In this article we will discuss about:- 1. Definition, Object and Need of Pasteurization 2. Formulation of Standards for Pasteurization 3. Pasteurizing Process and Equipment.
Definition, Object and Need of Pasteurization:
The term pasteurization, as applied to market milk today, refers to the process of heating every particle of milk to at least 63°C*(145°F) for 30 minutes, or 72°C (161°F) for 15 seconds (or to any temperature-time combination which is equally efficient), in approved and properly operated equipment. After pasteurization, the milk is immediately cooled to 5°C (41°F) or below.
Object (Purpose):
(a) To render milk safe for human consumption by destruction of cent per cent pathogenic micro-organisms;
(b) To improve the keeping quality of milk by destruction of almost all spoilage organisms (85 to 99 per cent).
Need:
As it is difficult to exercise strict supervision over all milk supplies, it becomes necessary to pasteurize milk so as to make it safe for human consumption. Any impairment of nutritive value is of the slightest extent.
Objections:
(a) Pasteurization encourages slackening of efforts for sanitary milk production;
(b) It may be used to mask low-quality milk;
(c) It diminishes significantly the nutritive value of milk;
(d) It reduces the ‘cream line’ or ‘cream volume’;
(e) Pasteurized milk will not clot with rennet;
(f) Pasteurization may be carelessly done; it gives a false sense of security;
(g) It fails to destroy bacterial toxins in milk;
(h) In India pasteurization is not necessary, as milk is invariably boiled on receipt by the consumer.
Formulation of Standards for Pasteurization:
The following considerations were involved in the formulation of the standards of pasteurization:
(a) Bacterial Destruction:
Cent per cent for pathogens. Mycobacterium tuberculosis, being considered the most heat-resistant among pathogens, was chosen as the index organism for pasteurization. Any heat treatment (i.e., temperature-time combination), which killed T.B. germs, also destroyed all other pathogens in milk.
(b) Cream Line Reduction:
The cream line or cream volume is reduced progressively with increase in the temperature-time of heating. (The consumer judges the quality of milk on the basis of the cream line.)
(c) Phosphatase Inactivation:
The complete destruction of phosphatase by pasteurization. (The phosphatase test is used to detect inadequate pasteurization.)
Thus the standards of pasteurization were such as to ensure:
(a) Complete destruction of pathogens;
(b) Negative phosphatase test; and
(c) Least damage to the cream line.
As T.B. germs are destroyed by a heat-treatment slightly lower than that for phosphatase inactivation pasteurization is carried out at a heat-treatment temperature above that for phosphatase inactivation and yet below that for cream line reduction, as shown in Table 1.27.
Salient Remarks:
(a) Although pasteurization is now considered a health measure, it is actually a commercial expedient;
(b) Pasteurization is neither a cure-all, nor is it fool-proof;
(c) Post-pasteurization should be avoided.
Pasteurizing Process and Equipment:
The equipment for pasteurization and the needs or specifications for adequate heat-treatment of milk have been developed simultaneously. To ensure proper pasteurization with a minimum amount of equipment and controls and the least risk, holding methods or batch processes were developed.
As operations grew, it became evident that higher temperatures would reduce the necessary holding time for pasteurization; this would result in a continuous operation, and with more compact equipment less plant space would be necessary.
Principles of Heat-Exchange:
The general principles for efficient heat-exchange are:
(i) Rapid movement of film of fluids on both sides of the heat transfer surface;
(ii) Thorough and certain mixing of this film with the body of the fluids;
(iii) Use of the counter flow principle.
(iv) As great a temperature difference as possible, consistent with accurate temperature control and prevention of any deleterious effect on the product treated;
(v) The use of the least number of intermediate fluids as possible;
(vi) As thin a sheet of heat-transfer wall as possible, consistent with proper mechanical strength;
(vii) Use of metals of good conductivity.
Considerations Involved in the Heating of Milk:
(i) The problem in heating milk is to obtain quick heat transfer without imparting a cooked flavour (and also impairing the creaming properties of milk).
(ii) The rate of heat transfer is mainly dependent on- the temperature gradient, renewal of surface films, thermal conductivity of the heat-transfer wall, heat-transfer area, etc.
