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Pasteurization

High Temperature Short Time (HTST) Pasteurization

Notes for guidance only - Check local country legislation


1.0 Standard Operating Procedure (SOP)

Pasteurisation is an important control measure ensuring product safety.

2.0 Legal requirements for drinking milk
The legal requirements set out in The Dairy Products (Hygiene) Regulations 1995 which are based on Council Directive 92/46/EEC are summarised below :


2.1 Plant and equipment
The pasteurising equipment must be approved or authorised by the approving authority and be fitted with:
i) An automatic temperature control
ii) A recording thermometer
iii) An automatic safety device preventing insufficient heating (Flow Diversion Valve)
iv) An adequate safety system preventing the mixture of heat-treated drinking milk with milk which has not been fully heat-treated; and
v) An automatic recording device which records the operation of the safety system referred to in iv) above or a procedure for monitoring the system's effectiveness.


2.2 Identification of critical points
The operator or manager of the establishment must constantly carry out his own checks based on the following principles:
i) Identification of critical points
ii) Monitoring and checking of such critical points by appropriate methods
iii) Taking samples for analysis
iv) Keeping written records of the different checks and tests


2.3 Pasteurisation conditions
i) Pasteurised milk must have been obtained by means of a heat treatment of at least 71.7ºC for 15 seconds or equivalent combination.
ii) Pasteurised milk must give a negative result to the phosphatase test.
iii) Immediately after pasteurisation the milk must be cooled to a temperature not exceeding 6ºC as soon as practicable.


2.4 Microbiological requirements
At the time of manufacture pasteurised milk must be free from pathogens and coliforms, and capable of retaining its properties for several days if kept refrigerated. The formal legal requirements are as follows:
In the random sampling checks carried out in the treatment establishment pasteurised milk must meet the following microbiological standards:
Pathogenic micro-organisms must be absent from 25 gram samples.
Coliforms (per ml): n = 5, c = 1, m = 0, M = 5
After incubation at 6ºC for five days
Plate count at 21ºC (per ml): n = 5, c=1, m = 50000, M = 500000
where :
n = number of sample units comprising the sample
m = threshold value; the result is satisfactory if the number of bacteria in all samples does not exceed ‘m’
M = maximum value; the result is unsatisfactory if the number of bacteria in any sample is ‘M’ or more
c = number of samples where the bacterial count may be between ‘m’ and ‘M’, the sample being acceptable if the bacterial count of the other samples is ‘m’ or less.


3.0 HACCP
A requirement of the Dairy Products (Hygiene) Regulations 1995 is that critical points relative to the process in the dairy establishment are identified, monitored and controlled.
One means of doing this is by means of the Hazard Analysis Critical Control Point (HACCP) system, which is likely to become a legal requirement when the revision of the EU hygiene legislation is completed. Best practice requires the provision of a formal HACCP plan.
To apply HACCP a flow diagram should be drawn up which includes all key steps of the process. The hazards at each step on the flow diagram should be identified and analysed so that the operator can be certain that appropriate control measures have been put in place for them. These control measures, together with critical limits and monitoring
procedures, should be documented for possible inspection by food law enforcement officers. All changes to the process and the introduction of new products necessitate a review of the HACCP plan. Further information on HACCP is available from the references.


4.0 Critical factors for safe operation
Pasteurisation is a critical control point in ensuring the safety of milk and dairy products.
To ensure that the pasteurisation process is being carried out satisfactorily attention should be paid to the following:


4.1 Constructional Safeguards Additional to Legal Requirements
i) Before and during installation of any new plant or modification of an existing one a commissioning process should take place to ensure that design parameters have been met. A formal HACCP plan is essential; section 3 provides an introduction to this process.
ii) Although the minimum legally required holding time at 71.7ºC is 15 seconds, it is recommended that a holding time of 25 seconds is used for drinking milk since there is some experimental evidence that this increases the effectiveness of the destruction of Mycobacterium paratuberculosis (MAP).
iii) Plant for pasteurisation must include a means of controlling the product flow rate to ensure that the minimum specified holding time is maintained at all times. Means of controlling flow rates include positive displacement
pumps and flow controllers. Homogenisers must not be used as flow controllers since flow rates will increase when the homogeniser is not being used.
iv) All possible ways of contaminating heat-treated product by raw milk must be eliminated. Any pipework connections which link product and raw milk, including those nominally there only for cleaning/CIP purposes, might cause such contamination if not designed with specific safeguards to prevent it.
v) The safety of the process depends on the performance of the automatic safety device (divert system). The recorder must indicate whether a divert event has occurred and whether the plant is running in automatic or manual modes.
vi) Consideration should be given to the design and set up of equipment to ensure that the pressure of pasteurised product is greater than that of the heating/cooling mediums within the plate pack. This will help to provide an additional safeguard against leakage due to damaged heat exchanger plates. It is not however an absolute safeguard nor is it suitable for all products. For example, the properties of whipping cream would almost certainly be adversely affected by this procedure.
vii) Consideration must be given to filtration of dairy products somewhere within the process of transfer from road tanker to sale container. If separation or clarification is included in the process, this acts as a filter. If not, a suitable filter must be incorporated, preferably prior to heat treatment. Cloth "bag" filters are commonly used.
viii) Ancillary processing equipment (e.g. separators) must be placed before the holding section of the pasteuriser.


