OPERATION OF HTST SYSTEMS

HTST pasteurization has become important to the dairy industry because of the operating efficiencies which it affords. Properly operated, these units allow high volume of production in a minimum of processing space.

The ability of HTST pasteurizers to assure a safe, finished product hinges on the reliability of the time-temperature-pressure relationships which must prevail whenever the system is in operation. It is important that the plant operator understand the HTST process in order to maintain proper surveillance over the equipment. The basic flow pattern is described below:

1. Cold raw milk, in a constant level supply tank, is drawn into the regenerator section of the HTST pasteurizer.

Note:—Some operators prefer to bypass the regenerator when starting. Under this system, cold milk is drawn directly through the timing pump (step 3) and into the heater section. The remaining steps are performed without exception. This bypass arrangement facilitates and speeds up the starting operation. After forward flow is established at the flow-diversion device, the bypass, which may be manually or automatically controlled, is not used and the raw milk flows through the regenerator. A second start-up technique involves the use of sanitizer solution at 170°F (77° C). This is passed through the complete unit and followed immediately by milk. Dilution of the first milk does occur, however, and care must be taken to prevent this from being packaged.

2. In the regenerator section, the cold raw milk is warmed by hot pasteurized milk flowing in a counter current direction on the opposite sides of thin stainless steel surfaces.

3. The raw milk, still under suction, passes through a positive displacement timing pump which delivers it under pressure through the rest of the HTST pasteurization system.

4. The raw milk is pumped through the heater section where hot water or steam on opposite sides of thin stainless steel surfaces heats the milk to a temperature of at least 161°F (72°C).

5. The milk, at pasteurization temperature, and under pressure, flows through the holding tube where it is held for at least 15 seconds. (The maximum velocity of the milk through the holding tube is governed by the speed of the timing pump, the diameter and length of the holding tube and surface friction.)

6. After passing the sensing bulbs of an indicating thermometer and a recorder/controller, the milk passes into the flow-diversion device which automatically assumes a forward-flow position if the milk passes the recorder/controller bulb at the preset cut-in temperature (i.e. 161°F (72°C) or higher). The valve automatically assumes the diverted-flow position if the milk passes the recorder/controller bulb below the preset cut-out temperature (i.e. 161°F (72°C)).

7. Improperly heated milk flows through the diverted-flow line back to the raw milk constant level supply tank.

8. Properly heated milk flows through the forward-flow line to the pasteurized milk regenerator section where it serves to warm the cold raw milk and, in turn, is cooled.

9. The warm milk passes through the cooling section, where coolant, on the sides of thin stainless steel surfaces opposite the pasteurized milk, reduces its temperature to 40°F (4°C) and below.

10. The cold pasteurized milk then passes to a storage tank or vat to await packaging.

HTST PASTEURIZERS EMPLOYING MILK-TO-MILK REGENERATORS WITH BOTH SIDES CLOSED TO THE ATMOSPHERE

Item 16p(C) establishes standards for regenerators. These standards insure that the raw milk will always be under less pressure than pasteurized milk in order to prevent contamination of the pasteurized milk in the event flaws should develop in the metal or joints separating it from the raw milk. An explanation of regenerator specifications is given below.

During normal operation (i.e., while the timing pump is operating), raw milk will be drawn through the regenerator at subatmospheric pressure. The pasteurized milk in the milk-to-milk regenerator will be above atmospheric pressure. The required pressure differential will be assured when there is no flow-promoting device downstream from the pasteurized milk side of the regenerator to draw the pasteurized milk through the regenerator, and the pasteurized milk downstream from the regenerator rises to at least a 1-foot elevation above the highest raw milk level downstream from the constant-level tank, and is open to the atmosphere at this or a higher elevation, as required in Item 16p(C)2.

During a shutdown (i.e., when the timing pump stops), the raw milk in the regenerator will be retained under suction, except as this suction may be gradually relieved by possible entrance of air drawn through the regenerator plate gaskets from the higher outside atmospheric pressure. With a free draining regenerator, as required under Item 16p(C)7, the raw milk level in the regenerator may drop slowly, depending on the tightness of the gaskets, ultimately falling below the level of the plates to the product level in the raw milk supply tank. However, under these conditions, as long as any raw milk remains in the regenerator, it will be at subatmospheric pressure.

