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Dairy processing: Higher heat shorter time (HHST) pasteurization systems and extended shelf life (ESL)

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Introduction

The following provides recommended practices for performing higher heat shorter time processing to extend the shelf life of dairy products.

Higher heat shorter time (HHST) treatment of fluid milk and milk products is the application of heat to a continuously flowing product using high temperatures, generally above 100°C, for such time to extend the shelf-life of the product under refrigerated conditions. This type of heat process can be used to produce dairy products with extended shelf life, generally referred to as “ESL”.

ESL means the ability to extend the shelf-life of a product beyond its traditional life by reducing the major sources of re-infection and maintaining the quality of the product all the way to the consumer (Dairy processing handbook, 2015).

ESL products are not considered to be commercially sterile products and, as such, must be cooled immediately after pasteurization to a temperature of 4°C or less and stored continuously under refrigeration at a temperature of 4°C or less.

Definitions

For the purposes of this document, the following definitions apply.

Forward flow
Means the normal flow conditions of pasteurized product towards pasteurized or sterile surge tanks and/or fillers.
Scheduled extended shelf life (ESL) process
Means all the conditions pertaining to the processing and packaging equipment, containers and products needed to achieve and maintain the required extended shelf-life under refrigerated storage conditions.

Higher heat shorter time (HHST) flow schematic

The HHST system, although similar to a High temperature short time (HTST) pasteurizer, operates at higher temperatures (above 100°C) and pressures. It also uses a pasteurization or sterilization cycle to pasteurize or sterilize the entire system prior to commencing production.

A flow schematic, or process and instrumentation diagram (PID), is a valuable tool for assessing the impact of any changes to the HHST system. Even slight modifications made to the HHST system may have an impact on its operation and 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 (Clean-in-place (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 piping, and the FDD, are areas where potential cross-connections could exist if the design or installation is improper. Refer to the Constant level tank and Flow diversion device (FDD) sections for more details.

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

Scheduled extended shelf life (ESL) process

To achieve the required pasteurization of ESL products in HHST systems, the generally accepted best practice is to design the scheduled ESL process to provide a thermal destruction of the target microorganism equivalent to that achieved by a process with a minimum lethality value F0=0.1.

Note: F0 is associated with commercially sterile products targeting a 12 log reduction in Clostridium botulinum spores; nevertheless, F0 was chosen in this case because it is the preferred method used by process authorities for calculating process kill, as opposed to the use of pasteurization value (P), which is more complex as it deals with varying reference temperatures and z-values.

Scheduled ESL process

Operating instructions

A process deviation occurs whenever any process is less than the scheduled ESL process or when critical factors are outside of specified limits.

Critical factor adherence

The critical factors are those factors specified in the scheduled ESL process as being necessary for the achievement of pasteurization of the ESL product. If any of these critical factors are not within the limits documented in the scheduled ESL process, this constitutes a process deviation and the product cannot be considered pasteurized ESL until process deviation procedures are completed.

Critical factor records

Processing records are part of the preventive control plan. They indicate whether the products were processed within the acceptable limits for the critical factors (no process deviations). Detailed documentation of process deviations permit follow up to determine the cause and corrective action for the deviation and to ensure any compromised product is properly identified and handled to prevent distribution or sale.

Process control records are part of the critical factors records.

  1. Ensure process control records for HHST systems provide the following data on every chart (use a 12-hour chart for the processing operation):
    • establishment name and address or licence number
    • date, shift and batch number where applicable
    • recorder unit identification when more than one is used
    • product type and amount of product processed (may be recorded in production records)
    • identification of sterilization/pasteurization cycles (for example, indicate when water or product is being run)
    • identification of CIP, “mini-wash” (if used)
    • unusual occurrences and operator comments (including time of occurrence)
    • signature or initials of the operators
    • chart pen markings (note: they should not overlap)
  2. Safety thermal limit recorder (STLR):
    • Take a reading of the official indicating thermometer during processing.
      • Ensure this reading is not lower than the recording thermometer reading.
    • Record the time the FDD is in the forward flow position, as indicated by the event pen.
    • Provide the recording thermometer tracing.
    • Provide the set point tracing, when multiple set points are used.
    • All of (1) above.
  3. Systems equipped with a Meter Based Timing System (MBTS):
    • Record the synchronized time with STLR chart.
    • Record the time the flow alarm is activated, as indicated by an event pen.
    • Provide the flow rate tracing.
    • All of (1) above.
  4. Pressure differential controller-recorder:
    • Record the synchronized time with STLR chart.
      • This information can be collected and stored in hard copy or electronically.
    • Provide the raw product or media side pressure tracing.
    • Provide the sterilized product side pressure tracing.
    • In lieu of raw product or media side pressure tracing and sterilized product side pressure tracing, take the pressure differential recording between them.
    • All of (1) above.
  5. Pressure limit recorder:
    • Record the synchronized time with STLR chart.
    • This information can be collected and stored in hard copy or electronically.
    • Record the holding tube operation pressure.
    • All of (1) above.
  6. Optional additional temperature recorders and controllers on the system:
    • Record the synchronized time with STLR chart.
    • Provide the recording thermometer tracing.
    • All of (1) above; note especially the identification of pasteurization/sterilization cycles.
  7. Include in the process deviation records:
    • Date and time of the process deviation.
    • Amount of product involved.
    • Product quarantine and release of affected product.
    • Investigation into the cause of the process deviation (for example, equipment breakdown, power failure, low temperature at outlet of holding tube.)
    • Action taken (for example, line cleared, repairs performed, system re-pasteurized/re-sterilized.)
    • Review by competent personnel.

