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Dairy processing: High temperature short time (HTST) pasteurization systems

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The following provides recommended practices for high temperature short time pasteurization of dairy products.

High temperature short time (HTST) pasteurization offers significant operating efficiencies compared to traditional batch pasteurization. HTST systems allow a high volume of production in a minimum of processing space.

The ability of HTST pasteurizers to assure a safe milk or milk product relies on the time-temperature-pressure relationships that must be maintained whenever the system is in operation. A hygienically designed system ensures that each particle of milk is pasteurized and that cross-contamination of pasteurized product is prevented.


Safe forward flow
A condition where the temperature of the product is above the divert set point and the flow control device is energized by the safety thermal limit recorder or the legal Programmable logic controller.

HTST system flow schematic

A flow schematic is a valuable tool for assessing the impact of any changes to the HTST pasteurization system. Even slight modifications to the pasteurization system or the Clean-in-place (CIP) system may have an impact on its processing operation and its safety.

Up-to-date and accurate

No cross connections

A cross-connection is a direct connection allowing one material to contaminate another.

For other applications CIP supply lines and return line circuits used for CIP cleaning and "mini-washes" on tanks, lines, pasteurizers or other equipment that may be washed while connected to product lines containing milk products or potable water and lines for final rinse):

The design of the constant level tank and inlet piping and the flow diversion device (FDD) area and product piping are areas where potential cross-connections could exist if the design or installation is improper. Refer to the Constant level tank and the Flow control device (FCD) sections for more details.

Refer to the Preventing cross-contamination and Appendix G: Preventing cross connections for more information on preventing cross connections in dairy establishments.

Critical control records

Pasteurization records contain processing information and indicate whether the products have been adequately pasteurized.

Temperature and time

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

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

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

Frozen dairy product mixes and egg nog:


Operating instructions

Process control records

Process control records are part of the preventive control plan. They are a historical record of the exact happenings of the pasteurization of each product and it is very important that they adequately and accurately reflect the heating process. They assist in the tracking down of quality and safety problems.

Retention of process control records

Process control records substantiate that the products were adequately pasteurized.

HTST Pasteurization System Criteria

HTST pasteurization systems should meet the criteria in the 3-A Accepted practices for sanitary construction, installation, testing, and operation of HTST and higher heat shorter time (HHST) pasteurization systems (Number 603).

Constant level tank

The constant level tank (CLT) is a reservoir for a supply, at atmospheric pressure, of raw or recirculated product to the pasteurizer to permit continuous operation of the HTST pasteurization system. It is located at the start of the pasteurization system. It controls the milk level and provides a uniform head pressure to the product leaving the tank.

Air in the pasteurizer may allow the milk particles to move more rapidly through the system and therefore not receive the heat treatment for the required time. Appendix B: Constant level tank design illustrates some example CLT designs.

General conditions



Overflow (point and diameter)

Air space

Level control device

The level control device controls the flow of milk to the constant level tank and therefore provides constant head pressure to the product leaving the tank.

Booster pump

A raw product booster pump may be installed in a conventional HTST pasteurization system under specific conditions. The booster pump is used to supplement the flow control device in moving raw milk from the constant level tank through the regeneration section. It may be used to remove excessive vacuum, and subsequent "flashing" or vaporization, in the regenerator section (particularly when the constant level tank is located an unusual distance from the timing pump).

General conditions

Positive displacement pumps, for example lobe-rotor pumps, piston pumps, etc., are not suitable in this type of application as they are not designed to allow the product to drain freely from the regeneration plates back to the CLT and could result in a higher pressure on the raw product side of the regeneration section during shutdown.

The raw product side of the regenerator may be by-passed when the booster pump is not in operation, such as during start-up of the system. This by-pass permits the cold product to be drawn directly to the flow control device from the constant level tank. When the required conditions are met (that is, the flow control device is operating, the flow diversion device is in forward flow and there is product pressure in the pasteurized regenerator section) the booster pump will start to operate, feeding raw product to the regenerator.

The by-pass line, which may be manually or automatically controlled by a valve, is not normally used when the booster pump is in operation. To preclude the entrapment of the product in the by-pass line during periods when the booster pump is in operation:



Regeneration section

Typically, the regeneration section is that part of the HTST unit where the cold raw product is warmed by hot pasteurized product flowing in a counter current direction on the opposite sides of thin stainless steel plates. The pasteurized product will in turn, be partially cooled.

Basic criteria for the regeneration section:

General conditions

Since the physical distance between the various liquids in the pasteurization plates is extremely small, the liquids have the potential to move through the plates and cross-contaminate the product if pin holes exist.

Shut-down capability

When the system is shut down, the raw milk flows back to the constant level tank.

Pressure differentials

Failure to maintain the required pressure differential in any section of the regenerator causes all flow promoting devices upstream of any raw regeneration section to be de-energized or isolated from the system.

Failure to maintain the required pressure differential in the pasteurized milk section of the regenerator causes all flow promoting devices upstream of any raw regeneration section to be de-energized or isolated from the system and vented to the atmosphere.