During shutdown, the pasteurized milk in the regenerator is maintained at atmospheric pressure or above by meeting the elevation requirement of Item 16p(C)2. Pressure greater than atmospheric is maintained when the level of pasteurized milk is at or above the required elevation; loss of pressure due to suction is prevented by prohibiting a downstream pump.

Any backflow of milk through the flow-diversion device would lower the pasteurized milk level during pump shutdowns, thus tending to reduce the pressure on the pasteurized milk side of the regenerator. A flow-diversion valve cannot be relied upon to prevent backflow in such instances, because during the first few minutes following a pump shutdown, the milk is still at a sufficiently high temperature to keep the diversion valve in the forward-flow position. Compliance with the provisions of Item 16p(C)2 and 3, however, will insure a proper pressure differential in the regenerator.

At the beginning of a run, from the time raw milk or water is drawn through the regenerator, until the pasteurized milk or water has risen to the elevation specified in Item 16p(C)2, the pasteurized milk side of the regenerator is at atmospheric pressure or higher. Even if the metering pump should stop during this period, the pressure on the pasteurized milk side of the regenerator will be greater than the subatmospheric pressure on the raw milk side, assured by compliance with Item 16p(C)2 and 3, as long as any raw milk remains in the regenerator.

When a raw milk booster pump is incorporated into the HTST system, Item 16p(C)5 requires, in part, that automatic means shall be provided to assure, at all times, the required pressure differential between raw and pasteurized milk in the regenerator before the booster pump can operate. The most common control employed to accomplish this is a sanitary pressure switch installed at or downstream from the pasteurized milk outlet of the regenerator. The pressure switch is adjusted to energize the booster pump only after the pasteurized milk pressure in the regenerator exceeds, by at least 1 pound per square inch, the maximum operating pressure developed by the booster pump.

The setting and checking of the pressure switch used to control the proper operation of the raw milk booster pump is described in Appendix 4, Test 9,1.

As an alternative control to the use of the pressure switch, the adjustable time delay relay in conjunction with a hydrostatic head, has been effectively used in HTST systems equipped with raw milk booster pumps of relatively low capacity. Such time delay relay provides a predetermined time lapse between the moment the flow-diversion device assumes the forward-flow position and the moment the booster pump is energized. The time lapse required is that necessary for the forward flow of milk through the regenerator and cooler to rise to a height sufficiently above the booster pump outlet to provide a pressure at least one pound greater than the maximum pressure developed by the booster pump. The pasteurized milk pipeline is vented to the atmosphere at or above the necessary vertical rise.

The setting and checking of the time delay relay and hydrostatic head used to control the proper operation of the raw milk booster pump is described in Appendix 4, Test 9,2.

Filter Media.—Air intake and pipeline filters shall consist of fiberglass, cotton flannel, wool flannel, spun metal, electrostatic material, or other equally acceptable filtering media, which are nonshedding and which do not release to the air, toxic volatiles, or volatiles which may impart any flavor or odor to the product.

Disposable media filters shall consist of cotton flannel, wool flannel, spun metal, nonwoven fabric U.S.P absorbent cotton fiber, or suitable inorganic materials which, under conditions of use, are nontoxic and nonshedding. Chemical bonding material contained in the media shall be nontoxic, nonvolatile, and insoluble under all conditions of use. Disposable media shall not be cleaned and reused.

Filter Performance.—The efficiency of intake filters shall be at least 50 percent as measured by the National Bureau of Standards' "Dust Spot Method"a using atmospheric dust as the lest aerosol.

The efficiency of either air pipeline filters or disposable filters shall be at least 50 percent as measured by the DOP (Dioctyl 1-phthalate fog)b test.

Piping.—Air distribution piping, fittings, and gaskets between the terminal filter and any product or product-contact surface shall be sanitary milk piping, except where the compressing equipment is of the fan or blower type. When the air is used for such operations as removing containers from mandrels, other nontoxic materials may be used.

