Preservation of Milk by Pasteurization & its storage; Methods of Pasteurization – LTH, HTST, UHT

Preservation of Milk by Pasteurization & its storage

a.  Methods of Pasteurization – LTH, HTST, UHT 

b.  Storage specifications after pasteurization 

c.  Phosphatase test and its significance 

Methods of Pasteurization – LTH, HTST, UHT

History:      

The term  pasteurization has been coined after the name of Louis Paster of France, who in 1860-18 demonstrated that heating wine at a temperature between 122 to 140⁰ F ( 50 to 60⁰ C) killed the spoilage organisms and helped in its preservation . 

The application of this process gave rise to the new term ‘Pasteurization’ which soon become current in technical language . 

Although Louis Paster pioneered studies on heat-treatment for preservation, pasteurization of milk was first attributed to Dr Soxhlet of Germany in 1886.

B.     Defination:

The  term pasteurization , as applied to market milk today, refers to the process of heating every particles of  milk to at least 63⁰ C (145⁰F) for 30 minutes or 72⁰C(161⁰F) for 15 seconds (or to any temperature-time combination which is equally efficient), in approved and operated equipment . After pasteurization , the milk is immediately cooled to 5 ⁰ C ( 41⁰ F ) or below.

C.     Objects ( purpose): 

(a) To render milk safe for human consumption by destruction of per cent pathogenic micro-orgnism.

 (b) To improve the keeping quality of milk by destruction of almost all spoilage organism (85 to 99 %).

 

D.    Need: 

As it is difficult to exercise strict supervision over all milk supplies, it become necessary to pasteurize milk so as to make it safe for human consumption.

 Any impairment of nutritive vale is of the slightest extent.

E.   Objections:

(a)    Pasteurization encourage 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 reduce the ‘cream line’ or ‘cream volume’

(e)   Pasteurization 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 boilrd on receipt by the consumer.

 

   Formulation of standards:   The following considerations were involved in the formulation of standards of pasteurization :

(a)    Bacterial destruction :  Cent per cent pathogen. Mycobacterium tuberculosis, being considered the most heat-resistance among pathogen, was chosen as the index organism for pasteurization . Any heat treatment (i.s. temperature –time commbination) , which killed T.B germs, also destroyed all other pathogen in milk.

(b)   Cream line reduction:  The creamline or cream volume is reduced progressively with increased in the temperature –time of heating

(c)    Phosphatase inactivation: The complet destruction of phosphatase by pasteurization ( The phosphatase test is used to detect inadequate pasteurization  ).

Thus the standard 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 .

G.    Salient remark:

(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.

 

H.     Pasteurizing  process and equipment:

1.Introduction:

The equipment for pasteurization and the needs or specifications for adequate heat-treatment of milk have been developed significantly. 

To ensure proper pasteurization with a minimum amount of equipment and control and the least risk, holding methods or batch processes were developed. 

As operation grew , it become evident that high temperature would reduce the necessary operation, and with more compact equipment less plant space would be necessary.

 

2. Principles of heat-exchange :

The general principles for efficient heat-exchange are:

(a)    Rapid movement of film of fluids on both sides of the heat transfer surface.

(b)   Thorough and certain mixing of this flim with the body of the fluid.

(c)    Use of the counterflow principle.

(d)   As  great a temperature difference as possible, consistent with accurate temperature control and prevention of any deleterious effect on the product treatment.

(e)   The usef of the least number of intermediates fluids as possible,

(f)     As thin a sheet of heat-transfer wall as possible , consistent with proper mechanical strength.

(g)    Use of metals of good conductivity

 

3.Consideration involvement in the heating of milk:

(a)  The problem in heating milk is to obtain quick heat transfer without imparting a cooked flavor .

(b) 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.

( c) The temperature gradient is initially quit high , but becomes smaller as milk approaches the desired pasteurization temperature.

4. Methods :

 1. HTST: High temperature  Short Time (HTST) Pasteurization.

1.Introduction:

This was first developed by A.P.V.Co. in the United Kingdom in 1922. 

is the modern method of pasteurizing milk and is invariably used where large volume of milk are handled. 