Retain all pertinent processing records as part of the preventive control plan. These records will assist in determining if these products were adequately pasteurized to meet the extended shelf-life.

HHST Pasteurization System Criteria

HHST pasteurization systems should meet the criteria in the 3-A Accepted Practices for Sanitary Construction, Installation, Testing, and Operation of HTST and HHST Pasteurization Systems (Number 603).

Constant level tank

The constant level tank (CLT) is a reservoir for a supply, at atmospheric pressure, of raw product to the pasteurizer to permit continuous operation of the HHST system. It is located at the start of the HHST 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 some example CLT designs.

General conditions

Design

Ensure the design and capacity of the tank does not permit air to be drawn into the pasteurizer with the product when operating at the maximum sealed capacity of the flow control device.

Cover

Airspace and overflow

Level control device

Feed pump

The feed pump is used to improve flow through the raw regenerator, and to supply the flow control device with milk from the constant level tank to prevent starving, especially if the flow control device is a homogenizer. It also helps to remove negative pressure and subsequent "flashing" or vaporization in the raw regenerator section. In HHST systems, the feed pump normally operates in both forward and diverted flow, as long as the flow control device is in operation.

General conditions

The raw product side of the regenerator may be by-passed at start up.

Location

Inter-wiring

Regeneration section

The regenerator section on HHST systems may either be of a milk-to-milk type or milk-to-heat transfer medium-to-milk. The cold raw product is warmed by hot pasteurized product flowing on the opposite sides of thin stainless steel plates or tubes. The pasteurized product will in turn, be partially cooled.

General conditions

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

Pressure differentials

Failure to maintain the required pressure differential in the pasteurized milk section of the regenerator causes the FDD to assume the divert flow position.

Flow control device (FCD)

The flow control device governs the uniform rate of flow through the holding tube so that every particle of product is held for the required period of time, as specified in the scheduled ESL process. This device is a positive displacement type pump or homogenizer. Other equally effective mechanisms such as a Meter Based Timing System (MBTS) with proper components (centrifugal pump, flow control device or variable speed motor, meter head, relays, alarms and flow recorder-controller, etc.) may also be used as a flow control device. Refer to Appendix C: Meter based timing system for more information on meter based timing systems.

General conditions

Set and sealed

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 hold time. Increasing the line resistance by the addition of plates or piping will decrease the flow rate, increasing 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 capability

When a MBTS replaces the positive displacement flow control device:

Heating section

The heating section of the HHST system provides rapid, uniform and controlled heating of the product up to sterilization temperature. The raw product is usually forced through this section by the flow control device. Heating may be by direct injection or infusion of steam, or indirect heating through tubes, plates, scraped-surface heat exchangers or other accepted systems.

General conditions

Ensure it is clean and in good condition.

Indirect heating

Direct heating

With direct heating, the steam injection process is an inherently unstable process. When steam is injected into a fluid, condensation of the steam may not be completed inside the injector, causing temperature variations in the holding tube that could lead to some milk particles being processed below the required temperature.

Heating medium

Any vapours in the holding tube can displace product, resulting in shorter holding times. Steam should be as as free as possible from non-condensable gases.

Pressure limit recorder controllers

For both direct and indirect heating systems, product pressures in the holding tube and across the steam injector are monitored and controlled to keep the product in a liquid phase and to ensure adequate isolation of the injection chamber.