FABRICATION AND INSTALLATION

Air Supply Equipment. —The compressing equipment shall be designed to preclude contamination of the air with lubricant vapors and fumes. Oil-free air may be produced by one of the following methods or their equivalent:

1. Use of a carbon ring piston compressor.

2. Use of oil-lubricated compressor with effective provision for removal of any oil vapor by cooling the compressed air.

3. Water-lubricated or nonlubricated blowers.

The air supply shall be taken from a clean space or from relatively clean outer air and shall pass through a filter upstream from the compressing equipment. This filter shall be located and constructed so that it is easily accessible for examination, and the filter media are easily removable for cleaning or replacing. The filter shall be protected from weather, drainage, water, product spillage, and physical damage.

Moisture Removal Equipment.—If it is necessary to cool the compressed air, an aftercooler shall be installed between the compressor and the air storage tank for the purpose of removing moisture from the compressed air.

Filters and Moisture Traps. —Filters shall be constructed so as to assure effective passage of air through the filter media only.

The air under pressure shall pass through an oil-free filter and moisture trap for removal of solids and liquids. The filter and trap shall be located in the air pipeline downstream from the compressing equipment and from the air tank, if one is used. Air pipeline filters and moisture traps downstream from compressing equipment shall not be required where the compressing equipment is of the fan or blower type.

A disposable media filter shall be located in the sanitary air pipelines upstream from and as close as possible, to each point of application or ultimate use of the air.

Air Piping. —The air piping from the compressing equipment to the filter and moisture trap shall be readily drainable.

A product-check valve of sanitary design shall be installed in the air piping downstream from the disposable media filter to prevent backflow of product into the air pipeline, except that a check valve shall not be required if the air piping enters the product zone from a point higher than the product overflow level which is open to the atmosphere.

The requirements of this section do not apply when the compressing equipment is of the fan or blower type. See illustrations depicting various air supply systems.

Note—For additional details, see 3-A Accepted Practices for Supplying Air Under Pressure in Contact With Milk, Milk Products, and Product-Contact Surfaces.

The following methods and procedures will provide steam of culinary quality for use in the processing of milk and milk products.

SOURCE OF BOILER FEED WATER

Potable water or water supplies acceptable to the regulatory agency shall be used.

FEED WATER TREATMENT

Feed waters may be treated, if necessary, for proper boiler care and operation. Boiler feed water treatment and control shall be under the supervision of trained personnel or a firm specializing in industrial water conditioning. Such personnel shall be informed that the steam is to be used for culinary purposes. Pretreatment of feed waters for boilers or steam generating systems to reduce water hardness, before entering the boiler or steam generator by ion exchange or other acceptable procedures, is preferable to addition of conditioning compounds to boiler waters. Only compounds complying with Section 173.310 of Title 21 of the Code of Federal Regulations may be used to prevent corrosion and scale in boilers or to facilitate sludge removal.

Greater amounts shall not be used of the boiler water treatment compounds than the minimum necessary for controlling boiler scale or other boiler water treatment purposes. No greater amount of steam shall be used for the treatment and/or pasteurization of milk and milk products than necessary.

It should be noted that tannin, which is also frequently added to boiler water to facilitate sludge removal during boiler blow-down, has been reported to give rise to odor problems, and should be used with caution.

Boiler compounds containing cyclohexylamine, morpholine, octadecylamine, diethylaminoethanol, trisodium nitrilotriacetate, and hydrazine shall not be permitted for use in steam in contact with milk and milk products.

BOILER OPERATION

A supply of clean, dry saturated steam is necessary for proper equipment operation; boilers and steam generation equipment shall be operated in such a manner as to prevent foaming, priming, carryover, and excessive entrainment of boiler water into the steam. Carryover of boiler water additives can result in the production of milk off-flavors. Manufacturers' instructions regarding recommended water level and blow-down should be consulted and rigorously followed. The blow-down of the boiler should be carefully watched, so that an overconcentration of the boiler water solids and foaming is avoided. It is recommended that periodic analyses be made of condensate samples. Such samples should be taken from the line between the final steam separating equipment and the point of the introduction of steam into the product.

PIPING ASSEMBLIES

Suggested piping assemblies for steam infusion or injection are shown on Figure 29.

Other assemblies which will assure a clean, dry saturated steam are acceptable.

INDICATING THERMOMETERS FOR BATCH PASTEURIZERS

Mercury-actuated; direct-reading; contained in a corrosion-resistant case which protects against breakage and permits easy observation of column and scale; filling above mercury, nitrogen, or other suitable gas.

