Dairy processing: Batch pasteurization systems

On this page

• Introduction
• Batch pasteurization records
• Batch pasteurizer
• Indicating thermometer (batch pasteurization)
• Indicating thermometer (air space)
• Recording thermometer (batch)
• Coil vats

Introduction

The following provides recommended practices for batch pasteurization of dairy products.

Batch (or vat) pasteurization is one of the first methods used to process milk. This system requires batch processing of small volumes of milk at a specific temperature for a long period of time. This method of pasteurization still exists in small plants to process fluid milk, and in larger plants to manufacture cultured products.

Batch pasteurization records

Temperature and time requirements

Temperature and time are critical factors required to achieve pasteurization. Failure to achieve pasteurization could result in a microbiological hazard in the dairy product. The following are the generally accepted pasteurization schedules for dairy products produced by batch pasteurization.

Milk-based products containing below 10  % milk fat (fluid milk, goat milk, whey):

• Process at 63°C for 30 minutes.

Milk-based products containing 10 % milk fat or higher, or added sugar (fluid cream, cream for butter, chocolate milk, flavoured milk, etc.):

• Process at 66°C for 30 minutes.

Frozen dairy product mixes and egg nog:

• Process at 69°C for 30 minutes (unless the milk ingredients going into the mix for the frozen dairy product are already pasteurized and all other components being added to the mix are microbiologically safe and ready to eat).

Other:

• Use any equivalent time-temperature process that is validated to meet the requirements of B.08.002.2(1) of the Food and Drug Regulations with respect to the reduction of alkaline phosphatase activity.
• Use any time-temperature process required under provincial regulations.

Process control records

The process control record is the legal record of the pasteurization process and therefore it is very important that it adequately and accurately reflect the heating process. Process control records also assist in tracking down quality and safety problems.

• Record process control information in pen to provide a permanent record.
• Replace the process control record daily.
  • Note on the record any unusual occurrences, including the reasons for and time of occurrence.
• Where process control information is in separate documents, ensure there is a cross-reference to link them, such as a production code and/or date.
• Review the process control record(s) on a timely basis.
  • Indicate on the record any follow-up for out of specification findings and the actions taken.
• Ensure the process control records provide the following data once every twelve hours:
  • plant name and address or licence number
  • date, shift and batch number (where applicable)
  • vat number
  • time of filling and emptying of the vat (if applicable) and record of holding period
  • reading of air space thermometer at the start of the holding period at a given time or reference point indicated on the chart and at the end of the holding period
  • recording of indicating thermometer during the holding period at a given time or reference point indicated on the chart (this reading is never lower than the recording thermometer reading)
  • number or location of recorder when more than one is used
  • amount and name of product represented by each batch or run on the chart
  • operator`s comments and reasons for all unusual occurrences (including time of occurrence)
  • signature or initials of operator

Verification of temperature and time

Process control records should substantiate that the minimum holding time is achieved by observing the batch pasteurization cycle.

• Verify the required minimum holding time and temperatures (product and air space) are met.
• Verify that the holding time does not start until the indicating thermometer reading has reached the pasteurization temperature, and the air space thermometer reads at least 3°C (5°F) higher than the minimum pasteurization temperature for the product.
• When the milk is heated to pasteurization temperature in the vat and is partially cooled in the vat before opening the outlet valve, have the records show at least 30 minutes at or above the minimum pasteurization temperature.
• When the milk is pre-heated to pasteurization temperature before entering the vat, have the records show a holding time of 30 minutes plus the time of filling the vat from the level of the recording thermometer bulb.
• When cooling starts after the outlet valve is opened or is done entirely outside the vat, have the records show a holding time of 30 minutes at or above minimum pasteurization temperature plus the time necessary to empty the vat to the level of the recording thermometer bulb.
• When the recorded time interval on the process control record at the pasteurization temperature includes filling and/or emptying time, indicate these intervals on the record by inscribing the holding time on the record
• Determine the filling time and emptying time for each vat initially and after any change which may affect these times.
• Do not add milk or raw product to the vat after the start-up of the holding period.

Retention of process control records

Process control records substantiate that the products are adequately pasteurized.

• Retain all pertinent processing records as part of the preventive control plan.

Batch pasteurizer

Batch pasteurizers require specific design features and auxiliary equipment to ensure that every particle of milk is pasteurized. Milk is heated in the vat to the desired temperature by means of hot water either sprayed on the sides of the vat or circulated around the double jacket or by means of heating coils surrounding the inner jacket.

