Dairy Establishment Inspection Manual – Chapter 18
B. Holding Time

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Test 8: Salt Conductivity Test (Procedure Under Review)

Application:

To all HTST pasteurizers employing a holding time of fifteen (15) seconds or longer.

Note: Certain regulatory agencies may grant an individual plant permission to use a shorter hold time in conjunction with a required higher pasteurization temperature, or permission to use a lower pasteurization temperature in conjunction with a longer required hold time. In these cases the specified alternate hold time is substituted in place of fifteen (15) seconds.

Frequency:

  1. Upon installation and annually thereafter.
  2. Whenever seal on speed setting is broken.
  3. Whenever any alteration is made affecting the holding time, the velocity of the flow (such as replacement of pump, motor, belt, driver or driven pulleys, or decrease in number of HTST plates), or the capacity of holding tube.
  4. Whenever a check of the capacity indicates a speed up.

Criteria:

Every particle of milk shall be held for the minimum legal hold time at least fifteen (15) seconds in both the forward and diverted flow positions.

Apparatus:

  1. Electrical conductivity measuring device.
  2. Table salt (sodium chloride).
  3. Device used for injecting brine into the holding tube.
  4. Stopwatch.
  5. 36 litre (8 gallon) can.
  6. Wrenches.

Method:

The holding time is determined by timing the interval for an added trace substance to pass through the holder. Although the time interval of the fastest particle of milk is desired, the conductivity test is made with water. The results found with water are converted to the milk flow time by formulation since a pump may not deliver the same amount of milk as it does water. See test 9 for the calculated method.

Procedure A: For systems with a positive displacement pump used as the Flow Control Device.

  1. Examine the entire system to insure that all flow promoting equipment is operating at maximum capacity and all flow impeding equipment is so adjusted or bypassed as to provide minimum resistance to the flow. There shall be no leakage on the suction side of the timing pump.
  2. Adjust variable speed pump to its maximum capacity (preferably with a new belt and full size impellers).
  3. Check homogenizers for seals and/or gears or pulley identification.
  4. Install one electrode at the inlet (lowest point) to the holding tube and the other electrode at the holding tube outlet. Close the circuit to the electrode located at the holding tube inlet.
  5. Operate the pasteurizer using water at pasteurization temperature, with flow diversion device in forward flow position.
  6. Quickly inject saturated sodium chloride solution (approximately 50 mL) into the holding tube inlet.
  7. Start the stopwatch with the first movement of the indicator of a change in conductivity. Open the circuit to the inlet electrode and close the circuit to the electrode at the holding tube outlet.
  8. Stop the stopwatch with the first movement of the indicator of a change in conductivity.
  9. Record the holding time.
  10. Repeat the test six or more times, until successive results are within 0.5 seconds of each other. The average of these tests is the holding time for water in forward flow. When consistent results cannot be obtained, purge the equipment, check instruments and connections, and check for air leakage on suction side. Repeat tests. Should consistent readings not be obtained, use the fastest time as the holding time for water.
  11. Repeat steps (4) through (10) for the holding time in diverted flow.
  12. With the pump at the same speed and equipment adjusted as in (1) above, time the filling of a 36 litre (8 gallon) can with a measured weight of water using the discharge outlet with the same head pressure as in normal operation. Average the time of several trials. (Since flow rates of the large capacity units make it very difficult to check by filling a 36 litre can, it is suggested that a magnetic flow meter be hooked up into the system or a calibrated tank of considerable size be used).
  13. Repeat procedure (12) using milk or the dairy products to be pasteurized through this unit.
  14. Compute the holding time for milk from the following formula by weight, using the average specific gravity. Compute separately for forward flow and diverted flow.

    Holding time for milk = (1.032(TMw)/Ww) (by weight), in which:

    • 1.032 = specific gravity for milk.
    • T = average holding time for water.
    • Mw  = average time required to deliver a measured weight of product.
    • Ww  = average time required to deliver an equal weight of water.
    • The holding time for milk may also be computed from the following formula by volume. Compute separately for forward flow and diverted flow.

