1. Pasteurization and Heat sterilization
ERT Food Engineering
Semester 1 Academic Session 2017/18

2. Subtopics
1. Introduction. 1.1 Processing systems 1.2 Pasteurization & Blanching Systems 1.3 Commercial Sterilization Systems Batch Continuous Retort Systems Aseptic processing system Ultra-High Pressure Systems
ERT 426 Food Engineering

3. Subtopics 2. Microbial Survivor Curves 3. Influence of External Agents
4. Thermal Death Time
5. Spoilage Probability
6. General method calculation
6.1 Application to Pasteurization
6.2 Commercial Sterilization
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4. 1. Introduction
In food industry, the processing steps required to eliminate the potential for foodborne illness is known as preservation processes .
Pasteurization is one of the traditional preservation processes, and uses thermal energy to increase the product temperature and inactivate specific pathogenic microorganisms.
Pasteurization results in a shelf-stable product with refrigeration.
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5. Introduction
Commercial sterilization is a more intense thermal process to reduce the population of all microorganisms in the product and leads to shelf-stable products in cans and similar containers.
Recently, technologies such as high pressure and pulsed electric fields have been investigated to reduce microbial populations in foods without the need for thermal energy.
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6. 1.1 Processing Systems
The design of all processing systems is unique for the specific food products being processed.
Traditional thermal processing systems are designed to provide the desired increase in product temperature, followed by a period of holding time and cooling from the elevated temperature.
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7. Processing Systems
Systems for alternative preservation processes involve bringing a treatment agent into contact with the food product for the period of time needed to reduce the action of deterioration reactions within the product.
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8. 1.2 Pasteurization and Blanching Systems
Pasteurization is a mild thermal process used to eliminate specific pathogenic microorganisms from a food.
Blanching is a similar thermal process used to inactivate enzymes in foods and prevent the deterioration reactions in the product.
Both processes accomplish the desired result without using the high temperatures normally associated with commercial sterilization.
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9. Pasteurization and Blanching Systems
Most pasteurization systems are designed for liquid foods, and with specific attention to achieving a specific time–temperature process.
Figure 1:
A milk pasteurization system.
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10. Pasteurization and Blanching Systems
The continuous high temperature-short-time (HTST) pasteurization system has several basic components, including the following:
Heat exchangers for product heating/cooling.
Holding tube.
Pumps and flow control.
Flow diversion valve.
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11. Pasteurization and Blanching Systems
A blanching system achieves a process similar to pasteurization, but with application to a solid food and to inactivate an enzyme system.
Figure 2: A steam blanching system
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12. Pasteurization and Blanching Systems
Since these systems are designed for solid foods, conveying systems are used to carry the product through the system.
The temperature-time profile at the slowest heating/cooling location of the product pieces are critical in ensuring the process, and are established by the speed of the conveyor through both stages of the system
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13. 1.3 Commercial Sterilization Systems
The use of a thermal process to achieve a shelf-stable food product is referred to as commercial sterilization.
The purpose of these processes is the reduction of microbial populations by sufficient magnitudes to create a shelf-stable food, without refrigeration.
The systems used for commercial sterilization are in 3 categories: batch, continuous, & aseptic.
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14. Commercial Sterilization Systems
When considering these categories, the batch and continuous systems accomplish the thermal treatment after the product is placed in the container or package.
For systems in the aseptic category, the process is accomplished before the product is placed in the container or package, and the container or package requires a separate process.
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15. 1.3.1 Batch system
A steel retort is a vessel designed to expose the product to temperatures above the boiling point of water.
The vessel must maintain pressures up to 475 kPa, or the pressures needed to maintain steam temperature as high as 135° to 150C.
The control system is designed to allow the environment (pressure and temperature) within the vessel to be increased to some desired level for a specified time, held at the desired condition for a specified time, and then returned to ambient pressure and temperature conditions.
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16. Batch system
For these systems, the product is introduced into the vessel after being placed into a container or package and sealed.
Figure 3:
A typical batch retort system for commercial sterilization of foods.
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17. 1.3.2 Continuous Retort Systems
Various systems have been developed to allow commercial sterilization to be accomplished in a retort and in a continuous manner.
The hydrostatic system uses a tower and two columns of water to maintain a high pressure steam environment for the product containers to move through.
The height of the water columns is sufficient to maintain the desired steam pressure and temperature.
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18. Continuous Retort Systems
The product conveyor carries the containers through the system in a continuous manner.
The product containers enter the system through a column of hot water and heating of product is initiated.