Flow control device (FCD)

The flow control device, also called the timing pump, is the heart of the HTST pasteurizer. It controls the rate of flow through the holding tube so that every particle of product is held for the minimum period of time required for pasteurization. This device is a positive displacement type pump (may be a homogenizer). Other equally effective mechanisms such as a MBTS with proper components (pump or flow control valve, relays, alarms and flow recorder) may also be used as a flow control device. Refer to Appendix C: Meter based timing system for more information.

General conditions

Set and sealed

If the device is of the variable speed type or single speed (but capable of being altered, belt and pulleys changed), for example a homogenizer:

Any change in the line resistance of the system after maximum speed of the pump has been set will alter the flow rate and corresponding holding time. Increasing the line resistance by the addition of plates or piping will decrease the flow rate, increasing the holding time. This increase in flow resistance in effect reduces the efficiency of the pasteurizer. Decreasing the line resistance by the removal of plates, pipes, or auxiliary units will increase the flow rate, decreasing the holding time. Wear of the drive belts and pump impellers due to normal operation will gradually decrease the rate of flow through the system, thereby increasing the holding time.

Fail safe (operation) capability

A time delay relay may be installed to permit the flow control device to continue operating during the normal time it takes for the flow diversion device to move from forward flow to diverted flow. This type of time delay relay is most common when homogenizers are used as flow control device.

Appendix C: Meter based timing system describes the criteria for a MBTS.

Heating section

The heating section of the HTST pasteurizer provides rapid, uniform and controlled heating of the product up to pasteurization temperature. The raw product is usually forced through this section by the flow control device.

General conditions

Pressure differentials and heating

Heating medium

Cooling section

The cooling section of the HTST pasteurizer uses chilled water and/or glycol to provide rapid, uniform and controlled cooling of the pre-cooled pasteurized product coming from the pasteurized regenerator section.

Since milk for cheese making is usually not cooled, the HTST unit in these types of operations may not have a cooling section.

General conditions

Pressure differentials and cooling

An automated mechanism is an effective means of achieving the correct pressure relationship in the cooling section during forward flow, divert and shutdown conditions so that the pressure on the pasteurized product side is greater than the cooling media side.

Cooling medium

Holding section

This is the part of the HTST pasteurizer system in which fully heated milk is held for at least the minimum required holding time. This section which consists of a holding tube and sensing chamber is located between the heating section of the HTST and the inlet of the flow diversion device.

General conditions

To attain the minimum holding time it is critical that the design of the holding tube prohibits air from being incorporated into the system. Air in the system will allow individual milk particles to move faster through the holding tubes, thereby reducing the holding time.

Slope and supports

A slope eliminates any air entrapment in the holding tube, which would displace product and reduce the holding time.

Holding time provisions

Holding verification

Sensing chamber

The sensing chamber is that portion of the holding tube which houses both the indicating thermometer and the STLR hot milk temperature sensors and is located at the outlet of the holding tube.

Extended hold

Some HTST pasteurizers have an extension installed on the holding tube to provide an "extended hold" for some products. The extension to the holding tube can be upstream or downstream from the flow diversion device. Systems using an extended hold generally have two air operated valves, one at the inlet and one at the outlet of the extended hold line, that are controlled by the microprocessor in the HTST panel through a switch on the panel.

If the extended hold is part of the official holding tube (between the fittings for checking the holding time by salt conductivity test):

Due to the contamination potential of the extended holds set up:

Flow diversion device (FDD)

The flow diversion device (FDD) controls the direction of product flow according to the temperature of the product leaving the holding tube.

Two common types of FDDs used are:

  1. Single stem - one valve system
  2. Dual stem - two valve system

General conditions

Divert line

Leak detect capabilities

Single stem flow diversion device

Leak detector ports or leak escape ports permit the escape, to the atmosphere, of product at sub-legal temperature which may have leaked past the first gasket seal on the forward flow portion of the valve. They prevent sub-legal milk from entering the forward flow line. Leakage at this point is a warning to the operator that the valve "O" rings are faulty.

Dual stem flow diversion device


Fail safe divert capability

Time delay relays

The time delay relay is a unit which defers a function by a set period of time.

Dual stem flow diversion devices

Meter based timing systems (MBTS)

The following are additional controls when a MBTS is the flow control device.


Sealing the flow diversion device will prevent any tampering with control switches and time delay relays.

Indicating thermometer

The pasteurizing indicating thermometer provides the official processing temperature of the product.

General conditions

Location and accessibility



Perform the following tests upon installation and at least every 6 months.

Refer to Critical process test procedures - Thermometers for information on testing the accuracy of indicating thermometers.


Safety thermal limit recorder (STLR)

The function of this device is to:

General conditions

This unit, more commonly referred to as the recorder controller, should meet the 3-A Accepted practice for the sanitary construction, installation, testing, and operation of HTST and HHST pasteurizer systems (Number 603).