    Holding time for milk = T(Mv)/Wv) (by volume), in which:

    • T = average holding time for water.
    • Mv  = average time required to deliver a measured volume of product.
    • Wv  = average time required to deliver an equal volume of water.
  15. Record results.

Corrective Action:

When the computed holding time for milk is less than that required, either in forward flow or diverted flow, the speed of the flow control device shall be reduced or an adjustment made in the holding tube and the timing test repeated until satisfactory holding time is achieved. Should an orifice be used, to correct the holding time in diverted flow, there should be no excessive pressure exerted on the underside of the valve seat of the flow-diversion device.

Procedure B: For systems with a Magnetic Flow Meter System used as a Flow Control Device.

  1. Examine the entire system to insure that all flow promoting equipment is operating at maximum capacity and all flow impeding equipment is so adjusted or bypassed as to provide minimum resistance to the flow.
  2. Adjust the set point on the flow alarm to its highest possible setting.
  3. Adjust the set point on the flow controller to a flow rate estimated to yield an acceptable holding time.
  4. Install one electrode at the inlet (lowest point) to the holding tube and the other electrode at the holding tube outlet. Close the circuit to the electrode located at the holding tube inlet.
  5. Operate the pasteurizer using water at pasteurization temperature, with Flow Diversion Device in forward flow position.
  6. Quickly inject saturated sodium chloride solution (approximately 50 mL) into the holding tube inlet.
  7. Start the stopwatch with the first movement of the indicator of a change in conductivity. Open the circuit to the inlet electrode and close the circuit to the electrode at the holding tube outlet.
  8. Stop the stopwatch with the first movement of the indicator of a change in conductivity.
  9. Record the holding time.
  10. Repeat the test six (6) or more times, until six (6) successive results are within 0.5 seconds of each other. The average of these six (6) tests is the holding time for water in forward flow. When consistent results cannot be obtained, purge the equipment, check instruments and connections, and check for air leakage on suction side of the pump, located at the raw product supply tank. Repeat tests. If six (6) consecutive readings cannot be achieved within 0.5 seconds, the pasteurizing system is need of repair.
  11. With the flow controller at the same set point as in (3) above, time the filling of a 36 litre (8 gallon) can with a measured weight of water using the discharge outlet with the same head pressure as in normal operation. Average the time of several trials. (Since flow rates of the large capacity units make it very difficult to check by filling a 36 litre can, it is suggested that a magnetic flow meter be hooked up into the system or a calibrated tank of considerable size be used).
  12. Record results.

Corrective Action:

When the computed holding time for milk is less than that required the set point on the Flow Controller shall be decreased, or adjustment made in the holding tube and the timing test repeated until a satisfactory holding time is achieved. Should an orifice be used, to correct the holding time in diverted flow, there should be no excessive pressure exerted on the underside of the valve seat of the flow diversion device.

Test 9: Calculated Method (Procedure Under Review)

Application:

To all HTST pasteurizers employing a holding time of 15 seconds or longer.

Frequency:

  1. Upon installation and annually thereafter;
  2. Whenever seal on speed setting is broken;
  3. Whenever any alteration is made affecting the holding time, the velocity of the flow (such as replacement of pump, motor, belt, driver or driven pulleys, or decrease in number of HTST plates), or the capacity of holding tube;
  4. Whenever a check of the capacity indicates a speed up.

Criteria:

Every particle of milk shall be held for the minimum legal hold time in both the forward and diverted flow positions.

Apparatus:

Tape Measure.

Method:

Determine the efficiency factor using the Reynolds number for water and all products to be processed at the maximum flow rate. Also determine flow rate ratio (product:water). Use the smallest efficiency factor and the determined flow rate ratio to calculate the required holding tube length.