The heating of the product is completed as the product is conveyed through the steam environment.
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19. Continuous Retort Systems
The final stage of the process is accomplished as the product containers are conveyed through a column of cold water.
The residence time for product within the system is a function of the conveyor speed.
Ultimately, the desired process for the product is a function of the steam temperature and the time required for product to be conveyed through the system.
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20. Continuous Retort Systems
Figure 4:
Schematic diagram of a hydrostatic sterilization system for canned foods
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21. 1.3.3 Aseptic Processing Systems
The unique aspect of the aseptic processing systems is that the product is thermally processed prior to being placed in a container.
The systems require independent sterilization of the container, and placement of the product into the container while in an aseptic environment.
These systems are limited to products that can be pumped through a heat exchanger for both heating and cooling.
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22. Aseptic Processing Systems
Figure 5: An aseptic processing system.
(1) feed tank with pump; (2) scraped surface heaters; (3) steam-pressurized hot-hold vessel with aseptic pump; (4) master process control panel; (5) aseptic scraped surface cooler; (6) nitrogen-pressurized cold surge; (7) aseptic low-pressure drum filler; (8) empty drum feed conveyor; (9) uppender; (10) palletized empty drum feed; (11) manual drum depalletizer; (12) semiautomatic drum palletizer; (13) palletized full drum discharge.
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23. Aseptic Processing Systems
By using high pressure steam as a heating medium in a heat exchanger, the product can be heated to temperatures in excess of 1000C.
Following the heating step, the product is pumped through a holding tube for residence time needed to achieve the desired thermal process.
Product cooling is accomplished in a heat exchanger using cold water as a cooling medium.
These systems cannot be used for solid foods, but have been adapted for high viscosity foods and liquid products containing solid particles.
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24. 1.3.4 Ultra-High Pressure Systems
The use of high pressures to achieve food preservation has evolved as a potential commercial process.
Historically, there have been demonstrations of inactivation of microbial populations by using ultra-high pressures in the range of 300 to 800 MPa.
More recently, systems have been developed and used to expose food products and achieve significant reductions in microbial populations in the product.
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25. Ultra-High Pressure Systems
Figure 6:
Schematic of an ultra-high pressure processing system.
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26. Ultra-High Pressure Systems
Figure 6:
The primary component of the system is a vessel designed to maintain the high pressures required for the process.
A transmitting medium within the vessel is in contact with the product and delivers the impact of the agent to the product and microbial population within the product.
Current systems operate in a batch or semicontinuous mode.
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27. Ultra-High Pressure Systems
A typical UHP process for a solid food or product in a container would be accomplished by placing the product in the system vessel, followed by filling the space around the product with the transmitting medium.
Pressure is increased by high pressure pumps for the transmitting liquid or by activating a piston to reduce the volume of the medium surrounding the product.
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28. Ultra-High Pressure Systems
After the desired pressure is reached in the vessel, the pressure is maintained for the period of time needed to accomplish the required reduction in microbial population in the product.
At the end of the holding period, the pressure is released and the process is completed.
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29. Ultra-High Pressure Systems
A system for pumpable product would use a high-pressure transfer value to introduce product into the vessel.
For these types of semicontinuous systems, the pressure is increased by introducing water behind a free piston applied directly to the product.
Following the process, the product is pumped out of the vessel, and the process cycle is repeated.
The processed product must be filled into packages or containers in an aseptic environment.
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30. Ultra-High Pressure Systems
During the UHP process, the product temperature will be increased further to adiabatic heating.
A typical product would experience an increase of 30C per 100 MPa, although the magnitude of increase would vary with product composition.
This temperature increase may or may not influence the process depending on the temperature of the product entering the UHP system.
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31. 2. Microbial survivor curves
During preservation processes for foods, an external agent is used to reduce the population of microorganisms present in the food.
The population of vegetative cells such as E. coli, Salmonella, or Listeria monocytogenes will decrease in a pattern as shown in Figure 7.
The population of microbial spores will decrease in a similar manner, but after an initial lag period.
These curves are referred to as microbial survivor curves.
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32. Microbial survivor curves
Figure 7: A survivor curve for a microbial population.
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33. Microbial survivor curves
Although the shape of these curves is often described by a first-order model, there is increasing evidence that alternate models are more appropriate when the application is the design of a preservation process.
A general model for description of the microbial curve would be:
where k is the rate constant and n is the order of the model.
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34. Microbial survivor curves
This general model describes the reduction in the microbial population ( N ) as a function of time.
First-order kinetic model (n=1) has been used to describe survivor curves obtained when microbial populations are exposed to elevated temperatures.