Magnification of Mercury Column.—To apparent width of not less than 0.0625 of an inch.

Scale. —Shall have a span of not less than 25 Fahrenheit degrees (14 Celsius degrees) including the pasteurization temperature plus and minus 5°F (3°C); graduated in 1°F (0.5°C) divisions with not more than 16 Fahrenheit degrees (9 Celsius degrees) per inch of span; protected against damage at 220°F (105°C).

Accuracy. —Within 0.5°F (0.2°C), plus or minus through the specified scale span. (Appendix 4, Test 1)

Submerged Stem Fittings.—Pressure-tight seat against inside wall of holder; no threads exposed to milk; location of seat to conform to that of a 3-A Sanitary Standard wall-type fitting or other equivalent sanitary fitting.

Bulb.—Corning normal or equally suitable thermometric glass.

INDICATING THERMOMETERS LOCATED ON PASTEURIZATION PIPELINES

Type.—Mercury-actuated; direct-reading; contained in corrosion-resistant case which protects against breakage and permits easy observation of column and scale; filling above mercury, nitrogen, or equally suitable gas.

Magnification of Mercury Column.—To apparent width of not less than 0.0625 of an inch.

Scale. —Shall have a span of not less: than 25 Fahrenheit degrees (14 Celsius degrees) including the pasteurization temperature plus and minus 5°F (3°C); graduated in 0.5°F (0.25° C) divisions with not more than 8 Fahrenheit degrees (4 Celsius degrees) per inch of scale; protected against damage at 220°F (105°C), and in the case of thermometers used on HHST systems protected against damage at 300°F (149°C).

Accuracy.—Within 0.5°F (0.2°C) plus or minus throughout specified scale span. (Appendix 4, Test 1)

Stem Fittings.—Pressure-tight seat against inside wall of fittings; no threads exposed to milk; distance from underside of ferrule to top of the sensitive portion of bulb not less than 3 inches.

Thermometric Response.—When the thermometer is at room temperature and then is immersed in a well-stirred water bath 19°F (11°C) or less above the pasteurization temperature, the time required for the reading to increase from water bath temperature minus 19°F (11°C) to water bath temperature minus 7°F (4°C) shall not exceed 4 seconds. (Appendix 4,Test 7)

Bulb—Corning normal, or equally suitable thermometric glass.

Type.—Mercury-actuated; direct-reading; contained in corrosion-resistant case which protects against breakage and permits easy observation of column and scale; bottom of bulb chamber not less than 2 inches, and not more than 3.5 inches, below underside of cover; filling above mercury, nitrogen, or equally suitable gas.

Magnification of Mercury Column.—To apparent width of not less than 0.0625 of an inch.

Scale. —Shall have a span of not less than 25 Fahrenheit degrees (14 Celsius degrees), including 150°F (66°C) plus and minus 5°F (3°C); graduated in not more than 2°F (1°C) divisions, with not more than 16 Fahrenheit degrees (9 Celsius degrees) per inch of scale; protected against damage at 220°F (105°C).

Accuracy.—Within 1°F (0.5°C), plus or minus, throughout the specified scale span. (Appendix 4, Test 1)

Stem Fittings.—Pressure-tight seat or other suitable sanitary fittings. No threads exposed.

Case.—Moisture-proof under normal operating conditions in pasteurization plants.

Scale. —Shall have a span of not less than 20 Fahrenheit degrees (11 Celsius degrees), including pasteurization temperature, plus and minus 5.0°F (3°C), graduated in temperature-scale divisions of 1°F (0.5°C) spaced not less than 0.0625 of an inch apart between 140°F and 155°F (60°C and 69°C): Provided, That temperature-scale divisions of 1F (0.5C) spaced not less than 0.040 of an inch apart are permitted when the ink line is thin enough to be easily distinguished from the printed line, graduated in time-scale divisions of not more than 10 minutes, having a chord or straight-line length of not less than 0.25 inch between 145°F and 150°F (63°C and 66°C): Provided, That on vats used solely for 30-minute pasteurization of milk products at temperatures above 160°F (71°C), 2°F (1°C) divisions may be used 0.0625 of an inch apart, with temperature accuracy 2°F (1°C) plus or minus.

