PASTEURISATION OF MILK

Pasteurization

• Pasteurization is one of the most important heat treatment processes. The term as applied to market milk refers to the process of heating every particle of milk to a temperature of at least 63°C (145.4°F) for 30 minutes or 71.7°C (161°F) for 15 seconds (or to the temp-time combination which is equally efficient) in properly designed equipment. Milk is immediately cooled to 4°C and stored in cold storage maintained at 4°+1°C.
• Three aspects emerging from the definition are:(i) level and degree of heat treatment, (ii) minimum chemical, physical and organoleptic changes, and (iii) minimum health hazards.

i. Time-Temperature Combination

The time-temperature combinations normally used for pasteurization of fluid milk are as follows:

•  63°C (145.4°F) and held at that temperature for at least 30 minutes
•  72°C (161.6°F) and held at that temperature for at least 15 seconds.

The milk is then immediately cooled to a temperature not greater than 4°C. The selected heat treatment shall be applied only once. This means pasteurization includes heating to a specific time-temperature combination followed by immediate cooling to 4°C.

 

ii. Purpose

Milk is pasteurized for two purposes:

•  To make safe for human consumption by destroying pathogenic microorganisms present in milk.
•  To improve its keeping quality.

The most heat resistant pathogenic organism at pasteurization temperature is the Mycobacterium tuberculosis and hence this has been made as an index organism to achieve complete safety of milk. Any heat treatment, which will destroy this organism, can be relied upon to destroy all other pathogenic organisms as well as other organisms involved in milk spoilage. Some bacteria, call thermodurics (heat resisting) may survive during pasteurization but immediately cooling slows down their growth and thus prevents them causing spoilage such as flavour taint or souring. Although, the main purpose of heat treatment is to destroy all microorganisms capable of causing disease in humans but pasteurization has two additional benefits,i.e. the destruction of a large number of spoilage microorganisms present in raw milk and deactivation of some natural enzymes like lipases, which can adversely affect the quality of manufactured products, i.e. lipolysis or breakdown of fat into glycerol and free fatty acid. However, we must be clear that pasteurization is not a substitute for cleanliness during milk production. The pasteurization process should only be applied to raw milk obtained from healthy cow, which is clean, sweet and has a low bacterial count.

Types of Heat Treatment

The heat treatment given in form of (i) holding and (ii) continuous correspondingly relate with two methods of pasteurization i.e.

• Batch, holding or Low Temperature Long Time (LTLT) method and
•  Continuous, High Temperature Short Time (HTST) method.

In the batch method, the milk is heated to 63°C in a tank or vat equipped with a hot water or steam jacket and agitators to keep the milk agitated; held for 30 minutes and then partly cooled in the batch pasteurizer. The further cooling is done by surface/plate cooler. This method is mostly used for processing of around 5000 liters of milk.

High Temperature-Short Time (HTST) pasteurization is the process, which is commonly used now a day all over the world. Plate Heat Exchanger (PHE) is used to heat, hold and cool the milk. Milk is heated to a temperature of at least 72°C and held at that temperature for not less than 15 seconds and then immediately cooled to a temperature not greater than 4°C.

Batch Pasteurizer

The parts of a typical batch pasteurizer are following:

•  Insulated outer casing
•  Insulated hinged cover
•  Stainless steel inner vessel
•  Agitator and its motor
•  Outlet cock and heating water distribution pipe.

Batch Pasteurizer

This system is well suited for small-scale operation, where less than 3000 to 5000 litres of milk are available. The vat may be rectangular, but a vertical, cylindrical design is preferred for practical reasons. The vat normally consists of an inner vessel, surrounded by an insulated outer casting, thus forming a jacket, through which hot water or steam is passed (Figure 5.1). After the milk has reached the required temperature (63.0°C), it is usually held at that temperature for a certain fixed period (30 minutes). Thereafter, it is cooled as quickly as possible either by circulating refrigerant/chilled water or through plate/surface chiller. Cooling the milk after pasteurization by circulating a refrigerant – in most cases cold water through the jacket or the vat may take much time. Therefore, a separate small-capacity surface, tubular or plate cooler may be used to rapidly cool the milk to the required temperature. This system also has the advantage that the vat will be available sooner for the pasteurization of another batch of milk.

