spring.
8. Regeneration (cooling): The pasteurized hot outgoing milk is partially and indirectly cooled by the incoming cold milk. This again adds to the economy of the HTST process.
9. Control panel: Contains instruments, controls, FDV mechanism and holding system, all centralized in one moisture proof panel.
10. Hot water set: Circulates hot water through the heating section of the machine to maintain the correct milk temperature within very fine limits.
11. Automatic control devices: These include steam pressure controller and water temperature controller.
Steam pressure controller: Maintains a constant hot water temperature for heating milk accurately to the required pasteurization temperature.
Water temperature controller: Regulates the amount of milk leaving the holding tube/ plate. This is an electric contact instrument that operates either a FDV or a milk pump, automatically preventing milk from leaving the holding section at required temperature. Both the frequency and duration of the flow diversion and the temperature of the milk leaving the holder are recorded on the thermograph (recording chart) by means of two separate pens (De, 2001).
Advantages of HTST pasteurization
Ø Large volume of milk may be processed continuously.
Ø Automatic precision controls assure positive pasteurization.
Ø The equipment requires a relatively small amount of floor and plant space.
Ø The system adapts itself well to CIP cleaning.
Ø Filling operations may begin almost simultaneously.
Ø The HTST method is economical, as it uses regenerator.
Ø The entire system is simple, requiring little supervisory attention.
Ø The capacity may be increased by increasing the number of plates without sacrificing floor space.
Ø It is well suited for regenerative heating and cooling.
Ø The closed unit keeps the processing losses to a minimum.
Disadvantages of HTST system
Ø The system is not well-adapted to handling small quantities of several liquid milk products.
Ø Gaskets require constant observation for possible leakage and lack of sanitation.
Ø It requires precision instruments of control.
Ø Complete drainage is not possible without losses.
Ø Long run pasteurisers may give rise to serious bacteriological problems; plant should be efficiently cleaned every six to eight hours to avoid these.
Tunnel (Spray) Pasteurizers
Tunnel (Spray) Pasteurizers are widely used in the beverage industry for continuous heating and cooling of products in sealed containers. They are ideal for high-volume throughput. Examples of such products are soft and carbonated drinks, juices, beers, and sauces. Using this procedure, post-processing contamination should be very much reduced, the major cause being defective seams on the container. There are three main stages in the tunnel: heating, holding, and cooling. In each stage water at the appropriate temperature is sprayed onto the container. Since heating rates are not as high as for plate or tubular heat exchangers, these processes are more suited to longer time/ lower temperature processes. The total transit time may be about 1 h, with holding temperatures between 60 and 700C for about 20 min (Brennan and Grandison, 2008).
Alkaline Phosphatase Test for Checking Efficiency of Pasteurization in Liquid Milk
Alkaline Phosphatase is an indigenous milk enzyme. The enzyme activity is destroyed at pasteurization temperature and has been adopted as an index of the efficiency of pasteurization. Since milk is a proven vector for a number of pathogenic bacteria, including Salmonella, Compylobactor and Listeria, the test is of very great significance to the dairy industry as a means of policing the thoroughness of heat treatments. In the following method, a solution of disodium p-nitrophenyl phosphate in a buffer of pH 10.2 is used as substrate. This compound, colourless in solution, is hydrolysed by alkaline phosphate of milk to liberate p-nitrophenol, which under alkaline condition gives an intense yellow colouration to the solution. The liberated p-nitrophenol is measured by direct comparison with standard colour discs in a Lovibond comparator. The test does not apply to sour milk and milk preserved with chemical preservatives (FSSA, 2012).
Into a test tube pipette 5 ml of buffer substrate solution, stopper and bring the temperature to 370C. Add 1 ml of test milk to it and shake and replace stopper, incubate at 370C for 2 hr. Incubate one blank prepared from boiled milk of the same type as that undergoing the test with each series of sample. Remove the tubes after 2 h and the content should be well mixed. Place the boiled milk blank on left hand side of the Comparator stand and test sample on the right. Take the reading in reflected light by revolving the disc until the test sample is matched. Record the readings falling between two standards by affixing a plus or minus sign to the figure for the nearest standard.
The test is considered satisfactory if it gives a reading of 10 µg or less of p-nitrophenyl per ml of milk. Properly pasteurized milk gives no discernible colour.
Ahmad T (2012) Pasteurization. In: Dairy Plant Engineering and Management. 8th chapter. Kitab Mahal, Allahabad, India. pp. 196-221.
De S (2001) Market milk. In: Outlines of Technology. 1st Ed., Oxford University Press- New Delhi. pp 57-64.
FSSA (2013) The Food Safety and Standards (Food Products Standards and Food Additives) Regulations, 2011. 3rd Ed., Commercial Law Publishers (India) Pvt. Ltd., p 169.
FSSAI (2012) Manual of Methods of Analysis of Foods. Milk and Milk Products. Lab Manual 1. Ministry of health and Family Welfare, Government of India, New Delhi. pp. 36-38.
Lewis M J and Jun S (2012) Thermal Processing. In: Food processing Handbook. 2nd ed. Brennan J G and Grandison A S (Eds.). Wiley-VCH Verlag GmbH & Co. KGaA. pp. 31-76.
Brennan J G and Grandison A S (2008) Thermal processing. In: Food Processing Handbook. 2nd chapter, Second Edn., Vol 1. Wiley-VCH Verlag GmbH & Co. KGaA. pp. 47-52.