USDA 

 

THERMAL PROCESSING 

TRAINING 

Module 11. Retorts— Processing 

With Overpressure 

Thermal Processing for Meat and Poultry 

Products Training 

1 
32 . ps I 9 

/ 

 

THERMAL PROCESSING 

TRAINING 

What is Overpressure? 

Overpressure: 

 Pressure supplied to 

a retort in excess of 

the normal pressure 

exerted by the 

heating medium at a 

given temperature 

20 

2 
AIR PRESSURE 

' ' ' l l - -==--=-- . - --- -

t~ 

HEAT 

EXPANSION 

 

Purpose of Overpressure 

THERMAL PROCESSING 

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Overpressure 

 To maintain container 

integrity 

 To permit adequate 

processing 

3 
 

 

 

 

 

THERMAL PROCESSING 

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Overpressure Requirements 

Overpressure requirements vary: 

 Too much at the start of the process could distort 

containers (crush containers) or damage seals 

 Too little during heating could lead to container 

rupture or seal damage, slow heat penetration, or 

interfere with water circulation patterns in the retort 

 Too little during cooling could lead to container rupture 

or seal damage 

4 
 

THERMAL PROCESSING 

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Factors Affecting Overpressure Requirements 

 Product fill temperature 

 Container headspace 

 Container vacuum 

 Entrapped air 

 Processing temperature 

5 
Overpressure Applications 

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 Plastic containers 

 Flexible pouches 

 Metal trays 

 Glass jars 

6 
 

 

 

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General Characteristics of Retorts that 

Provide Overpressure 

 Introduced steam or air is the source of 

overpressure 

 Batch processing, not continuous container 

handling 

 Static (still), rotary (end-over-end), and back 

and forth (Shaka®process) agitation models 

7 
rotation 

 

 

THERMAL PROCESSING 

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End-Over-End Agitation 

 Containers are held 

in place in the basket 

 Rotating framework 

holds baskets 

 Variable rotational 

speed 

 Custom racking 

system 

8 
 

End-Over-End Agitation 

THERMAL PROCESSING 

TRAINING 

Lock Down 

Mechanism 

and Plate 

Crate to 

Hold 

Containers 

Rotating Reel 

9 
 

 

Back and Forth (Shaka®process) Agitation 

THERMAL PROCESSING 

TRAINING 

 Containers are held in 

place in the basket 

 Variable double 

reciprocating strokes 

 Custom racking/basket 

system 

10 
Retorts that Provide Overpressure 

THERMAL PROCESSING 

TRAINING 

 Processing medium: 

• water immersion, 

• cascading water, 

• water spray, or 

• steam/air mixture 

Water Spray 

11 
 

 

 

Installation and Operation of 

Overpressure Retorts 

THERMAL PROCESSING 

TRAINING 

Common Considerations: 

 Installation, operation, and 

controls vary based on the 

processing medium 

 Usually equipped with 

various alarms and 

warning devices to 

monitor critical operating 

procedures 

12 
Instrumentation-Pressure Control 

THERMAL PROCESSING 

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 Each retort must 

have a pressure 

recording device 

 Each retort should 

have pressure gauge 

13 
 

 

 

 

 

 

THERMAL PROCESSING 

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Processing Medium Circulation 

 Each retort must have a means of providing 

uniform HD/TD during processing 

 The efficiency of the circulation system must 

be documented in HD/TD data or other 

documentation from a PA 

 HD/TD data must be on file at the 

establishment to support the retort operating 

procedures 

14 
 

 

THERMAL PROCESSING 

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Factors that may Affect Heat Distribution or 

Processing Medium Circulation 

 Crate and rack design 

 Loading configuration 

 Container type, size, 

or position 

Nested Pouches 

15 
 

 

THERMAL PROCESSING 

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Factors that may Affect Heat Distribution or 

Processing Medium Circulation 

 Operating pressure 

 Come-up procedures 

 Partial loads 

 Fan or pump off or not functioning properly 

16 
THERMAL PROCESSING 

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Full Water Immersion Retorts 

17 
 

 

THERMAL PROCESSING 

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Water Immersion Retort Characteristics 

 Water is the heating/cooling medium which 

completely covers the containers 

 Horizontal and vertical configurations 

 Introduced air or steam provides overpressure 

 Several manufacturers 

18 
 

 

 

THERMAL PROCESSING 

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Instrumentation - Temperature Indicating Device 

 Sensor (probe) located beneath water surface 

 Sensor (probe) must extend at least 2" into 

water 

 Vertical retorts – sensor (probe) may be 

located in thermometer pocket 

19 
 

 

 

THERMAL PROCESSING 

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Instrumentation - Temperature Indicating Device 

