By Sabbu Sangeeta & Sweta Rai
The pasteurized or market milk refers to fluid whole milk that is sold to individuals usually for direct consumption.
CONSTITUENTS OF MILK
i. Major constituents:It includes water, fat, protein, lactose and ash/mineral matters.
ii. Minor constituents:it includes phospholipids, sterols, vitamins, enzyme and pigments etc.
iii. True constituents: it includes milk fat, casein and lactose.
PROCESSING OF PASTEURIZED MILK
A. Buying and collection of milk
- Various methods for buying milk are employed, singly or in combination and payment based on weight or volume, the fat content of milk, use, quality (flavour, colour, sediment test, bacterial count etc) and cost of production of milk.
- In almost all countries, the production of milk is confined to rural areas, while demand is mostly urban in nature. Hence milk has to be collected and transported from the production area to processing and distribution point. Milk can be collected from a different co-operative organization, through contractors and by individual producers.
B. Cooling and transportation of milk
- Milk should be cooled to below 5ºC at farm or at chilling centre because common milk micro-organisms grow best between 20 to 40ºC, so prompt cooling is required and it is essential for safe transportation. Different methods can be employed for cooling of milk such as can-immersion method, surface cooler and bulk-tank cooler etc.
- Milk should be transported from chilling centre to processing unit in morning or in evening hours. Mode of transport depends upon the carrying load, the distance of collection and local conditions. Refrigerated vehicles should be used to prevent the increase in acidity and microbial content of milk.
C. Receiving of milk
When milk is received at the milk plant/dairy, it should be at free from off-flavours and extraneous matter. Milk should be at 5ºC or below with clean, sweet and pleasant flavour. Several procedures are following at receiving plate-form as enlisted below:
i. Unloading:The milk cans are unloaded manually.
ii. Grading:The principle of grading is based on different platform test such as organoleptic tests (odour, taste, appearance and touch), acidity, sediment, lactometer reading etc.
iii. Sampling: It is done for chemical and microbial test of milk. Samples may be individual, composite (mixture of two or more individual lots of milk), drip (representing the entire day’s supply) etc.
iv. Weighing:This is the essential step in accounting for milk receipts and disposal, making payments for milk, etc. The milk-in-cans is dumped into the weigh tank, either manually or mechanically. The milk can be measured by weight or by volume.
v. Testing: Apart from initially accepted/rejected lots of milk, there is always some of doubtful quality. All the accepted lots have already been properly sampled; these, together with samples of the remaining two categories, have to be tested in the quality control laboratory for the final verdict of acceptance/rejection. Further, a record of the chemical and bacteriological quality of all accepted milk has to be maintained for making payments, etc.
- The milk is pre-heating for efficient filtration/clarification. As the temperature of the milk increases, the viscosity decreases and more efficient filtration/clarification results. The usual temperature of pre-heating is 35-40ºC.
- It is done to improve the aesthetic quality of milk by removing visible foreign matter. In filtration remove the suspended matter by straining process while in clarification remove the same by centrifugal sedimentation.
F. Cooling and storage of raw milk
- As soon as milk is received in the plant, it is chilled to 5ºC or below and stored cool till used, to prevent deterioration in its bacteriological quality during the interim period. Different methods such as plate cooler, surface cooler, internal tubular cooler, jacketed vat tanks can be employed for cooling purpose.
- Morden milk plants hold both raw and pasteurized milk for a much longer period. Normally the milk storage capacity is equal to one day’s intake. This allows a more nearly uniform work-day for processing and bottling operation with less dependence on time for receiving raw milk.
- It refers to the adjustment of fat and/or solid-not-fat (SNF) percentage of milk to a desired value according to legal standards.
- Different milk which is available in market has different fat and SNF ratio such as 3% fat and 8.5% SNF of tonned milk etc.