When survivor curve data are presented on semilog coordinates, a straight line is obtained, as shown in Figure 8.
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35. Microbial survivor curves
Figure 8: Microbial survivor curve on semilogarithmic coordinates.
The slope of the straight line is the first-order rate constant ( k ), and is inversely related to the decimal reduction time, D .
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36. Microbial survivor curves
The decimal reduction time (D) is defined as the time necessary for a 90% reduction in the microbial population.
Alternatively, the D value is the time required for a one log-cycle reduction in the population of microorganisms.
Based on the definition of decimal reduction time, the following equation would describe the survivor curve:
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37. Microbial survivor curves
It is evident that:
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38. 3. Influence of External Agents
The survivor curves for microbial populations are influenced by external agents.
As the magnitude of preservation agents such as temperature, pressure, and pulsed electric fields increase, the rate of the microbial population reduction increases.
Exposure of a microbial population to an array of higher temperatures results in an increasing slope for the first-order curves.
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39. Influence of External Agents
Traditional thermal processing has used the thermal resistance constant (z) to describe the influence of temperature on decimal reduction time (D) for microbial populations.
The thermal resistance constant (z) is defined as the increase in temperature necessary to cause a 90% reduction in the decimal reduction time D.
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40. Influence of External Agents
A plot of the logarithm of D versus temperature, used to determine the thermal resistance constant (z).
The D values for different temperatures are plotted on semilog coordinates, and the temperature increase for a one log-cycle change in D values is the z value.
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41. 4. Thermal Death Time
The thermal death time (F) is the total time required to accomplish a stated reduction in a population of vegetative cells or spores.
This time can be expressed as a multiple of D values, as long as the survivor curve follows a first-order model.
A typical thermal death time in thermal processing of shelf-stable foods is F = 12D, with the D value for Clostridium botulinum.
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42. 5. Spoilage Probability
When considering shelf-stable food products, we can design the preservation process to eliminate spoilage in addition to ensuring microbial safety.
The spoilage probability is used to estimate the number of spoiled containers within a total batch of processed product.
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43. Thermal Death Time
If the goal of the preservation process is to achieve a probability of one survivor from the microbial population for all containers processed, then
The initial population of spoilage microorganisms in each container
Decimal reduction time
Number of containers exposed to the preservation process
Thermal death time
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44. 6. General method calculation
Major requirement of the general method is that the thermal death time, F, for the microbial population considered must be known at all temperatures to which the product is exposed during the preservation process.
It should be noted that the thermal death time decreases as temperature increases.
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45. 6.1 Application to Pasteurization
During pasteurization, the food is heated to a defined temperature, and held at that temperature for a defined time period.
The lethality associated with the pasteurization process is based on the holding period only; impact of elevated temperatures on lethality during heating and cooling are not significant or are not considered.
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46. Application to Pasteurization
The traditional batch pasteurization process is accomplished using a holding time of 30 min at 63°C.
By using a reference temperature of 63°C, the lethal rate is 1.0 for the entire holding period of 30 min.
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47. Application to Pasteurization
As illustrated in Figure below, the lethality for the process is the area under the lethal rate curve, or 30 min.
A lethal rate curve for a pasteurization process.
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48. Application to Pasteurization
The HTST pasteurization process is a continuous process accomplished by heating the product to 71.5°C and passing through a holding tube at a rate that ensures the required holding time.
When the lethal rate for the HTST process is based on the reference temperature of 63°C, the magnitude is 120.
The holding time needed to achieve the same lethality as the batch process is 15 s.
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49. 6.2 Commercial Sterilization
In order to express the process in terms of the application, the actual time for the product to be exposed to the retort environment must be defined.
As illustrated in Figure below, a finite amount of time is required for the temperature within the retort to reach the final and stable condition ( T M).
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50. Commercial Sterilization
The temperature history at the slowest-heating location would follow the curve indicated.
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51. Commercial Sterilization
Figure: Typical temperature history curves for the heating medium and the product during a thermal process.
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52. Commercial Sterilization
The temperature does not reach TM, and the decrease in temperature at the slowest-heating location occurs significantly after the container is exposed to the cooling medium.
The impact of the thermal process is evaluated by using the measured temperature history to create the lethal rate curve.
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53. Commercial Sterilization
In many situations, the purpose of the process calculation is to estimate the actual process time required to achieve a target thermal death time, F.
In applications, the operator time ( tp) is the difference between the time at the end of product heating (beginning of cooling) and the time when the retort reaches a stable heating medium temperature.
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