 Horizontal retorts – sensor (probe) must be 

directly in shell for a retort using a steam 

spreader 

 Horizontal retorts using a heat exchanger – 

sensor (probe) is located on the water return 

pipe before the entry to the exchanger 

20 
 

 

 

 

THERMAL PROCESSING 

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Instrumentation - Temperature/Time 

Recording Device 

 Recorder sensor (probe) usually adjacent to 

temperature indicating device sensor 

 May be combined with the steam controller to 

be a recorder-controller 

 Vertical retort – sensor (probe) must be below 

lowest crate support where steam does not 

strike it directly 

21 
 

 

 

 

THERMAL PROCESSING 

TRAINING 

Instrumentation – Recorder/Controller 

Sensor Location 

 Horizontal retort – sensor (probe) must be 

between water surface and center line in a 

retort using a steam spreader 

 Horizontal retorts using a heat exchanger – 

sensor (probe) is located on the water return 

pipe before the entry to the exchanger 

22 
,- -

 

 

 

Instrumentation - Water Level Indicator 

THERMAL PROCESSING 

TRAINING 

 Each retort must be 

equipped with a 

water level indicator, 

e.g., 

• Water sight glass 

tube 

• Water level alarm 

(visual and audible) 

• Petcock valves 

WATER 

LEVEL 

SIGHT 

GLASS 

23 
AIR 

61NCHES 

! SIGHT GLASS 

Instrumentation - Water Level Indicator 

24 

THERMAL PROCESSING 

TRAINING 
Instrumentation - Water Level Indicator 

THERMAL PROCESSING 

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Petcocks 

25 
Instrumentation - Water Level Indicator 

THERMAL PROCESSING 

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26 
 

 

 

 

THERMAL PROCESSING 

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Instrumentation - Drain Valves 

 Water immersion retorts must be equipped 

with non-clogging, watertight drain valve to 

ensure minimum water level 

 Must have screens over drain openings to 

prevent loose containers and debris from 

entering circulation system 

27 
THERMAL PROCESSING 

TRAINING 

Instrumentation - Crate Supports 

 For vertical water immersion retorts: 

• Bottom crate supports are required 

• Crate centering guides are recommended 

• Bottom baffle plates prohibited 

28 
APPROX 

SAME 

EMP. 

Vertical Retorts 

THERMAL PROCESSING 

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29 
 

 

THERMAL PROCESSING 

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Vertical Retorts - Water Circulation 

 By compressed air or 

 By mechanical means such as a pump 

30 
 

 

 

THERMAL PROCESSING 

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Vertical Retorts - Compressed Air 

Water Circulation 

 Air is introduced with steam through bottom 

spreader 

 Air bubbles agitate water as they rise upward 

 During CUT, a greater volume of air is needed 

to prevent steam hammer (noise) 

31 
 

THERMAL PROCESSING 

TRAINING 

Vertical Retorts - Compressed Air 

Water Circulation 

 During processing, the air volume is reduced 

to recommended level 

 The air in the retort headspace maintains the 

overpressure 

 Two steam spreader designs 

32 
Vertical Retorts - Spreader Configurations 

THERMAL PROCESSING 

TRAINING 

33 
Horizontal Retorts - Design 

THERMAL PROCESSING 

TRAINING 

AllPax Stock Rotomat 

34 
THERMAL PROCESSING 

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Horizontal Retorts - Design 

AllPax or Stock Rotomat 

Steam pressure 

H.E. 

 

35 
 

 

 

 

 

 

THERMAL PROCESSING 

TRAINING 

Horizontal Retorts - Mechanical Water 

Circulation 

 Horizontal retorts need a water recirculation 

system which is usually a pump 

 Suction manifold in the bottom of the retort 

 Distribution system (spreader) along the top of 

the retort 

 Suction outlets must be protected with screens 

 Pump must be equipped with warning device 

to indicate when it is not functioning 

36 
 

 

 

Water Immersion Retort - Crates/Container 

Orientation/Loading Configuration 

THERMAL PROCESSING 

TRAINING 

 Racks designed to 

positively hold 

containers 

 Must provide free 

movement of the 

water 

 Adequacy must be 

documented in 

HD/TD data 

37 
 

 

 

 

THERMAL PROCESSING 

TRAINING 

Water Immersion Retort - Cooling Water 

Supply 

 Should not strike jars directly 

 Vertical retorts - introduced into process water 

about 4 inches above top layer of jars 

 Horizontal retorts - introduced into suction 

side of water circulation system 

38 
 

 

 

 

Water Immersion Retort – Critical 

Operating Parameters 

THERMAL PROCESSING 

TRAINING 

 Water level 

 Air circulation rate for 

vertical retorts 

 Water recirculation 

rate for retorts using 

pumps 

 Overpressure 

 Reel speed timing for 

agitating processes 

39 
 

 