For HHST systems that are capable of operating with less than 518 kPa (75 psi) pressure in the holding tube:

Sealed

Once all tests have been completed, seal the controllers and settings to prevent unauthorized adjustments.

Ratio controller (direct heating systems)

Use a ratio controller for systems applying direct heat to the product to prevent water adulteration of the product being processed.

Holding section

This is the part of the HHST processing system in which heated product is held for the specified time required in the scheduled ESL process. This section is located after the final heating section of the HHST processing system, and may include the sensing chamber at the end. The sensing chamber is that portion which houses both the official indicating thermometer and the STLR hot milk temperature sensors.

General conditions

Slope and support

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

Holding verification

The calculated holding time is used to determine the minimum length of the holding tube, based on the flow rate used.

Flow diversion device (FDD)

The FDD controls the direction of product flow according to the establishment of safe conditions within the processing system. It is located downstream from the regenerator section, and is designed to automatically divert flow away from the surge tank or filler.

General conditions

When pasteurizing ESL products, use one of the following FDD designs:

1. Dual-stem type FDD: incorporates two three-way valves in series.

2. Steam-block type FDD system: incorporates a divert valve and one or more steam-block valves.

Installations on HHST processing systems often have operating parameters for the FDD that are so complex they can only be handled by a micro-processor or programmable logic controller (PLC).

Return line

A flash cooler may be installed on the return line to prevent injury to bystanders during divert events when pasteurizing/sterilizing the system.

Location

Locate the FDD downstream from the regeneration and before the surge tanks or fillers.

Fail safe divert capability

Indirect heating systems

Direct heating systems

After an event causing a flow diversion, hold all product contact surfaces between the holding tube and the FDD at or above the required pasteurization or sterilization temperature continuously and simultaneously for at least the required pasteurization or sterilization time, as outlined in the scheduled ESL process (see also the sub-section Thermal limit controller sequence logic).

Leak detect

In HHST systems where the filler continues to operate from a surge tank while the FDD is in the divert position, and the FDD is a steam-block type:

Sealed

Indicating thermometer

The indicating thermometer provides the official processing temperature of the product, which is a critical factor in the scheduled ESL process.

General conditions

Location and accessibility

Specifications

Calibration

Sealed

Safety thermal limit recorder (STLR)

The function of this device is to:

General conditions

Location

Specifications

Temperature recording pen

Frequency (Event or Divert) pen

This pen records the position of the FDD with a line on the outer edge of the chart. Some systems may be designed so that the event pen indicates the critical factors required to enable forward or diverted flow. In such cases, the event pen will indicate when at least one of those pre-determined critical factors is not met.

Third pen

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

Thermal limit controller sequence logic

Since the FDD is located downstream from the regeneration and cooling sections on a HHST systems, forward flow conditions cannot occur until all product contact surfaces from the holding tube to the FDD have been held at or above the required system pasteurization temperature for the time specified in the scheduled process.

The thermal limit controller unit uses a sequence of electrical inputs and timers to ensure the HHST processing system is pasteurized or sterilized before allowing the FDD to assume the forward flow position.

Indirect heating systems

Direct heating systems

This assures that all parts of the system have been properly pasteurized or sterilized before allowing the FDD to move into the forward flow position. Once the minimum times and temperatures have been satisfied for system pasteurization or sterilization, the two auxiliary controllers (see Auxiliary temperature recorders and controllers (at the FDD, and at the vacuum chamber on direct heating systems) will then "drop out” of the control loop, and the primary recorder-controller (STLR) at the holding tube outlet (sensing chamber) resumes its function for normal product processing temperature control.

Calibration

Sealed

Programmable logic controllers and computers

Control of non-food safety functions

Programmable logic controllers or computers installed on an HHST processing system for operational convenience (in other words, have no impact on food safety) meet the following criteria.

Control of food safety functions

Computers for the operation of food safety controls on HHST processors have additional considerations. 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 recorder controllers

This section covers the actual pressure devices used to maintain proper pressure relationships. As explained in the Regeneration section and the Cooling section, proper pressure relationships must exist across all media to prevent contamination of the pasteurized product by raw product, heating medium and cooling medium. These pressure relationships must be maintained under forward flow, divert flow and shutdown.

General conditions

A 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 FDD, pressure indicating and recording capabilities, and set-point indication.

Pressure gauges may be used to verify the pressure display for the pressure differential recorder controller.

Location

Two types of regeneration are used in HHST systems, product-to-product regenerators, and product-to-heat transfer medium-to-product regeneration systems. The latter system is often preferred for some products, because it allows more even heat transfer and prevents burn-on.

