Dairy processing: Aseptic processing and packaging systems
Heating section

General conditions

• Has a sanitary design
• Is in clean and in good condition
• Is constructed of stainless steel or other corrosion resistant material

Indirect heating:

• Ensure there are no leaks at gaskets, seals, joints or connections during operation

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.

• Use a direct steam injector that is designed to provide complete condensing of the steam inside the injector
  • see Appendix L: Examples of steam injectors for examples of some suitable steam injectors for use in steam injection systems

Heating medium:

Steam used as a heating medium is free of harmful substances or extraneous matter.

• Use boiler and water treatment chemicals and other additives that are safe and suitable for use in dairy processing
• Use culinary steam for direct steam injection or infusion (see preventive controls for Culinary steam)

Any vapours in the holding tube can displace product, resulting in shorter holding times.

• Ensure steam is as free as possible from non-condensable gases
  • a de-aerator installed on the boiler helps to keep the holding tube free of 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 systems that are capable of operating with less than 518 kPa (75 psi) pressure in the holding tube:

• use a pressure limit recorder controller to monitor product pressure in the holding tube
  • this instrument has a pressure switch that causes the flow diversion device to move to the divert position if the product pressure falls below a prescribed value, for example, if the operating temperature is 100°C (212°F), the pressure switch is set at 69 kPa (10 psi); If the operating temperature is 116°C (240°F), the switch is set at 140 kPa (20 psi)
• determine the pressure switch settings during the set up and testing procedures (see Test 30 in Critical process test procedures)
• Appendix N: Vacuum breakers shows the pressure switch settings for corresponding operating temperatures

For direct heating systems with steam injection only:

• Use a differential pressure limit indicator to ensure adequate isolation (supplementary orifices) of the injection chamber so that product is uniformly heated in the chamber
  • ensure it has a differential pressure switch so that the flow diversion device will move to the divert position if the pressure drop across the injectors is below 69 kPa (10 psi)
• Keep records of the holding tube operational pressures, the pressure switch settings, the results of tests
  • follow up on out-of-specification findings

Controllers and settings sealed

• Seal the controllers and settings once all tests have been completed 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
• Locate one sensor immediately prior to the point of steam injection (incoming product), and the other immediately after the product exits the vacuum chamber (outgoing product)
• Determine the optimum temperature differential between the incoming and outgoing product by total solids analysis
  • set this differential on the ratio controller
• Use the ratio controller to automatically control the pre-heat steam supply or the flash chamber vacuum to prevent water adulteration of the product
  • the ratio controller is interlocked with the vacuum pump and/or steam controller and automatically monitors and controls the amount of vacuum applied and/or the amount of steam injected
• Make adjustments to the system when a water feed line is connected to a vacuum condenser, and the vacuum chamber is not physically separated from the vacuum condenser, to prevent the back up and overflow of water from the vacuum condenser into the vacuum chamber
  • this prevents adulteration of the product with water