 

 

THERMAL PROCESSING 

TRAINING 

Agitating Water Immersion Retorts -

Rotational Speed 

 Must be checked before process timing starts 

and, if needed, adjusted as specified in the 

process schedule 

 Must be determined and recorded for each 

retort load 

 Prevent unauthorized changes 

• Lock or Posted Sign 

40 
Agitating Water Immersion Retorts -

Rotational Speed 

THERMAL PROCESSING 

TRAINING 

Crate Position Rotational 

Speed is Determined 

Electronically by Sensors 

on the Reel Shaft 

41 
 

THERMAL PROCESSING 

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Cascading Water Retorts 

42 
 

 

THERMAL PROCESSING 

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Cascading Water Retort Characteristics 

 Water is the heating/cooling medium that rains 

(cascade) down over and through the 

containers 

 Horizontal configuration 

 Introduced air provides overpressure 

 Several manufacturers 

43 
Exploded View of Cascading Water Retort 

THERMAL PROCESSING 

TRAINING 

H.E. Water Distribution 

Manifold 

Water Pump 

Screened Water Return 

Water Level 

Sensor 

44 
Cascading Water Retort Design 

THERMAL PROCESSING 

TRAINING 

45 

Water is Recirculated with a Pump 

WATER PUMP - MUST INDICATE OPERATION 
Cascading Water Retort Design 

THERMAL PROCESSING 

TRAINING 

WATER 

RETURN 

TROUGH 

46 
Cascading Water Retort Design 

THERMAL PROCESSING 

TRAINING Screen in Water Return 

Trough on Retort- 1.5 mm Holes 

47 
 

 

Cascading Water Retort Design 

THERMAL PROCESSING 

TRAINING 

H.E. 

Pump 

Rear View with 

Heat Exchanger 

and Water Return 

at Rear of Retort 

48 
Cascading Water Retort Design 

THERMAL PROCESSING 

TRAINING Water Shower 

49 

Distribution Manifold 
 

Cascading Water Retort Design 

THERMAL PROCESSING 

TRAINING Separate Water Distribution Manifolds 

50 
Cascading Water Retort - Instrumentation 

THERMAL PROCESSING 

TRAINING 

MIG 

RECORDER 

RTD 

PRESSURE 

DIFFERENTIAL 

SENSOR 

AIR PRESSURE 

CONTROL VALVES 

H.E. 

CONTROL RTD 

51 
 

 

Cascading Water Retort – Water Pressure 

Monitoring 

THERMAL PROCESSING 

TRAINING 

52 

Pressure Differential 

Sensor is used to 

Alert Operator to 

Changes in Pressure 

from One Side of 

Pump to the Other 

Side 
Cascading Water Retort – Compressed Air Entry 

THERMAL PROCESSING 

TRAINING 

AIR ENTRY 

53 
stacked in columns 

layer 

pad 

off-set 

 

 

Cascading Water Retort - Crates/Container 

Orientation/Loading Configuration 

THERMAL PROCESSING 

TRAINING 

 Must provide free movement of the water and 

allow water to contact the containers 

 Adequacy must be documented in HD/TD data 

54 
Cascading Water Retort - Water Distribution 

Manifold Maintenance 

THERMAL PROCESSING 

TRAINING Mineral Scale Buildup 

55 
 

 

 

Cascading Water Retort – Water Level 

THERMAL PROCESSING 

TRAINING 

 Water level must be maintained within the 

range specified by retort manufacturer or PA 

during come-up, thermal processing, and 

cooling periods 

Water Level? 

56 
 

 

 

 

 

THERMAL PROCESSING 

TRAINING 

Cascading Water Retort – Water Recirculation 

 Water recirculation (flow rate, e.g., gallons per 

minute) for maintaining uniform heat 

distribution in the retort must be documented 

by HD/TD data or other documentation from 

the PA 

57 
 

Cascading Water Retort – Critical 

Operating Parameters 

THERMAL PROCESSING 

TRAINING 

 Stepped come-up 

procedures 

 Water recirculation 

(flow) rate 

 Overpressure 

 Reel speed timing for 

agitating processes 

58 
THERMAL PROCESSING 

TRAINING 

Water Spray Retorts 

59 
 

 

 

 

 