General conditions

A single- or two- speed agitator, and a vertical baffle plate inside the vat are used to create efficient agitation for proper heat transfer. Effective agitation reduces the chance of burn-on on the inside wall.

• Operate the agitator during pasteurization.
• The exterior and interior of the vat is constructed of stainless steel
• Keep clean and in good mechanical condition.
• Slope the bottom of the vat to permit free drainage.
• Remove and clean sanitary seals, coils, and valves daily.
• Ensure the agitation is such that the temperature difference between the milk in the centre of the vat and the coldest milk in the vat does not exceed 0.5°C (1°F) at any time during the holding operation.

Inlet/outlet valves and connections

Conditions for valves used on vat pasteurizers:

1. Close-coupled vat outlet valve: The seat is either flush with the inner wall of the pasteurizer or so closely coupled that no milk or milk product in the valve is more than 0.5 °C (1°F) colder than the milk or milk product at the centre of the pasteurizer at any time during the holding period.
   • A close-coupled valve, which is not truly flush, satisfies this condition when:
   • The vat outlet is flared so that the smallest diameter of the large end of the flare is not less than the diameter of the outlet line plus the depth of the flare, and
   • The greatest distance from the valve seat to the small end of the flare is no greater than the diameter of the outlet line, and
   • The outlet and the agitator are placed to ensure that milk current will be swept into the outlet.
2. Leak-protector vat outlet valve: Has a leak-detecting device to prevent the contamination of pasteurized product surfaces including the valve outlet connection. When the valve is in any closed position, the device will prevent (redirect) leakage of milk or milk product past the valve.

Manual valves and the automatic valve system described below meet the above conditions.

For Air operated (Automatic) valve systems:

• Use a double seat mix-proof valve which has two seats (on separate stems) with a leakage chamber (vent or leak port) between them.
  • Vent the leakage chamber to the atmosphere with a leak detect tube having a hydraulic diameter greater than the hydraulic diameter of the supply.
  • Ensure the leak detect vent is always fully open to the atmosphere with no restrictions and the valves are installed such that a leak can be observed.
  • Define the valve’s fail safe position as closed.
• Use valves that have been shown through testing and validation to meet the criteria in Appendix G: Preventing cross connections.
• Ensure the valve has at least one micro-switch or other sensor to signal that it is closed (inactivated position) prior to beginning the holding period required for pasteurization of the product in the vat.
• Keep the valve closed (inactivated position) for the entire holding period.
• Design and operate the leak detector chamber and vent to be pasteurized by flushing with culinary steam and to be cleaned and sanitized by clean-in-place (CIP).
  • Ensure the steam reaches all points within the leak detector chamber so that all internal portions of the chamber reach the minimum pasteurization time/temperature for the product being processed.
  • Equip the leak detector chamber and vent with an air space indicating thermometer.
• Do not start the minimum time requirement for the steam pasteurization of the leak detector chamber until the minimum time and temperature requirement for pasteurization of the product in the vat has been completed.
• Interlock the vat pasteurizer and the outlet valve so that the outlet valve remains closed throughout the pasteurization of the product in the vat and the steam pasteurization of the leak detection chamber.

Note: Air operated (automatic) valve systems can be used to separate raw and pasteurized products on the outlet line of a vat pasteurizer, but not to separate raw and pasteurized product elsewhere in the system (see Appendix G: Preventing cross connections).

For plug-type outlet valves:

• Prevent clogging and promote drainage.
  • Ensure plug-type outlet valves have leak-protector grooves at least 0.5 cm (3/16 inch) wide and 0.25 cm (3/32 inch) deep at the centre.
  • Install the valves in the proper position to ensure the function of the leak protector grooves and the drainage of the leak-detector valve.
• Ensure plug-type outlet valves have stops on the plug to guide the operator in closing the valve so that unpasteurized milk or milk product does not inadvertently enter the outlet line.
  • Design the stop so that the plug will be irreversible when the plug is provided with any grooves or their equivalent, unless duplicate, diametrically opposite grooves are also provided.
  • Design the stop so that the operator cannot turn the valve beyond the stop position, either by raising the plug or by any other means.

Note: Butterfly valves do not meet the above criteria.