(iii) The temperature gradient is initially quite high, but becomes smaller as milk approaches the desired pasteurization temperature. (A gradient which is too high at the end may injure the flavour and creaming properties of milk.)
(iv) Efficient renewal of surface films hastens heat transfer, so that a high temperature gradient is unnecessary. (Renewal of surface films involves flow or agitation of two liquids, viz., milk and water, Agitation of milk is limited, as at high temperatures it may cause partial ‘homogenization’ of fat globules; while that of water is limited only by considerations of design, cost of pumping, etc.)
(v) In case of two liquids, viz., milk and water, the principle of a counter-current flow is invariably used. (Only by a counter- current flow can milk fully attain the temperature of the heating/ cooling medium; if the two are co-current, an intermediate temperature will result.)
(vi) Efficiency of renewal of surface films can eliminate use of metals of high thermal conductivity.
(vii) Provision of as large a heat-transfer area as possible contributes towards rapid heat transfer. For this purpose, the plates of the heat-exchanger are corrugated in various ways to increase their surface area. (The corrugations also give high efficiency turbulent flow.)
(viii) The heating medium is almost invariably hot water; sometimes steam under partial vacuum is also used. The hot water is circulated under pump pressure.
Methods:
(a) In-the-Bottle Pasteurization:
Bottles filled with raw milk and tightly sealed with special caps are held at 63-66°C (145-150°F) for 30 minutes. Then the bottles pass through water sprays of decreasing temperatures which cool both the product and the bottle.
Advantage:
Prevents possibility of post-pasteurization contamination.
Disadvantages:
(i) Transfers heat very slowly;
(ii) There is greater risk of bottle breakage;
(iii) Oversized bottles have to be used, to allow for milk expansion during heating;
(iv) Special types of water-tight caps have to be used.
This method is at present outdated, although in-the-bottle sterilization of milk is widely prevalent.
(b) Batch/Holding Pasteurization:
This is also called the Low- Temperature-Long-Time (LTLT) method. The milk is heated to 63°C/145°F for 30 minutes and promptly cooled to 5°C or below. In this system, heating is done indirectly; the heat moves through a metal wall into the product for heating, and out of the product for cooling.
The pasteurizer may be of three types:
(i) 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) paddles/propellors. 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.
Advantage:
Flexibility in use. (It is also known as a multipurpose or multi-process vat.)
(ii) 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.
Advantage:
The same as above.
(iii) 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).
Disadvantage:
Coils are difficult to clean, which accounts for the decline in their use.
Note:
(i) As vat heating/cooling is rather slow and involves too much agitation, causing churning and impairment of creaming properties, this operation can be efficiently carried out by Plate or Tubular Heat Exchangers,
(ii) With the vat holding system, a continuous flow of milk can be obtained with multiple vat installation; however, this may encourage the growth of thermophilic organisms over long periods.
(c) High Temperature Short Time (HTST) Pasteurization:
Introduction:
This was first developed by A. P. V. Co. in the United Kingdom in 1922. It is the modern method of pasteurizing milk and is invariably used where large volumes of milk are handled. The HTST pasteurizer gives a continuous flow of milk which is heated to 72°C (161°F) for 15 seconds and promptly cooled to 5°C or below.
Advantages:
(i) Capacity to heat-treat milk quickly and adequately, while maintaining rigid quality control over both the raw and finished product;
(ii) Less floor space required;
(iii) Lower initial cost;
(iv) Milk packaging can start as soon as pasteurization begins, thus permitting more efficient use of labour for packaging and distribution;
(v) Easily cleaned and sanitized (system adapts itself well to CIP-cleaning);
(vi) Lower operating cost (due to fullest use of regeneration);
(vii) Pasteurizing capacity can be increased at nominal cost;
(viii) Reduced milk losses;
(ix) Development of thermophiles not a problem;
(x) The process can be interrupted and quickly restarted;
(xi) Automatic precision controls ensure positive pasteurization.
Disadvantages:
(i) The system is not well-adapted to handling small quantities of several liquid milk products;
(ii) Gaskets require constant attention for possible damage and lack of sanitation;
(iii) Complete drainage is not possible (without losses exceeding those from the holder system);
(iv) Margins of safety in product sanitary control are so narrow that automatic control precision instruments are required in its operation;
(v) Pasteurization efficiency of high-thermoduric count raw milk is not as great as it is when the holder system is used;
(vi) Greater accumulation of milk-stone in the heating section (due to higher temperature of heating).