It is the process of heating below 100 ºC to improve the quality of milk by destruction of almost all spoilage organisms followed by prompt cooling to 5ºC. Pasteurization of milk was first attributed by Dr. Soxhlet of Germany in 1886. Different methods are used for pasteurizing the milk as described below. Index micro-organism of milk pasteurization is Coxiella burnetii and index enzyme is alkaline phosphatase.
a) In-bottle-pasteurization:Bottles filled with raw milk and tightly sealed with special caps are held at 63-66 ºC for 30 minutes. Then the bottles pass through water sprays of decreasing temperatures which cool both the product and the bottle. This method prevents the possibility of post-pasteurization contamination. This method has some loophole such as slow method, greater risk of bottle breakage, required high-quality water tight caps etc.
b) Batch/Holding pasteurization/Low-Temperature-Long-Time (LTLT):The milk is heated to 63ºC for 30 min and promptly cooled to 5 ºC or below. Heating is done indirectly through a metal wall into the product. The pasteurizer may be of different types like water-jacketed vat, water-spray type and coil-vat type used in this method. This method is rather slow and involves too much agitation, causing churning and impairment of creaming properties.
c) High-Temperature-Short-Time Pasteurization (HTST)/ Flash Pasteurization:This method was first introduced by A.P.V.Co. in the United Kingdom in 1922. It gives the continuous flow of milk which is heated to 72 ºC for 15 seconds and promptly cooled to 5 ºC or below. This method has many advantages as listed below:
- It is a quick and continuous method.
- It has a low initial and operating cost.
- It required less floor space area.
- Milk packaging can start as soon as pasteurization begins
- Easily clean and sanitized (CIP-cleaning)
- Pasteurization capacity can be increased at minimal cost
- Reduce milk losses
- Development of thermophiles is not a problem
- This process can be interrupted and restarted quickly
- Automatic precision controls ensure positive pasteurization.
This method has some limitation as listed below:
- The system is not well-adopted to handling small quantities of liquid milk.
- Gaskets require constant attention for possible damage and lack of sanitation
- Greater accumulation of milk stone in the heating system
- Greater attention towards the cleaning and sanitization
Process of HTST
Fig 1 depicts the flow process of pasteurization by HTST method. It includes:
i. Dump Tank:It contains a known volume of raw milk for processing.
ii. Flow-Control-Balance Tank:Maintains a constant head of milk for feeding the raw milk pump and also receives any unpasteurized milk diverted by FDV.
iii. Pump: Centrifugal pump with flow control device can be used to ensure constant input and output.
iv. Plates:The plate heat exchanger (also called para flow) is commonly used in the HTST system 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. 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 absorbant rubber gasket or seal.
v. 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 incoming milk requires less heating by hot water to raise its temperature for holding.
vi. Filter and Homogenization: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 sieves, are usually cylindrical in shape. Milk is homogenized to reduce cream separation and uniformly distribution of fat throughout the milk.
vii. Holding: the holding tube or plate ensure that the milk is held for a specific time, not less than 15 seconds at the pasteurization temperature of 72 ºC (161 ºF).
viii. Flow Diversion Valve (FDV): this routes the milk after heat treatment. If the milk has been properly pasteuri-zed, it flows forward through the unit. If the milk is unpasteurized (means milk temperature does not reach to 72 ºC) it automatically diverted back to flow control balance tank.
ix. 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.
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 includes steam pressure controller, milk temperature recorder etc. to ensure proper pasteurization.
xii. Pressure in the System: The normal pressures maintained in the system are:
- Pasteurized milk – 15 psi
- Raw milk – 14 psi
- Heating/cooling medium – 12-13 psi
xiii. Holding test time: Several methods are used for determination of the holding time such as electrical conductivity method, dye injection method, electronic timer method etc.
d) Electric Pasteurization
e) Vacuum Pasteurization: Milk is pasteurized under reduced pressure by direct steam. This process is also called ‘Vacreation’.
f) Stassanization: In this method, pasteurization is carried out in a tubular heat exchanger. Milk is heated to the desired temperature by passing it between two water heated pipes through narrow space of 0.6-0.8 mm. The milk is heated to about 74 ºC for 7 seconds and immediately cooled.
g)Ultra-High-Temperature pasteurization: The milk is heated to about 135-150 ºC for a fraction of second (no-hold). This method required immediate aseptic packaging.
h) Uperization/ Ultra-pasteurization: In this method milk is heated with direct steam up to 150 ºC for a fraction of second.
I. Bottling / Packaging: The pasteurized and cooled milk is immediately bottled /packaged to protect the milk against contamination, loss, damage or degradation due to micro-organisms, exposure to heat, light, moisture and oxygen etc.