Additional criteria for valves and connections of a vat pasteurizer (see Appendix E: Diagram of valves):

• Design outlet valves to prevent the accumulation of unpasteurized milk or milk product in the milk or milk product passages of the valve when the valve is in the closed position (the position of the valve seat that stops the flow of milk into and out of the pasteurizer).
  • During the heating or holding step, milk in this passage may not reach pasteurization temperature and when the valve is re-opened this unpasteurized milk would contaminate the pasteurized product surfaces.
• Ensure the inlet connections do not enter the pasteurizer below the level of milk.
• When the inlet line enters the vat above the milk level and is submerged in the milk, provide the inlet line with an automatic air-relief or vent located at the valve, or drill a hole of at least 0.32 cm (1/8 inch) in diameter in the fill pipe below the vat cover, but above the maximum milk level.
• Do not locate inlet valves in the vertical pipeline.
• Slope the pipeline between the inlet valve and vat to assure free drainage.
• Ensure close-coupled vat outlet valve bodies and plugs are made of stainless steel or of other materials that have heat transfer properties at least equal to stainless steel.
• Remove milk inlet piping from the vat during pasteurization, holding, cooling and emptying.
• On systems using a plug type outlet valve on the vat pasteurizer, remove the pasteurized milk outlet piping at the outlet valve during vat filling, heating and holding.

Cover (ports)

• Construct covers in a way that prevents the entrance of surface drainage.
  • For example, have all openings protected by raised edges and overlapping edges on the cover.
• Locate condensate-diverting aprons, if used, as close to the vat as possible and on all pipes, thermometers and other equipment extending into the vat.
  • These are not needed if a water-tight joint is used.
• Keep the cover closed during operation to assure proper pasteurization of the air space during the holding time.

Air space heating

Air space heating can be achieved by using an elevated pasteurization temperature or using an air space heater.

• Equip each vat that is operated at the minimum pasteurization temperature with an air space heater using culinary steam and an air space indicating thermometer.
• If the air space temperature is at 3°C (5°F) or more above the minimum pasteurization temperature, the air space heater is not needed but the air space indicating thermometer is necessary.

Indicating thermometer (batch pasteurization)

Indicating thermometers on batch pasteurizers provide the official processing temperature of the product.

General conditions

Batch indicating thermometers may be long stemmed and inserted directly into the product or inserted into a projectile well located in the side of the vat.

• Ensure the indicating thermometer is of sanitary design, is mercury actuated, has direct reading, has no column splitting and is contained in a corrosion resistant case which permits easy observation of column and scale.
• Use nitrogen (or another suitable gas) filling above the mercury.
• Protect the thermometer against elevated temperatures used during cleaning and sanitizing cycles.
• Use a Corning normal bulb or equally suitable thermometric glass.

Types other than mercury actuated (for example, resistance temperature detectors (RTD)) may be used if they are of sufficient accuracy and precision. The criteria in Appendix J: Design criteria for digital thermometers for use in critical processes can be used to evaluate RTDs when used as alternatives to mercury actuated direct reading thermometers.

Specifications

• Magnify the mercury column width to an apparent width of at least 1.6 mm (.0625 inch).
• Ensure the scale has a span of at least 14°C (25°F) including the pasteurization temperature plus or minus 3°C (5°F), graduated in 0.5°C (1°F) divisions.
• On batch pasteurizers used solely for thirty (30) minute pasteurization of milk and milk products above 72°C, indicating thermometers with 1°C (2°F) scale graduations, with not more than 6°C (11°F) per 2.54 centimetres (1 inch) of scale can be used.
• Ensure the indicating thermometer is in the same unit of measure as the recording thermometer (either both are Celsius or both are Fahrenheit).

Calibration

• Calibrate the indicating thermometer upon installation and at least every 6 months thereafter using a thermometer of known accuracy.
  • Verify it is accurate to within 0.25°C (0.5°F) throughout the specified scale range.
• Keep records of tests performed to determine the thermometer's calibration.
• Increase the frequency of testing if the calibration is consistently found to be out of adjustment.
  • If the calibration is consistently found to be out of adjustment, immediately identify and rectify the reason for the calibration problems.
• Use the testing methods in Critical process test procedures – Thermometers.
  • Follow-up on out of specification findings.
  • Investigate the safety of the product produced with out of calibration equipment. If the indicating thermometer is reading higher than the calibration standard, the product may have been under processed.