HTST pasteurization gives a continuous flow of milk which is heated to 72 ⁰ C (161 ⁰ F) for 15 seconds and promptly cooled to 5 ⁰ C or below.

2. Advantages:

(i)      Capacity to heat –treat milk quickly and adequately , while maintaining right 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 distrubition.

(v)    Easily cleaned and sanitized

(vi)  Lower operating cost.

(vii) Pasteurizing capacity can be increased at normal  cost.

(viii)   Reduced milk losses.

(ix)  Development of thermophiles not a problem.

(x)    The process can be interrupted and quickly restarted.

(xi)  Automatic precision control ensure positive pasteurization.

3. Disadvantages:

(i)       The system is not well- adapted to handling small quantities of several liquid milk product.

(ii)    Gaskets require constant attention for possible damage and lack of sanitation.

(iii)   Complete drainage is not possible.

(iv)  Margins of safety in product sanitary control are so narrow that automatic control precision instruments are required in its operation.

(v)    Pasteurization efficiently of high- thermoduric count raw milk is not aas great as it is when the holder system is used.

(vi)  Greater accumulation of milk- stone in the heating section .

4. Milk flow: The following steps or stages are involved as milk passes through the HTST pasteurization  system:

Balance tank; regenerative heating; holding; regenerative cooling; and cooling by chill water or brine. An  arrangement for incorporationof the filter/clarifer , homogenizer , etc. in the circuit is also made desired . There is some variation in the use or order of these steps in different milk processing plants.

 

 Construction of holding pasteurization:

There are several different type's of holding pasteurization on the market.

These machines differ from one another primarily in their shape,general construction, and the method employed in agitating the Milk or cream during the heating and cooling process.

(1) Coil Vats.:

The most common type of holding pasteurizer has been the coil vat.

This machine consists of a rectangular or square tank constructed of tinned copper, glass-lined steel, or stainless steel.

The sides and the bottom are insulated with Cork or other insulating material which is encased in a wooden or a metal covering.

A lid of the same construction is provided to cover the entire tank.

A coil is usually constructed of the same material as the tank and consists of hollow tubing, 2 to 4 inches in diameter, and encircles spirally a central shaft.

The shaft is set on a line representing the center of the tank from end to end.

The coiled tubing is placed at a fixed distance from the shaft in such a manner as to carry the coil to a point within 2 or 3  inches of the sides of the tank.

The coil is connected directly with the shaft at both end's so that water may enter from the outside of the vat through the shaft, into the coil and out of the vat through the shaft at the opposite end.

By revolving the shaft by means of the power-drive gearing at the end of the pasteurizer, the Milk or cream can be agitated and by passing hot or cold water through the coil, the milk or cream may be heated or cooled.

A relatively small tank is provided, generally at the end of the vat, to hold the water used in heating and cooling.

The square type of pasteurizer is constructed in much the same manner as the rectangular vat except that the coil is suspended vertically in the tank.

(2) Glass- lined tanks:

Another type of holding pasteurizer is a kettle-shaped tank constructed of steel, which is enameled with glass on the inside.

Surrounding the glass- lined tank is a second tank of steel with an air space between the two.

The entire vat is often covered with an insulating material.

Milk is agitated in the vat by means of a propeller set at one side and near the bottom of the tank.

Milk or cream is heated  by passing hot water or live steam into the space between the two tanks.

At the same time, the propeller is kept in motion to stir the Milk.

Cooling is very slow in such tanks and for this reason a surface or tubular cooler generally is used in connection with this type of pasteurizer.

(3) Stainless-steel  Tanks:

With the development of chromium- nickel-steel alloys, holding tanks have been placed on the market in a great variety of design.

Some types of agitator is suspended from the cover as show in figure.

The heating or cooling medium circulates in various ways in the space between the outer and inner jackets.

The resistance of these alloys to corrosion or attack by dairy products has made them extremely popular in the Milk industry in other equipment as well as holders used in the pasteurization process.