Batch pasteurizers have a small heating surface area relative to their contents.Heat transfer is greatly improved by agitating the milk. Agitators of different design are used for this purpose. They may even consist of double-walled paddles or other devices with internal steam or water circulation. Care must be taken to avoid foam formation during filling of vat. It is very difficult to heat the milk and foam together uniformly and consequently microorganisms present in the foam may survive pasteurization. If the inlet valve is at the bottom of the vat, foam formation can easily be prevented. A lid or cover on top of the vat promotes a uniform temperature of the contents and prevents skin formation on the milk.

HTST Pasteurizer

The HTST system is the most common method used by the dairy plants for pasteurization of milk. The main advantage of HTST pasteurization is its capacity to heat treat milk quickly and adequately with built-in safeguards that prevent improper pasteurization due to under heating of milk. The HTST system employs plate heat exchangers for heating, regeneration and cooling. The system consists of feed pump, plate heat exchanger, holding section, flow diversion valve,instrumentation, essential services and piping system. The entire process is automatic and is ideal for handling of 5000 litres per hour (lph) or higher quantity of milk.

i. Flow diagram of pasteurization process

The schematic flow diagram of HTST pasteurization is given in below Figure.Raw milk enters the constant heat tank (balance tank), passes to the milk pump and then through a flow controller to the plate heat exchanger. The plate heat exchanger consists of regeneration section, heating, holding and cooling sections.

Flow Diagram of Pasteurization

The raw milk enters the pre-heating (regeneration section), where hot pasteurized milk (72°C) flows counter current to the raw cold milk, within adjacent plates,transferring heat for pre-heating of raw milk and pre-cooling of pasteurizing milk resulting in energy saving. The partially heated raw milk passes through a filter or clarifier and homogenizer. It then enters the heating section where it is heated to at least 72°C. The hot milk then passes through the holding section to ensure that the fastest moving particles of milk are held at 72°C for at least 15 seconds.

The flow diversion valve diverts the milk to constant head tank if it is not properly heated to pasteurization temperature. Properly pasteurized milk passes forward through the flow diversion valve into the regeneration section where it is cooled by incoming cold raw milk passing in the opposite direction on the other side of the plates. Milk enters the cooling section and is cooled at 4°C before storage.

An indicating thermometer situated at the outlet of the holding section measures the temperature of the hot milk and this is recorded on a revolving thermograph. If the temperature of the milk falls below 72°C, the hot milk-recording pen drops past the set pointer on the thermograph and this activates the flow diversion value, the safe-guard pen and an alarm bell. The flow diversion valve diverts the unheated milk into the constant head tank for re-circulation until the milk reaches the correcting temperature.

 

ii. Components of a HTST Pasteurization Plant

The complete pasteurizer plant consist of:

•  Constant head tank
•  Milk feed pump
•  Flow controller
•  Filters
•  Clarifier
•  Homogenizer
•  Plate heat exchanger consisting of bank of plates compartmentalized into regeneration, heating, holding and cooling sections,
•  Flow diversion valve
•  Instruments associated with indicating controlling and/or recorded functions,
•  Systems for providing steam, air, water, heating and cooling arrangements, and
•  Piping system to link various components

 iii. Plate Heat Exchanger (PHE)

The Plate Heat Exchanger consists of a bank of plates inter-connected (sections) held in a rigid frame (figure). The main function of the PHE is the exchange or transfer of heat from a hot liquid (hot water or hot pasteurized milk) to a cooler one (cold water, chilled water brine or raw milk) across a metal plate. Let us see how the heat is transferred through plates.

Plates: The plates are thin stainless steel sheets usually rectangular in shape. The plates are corrugated and cause a turbulent flow, which increases rate of heat exchange. The rate of heat exchange also depends on the surface area of the plate,the thickness and type of metal used in the plates, the rate and direction of flow of the liquids and the difference in temperature between the two liquids involved in the heat exchange process.

An approximate 3-8 mm space is maintained between the plates by a non-absorbent rubber seal, which is bonded around the edges of the plate. The liquids, which are sandwiched among the plates, enter and leave the interspaces through holes in the corners of the plates. Open and blind holes route the liquids from one set of plates to another. The capacity of the pasteurizer is secured by a corresponding number of plates.

The heat used during the UHT process can cause Maillard browning and change the taste and smell of dairy products. An alternative process is HTST pasteurization (high temperature/short time), in which the milk is heated to 72 °C (162 °F) for at least 15 seconds.
UHT milk, if not opened, has a typical unrefrigerated shelf life of six to nine months. HTST pasteurized milk has a shelf life of about 12 months, and pasteurized milk about two weeks from processing, or about one week from being put on sale.

Plate Heat Exchanger

Regeneration sections: The bank of plates is usually divided into four sections separated by connector grids with inlet and outlet bosses. In the regeneration section, the incoming cold milk is heated by the hot pasteurized milk and the pasteurized milk is cooled by transferring heat to the cooling medium. This heat transfers process work most effectively when the two liquids involved flow in
opposite direction, i.e. counter current flow on either side of the plates. Regeneration section raises the raw milk temperature from 4°C to 67°C and cools the pasteurized milk from 72°C to 10°C. Thus, PHE saves about 92% of heating and cooling energy. Theregeneration efficiency is calculated by using the following formula:

% Regeneration = temperature increase due to regeneration/ total temperature increase

For example: The cold milk enters the pasteurizer at 4°C and attains a temperature of 60°C after regeneration. The final pasteurization temperature is 72°C. Calculate the regeneration efficiency.

Increase in Temperature due to regeneration: 600C-40C=560C

Total Temperature Increase: 720C-40C= 680C

% Regeneration efficiency: 560C/680C = 82.36%

Steam-heated hot water or vacuum steam is used in heating section to raise the partly heated raw milk to pasteurization temperature. The holding section is either plate type or tube type. The plate type will have a number of plates. The partly cooled pasteurized milk is further cooled in cooling section to 4°C.

 

iv. Instrumentation

The instruments associated with the pasteurization plant are used for performing three functions in below Table

Uperization :

This is the shortened form of ultra pasteurisation which has been developed in the 1950s.

Ultra-high temperature processing (UHT), or ultra-heat treatment, sterilizes food by heating it above 135 °C (275 °F) – the temperature required to kill spores in milk - for 1 to 2 seconds.[1] UHT is most commonly used in milk production, but the process is also used for fruit juices, cream, soy milk, yogurt, wine, soups, honey, and stews.UHT milk was first developed in the 1960s and became generally available for consumption in the 1970s.

Sterilization

Definition

Sterilized milk refers to a product obtained by heating milk in a container in a commercial cooker/ retort to temperatures of 110-130OC for 10-30 min. The process is also referred as in-container sterilization. Sterilized milk is generally intended for prolonged storage at room temperature (up to 6 months). The major objective of heat sterilization is to destroy microbial and enzymatic activity. The length of time and magnitude of temperature employed during processing depend on the type of the product, number and heat resistance of microorganisms and enzymes present in milk. The heat resistance of microorganisms or enzymes is generally evaluated in terms of D-value or Z-value. Sterilization load or heat load for sterilization is generally expressed in terms of Fo value.

Theoretical Basis

Clostridium botulinum is considered as the index organism for assessing thermal sterility in foods. Under anaerobic conditions, inside a sealed container, it can produce botulin, a toxin, which can be 65% fatal to humans. Therefore, destruction of this organism is a minimum requirement of heat sterilization. As milk is a low acid (pH>4.5) food, it is recommended to achieve 12 decimal reductions for C.botulinum. This can be achieved by heating the product at 121OC for 3 min(Fo = 3). However, this minimum treatment may produce milk that is safe but not necessarily commercially sterile. This is so because there are more heat-resistant spores present in milk. There is B. stearothermophilus or B. sporothermodurans.These spores are not pathogenic. Their presence may require heat treatment equivalent to two (2) or more decimal reductions. This may correspond to an F0 value of 8.Target spoilage rates should be less than one survivor in every 10,000 containers.

Types of Sterilization Plants

Sterilizing retorts are either batch type or continuous in operation. Batch type sterilizers may be either vertical or horizontal. Horizontal retorts are easier to load or unload. They have facilities for agitating containers/cages. However, they require more floor space. Typically such horizontal retorts contain concentric cages. Cans are loaded horizontally into the annular space between the cages. When cages are full, the retort is sealed. The cages are supported by guide rails, which slowly rotate them. This stirring of the contents in cans facilitate proper heating. Continuous retorts are generally equipped with better controls. They cause very gradual change in pressure inside the cans. Thus products are heated more uniformly. Can seams are also subjected to less strain in comparison to batch process.

Continuous sterilizers: They are mainly of three types: (a) cooker-coolers; (b)hydrostatic sterilizers; and (c) rotary sterilizers. Cooker-coolers carry cans on a conveyor which pass through three sections of a tunnel. These sections are maintained at different pressures for preheating, sterilization and cooling. The hydrostatic sterilizer consists of a chamber equipped with provision for steam injection. The chamber that is partially full of water is connected to two water columns (12 to 18 meter tall, barometric leg) which are used to adjust pressure in the chamber. If the height of the water columns is changed, the steam pressure is changed and therefore the maximum attainable temperature changes. For example,to get a temperature of 116oC, a difference in height between the two water columns should be 10.7 m while for attaining 121oC temperature in the chamber,the water column difference should be 13.7 m. A conveyor with provision to accommodate cans of different sizes moves through the steam chamber carrying the food cans. The heating time could be regulated by varying the speed of the conveyor. Hydrostatic sterilizers are very flexible and suitable for large capacity plants. However, size of the structure and high capital costs are the major disadvantages of this system.

Continuous rotary sterilizer consists of several horizontal inter linked cylinders which allow for preheating, heating, precooling and cooling in upto four continuous stages.The vessel has a spiral track on the inner wall. A spoke or reel within the centre of the cooker causes the cans to roll along the spiral track. Rotary valves used to interconnect the shells, maintain pressure in the heating and cooling sections. Sealed cans are introduced directly from the sealing machines. The contents inside the cans are mixed as cans travel along the helix and therefore enhance heat transfer and ensure less heat damage to the product. Cans coming out of the cooker are directly taken to labelling and palletizing machine. Rotary sterilizers are particularly suitable for processing of milk and milk based products, which are extremely heat sensitive and susceptible to browning.

Quality of Sterilized Milk

Sterilized milk has a rich creamy appearance and a distinct cooked flavour (rich,nutty, caramelized). It is considerably browner in colour than raw milk. The brown colour develops due to formation of coloured pigments resulting from interactions between free amino groups of proteins and aldehyde group of lactose through Maillard reactions. The intensity of cooked flavour and brown colour depends upon the severity of heat treatment. In-container sterilization causes loss of nearly half of the ascorbic acid (Vitamin C) and sizeable loss of thiamine (30-40%). Vitamin B12 is almost completely destroyed. Fat soluble vitamin A, carotene, riboflavin and nicotinic acid are not affected. Biological value of proteins is only marginally affected.Sterilized milk cannot be coagulated with rennet unless calcium chloride is added externally.