Indicating thermometer (air space)

An air space indicating thermometer ensures a temperature of at least 3°C (5°F) higher than the required minimum pasteurization temperature is maintained in the air space above the milk. This will guarantee that every particle of milk, including milk at the surface, will receive the full heat treatment.

General conditions

• Ensure the air space indicating thermometer is of sanitary design, is mercury actuated, has direct reading, and is contained in a corrosion resistant case which permits easy observation of column and scale.
• Use nitrogen (or another suitable gas) filling above the mercury.
• Protect the thermometer against elevated temperatures used during cleaning and sanitizing cycles.
• Use a Corning normal bulb or equally suitable thermometric glass.
• Do not submerge the thermometer bulb in the product.

Types other than mercury actuated (for example, resistance temperature detectors (RTD)) may be used if they are of sufficient accuracy and precision. The criteria in Appendix J: Design criteria for digital thermometers for use in critical processes can be used to evaluate RTDs when used as alternatives to mercury actuated direct reading thermometers.

Specifications

• Ensure the bulb chamber does not sit less than 5 cm (2 inches) and not more than 8.75 cm (3.5 inches) below the underside of the cover, and is at least 2.5 cm (1 inch) above the product level.
• Ensure the scale has a span of at least 14°C (25°F) including the pasteurization temperature plus or minus 3°C (5°F), graduated in 1°C (2°F) divisions.

Calibration

• Calibrate the thermometer upon installation and at least every 6 months thereafter using a thermometer of known accuracy.
  • It should be accurate to within 0.5°C (1°F) throughout the specified scale range.
• Keep records of tests performed to determine the thermometer's calibration.
• Increase the frequency of testing if the calibration is consistently found to be out of adjustment.
  • If the calibration is consistently found to be out of adjustment, immediately identify and rectify the reason for the calibration problems.
• Use the testing methods in Critical process test procedures – Thermometers.
  • Follow-up on out of specification findings.
  • Investigate the safety of the product produced with out of calibration equipment. If the air space indicating thermometer is reading higher than the calibration standard, the product may have been under processed.

Recording thermometer (batch)

The recording thermometer automatically records the temperature of the product on a chart that also indicates the time of day, thus providing a record of the process and processing time.

General conditions

• Ensure the recording thermometer unit is clean and in good mechanical condition.
• Ensure it is moisture-proof under normal operating conditions, spring- or electrically-operated and protected against damage at 105°C.
• Equip the chart positive drive mechanism with a system to prevent slippage and manual rotation (for example, pin to puncture chart paper).
• Produce a continuous permanent record of all pertinent information (time of day and temperature).
• Service the unit at least once a year and keep records of this servicing.

Recording pen

• Ensure the recording thermometer has a functioning temperature recording pen.
  • Use a recording pen with a thickness no greater than 0.7mm (0.025 inch).
  • Verify the recording pen arm follows the reference arc etched on the recorder casing.
• Adjust the pen reading to coincide with that of the indicating thermometer.

Sensor location

• Equip each vat with an indicating and a recording thermometer.
  • Double resistance temperature detector (RTD) sensors can also be used, one to the indicating and one to the recording thermometer.
• Ensure the recording and indicating thermometer bulbs accurately monitor the processing temperature.
  • Locate the sensors as close as possible to each other.
  • Submerge the sensors in the product being processed.

Chart specifications

• Ensure the recording thermometer is in the same unit of measure as the indicating thermometer (either both are Celsius or both are Fahrenheit).

Batch pasteurizers using temperatures above 72 °C:

• Recording thermometers use charts graduated in temperature scale divisions of 1°C (2°F), spaced not less than 1mm (0.040 inch) apart between 65°C and 77°C, including pasteurization temperature, plus or minus 3°C (5°F), graduated in time scale divisions of not more than 15 minutes.
• Ensure the circular chart makes no more than one revolution in 24 hours.

Batch pasteurizers using temperatures below 72 °C:

• Recording thermometers use charts graduated in temperature scale divisions of 0.5°C (1°F), spaced not less than 1.6 mm (0.0625 inch) apart between 60°C and 69°C, including pasteurization temperature, plus or minus 3°C (5°F), graduated in time scale divisions of not more than 10 minutes.
• If the ink line is thin enough to distinguish it from the chart line, space the temperature scale divisions of 0.5°C (1°F) at least 1 mm (0.040 inch) apart.
• Ensure the circular chart makes no more than one revolution in 12 hours.