Temperature Accuracy. —Within 1°F (0.5°C), plus or minus, between 140°F and 155°F (60°C and 69°C) (Appendix 4, Test 2).

Time Accuracy.—The recorded elapsed time, as indicated by the chart rotation, shall not exceed the true elapsed time, as compared to an accurate watch, over a period of at least 30 minutes at pasteurization temperatue. Recorders for batch pasteurizers may be equipped with spring operated or electrically operated clocks (Appendix 4, Test 3).

Pen-Arm Setting Device.—Easily accessible; simple to adjust.

Pen and Chart Paper.—Pen designed to give line not over 0.25 of an inch wide; easy to maintain.

Temperature Sensing Device.—Protected against damage at temperature of 220°F (105°C).

Submerged Stem Fitting.—Pressure-tight seat against inside wall of holder, no threads exposed to milk or milk products. Distance from underside of ferrule to the sensitive portion of the bulb to be not less than 3 inches.

Chart Speed. —A circular chart shall make one revolution in not more than 12 hours. Two charts shall be used if operations extend beyond 12 hours in 1 day. Circular charts shall be graduated for a maximum record of 12 hours. Strip-charts may show a continuous recording over a 24-hour period.

Chart Support Drive.—The rotating chart support drive shall be provided with a pin to puncture the chart in a manner to prevent its fraudulent rotation.

RECORDER/CONTROLLERS FOR CONTINUOUS PASTEURIZERS

Case.—Moisture-proof under normal operating conditions in pasteurization plants.

Chart Scale.—Shall have a span of not less than 30 Fahrenheit degrees (17 Celsius degrees), including the temperature at which diversion is set, plus and minus 12°F (7°C), graduated in temperature scale divisions of 1°F (0.5°C) spaced not less than 0.0625 of an inch apart at the diversion temperature, plus or minus 1°F (0.5°C): Provided, That temperature-scale divisions of 1°F (0.5°C) spaced not less than 0.040 of an inch apart are permitted when the ink line is thin enough to be easily distinguished from the printed line, graduated in time-scale divisions of not more than 15 minutes, having an equivalent 15 minute chord or straight-line length of not less than 0.25 of an inch at the diversion temperature, plus or minus 1°F (0.5°C).

Temperature Accuracy. —Within 1°F (0.5°C), plus or minus, at the temperature at which the controller is set to divert, plus and minus 5°F (3°C) (Appendix 4, Test 2).

Power Operated. —All recorder/controllers for continuous pasteurization shall be electrically operated.

Pen-Arm Device.—Easily accessible; simple to adjust.

Pen and Chart Paper.—Pen designed to give line not over 0.025 of an inch wide; easy to maintain.

Temperature Sensing Device.—(Bulb, tube, spring, thermister) protected against damage at temperature of 220°F (105°C): Provided, That recorder/controller temperature sensing devices used on HHST systems shall be protected against damage at temperatures of 300°F (149°C).

Submerged Stem Fitting.—Pressure-tight seat against inside wall of pipe; no threads exposed to milk or milk products; location from underside of ferrule to the sensitive portion of the bulb not less than 3 inches.

Chart Speed. —A circular chart shall make one revolution in not more than 12 hours. Two charts shall be used if operations extend beyond 12 hours in 1 day. Circular charts shall be graduated for a maximum record of 12 hours. Strip-charts may show a continuous recording over a 24-hour period.

Frequency Pen. —The recorder/controller shall be provided with an additional pen-arm for recording, on the outer edge of the chart, the record of the time at which the flow-control device is in the forward-flow, diverted-flow, or stopped position. The chart time line shall correspond with the reference arc, and the recording pen shall rest upon the time line matching the reference arc.

Controller.—Actuated by same sensor as recorder pen but cut-in and cut-out response independent of pen-arm movement.

Controller Adjustment.—Mechanism for adjustment of response temperature simple, and so designed that the temperature setting cannot be changed or the controller manipulated without detection.

Thermometric Response.—With the recorder/controller bulb at room temperature and then immersed in a well stirred water or oil bath at 7°F (4°C) above the cut-in point, the interval between the moment when the recording thermometer reads 12°F (7°C) below the cut-in temperature and the moment of power cut-in shall be not more than 5 seconds (Appendix 4, Test 8).