Milk Flow:
The following steps or stages are involved as milk passes through the HTST pasteurization system- balance tank; pump; regenerative heating; holding; regenerative cooling; and cooling by chill water or brine. An arrangement for incorporation of the filter/clarifier, homogenizer, etc., in the circuit is also made when desired. There is some variation in the use or order of these steps in different milk processing plants.
Functions of the Important Parts:
(i) 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 FDV.
(ii) Pump:
Either a rotary positive pump between the regenerator and heater (USA), or a centrifugal pump with a flow control device to ensure constant output, after FCBT (UK. and Europe) is used.
(iii) Plates:
The Plate Heat Exchanger (also called Para flow) 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.
These plates are supported in a press between a terminal block in each heating and cooling section. The heat moves from a warm to a cold medium through stainless steel plates. A space of approximately 3 mm. is maintained between the plates by a non- absorbent rubber gasket or seal which can be vulcanized to them.
The plates are numbered and must be properly assembled. They are tightened into place, and are so designed as to provide a uniform but not excessively turbulent flow of products with rapid heat transfer. Raised sections (corrugations) on the plates in the form of knobs, diamonds and channels, help provide the turbulent action required.
Greater capacity is secured by adding more plates. Ports are provided in appropriate places, both at the top and bottom of the plates, to permit both the product and the heating/cooling medium to flow without mixing.
(iv) Regeneration (Heating):
The (raw) cold incoming milk is partially and indirectly heated by the hot outgoing milk (milk-to- milk regeneration). This adds to the economy of the HTST process, as the incoming milk requires less heating by hot water to raise its temperature for holding.
(v) 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 one at a time. This permits continuous operation, the flow being switched from one to the other while replacing a filter.
(vi) Holding:
The holding tube or plate ensures that the milk is held for a specified time, not less than 15 seconds, at the pasteurization temperature of 72°C (161°F) or more.
(vii) 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 (i.e., in which the temperature does not reach the legal limit) is automatically diverted back to the FCBT for reprocessing. It is usually operated by air pressure working against a strong spring.
Should the temperature fall, air pressure is released and the valve snaps shut immediately. When the temperature is regained, air pressure builds up and the valve opens to forward flow. The system is so arranged that any failure of air or electricity moves the valve in the diverted position.
Note:
The flow of unpasteurized milk can also be stopped with a ‘pump stop’, which automatically stops the milk-pump motion if the product temperature drops below the desired level. When the proper temperature is reached, the pump stop restarts the operation and allows the flow of milk to continue.
(viii) Regeneration (Cooling):
The pasteurized hot outgoing milk is partially and indirectly cooled by the incoming cold milk (milk-to- milk regeneration). This again adds to the economy of the HTST process In fact, when pre-cooled (raw) milk is received, the high degree of regeneration (72 to 85%) allows water cooling to be dispensed with entirely.
(ix) Control Panel:
Contains instruments, controls, FDV- mechanism and holding system, all centralized in one moisture proof panel. The lower half of the panel forms an air-insulated chamber which carries the holding tube.
(x) Hot Water Set:
Circulates hot water through the heating section of the machine to maintain the correct milk temperature within very fine limits.
(xi) Automatic Control Devices:
These include:
a. Steam Pressure Controller:
Maintains a constant hot water temperature for heating milk accurately to the required pasteurization temperature. (Acts as a reducing valve in the steam supply line, so as to give a constant steam pressure.)
b. Water Temperature Controller:
Regulates the amount of steam entering the hot water circulating system.
c. Milk Temperature Recorder:
Records the temperature 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 sub legal temperatures.
Both the frequency and duration of the flow diversion and the temperature of milk leaving the holder are recorded on the thermograph (recording chart) by means of two separate pens. (The ‘check thermometer’ is placed near the milk temperature recorder.)
6. Pressure in the System:
The normal pressures maintained in the HTST system are:
Pasteurized milk – 15 psi
Raw milk – 14 psi
Heating/cooling medium – 12 to 13 psi
7. Holding Time Test:
The holding time of a HTST pasteurizer is the flow time of the fastest particle of milk at a prescribed temperature through the holding section. The holding time is calculated between the points at which the heated milk leaves the heating section and reaches the FDV.