Procedure:

  1. Determine the inside diameter in metres of the holding tube (Table 1).
  2. Calculate the velocity of the product using the following equation:
    • V = F/A
    • Where: V = Velocity (m/s).
    • F = Flow rate (litres/hr or litres/sec or m3/s).
    • A = Area (m2)Footnote 3
Table 1 - Holding Tube Dimensions
Outside Diameter Inside Diameter (d)Table note 1 Area (a) Volume (q)
in. cm in. ft cm ft2 cm2 Imp. gal./ft Table note 2 l/m
1 2.54 0.902 0.0752 2.291 0.0044 4.122 0.0277 0.4122
1.5 3.81 1.402 0.117 3.561 0.0107 9.959 0.0669 0.9959
2 5.08 1.87 0.156 4.749 0.0191 17.713 0.1190 1.7713
2.5 6.35 2.37 0.198 6.019 0.0306 28.454 0.1912 2.8454
3 7.62 2.87 0.239 7.289 0.0449 41.728 0.2803 4.1728
4 10.16 3.834 0.32 9.739 0.0802 74.494 0.5003 7.4 494

Table notes

Table note 1

"Fundamentals of Food Process Engineering – 3rd Edition", Romeo T. Toledo, Department of Food Science and Technology, University of Georgia, Athens, Georgia, 2007.

Return to table note 1 referrer

Table note 2

1 imp. gal. = 0.16026 ft3

Return to table note 2 referrer

Note: Holding tube dimensions: 1" & 1½" is 18 standard wire gauge (SWG), 2", 2½" & 3" is 16 SWG, 4" is 14 SWG.

Table 2 - Density and Viscosity Values
Product Density (p) Viscosity (μ)
Type Temp g/l lb/ft3 cP lb/ft×s
Milk 72°C 1012 63.15 0.515 0.000346
Cream (40%) 75°C 982.6 61.3 3.4 0.00228
Ice Cream Mix 80°C 1100 68.64 150 0.1008

Note: These figures incorporate a safety factor in recognition of the potential variances in product formulations and batching procedures.

3. Determine the Reynolds number at the maximum flow rate for water and all products to be processed using the following formula:

  • Reynolds number (Re) = (p x V x d)
  • Where: p = fluid density (kg/m3).
  • V = velocity (m/s)
  • d = tube inside diameter (m).
  • μ = viscosity (kg/ (sec x m)). (Table 2)

4. Convert the Reynolds number obtained into logarithmic number.

5. Using the converted Reynolds number read the efficiency factor from the provided chart (Figure 1).

6. Determine the flow rate ratio (r) using the method outlined in Test 8, steps 11 and 12 and the following formula:

  • Flow rate ratio (r) = ((Mv)/Wv)
  • Where: Mv = average time required to deliver a measured volume of product.
  • Wv = average time required to deliver an equal volume of water.
  • For large pasteurizers this should be done by using a magnetic flow meter instead of 36 litre can.

7. Calculate the minimum holding tube length using the following formula:

  • L = (t x V)/(E x r)
  • Where: L = Length (m)
  • t = minimum holding time (s).
  • V = velocity (m/s).
  • E = efficiency factor.
  • r = flow rate ratio

8. Calculate the target salt test using the following formula:

  • Target salt test in seconds = L/V
  • Where: L = Length (m).
  • V = velocity (m/s).

Example:

A cheese plant wants to process cheese milk at 162°F for 16 seconds at a flow rate of 25,000 litres per hour. The determined flow ratio of milk/water is 0.88. The holding tube size to be installed is 2". What is the required holding tube length and target salt test?

Figure 1 - Efficiency Values Ration of Average to Maximum Velocity (V/V Max)
Figure - Efficiency Values Ration of Average to Maximum Velocity (V/V Max). Description follows.
Description of image - Effect of Reynolds number on the ratio of average to maximum velocity in smooth tubes
  • This graph is used to determine the efficiency value by using the Reynolds number to calculate the time in smooth holding tubes.
  • The x axis on the graph shows Efficiency Value for the Ratio of Average to Maximum Velocity (V/V max). The values for the scale range from 0.5 to 1.0.
  • The y axis shows Reynolds No. - Log Chart. The values range from 102 to 107.
  • The graph also shows the points that the laminar flow, transitional and turbulent flow occur.