Product-to-product regenerators:

Product-to-heat transfer medium-to-product regenerators:

Specifications

Calibration

Sealed

Auxiliary temperature recorders and controllers

These instruments may be used in several locations on the HHST processing system, to provide a record of start-up pasteurization/sterilization and product processing temperature, and to provide temperature signals to the thermal limit controller unit or other processing controls.

General conditions

Cooling section

This section of the pasteurizer uses chilled water and/or glycol to cool the hot product down to packaging and filling temperature. Since the FDD is located downstream from this section, the cooling section may become contaminated with potentially unpasteurized product during divert, and must be re-pasteurized/re-sterilized as part of the thermal limit controller sequence logic after a divert event.

Flash coolers are sometimes installed on the divert line to prevent injury to bystanders if a divert event occurs during the pasteurizing of the holding tube and cooling section, when there is no cooling turned on.

General conditions

Pressure differentials

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

Heating, pre-heating and chilled water media can be a potential source of contamination to the pasteurized product.

Homogenizer

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.

General conditions

Homogenizer larger than FCD

If a homogenizer located downstream from the flow control device has a capacity greater than the flow control device, the homogenizer is not a flow promoter. For example:

Surge tank

The surge tank acts as a pasteurized product balance tank for the fillers. This allows both the fillers and the HHST processing system to operate independently.

General conditions

Stuffing pump

Stuffing pumps may be used to improve the efficiency of other devices, such as homogenizers.

General conditions

Installation/operation

If the homogenizer is used as a flow control device, a centrifugal type stuffing 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.

Packaging

Packaging conditions

Packaging records

Regulatory Requirements

1. The standard for whey in the Food and Drug Regulations (FDR) states:

Health Canada's List of permitted food additives with other accepted uses outlines the permitted use of benzoyl peroxide and hydrogen peroxide in whey.

B.2.1 Benzoyl Peroxide: Liquid whey destined for the manufacture of dried whey products other than those for use in infant formula. Purpose of Use: To decolourize. Maximum level of use: 100 p.p.m.

H.1 Hydrogen Peroxide: Liquid whey destined for the manufacture of dried whey products. Purpose of Use: To decolourize and maintain pH. Maximum level of use: 100 p.p.m.

2. When whey products are used as an ingredient in other foods, FDR section B.01.009 exempts them from listing components in the ingredient list.

B.01.009 (1) Components of ingredients or of classes of ingredients set out in the following table are not required to be shown on a label:

3. FDR provides an exemption for the components of food additive preparations, such as starch used as a carrier for bleaching agents, from being declared in the ingredient listing.

B.01.009 (2) Subject to subsection (3), where a preparation or mixture set out in the table to this subsection is added to a food, the ingredients and components of the preparation or mixture are not required to be shown on the label of that food.

8. food additive preparations

(3) If the preparation or mixture contains:

The Food and Drug Act (FDA) provides specific exemptions for products that are destined for export.

37 (1) This Act does not apply to any packaged food, drug, cosmetic or device, not manufactured for consumption in Canada and not sold for consumption in Canada, if the package is marked in distinct overprinting with the word "Export" or "Exportation" and a certificate that the package and its contents do not contravene any known requirement of the law of the country to which it is or is about to be consigned has been issued in respect of the package and its contents in prescribed form and manner.

The Safe Food for Canadians Regulations (SFCR) also has exceptions for products destined for export.

16 (1) Any person may export a food that does not meet the requirements of these Regulations, other than a requirement of paragraph 8(1)(c) or (d) or subsection 15(1), if a label applied or attached to the food bears the word “Export” or “exportation” and

  1. if the foreign state to which the food is exported has a different requirement on the same matter as the unmet requirement, the person prepares a document that substantiates that the foreign state's requirement has been met; or
  2. if the foreign state to which the food is exported has no requirement on the same matter as the unmet requirement,
    1. the unmet requirement must be a requirement set out in any of subsection 9(1), sections 10, 188 to 192, 195, 197, 201, 210, 244 to 249, 253 and 255, subsection 257(2), paragraphs 258(c) and (d), sections 262, 264, 265, 267, 268, 272, 273, 275 and 280, paragraph 286(a), sections 288, 292 to 295, 306 to 308, 312, 313, 316, 322, 324 to 327, 329 and 331, and
    2. the person prepares a document that sets out the specifications for the unmet requirement as stipulated by the person in the foreign state for whom the exported food is intended.
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