Starter vats using temperatures above 80°C:

• Recording thermometers use charts graduated in temperature scale divisions of 1°C (2°F), spaced not less than 1mm (0.040 inch) apart between 73°C and 85°C, including treatment temperature, plus or minus 3°C (5°F), graduated in time scale division of not more than 15 minutes.
• Ensure the circular chart makes no more than one revolution in 24 hours.

Accuracy

• Verify the performance accuracy of the batch pasteurizer recording thermometer at least once a year.
• Keep records of tests performed to determine accuracy, such as:
  • Recorder temperature accuracy: Test the temperature recorded is accurate to within 0.5°C (1°F) for processing temperature between 60 – 71°C and within 1°C (2°F) for processing temperature above 71°C
  • Recorder time accuracy: Test the recorded time of pasteurization does not exceed the true elapsed time.
  • Recording thermometer check against indicating thermometer: Test the recording thermometer does not read higher than the corresponding indicating thermometer. Have the Operator mark the actual indicating thermometer reading daily onto the chart, with the time of verification marked as well.
• Use the testing methods in Critical process test procedures – Thermometers.
  • Follow-up on out of specification findings.
  • Investigate the safety of the product produced with out of calibration equipment.

Coil vats

A coil vat is a type of batch pasteurizer that may be used for pasteurizing cream for butter-making. A revolving heating coil gently agitates the cream to create uniform heating throughout the vat.

General conditions

• Constructed of stainless steel
• Good mechanical and sanitary condition
• Bottom is sloped to permit free drainage
• Shaft ends have removable sanitary seal assemblies that are cleaned daily
  • For old-style equipment without sanitary seals, maintain and operate the packing glands without adverse effects
• Vat liner, coils, centreboard, covers, outlet valve, fittings and exterior surfaces are clean and in good condition

Critical controls

• Equip coil vats with an indicating and a recording thermometer for control of pasteurization
• Use recording charts that show all the information necessary to assess if proper pasteurization has occurred
  • If there is no recording thermometer (older equipment only), use other means to prove that the product is properly pasteurized. For example, conduct a phosphatase test on the cream for each day's production. If more than one vat is produced per day, test a composite sample that contains cream from each vat
• Do not add product to the vat after the beginning of the hold period and do not remove the product before the hold period is completed

Air space heating:

• During the holding period, maintain the air space temperature above the product at least 3°C (5°F) higher than the minimum required temperature of pasteurization
  • This could be accomplished by installing an air space heater or elevating the pasteurizing temperature
• Equip each pasteurizing coil vat with an air space indicating thermometer
  • Ensure the bulb is not submerged in the product
  • For coil vats that are not equipped with an air space indicating thermometer a phosphatase test can be used to verify the product has been properly pasteurized
• Close covers prior to the holding period to assure proper pasteurization of the air space during the holding time, and keep covers closed during the holding and cooling period
  • Vat covers may be open while cream is being heated

Refer to Evaluation of dairy processing equipment for the general criteria for indicating and recording thermometers.

Records

The process control record is the legal record of the pasteurization process, so it is very important that it adequately and accurately reflects the heating process. Process control records also assist in tracking down quality and safety problems.

• Record process control information in pen to provide a permanent record
• Replace the process control record daily
  • Note in the record any unusual occurrences, reasons for and time of occurrence
• Where process control information is in separate documents, ensure there is a cross reference to link them, such as by production code and/or date
• Review the process control record(s) on a timely basis
  • Indicate on the record any follow-up for out of specification findings and the actions taken
• Ensure the process control records for the coil vats provide the following data for every batch:
  • Plant name and address or licence number
  • Date, shift and batch number where applicable
  • Vat number
  • Record of the holding period
  • Reading of air space thermometer at the start of the holding period at a given time or reference point indicated on the chart and at the end of the holding period
  • Reading of the indicating thermometer during the holding period at a particular referenced reading point on the chart
  • Amount and name of the product pasteurized (when more than one product is pasteurized)
  • Operator's comments and reasons for all unusual occurrences
  • Signature or initials of the operator
• If a phosphatase test is done (either on the cream or the butter), also keep the following records:
  • Results of the phosphatase test for each day's production (composite sampling is acceptable)
  • Temperature of pasteurization for each vat
  • Holding time of pasteurization for each vat
  • Signature of the responsible person

Refer to Record keeping for additional information.