The efficiency of pasteurization in the HTST system depends as much on the correct maintenance of temperature as on the holding time. Hence the latter should be checked periodically. Several methods are used for determination of the holding time, viz., the electrical conductivity method (of a salt solution); the dye injection method; the electronic timer method; etc.
(d) Vacuum Pasteurization (Vacreation):
This refers to pasteurization of milk/cream under reduced pressure by direct steam. The process was first developed in New Zealand by M/s Murray Deodorizers Ltd. who called the equipment a ‘Vacreator’ and the process ‘Vacreation’. It was designed to remove feed and other volatile flavours from cream, and to pasteurize it for butter- making.
The vacreator consists of three stainless steel chambers connected to one another for steam heating and vacuum treatment with continuous product flow. The product, in the form of fine droplets, enters the first chamber of the vacreator where pasteurization occurs. The chamber is operated under a vacuum of 5 inches Hg which maintains a temperature of 90 to 95°C, while steam, fed from the top, falls by gravity to the bottom of the chamber.
Then the product and some free steam are moved from the bottom of the first chamber to the top of the Second one. The temperature of the second chamber is maintained at 71 to 82°C under a vacuum of 15 to 20 inches Hg. A portion of the steam previously added is removed, and the product moves down through the chamber.
Some of the tainting substances and off-flavours are removed by heat and vacuum treatment. The product then moves on to the third chamber at 43°C by maintaining a vacuum of 26 to 28 inches Hg, and here more water and off-flavours are removed. A multi-stage centrifugal pump removes the product from the third chamber.
Altogether, it takes the product about 10 seconds to move continuously through the unit. (Steam of uniform pressure, normally ensured by a uniform pressure reducing and regulating valve, should be provided. It must not contain any boiler compounds which may contaminate the product.)
(e) Stassanization:
This method of pasteurization is carried out in a tubular heat exchanger consisting of three concentric tubes. It was invented in France by Henri Stassano and is used to a considerable extent in French, Danish, Italian and other milk plants.
The principle of its operation is the heating of milk to the desired temperature by passing it between two water- heated pipes through the narrow space of 0.6 to 0.8 mm. The milk is heated to about 74°C (165°F) for 7 seconds and then promptly cooled as usual.
(f) Ultra-High Temperature Pasteurization:
Ultra-high temperature (UHT) pasteurization was developed in the 1950s. This usually encompasses temperature-time combinations of 135 to 150°C (275 to 302°F) for no-hold (a fraction of a second). The success of UHT heat-treatment of milk depends on immediate aseptic packaging.
(g) Flash pasteurization:
In earlier days, this term was used for what is today called HTST.
(h) Uperization:
This is itself a shortened form of the word ‘Ultra-pasteurization’, which has been developed in Switzerland. In this process milk is heated with direct steam up to 150°C (302°F) for a fraction of a second. The process is continuous.
The method of manufacture of uperized milk is as follows- high quality raw milk is immediately clarified on receipt, then chilled and stored in tanks. In the first part of the uperization process, the milk is fore warmed to 50°C and de-aerated to remove most of the dissolved oxygen and volatile off-flavours by vacuum treatment.
In the second part, the milk is first pre-heated to about 80-90°C and then heated in the uperization chamber with high pressure steam to around 150°C for ⅓ to ¾ second. After this heating, the product moves into an expansion chamber at near atmospheric pressure, thereby forcing some evaporation of moisture. The product is then moved to a cooler and then into storage.
The advantages claimed for uperized milk are:
(i) Long keeping quality;
(ii) Removal of feed and other volatile off-flavours;
(iii) Appreciable homogenization effect;
(iv) Reduction in acidity;
(v) Efficient destruction of micro-organisms;
(vi) Effect of uperization on nutritive value and flavour no greater than that of pasteurization.
(i) Alternatives to Pasteurization:
Various types of treatment have been proposed from time to time as alternatives to pasteurization.
These include:
(a) The Hofius process;
(b) Electronic heating;
(c) Ultra-violet irradiation;
(d) Ultra-sonic vibration, etc.