Chart Support Drive.—The rotating chart support drive shall be provided with a pin to puncture the chart in a manner to prevent its fraudulent rotation.

Scale Range.—Shall have a span not less than 50 Fahrenheit degrees (28 Celsius degrees) including normal storage temperatures plus and minus 5°F (3°C) with extension of scale on either side permitted; graduated in not more than 2°F (1°C) divisions.

Temperature Scale Divisions.—Spaced not less than 0.0625 of an inch apart between 35°F and 55°F (2°C and 13°C).

Accuracy. —Within 2°F (1°C) plus or minus, throughout the specified scale range.

Stem Fitting.—Pressure-tight seat or other suitable sanitary fittings. No threads exposed.

Case.—Moisture-proof under operating conditions in processing plants.

Scale.—Shall have a scale span of not less than 50 Fahrenheit degrees (28 Celsius degrees), including normal storage temperature plus and minus 5°F (3°C), graduated in not more than 2°F (1°C) divisions with not more than 40 Fahrenheit degrees (23 Celsius degrees) per inch of scale; graduated in time scale divisions of not more than 1 hour having a chord or straight-line length of not less than 0.125 of an inch at 40°F (5°C). Chart must be capable of recording temperatures up to 180°F (83°C) (Span specifications do not apply to extensions beyond 100°F (38°C))

Temperature Accuracy.—Within 2°F (1°C) plus or minus, between specified range limits.

Pen-Arm Setting Device.—Easily accessible; simple to adjust.

Pen and Chart Paper.—Designed to give line nut over 0.025 of an inch thick when in proper adjustment; easy to maintain.

Temperature Sensor.—Protected against damage at 212°F (100°C)

Stem Fitting.—Pressure-tight seat or other suitable sanitary fitting. No threads exposed.

Chart Speed. —The circular chart shall make one revolution in not more than 7 days and shall be graduated for a maximum record of 7 days. Strip chart shall move not less than 1 inch per hour and may be used continuously for 1 calendar month.

Location.—Temperature sensor in the return line downstream from process.

Case.—Moisture-proof under operating conditions.

Scale.—Shall have a range from 60°F to 180°F (16°C to 83°C), with extensions of scale on either side permissible; graduated in time-scale divisions of not more than 15 minutes. Above 110°F (44°C), the chart is to be graduated in temperature divisions of not more than 2°F (1°C) spaced not less than 0.0625 of an inch apart:Provided, That temperature-scale divisions of 2°F (1°C) spaced not less than 0.040 of an inch apart are permitted when the ink line is thin enough to be easily distinguished from the printed line.

Temperature Accuracy.—Within 2°F (1°C) plus or minus, above 110°F (44°C).

Pen-Arm Setting Device.—Easily accessible; simple to adjust.

Pen and Chart Paper.—Designed to mark a line not over 0.025 of an inch wide; easy to maintain.

Temperature Sensor.—Protected against damage at 212°F (100°C).

Stem Fitting.—Pressure-tight seat against inside wall of pipe; no threads exposed to solution.

Chart Speed.—Circular charts shall make one revolution in not more than 24 hours. Strip charts shall not move less than 1 inch per hour. More than one record of the cleaning operation shall not overlap on the same section of the chart for either circular- or strip-type charts.

Indicating thermometers used in refrigerated rooms where milk and milk products are stored shall meet the following specifications:

Scale Range.—Shall have a span not less than 50 Fahrenheit degrees (28 Celsius degrees) including normal storage temperatures plus and minus 5°F (3°C) with extensions of scale on either side permitted, graduated in not more than 2°F (1°C) divisions.

Temperature Scale Divisions.—Spaced not less than 0.0625 of an inch apart between 32°F and 55°F (0°C and 13°C).

Accuracy.—Within 2°F (1°C) plus or minus, throughout the specified scale range.

(a) Dill, R. S., A Test Method for Air Filters.Transactions of the American Society of Heating and Ventilation Engineers, 44:379, 1938.

(b) DOP-Smoke Penetration and Air Resistance of Filters, Military Standard No. 282, Section 102.9.1, Naval Supply Depot, 5801 Tabor Avenue, Philadelphia, Pennsylvania 19120.