Convert the flow rate from litres per hour to litres per sec:

Flow rate = 25000/(3600) = 6.94 litres/sec

Convert the tube internal diameter from inches to metres:

Pipe diameter = 2"; therefore internal diameter from Table 1 = 1.872" = 0.0475 m

Calculate the cross-sectional area of the tube (internal):

Area = πr2 = 3.14 × (0.0475)/4 = 0.00177 m2

Using the flow rate and cross-sectional area, calculate the velocity:

  • Velocity = F/A; where F = flow rate and A = cross-sectional area;
  • Velocity = (6.94 × 0.001(m3/s)) /(0.00177(m2)) = 3.90 m/s

Calculate the Reynolds number for water and milk:

  • Re water = (p x V x d); where p = density; V = velocity; d = tube inside diameter; μ = viscosity of water;
  • Re water = (1000(kg/m3) × 3.90(m/s) × 0.0475(m))/0.0005(kg/sec×m)
  • = 370500 or 3.7 × 105
  • Using the calculated Reynolds number and Figure 1, the efficiency factor of water = 0.86
  • Re milk = (p x V x d)
  • = (1012(kg/m3) × 3.90(m/s) × 0.0475(m))/0.000515(kg/sec×m)
  • = 364025 or 3.6 × 105
  • Using the calculated Reynolds number and Figure 1, the efficiency factor of milk = 0.86
  • For holding tube length calculation use the lowest efficiency factor of the two = 0.86

Calculating the tube length:

  • Tube length = L = (t x V)/(E x r); where t = minimum holding time; V = velocity;
  • E = efficiency factor; r = flow rate ratio
  • Tube length = L = (16(s) × 3.90(m/s))/(0.86 × 0.88)
  • = 83.42 metres

Calculating the target salt test:

  • Target salt test = L/V; where L = length; V = velocity
  • = 83.42(m)/3.90(m/s)
  • = 21.39 seconds

References:

  1. A Research Note, Prediction Of Holding Times For Continuous Thermal Processing of Power-Law Fluids, Volume 41 (1976) Journal of Food Science, J.A. Palmer and V.A. Jones. Department of Food Science, North Carolina State University, Raleigh, NC 27607.
  2. Effect of Process Variables on the Holding Time in an Ultrahigh–Temperature Steam Injection System, Volume 53. No. 10 Journal of Dairy Science, E.R. Edgerton and V.A. Jones. Department of Food Science and J. A. Warren Department of Experimental Statistics, North Carolina State University, Raleigh 27607.

Test 9.1: Calculation of Holding Tube Length (Under Review)

Application:

To all APPS and HHST systems using direct and indirect heating.

Note: Because of the short holding times and holding tube length, the required minimum holding times for all APPS and HHST systems must be calculated from the pumping rate rather than the salt conductivity test. Laminar flow may occur in high viscosity products since the fastest particle can move at twice the speed of the average particle. Therefore, the holding tube lengths must be calculated as twice the length to compensate for laminar flow.

Frequency:

  1. Upon installation and annually thereafter.
  2. Whenever the seal on the speed setting is broken.
  3. Whenever any alteration is made affecting the holding time, the velocity of the flow (e.g. replacement of pump, motor, belt, driver or driven pulley, decrease in number of heat exchange plates), or the capacity of the holding tube.
  4. Whenever a check of the capacity indicated a speed up.

Criteria:

Every particle of product to be held for the minimum holding time in both forward and diverted flow positions.

Apparatus:

None.