Diversion capabilities

Cut-in and cut-out

The cut-in temperature is the temperature observed on the indicating thermometer, at the instant the flow diversion device begins to move to the forward flow position. The flow diversion valve responds to the signal sent out by the STLR when the STLR senses a product temperature at or above the set-point, and is therefore temperature dependent. For HTST systems equipped with dual stem flow diversion devices, the leak detect valve responds after a preset time delay, and is therefore time dependent. The cut-out temperature is the temperature (during descent) at which the flow diversion device assumes the divert flow position.

With recent technology, it is possible to perform automated cut-in/cut-out temperatures using Programmable Logic Controllers (PLC).


Temperature recording pen

Frequency (event or divert) pen

This pen records the position of the flow diversion device with a line on the outer edge of the chart.

Third pen

If the STLR requires a third pen, as with a multiple temperature divert unit:


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.


Tests which should be performed include the following:


Programmable logic controllers and computers

Computers are different from hard-wired controls in three major areas. The design of computerized food safety controls needs to address these areas to provide adequate public health protection.

Pressure differential controllers and gauges

The following section covers the actual pressure devices used. The appropriate pressure differential is covered in the Regeneration and Cooling sections.

General conditions

A legal PLC can be used to control the pressure differential in lieu of a pressure differential controller as long as the same control conditions are respected such as inter-wiring with the FDD, pressure indicating and recording capabilities, and set-point indication.






Recording thermometer (cooling)

This instrument automatically records the temperature of the product (for example cooled pasteurized milk) on a chart, thereby providing a record of the process.

General conditions

Pasteurized product discharge

This section discharges the cooled pasteurized product to a point beyond the HTST pasteurizing system in a way that maintains higher pressure on the pasteurized side of the regenerator during divert flow or shutdown to minimize the risk of re-contamination. This section is normally located downstream of the cooling section.


By creating a head pressure which opens to the pasteurized side of the regenerator, higher pressure is established on the pasteurized regenerator section.

Vacuum break

Vacuum breaker


The homogenizer is a high pressure pump that produces a homogenized product by reducing the size of fat globules as they are forced through a small orifice under high pressure. Since the homogenizer is a positive pump, it can be utilized as a flow control device. If the homogenizer is utilized as a flow control device, refer to the Flow control device section for recommendations. If the homogenizer is not the flow controlling device then this section may apply.

General conditions

When operated in conjunction with the HTST pasteurizer:

Recirculation line

If the homogenizer has a capacity greater than the flow control device then the homogenizer would normally be located downstream from the flow control device.

Relief line

If the homogenizer is of lower capacity than the flow control device, and the flow control device feeds product to the suction side of the homogenizer:


When the homogenizer is of lesser capacity than the flow control device:


The separator-clarifier is a piece of auxiliary equipment that mechanically separates milk into fat and skim milk by centrifugation. Self-cleaning separators also provide a clarifying function by regularly de-sludging the somatic cells, leucocytes and other inedible materials.

The centrifugal force created in the separator may be enough to promote flow, therefore all separators are potential flow promoting devices. Certain design criteria are necessary to ensure that they do not influence the pressure differential the regenerator.

General conditions


Separators may be located upstream or downstream of the flow diversion device.

Non-flow promoting

The separator is considered a flow promoting device.

Vacuum break

Flavour adjusting equipment

This equipment removes undesirable volatile odours and flavours by subjecting dairy products to a vacuum treatment. The product is passed through a vacuum chamber which acts to remove volatile flavours such as onion, alfalfa, silage, etc. from milk.

General conditions

Properly valved out

When culinary steam is introduced into the product downstream from the flow diversion device:

When a water feed line is connected to a direct water-vapour vacuum condenser:

When vacuum equipment is located downstream from the flow diversion device:

When vacuum equipment is located downstream from the flow diversion device:

Steam ratio control

When culinary steam is introduced directly into the product:

Stuffing pump and flow promoting devices

Stuffing pumps are used to force-feed certain equipment, such as homogenizers. The stuffing pump is necessary in large homogenizers where it will enable the product to be under positive pressure at the homogenizer suction intake manifold.

General conditions

Installation and operation

Install and operate the stuffing pump in a way that will not:

When the homogenizer is used as a flow control device, a centrifugal type pump may be installed between the raw product outlet of the regenerator and the inlet manifold of the homogenizer to supply the desired pressure to the homogenizer. These pumps may be installed to turn on prior to starting the homogenizer.

Supplementary milk solids and milk fat injection system (in-line standardization)

Injection systems may be used to increase the solids level of milk used in the cheese making process to improve overall cheese yield. This can be achieved by injecting milk fat and milk solids directly into the CLT or directly injecting milk fat and milk solids into the HTST system prior to pasteurization.

Injection pumps are generally positive displacement type pumps, but can also be centrifugal pumps in conjunction with a MBTS. They are flow promoting devices and depending on the point of injection are considered as part of the HTST system.

The following is based on two example methods of injection. There may be other injection system configurations that achieve the same outcomes.

General conditions

Installation and operation

When the injection occurs directly into the CLT:

When the injection occurs between the CLT and the FCD:

Another option is to valve the injection line out of the system by using sanitary mix-proof fail safe valves to isolate the injection from the HTST when the system is not running (FCD is de-energized).

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