Water Spray Retort Characteristics 

THERMAL PROCESSING 

TRAINING 

 Water is sprayed over the containers from 

several angles 

 Direct and indirect 

steam heating of the 

process water 

 Some inject steam into 

the retort 

 When steam is injected, 

a steam/water mixture 

heats the containers 

60 
Water Spray Retort Characteristics 

THERMAL PROCESSING 

TRAINING 

 Horizontal configuration 

 Introduced air provides overpressure 

 Several manufacturers 

61 
PROCESS WATEFt 

tJ~VEl PROBE DRAIN 

I

I CQOLING 

. I WATER 

I 

( 

St EAM 

PREHEAT 

PROCESS 

WATcR 

THERMAL PROCESSING 

TRAINING 

Water Spray Retort Design 

62 
HEAT 

EXCHA 'GER 

FILTER 

SJ20/98rn de 

THERMAL PROCESSING 

TRAINING 

Water Spray Retort Design 

63 
Water Spray Retort Design 

THERMAL PROCESSING 

TRAINING Steam Spreaders and Spray Bars 

SPRAY BARS 

STEAM 

SPREADERS 

WATER FILL 

SPREADER 

64 
Water Spray Retort Design 

THERMAL PROCESSING 

TRAINING 

65 

Water Return Through Screened Exit Ports 

WATER RETURNS 

WATER LEVEL 

SENSOR 
 

 

 

 

THERMAL PROCESSING 

TRAINING 

Water Spray Retort – Come-Up Procedures 

 Like cascading water retorts, a stepped 

program with a temperature overshoot is used 

to bring the retort’s cold spot up to process 

temperature 

66 
 

 

Water Spray Retort – Crates/Container 

Orientation/Loading Configuration 

THERMAL PROCESSING 

TRAINING 

 Must provide free movement of the water and 

allow water to contact containers 

 Adequacy must 

be documented 

in HD/TD data 

67 
 

 

 

 

Water Spray Retort – Water Level 

THERMAL PROCESSING 

TRAINING 

 Water level must be maintained within the 

range specified by retort manufacturer or PA 

during come-up, thermal processing, and 

cooling periods 

Water in Bottom? 

68 
 

 

 

 

 

 

THERMAL PROCESSING 

TRAINING 

Water Spray Retort – Water Recirculation 

 Water recirculation (flow rate, e.g., gallons 

per minute) for maintaining uniform heat 

distribution in the retort must be documented 

by HD/TD data or other documentation from 

the PA 

69 
 

 

 

THERMAL PROCESSING 

TRAINING 

Water Spray Retort – Critical Operating 

Parameters 

 Stepped come-up procedures 

 Water recirculation (flow) rate 

 Overpressure to maintain container integrity 

 Reel speed timing for agitating processes 

70 
THERMAL PROCESSING 

TRAINING 

Steam/Air Retorts 

71 
 

 

 

THERMAL PROCESSING 

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Steam/Air Retort Characteristics 

 A mixture of steam and air is the heating 

medium 

 Steam/air ratios range from 75% steam/25% 

air to 95% steam/5% air 

 A fan is used to maintain uniform steam/air 

circulation 

 Introduced air provides overpressure 

 Several manufacturers 

72 
Steam/Air Retorts 

THERMAL PROCESSING 

TRAINING 

Baffle Plates 

Plenum 

73 
Steam/Air Retorts 

THERMAL PROCESSING 

TRAINING 

FAN 

74 
 

 

 

 

 

THERMAL PROCESSING 

TRAINING 

Instrumentation-Temperature Device/Recorder 

 Temperature indicating device and recordercontroller probes are usually inserted directly 

into retort shell in such a position that steam 

does not strike them directly 

 The location of the probes on the retort may 

depend on the type of steam/air retort. 

75 
 

 

 

 

THERMAL PROCESSING 

TRAINING 

Steam/Air Retort – Come-Up Procedures 

 Like cascading water/water spray retorts, a 

stepped program with a temperature 

overshoot is used to bring the retort’s cold 

spot up to process temperature 

76 
 

 

 

 

THERMAL PROCESSING 

TRAINING 

Steam/Air Circulation 

 A method of circulating the steam/air mixture 

must be provided 

 The circulation system, usually a fan, must be 

checked for proper functioning and must be 

equipped with device to warn the operator 

when it is not functioning 

77 
 

 

 

 

THERMAL PROCESSING 

TRAINING 

Steam/Air Retort - Critical Operating 

Parameters 

 Stepped come-up procedures 

 Percent steam/air mixture (e.g., 90%/10%), or 

maximum pressure (e.g., 25 PSIG) 

 Steam/air mixture circulation and flow rate 

(e.g., 30 cubic feet/second) 

 Reel speed timing for agitating processes 

78 
 

 

 

 

THERMAL PROCESSING 

TRAINING 

Steam/Air Mixture 

 The steam/air mixture or the temperature 

and pressure (that represents a specific 

steam/air ratio) used to thermal process a 

product must be the same mixture or 

temperature and pressure documented in the 

heat penetration tests used to establish the 

product’s process schedule 

79 
Questions 

THERMAL PROCESSING 

TRAINING 

Questions?

80