 (4) Spray Vats:

Still another types of holding pasteurizer is known as the “spray pasteurizer”.

 This types of holder usually consists of a long, deep, doubled-walled tank, the general construction of which, in some instance, is somewhat similar to that of the coil vats.

The milk or cream in this types of pasteurizer is agitated very gently by slowing moving paddles which are suspended vertically in the tank containing the milk.

Heating or cooling is accomplished in this type of pasteurizer by circulating hot, or cold water through spraying devices against the side walls of the inner vat by means of a pump.

Construction of  High-temperature Short-time Pasteurization

There are several types of high-temperature short time, flash or continuous pasteurizer in the market.

These machines are constructed of heavy copper, tinned copper, or stainless steel.

The common types in use are the Danish style, The film types, The internal-tubular or external- tubular (surface), and the plate- type pasteurizer.

The latter types are often called “heat exchangers”and may involve regeneration.

(1). The Danish Heater:

The Danish heater consists of a double-walled cylinder placed on end, with a height about double the diameter.

The milk or cream passes continuously through the inner compartment while the heating medium, live stream or hot water, is allowed to enter the compartment between the two walls.

A rapidly revolving paddle keeps the milk or cream in  a thin layer against the heated wall of the inner compartment.

The time required for a particular portion of milk or cream to pass through the machine varies from a fraction of a minute to not more than 2 minutes.

A high temperature is ordinarily employed.

(2). Drum or Film heater:

This types of flash pasteurizer consists of two drums, on within the other, which are heated by means of stream or hot water.

Milk or cream to be heated passes in a thin film between the heated surface and thus is raised very quickly to the desired temperature.

 

(3). Internal-tubular Heater:

This consists of a series of two tubes, one within the other.

Milk flows through the inner tube while steam or hot water is introduced into the outer tube which surrounds the inner tube.

In this manner milk or cream flowing through the inner tube is heated to a desired temperature.

 

(4). External-tubular (surface) Heater:

This equipment usually involved the passage of milk or cream over the surface of the tubes with the heating medium flowing through the inside of the tube.

Either internal-tubular or external-tubular pasteurizer may also serve as regenerators with the milk or cream passing both inside and outside of the tubes but in opposite directions, thus providing for mutual heating and cooling.

 

(5). Plate- type Pasteurizer:

For a number of years the plate type of heat exchanger has been used for heating or cooling liquids.

Recently this system has been employed in connection with the pasteurization of milk and creams .

European employed pasteurization of milk and cream .

Europeans dairies employed these types of flash pasteurizers some time before they were accepted in America.

The plate pasteurizer consists of a compact group of relatively thin metal plates that are held tightly together in a press.

The plates are so designed as to permit the flow in opposite direction  of milk and water in a thin film between two plates.

Milk is brought to the pasteurization temperature in a few seconds.

Additional plates permits the cooling of milk very quickly.

By adding still more plates a regenerative system is included, which employs hot milk ,thus making very efficient use of the heat employed the process.

By almost instant heating and cooling the creaming ability and original flavour of milk are Process 

Flow diagram of High temperature  Short Time (HTST) :

Milk chilling

Pre-heating (regeneration) and Standardization Stage

Clarification stage

Standardization stage

Homogenization stage

Heating section

Holding section

Cooling/chilling section

 Steps of Pasteurization:

1. Milk chilling

• Chilling is not a pasteurization process but it is a necessary step when dealing with large volumes of milk.
• Milk leaves the cow’s udder at temperatures above the ambient, which encourages rapid bacterial multiplication that speeds up spoilage.
• However, reducing the temperatures to between 2° C to 5° C arrests bacterial growth and metabolism.
• This provides a head start at keeping the quality before proper pasteurization commences.

2. Pre-heating (regeneration) and Standardization Stage

• After bulking, the chilled milk is heated to about 40°C to facilitate easy separation of butterfat during standardization.
• The system uses regenerative heating, i.e., it uses the heat of the already pasteurized milk to heat up the incoming chilled milk. The chilled milk, in a counter current flow, cools down the pasteurized milk.
• The purpose of standardization is to obtain a product with uniform content of butter fat. 

3. Clarification stage

• Clarification is essential for removing all foreign matter from the product.
• Large solid particles are removed by straining the milk through tubular metallic filters.
• A centrifugal clarifier (not the one used for standardization) is used to remove all soil and sediments from milk.
• The filters, usually fitted in parallel twins permits continuous processing as one can be cleaned while the other is running.
• Clean the filters regularly (between 2 to 10 operational hours depending on the level dirt) to avoid growth of bacteria.

4. Standardization stage

• It is important to standardize milk fat to ensure that you end up with a product of consistent quality in the market. Different consumers prefer different products.
• There are customers who will consume skim milk only while there are those who will take low fat milk. There are those who will take standardized milk while there are those who prefer high fat milk.
• Standardization is necessary to ensure that all the customers are catered for. Again, it is during the process of standardization that you get to separate the butterfat that is used for making cream and other fat based products such as butter and ghee.

5. Homogenization stage

• Homogenization is a physical process of breaking down the the milk fat globules into tiny droplets to discourage cream separation.
• Tiny droplets of fat do not rise in a milk column since reducing their sizes also increases their density in the milk.
• A milk homogenize working at between 100 to 170 bars splits all the fat globules into very tiny droplets that increases the level of integration of the fat in the milk.
• As a result, the milk fat remains uniformly distributed in the milk.

6. Heating section

• Utilizes heat from steam to raise the temperatures of the milk from about 60°C to the required 72°C that is effective to kill the Clostridium botulinum spores.
• The steam exchanges heat with the milk across the PHE plates in a counter current motion.
• At the end if this section, there is a temperature sensor, which controls the flow diversion valve.
• Any milk that does not attain the required temperature is diverted back to the heating section until it attains the required temperatures.

7. Holding section

• After heating, milk flows into the holding tubes whose lengths have been calibrated with the milk flow rate to ensure that milk takes at least 16 seconds in the tubes. All the milk must maintain the required pasteurization temperatures at the end of the tubes.
• In case of a breach, a sensor will trigger the flow diversion valve to take the milk back to the heating section to bring the milk to the required temperature.
• Once the milk has attained the required temperatures at the end of the holding tubes, milk flows back to the regeneration section to heat the incoming chilled milk while in itself being cooled down to about 30°C.

8. Cooling/chilling section

• After regenerative cooling of pasteurized milk, it moves to the cooling section of the PHE where chilled water/PHE coolant lowers the temperature of pasteurized milk to 4°C.
• The chilled milk is then pumped to the packaging machines for aseptic packaging and subsequent storage in the cold room.

 

 

2.UHT : 

Ultra –high temperature (UTH) pasteurization was developed in the 1950s. 

This usually encompasses temperature- time combination of 135 to 150⁰ C ( 275 to 302 ⁰ F ) for no-hold ( A fraction of a second ). 

The success of UTH heat-treatment of milk depends on immediate aseptic packaging.

This process involves heating the milk using commercially sterile equipment and filling it under aseptic conditions into hermetically sealed packaging. 

The milk must be heated to 280 °F (138 °C) for at least two seconds, then rapidly cooling it down.

 UHT kills more bacteria (good and bad) and gives it a much longer shelf life

 

3. LTH : 

low temperature holding (LTH) method in which milk is heated to 62.8°C (145F) for 30 minutes in commercial pasteurizers (or) large closed vats which are heated by steam coils, hot water jackets etc

 

 References:

Clarence Henry Eckles, Willes Barnes Combs, Harold Macy (1943). Milk and milk products, 4th Ed. McGraw-Hill book Company, Incorporated.

Sukumar. De (2001). Outlines of Dairy Technology. 1st Ed. Oxford University Press Delhi.

b.  Storage specifications after pasteurization 

Increasing raw milk storage time prior to pasteurization may affect product shelf-life. Raw milk was stored at 4.5°C for 0, 2, 4, and 6 days prior to pasteurizing.

 Milk samples from each pasteurized lot were analyzed after continuous storage at 4.5°C for 0, 4, 8, 12, 16, and 20 days.

 Both raw and pasteurized samples were analyzed for coliforms, psychrotrophs, and total bacteria counts. Flavor scores were also determined.

 No correlations were significant between raw or pasteurized samples and total bacteria or coliform counts. Related were flavor score and days held raw, shelf-life of the resulting pasteurized product, and interaction of days held raw and shelf-Iife of the pasteurized product. 

Psychrotrophic counts and age of the raw milk were correlated. 

There is continuing interest among processors to extend the shelf-life of milk and milk products by higher processing temperatures and preventing recontamination by psychrotrophic bacteria after pasteurization

Psychrotrophic bacteria are difficult to exclude from raw milk and may grow and produce proteolytic enzymes during refrigerated storage. Many of these protease enzymes are heat-stable and survive pasteurization temperatures.

Hence, if sufficient heat-stable enzymes are in raw milk prior to pasteurization, defects may become more pronounced after pasteurization during refrigerated storage. Psychrotrophs isolated from raw milk produced proteases that survived 149°C for 10 s. Seventy to 90% of raw milks contained psychrotrophs capable of producing heat resistance proteases  

Reference:

https://www.sciencedirect.com/science/article/pii/S0022030282824910

c.  Phosphatase test and its significance 

Pasteurisation is an essential process in the production of milk which is safe and free from pathogens.

 Alkaline Phosphatase is an enzyme which is naturally present in milk, but is destroyed at a temperature just near to the pasteurization temperature. 

Alkaline Phosphatase test is used to indicate whether milk has been adequately pasteurised or whether it has been contaminated with raw milk after pasteurisation.

 This test is based on the principle that the alkaline phosphatase enzyme in raw milk liberates phenol from a disodium para-nitro phenyl phosphate and forms a yellow coloured complex at alkaline pH . 

The intensity of yellow colour produced is proportional to the activity of the enzyme.

 The colour intensity is measured by direct comparison with standard colour discs in a Lovibond comparator. 

The test is not applicable to sour milk and milk preserved with chemical preservatives.

Apparatus required

1.    Water-Bath -maintained at 37±l⁰C, thermostatically controlled.

2.    Comparator - with special discs of standard colour glasses calibrated in µg p-nitrophenol per ml milk, and 2 x 25 mm cells.

3.    Test Tubes - of size 16 x 1.50 mm and rubber stoppers to fit.

4.    Pipettes - 1, 5, and 10 ml.

5.    Filter Paper - Whatman No. 2 or equivalent.

6.    Litmus Paper

Reagants

1.    Sodium Carbonate-Bicarbonate Buffer - Dissolve 3.5 g of anhydrous sodium carbonate and 1.5 g of sodium bicarbonate in one litre of distilled water.

2.    Buffer Substrate - Dissolve 1.5 g of disodium p-nitrophenyl phosphate in one litre of sodium carbonate-bicarbonate buffer. This solution is stable if stored in a refrigerator at 4°C or less for one month but a colour control test should be carried out on such stored solutions

 

Procedure

1.    Pipette 5 ml of buffer substrate into a clean, dry test tube followed by 1 ml of the milk to be tested. Stopper the tube, mix by inversion and place in the water-bath

2.    At the same time place in the water-bath a control tube containing 5 ml of the buffer substrate and 1 ml of boiled milk of the same kind as that under test that is pasteurized homogenized, low fat.

3.    After 2 hours, remove the tubes from the bath, invert each and read the colour developed using the comparator and special disc, the tube containing the boiled milk control being placed on the left of the stand and the tube containing the sample under test on the right. Record readings which lie between two standard colour discs by adding a plus (+) or minus (-) sign to the figure of the nearest standard.

Reference : 

https://www.dairyknowledge.in/content/alkaline-phosphatase-test-pasteurized-milk#:~:text=Pasteurisation%20is%20an%20essential%20process,safe%20and%20free%20from%20pathogens.&text=Alkaline%20Phosphatase%20test%20is%20used,with%20raw%20milk%20after%20pasteurisation.