Method:

The holding tube length is determined by calculation and is specified in the scheduled process. Fully developed laminar flow is assumed. An experimental determination of pumping rate is required, which is accomplished by determining the time required for the pasteurizer to fill a vessel of known volume, converting this by division to obtain flow rate in gallons per second and multiplying this by the applicable number from the tables below to obtain the required length of the holding tube. The resulting calculations will provide the required length of the holding tube for the process. Holding tube lengths for pasteurizers with indirect heating for a pumping rate of 1 gallon (4.546 L)/second are:

Table 1 - Holding Tube Lengths (Inches/cm) for Indirect Heating Pasteurizers
Holding Time
(SEC)
1" (2.54 cm) Tube Diameter " (3.81 cm) Tube Diameter 2" (5.08 cm) Tube diameter " (6.35 cm) Tube diameter 3" (7.62 cm) Tube diameter
1 723.0 (1836.42) 300.0 (762) 168.0 (426.72) 105.0 (266.7) 71.4 (181.36)
0.5 362.0 (919.48) 150.0 (381) 84.0 (213.36) 52.4 (133.10) 35.7 (90.67)
0.1 72.3 (183.64) 30.0 (76.2) 16.8 (42.67) 10.5 (26.67) 7.14 (18.14)
0.05 36.2 (91.95) 15.0 (38.1) 8.4 (21.34) 5.24 (13.31) 3.57 (9.07)
0.01 7.23 (18.36) 3.0 (7.62) 1.68 (4.27) 1.05 (2.67) 0.714 (1.814)

Assumed Pumping rate = 1 US Gal/Sec (4.546 Litres/Sec) (1 US Gal = 3.785 Litres)

Note: These lengths assume fully developed laminar flow

With steam injection processes, the holding tube is adjusted since the product volume increases because of increased volumes in the holding tube. With a 120°F (48.89°C) temperature increase by steam injection, a volume increase of 12% will occur in the holding tube. The values in the table below reflect this volume increase. This surplus water is evaporated off as the pasteurized product is cooled in the vacuum chamber. The temperature-time standard is chosen by the processor and the required holding tube length is calculated from an experimental determination of the pumping rate.

Table 2 - Holding Tube Lengths (inches /cm) for Steam Injection Pasteurizers
Holding Time
(SEC)
1" (2.54 cm) tube diameter " (3.81 cm) tube diameter 2" (5.08 cm) tube diameter " (6.35 cm) tube diameter 3" (7.62 cm) tube diameter
1 810 (2057.4) 336 (853.44) 188 (477.52) 118 (299.72) 80.0 (203.2)
0.5 405 (1028.7) 168 (426.72) 94.0 (238.76) 59.0 (149.86) 40.0 (101.6)
0.1 81.0 (205.74) 33.6 (85.34) 18.8 (47.75) 11.8 (29.97) 8.00 (20.32)
0.05 40.5 (102.87) 16.6 (42.16) 9.40 (23.87) 5.90 (14.99) 4.00 (10.16)
0.01 8.10 (20.57) 3.36 (8.53) 1.88 (4.78) 1.18 (3.0) .80 (2.03)

Assumed Pumping Rate = 1 US Gal/Sec (4.546 Litres/Sec) (1 US Gal = 3.785 Litres)

Note: These lengths assume fully developed laminar flow and temperature increase of 120°F (48.89°C) by steam injection.

The calculations follow the equation: A = B X C

Where:

  • A = holding tube length (inches)
  • B = measured pumping rate (gallons per second)
  • C = holding tube length from Table (inches per gallon per second)

Example 1:

The health authority knows the time-temperature standard and flow rate and wants to know the required length for the holding tube. The pasteurizer has a nominal capacity of 10,000 pounds per hour. The time required to fill a 10 gallon can with water from the pasteurizer is 32.5 seconds. The temperature-time standard is 204°F (95.56°C) for 0.05 second, and the holding tube is 2 inches in diameter. The pumping rate is 10 gallons divided by 32.5 seconds, which is 0.308 gallon per second.

The required holding tube length, A is calculated from Equation 1 (A = B X C). The pumping rate, B, is 0.308 gallon per second, and from Table 1, the holding tube length, C, required for a holding time of 0.05 second with a pumping rate of 1 gallon per second in 2 inch diameter tubing is 9.4 inches.

For this example,

A = 0.308 × 9.4
A = 2.9 inches
Therefore the holding tube must be at least 2.9 inches long.