4.2 Operational Safeguards
i) Each HTST plant on site should have a plateage diagram and checks must be made that the plates have been assembled in the correct order. This must be checked again each time the plant is re-gasketed.
ii) Plate packs must not be over tightened since this can lead to plate-to-plate contact and plate damage.
iii) Since opening plate packs can cause damage plate packs should not be opened unnecessarily. Use of the correct seals and gasket materials is critical.
iv) Leakage of raw milk to pasteurised milk/dairy product or cooling water to pasteurised milk/dairy product can occur unless the plant has correctly designed pressure balances built into its design.
v) Up-to-date pipeline diagrams should be available on site for all processing operations.
vi) Plates should be examined visually for evidence of corrosion and pinholes, and the rubbers checked whenever the plant is opened. Tests of pack integrity must be carried out at least annually. A variety of methods for pin-hole testing exist including gas, pressure, and conductivity. The plant supplier can advise. Plates should be designed to encourage turbulent flow through the pasteuriser to discourage the deposition of milk components.
vii) Pipes, tanks and floats should be examined for cracks at intervals laid down in the preventative maintenance schedule. These intervals will depend on the type of plant, product and usage rate.
viii) All pipe-line systems need to be free from “dead legs”.
ix) Precautions need to be taken to prevent raw milk gaining access to processed milk through C.I.P systems valves. Separate C.I.P systems for raw and pasteurised milk are recommended.
x) Divert valves need to be inspected and maintained at intervals laid down in the preventative maintenance schedule. Checks on the operation of the divert valve should include a visual inspection of the valve as it operates.
xi) Records of all checks and maintenance carried out should be documented for verification and auditing purposes.


4.3 HTST Plant Operation
i) Calibrated thermometers must be fitted to the plant for indicating heating and cooling of product, and the calibration checked regularly. The thermograph must agree with the plant thermometer reading and this agreement should be verified periodically. The plant thermometer reading must be equal to or higher than the thermograph reading.
ii) The operation of the automatic safety device must be checked at least daily immediately prior to starting production and a record kept. When an automatic diversion valve is activated the temperature of its operation should be checked both when the device is activated and at the return to normal forward flow.
iii) The holding time must be checked at least annually and after changes to the plant. The holding time must be checked for each new product where the flow characteristics are different to previous products.
iv) A daily check should be carried out to ensure that any device for maintaining product flow at the required value is in place.
v) On conventional charts all pens, including the “events” pen need to be working and records need to be kept for at least six months
vi) Temperature charts should be examined daily at start-up to ensure that plants are never being run in manual mode. Manual mode is generally the cleaning/CIP setting.


4.4 Start-up procedures

Most dairy plants are designed to start up on water and once at operating temperatures milk is introduced. Procedures need to be in place to ensure that the interface between milk and water does not present a food quality or food safety
hazard.
The first milk through the plant and fillers should not be despatched without positive laboratory clearance.


4.5 Procedures following a plant diversion
The first milk through the plant when forward flow is resumed should not be despatched without positive laboratory clearance.


4.6 Laboratory checks
The following checks can confirm compliance with the legal requirements and also verify satisfactory operation of the pasteuriser:


4.6.1 Phosphatase
The enzyme alkaline phosphatase is naturally present in all raw milk and is destroyed by pasteurisation under the correct time/temperature conditions (71.7ºC for 15 seconds). Testing for the presence of phosphatase is a requirement of the Regulations. It is recommended that milk is not despatched from the processing dairy without positive laboratory clearance. As well as testing packed product consideration should be given to testing at the cooler exit and finished milk holding tank. Phosphatase tests should also be carried out on a positive release basis on milk leaving a pasteuriser immediately after returning to forward flow following diversion. The most commonly used phosphatase test is that of Aschaffenburg and Mullen. This is a colourimetric test giving a pass/fail level of about 0.1% raw milk in the product. Fluorometric or chemoluminescence methods are more rapid, and sensitive, being capable of detecting less than 0.01% raw milk. It is recommended that where rapid methods are used the baseline for each plant is established and any deviation investigated immediately.


4.6.2 Coliform
Any presence of coliform bacteria is an indicator of unsatisfactory operations since coliforms are destroyed by pasteurisation. Presence of coliforms is likely to be the result of contamination of the product after pasteurisation, cracked plates or poor pasteuriser cleaning. As well as testing packed product consideration should
be given, for trouble shooting purposes, to testing at the cooler exit and finished milk holding tank.
It should be noted that testing for Enterobacteriaceae is an alternative to testing for coliforms though not for legal compliance. Also the sensitivity of both Enterobacteriaceae and coliform testing can be improved by pre-incubating
samples.


5.0 Pasteurisation Time Temperature for liquid milk Products with higher fat or solids content, for instance cream, are generally heated to higher temperatures than indicated below

Alternative heat treatments
Equivalent heat treatments for pasteurisation of milk up to 10% fat are one of the combinations below or an interpolation between them:


PASTEURISER holding section holding time calculation


In calculating the holding time for produce in the holding section, the flow conditions existing in the holding section are taken into account by calculating the Reynolds Number of the product in the holding section at the heat treatment temperature.
Reynolds Number, Re = ρvD/μ
where:
ρ = density, kg/m3
v = velocity of flow, m/s
D = diameter, m
μ = viscosity of produce at the heat treatment temperature, Pa s
Laminar flow is assumed when the Reynolds number is less than 4000. For laminar flow, the holding time is calculated by assuming that the maximum velocity is twice the average velocity based on the maximum flow rate.
Turbulent flow is assumed where the Reynolds number exceeds 4000. For turbulent flow, the holding time is calculated from the actual measured fastest particle velocity based on the maximum flow rate. As the ratio of maximum velocity varies with Reynolds number it is recommended that the design is reviewed by a heat treatment validator or evaluator. As an initial guide for turbulent flow, the maximum velocity may be assumed to be 1.33 times the average velocity for Reynolds number=4000, and 1.25 times the average velocity when the Reynolds number exceeds 20000.


Holding Time (s) Temperature (oC)

0.01 ....................................100
0.05 ......................................96
0.1 ........................................94
0.5 ........................................90
1.0 ........................................89
8 ........................................73.4
9 ........................................73.1
10 ......................................72.8
11 ......................................72.7
12 ......................................72.5
13 ......................................72.3
14 ......................................72.1
15 ......................................72.0
16 ......................................71.9
17 ......................................71.8
18 ......................................71.7
19 ......................................71.6
20 ......................................71.5
22 ......................................71.3
24 ......................................71.1
26 ......................................70.9
28 ......................................70.8
30 ......................................70.7
35 ......................................70.4
40 ......................................70.1
45 ......................................69.9
50 ......................................69.7
55 ......................................69.5
60.......................................69.3

Equivalent minimum heat treatments for HTST pasteurisation of some specific dairy products are:

Dairy products (including cream) with a fat content of 10% or more
Increase the minimum temperature at any specified holding time by 3 oC, followed by immediate cooling to 7 oC or less, or to the processing temperature if the product is to be used immediately for further processing
Dairy products containing added sweeteners

Condensed or concentrated milk, whey or milk or whey products

Concentrated milk transported between plants
80 oC for 25 s, followed by immediate cooling to the processing or packaging temperature Equivalent heat treatments for other dairy products depend on such factors as the nature of the product, solids content, viscosity, etc. The holding time shall be calculated from maximum flow rate data.Some manufacturing processes (e.g. for AMF, butter and milk powder) may include adequate heat treatment to ensure product safety. A separate pasteurisation step is not required provided:the heat treatment is equivalent to those in the table above, allowing for the nature of the product;
all other standards for pasteurisation apply, including product diversion for inadequate heat treatment, instrumentation for temperature control and recording, checking, and independent evaluation.
In some countries, alternative heat treatments are not permitted. Products manufactured for export to these countries must be derived from milk that has been heat treated according to the minimum standards (commonly 72 oC for 15 seconds) specified by the health authorities, and a separate pasteurisation step may be required in manufacturing processes.


6.0 HTST Plant Cleaning
Cleaning in Place (CIP) is recommended for HTST processing plant. The critical elements are detergent strength, temperature, flow rate, and circulation time. The appropriate conditions for efficient cleaning need to be established, preferably with the detergent supplier, then properly controlled and monitored. A minimum sequence would
typically be Pre-rinse with cold water until visible product residue at the plant exit is clear.
Form a CIP circuit from the plant outlet into the top of a feed balance tank, add detergent recommended for HTST plants to produce the appropriate concentration and apply heat to bring the temperature up to the minimum required at the return. Circulate for as long as trials have shown is needed to ensure all product residues have been removed.
Rinse with cold water until visible detergent residues are undetectable at the exit.
Shortly before use circulate water, raising the temperature to at least 80ºC at the return for at least ten minutes. Ensure all parts of the product route achieve at least 80ºC. Cool to normal operating temperatures before introducing milk or product.
It should be noted that higher flow rates from those used in production runs may be required to get effective cleaning and it is necessary to ensure that any such modifications for cleaning (e.g. removal of the flow controller) are restored hygienically in normal operation.


7.0 Operator Training
It is a legal requirement that operators are given instruction and training with regard to hygiene. In particular:
i) It is essential that all operators of pasteurisers are trained in the operation of the plant, and have a basic understanding of the principles of pasteurisation. External courses are available if necessary.
ii) Operator competence must be assessed periodically and retraining carried out as necessary.
Written operating instructions must be in place. These must be based on the site HACCP and must include details of the critical control points and associated monitoring


8.0 Health and Safety
When considering the installation of any new plant or modification of an existing one,
consideration should also be given to health and safety implications including the
requirements of the Provision and Use of Work Equipment Regulations 1998

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