Example 2:

The health authority knows the temperature-time standard and the actual holding tube length and wants to know the maximum permissible pumping rate.

The pasteurizer has a nominal capacity of 60,000 pounds per hour, and the temperature-time standard is 204°F (95.56°C) for 0.05 second. The holding tube is 3 inches in diameter and 6 inches long.

The pumping rate is calculated from Equation 1 (A = B X C). The holding tube length, A, is 6 inches and from Table 1, the holding tube length, C, required for a holding time of 0.05 second with a pumping rate of 1 gallon per second in 3 inch diameter tubing is 4 inches. For this example:

6 = B × 4
B = 6/4
Therefore … B = 1.5 gallons per second.

The maximum permissible pumping rate is 1.5 gallons per second. At this pumping rate, the time required to fill a 100 gallon vat is 100 gallons divided by 1.5 gallons per second, or 66.6 seconds.

Procedure:

  1. Examine the entire system to insure that all flow promoting equipment is operating at maximum capacity and all flow impeding equipment is so adjusted or bypassed as to provide minimum resistance to the flow. In-line filters must be removed, booster pumps must be in operation and vacuum equipment in the system must be operating at a maximum vacuum.
  2. Before beginning, the pasteurizer is to be operated at maximum flow for a sufficient time to purge air from the system (about 15 minutes) and tighten pipe connections on the suction side of the metering pump enough to exclude the entrance of air. With the pasteurizer operating on water, adjust the metering pump to its maximum capacity, preferably with a new belt and full-size impellers.
  3. Make sure no flow exists in the diverted line and measure the time required to deliver a known volume of water at the discharge line of the pasteurizer in forward flow. Repeat this at least once to determine that the measurements are consistent.
  4. Repeat the above steps 1 to 3 of this procedure in diverted flow by collecting the water at the discharge of the divert line.
  5. Select the greatest flow rate (shortest delivery time for the known volume) and calculate the flow rate in gallons per second by dividing the known volume by the time required to collect the known volume. Multiply this value by the appropriate number in the above tables (Table 1 for indirect heating and Table 2 for direct heating with steam injectors).
  6. Determine the number and type of fittings in the holding tube and convert these to equivalent lengths of straight pipe with the use of Table 3. Determine the total length of the holding tube by adding the equivalent lengths of the fittings to the measured lengths of straight pipe. If the actual holding tube length is equivalent to or greater than the required holding tube length, record the number and type of fittings, the number and length of straight pipes and the holding tube configuration. Make sure the holding tube slopes upward at least 6.35 mm (0.25 inch) per foot. If the temperature sensor is located at the beginning of the holding tube, the holding tube shall be protected against heat loss by material that is impervious to water.
  7. Re-seal regulatory controls as necessary.
  8. Record results.
Table 3: Centerline Distances of 3-Fittings
3 A - Fitting Size Designation 1 1½ 2 2½ 3
2C 90° bend 3.4 4.8 6.2 8 9.7
2CG 90° bend 3.1 4.5 5.8 7.6 9.3
2F 90° bend 3.4 4.8 6.2 8 9.7
2FG 90° bend 3.1 4.5 5.8 7.6 9.3
2E 90° bend 3.4 4.8 6.2 8 9.7
2EG 90° bend 3.2 4.6 6 7.7 9.4

Alternate procedure for pasteurizers of large capacity:

  1. Remove the divert line from the raw product supply tank and turn off the product pump feeding the raw product supply tank. Suspend a sanitary dip stick in the raw product supply tank and operate the pasteurizer at maximum capacity.
  2. Record the time required for the water level to move between two graduations on the dip stick. The volume of water is calculated from the dimensions of the raw product supply tank and the drop in water level.
  3. Flow rate = the volume of water removed from the raw product supply tank divided by the time required to remove the water.

Corrective Action:

If the length of the holding tube is shorter than the calculated length, reseal the metering pump at a slower speed or lengthen the holding tube or both and repeat the above determination.

Date modified: