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Thermal treatment for products

Thermal treatment is normally applied to the product to reduce its microbial load and guarantee the required shelf life. Heat acts on the proteic structures inside the microorganism cells and can kill the microorganism or deactivate its reproduction capability. Effectiveness of a heat treatment is normally expressed in pasteurizing unit:


K = constant,
ts = sterilization time in minutes
Ts = sterilization temperature in °C. 

Heating and cooling time are not considered in the determination of the heat treatment time, although they contribute to the sterilization performance. Thus, holding time and not heating time is used as a treatment parameter. Temperature is the most effective microorganism killing agent and the most relevant treatment parameter. Its sterilizing effect is growing exponentially when temperature is raised. High efficiency can be reached from 80°C upwards. For example, 100 P.U. can be applied to orange juice by treating it at 130°C for 1 second holding time or by treating it at 90°C for 50 seconds holding time. It is normally necessary to make compromises between treatment parameters and product decay after treatment to determine a treatment that allows the product to reach the required shelf life at the lowest possible decay of organoleptic properties. P.U. are useful to compare different treatment and determine their overall efficiency, but analysis of the actual product according to reference microorganisms is necessary to determine the actual killing action of a given treatment.

 Product pasteurization is an important step in the whole aseptic process.
Fig.1. Product pasteurization is an important step in the whole aseptic process.
When a microbic colony is heated for a given time at a given temperature, the speed of the reduction in the number of microorganisms is directly proportional to their concentration
according to the relation:

Where K is the reaction speed constant, depending from reference microorganism, temperature and environment conditions.

By integrating between t1 = 0 and t2 = t we obtain:

or, in general terms:

Switching from natural logarithms to base 10 logarithms and highlighting N = N0 / 10 we obtain:


where D (decimal reduction time) represents the timeframe where the number of microorganisms is reduced by a factor of 10.

By substituting the found value we obtain the first Law of Bigelow:


Values for D change according to the different reference microorganism. The higher the value of D, the more resistant is the microorganism.

We can infer from the First Law of Bigelow that a longer time is required if N0 is a big number.

Moreover, it is not possible to completely kill all microorganisms, as this would imply an infinite treatment time. Killing time becomes shorter as the treatment temperature becomes higher (see "Graphic representation of Second Law of Bigelow").

The inclination of TDT is represented as z, equal to the number of degrees needed to reduce D by a factor of 10. The relation between D(t) and z can be expressed as:

If we assume the value D121°C we obtain the canonical form of the Second Law of Bigelow:

This second law enables us to know the value of D at the required temperature if we know D121°C and z.

For example, we know that for Clostridium botulinum z is equal to 10°C and D121°C value is 0.2'; we can then calculate the value for D at the temperature of 100°C:

D and z express the resistance of a reference microorganism; you have to take into account also the composition of the product, as the ingredients of a certain product feature a different resistance to heat. The time required (expressed in minutes) to reach a certain sterilization degree is called sterilizing effect FTz and can be calculated using the following formula:

n is defined as the number of log reductions.

For a temperature of 121°C we obtain


From a technical point of view FTz is the sum of the killing contribution of each temperature for the microorganisms hosted inside the foodstuff; in other words it is the integral of the heat penetration curve.

The killing action is defined as the relationship between F0 and FTz.

It is then necessary to know the thermal history T(t), that is the heat penetration curve into the product. The sterilization effect can be expressed as:
where tP is the sum of times due to heating time, contact time and cooling time.

Graphic representation of Second Law of Bigelow

Graphic representation of Second Law of Bigelow
Fig.2. Graphic representation of Second Law of Bigelow

Table of contents

  1. Introduction
  2. 1.Markets, opportunities, a comparison of the technologies
    1. 1.1. “High acid” and “Low acid” beverages
    2. 1.2. Juices and Nectars
    3. 1.3. Sport Drinks
    4. 1.4. Tea and infusions
    5. 1.5. Functional Beverages
    6. 1.6. Milk-based products
    7. 1.6.1. UHT Milk
    8. 1.7. Historical perspective: Evolution of the technology from the Roman era to our day and age
    9. 1.7.1. "Aseptic" technology in the Roman era
    10. 1.7.2. The Roman "filling, capping and storage process"
    11. 1.8. Technologies to meet market demand
    12. 1.8.1. Use of preservatives
    13. 1.8.2. Hot fill
    14. 1.8.3. Ultra-clean filling
    15. 1.8.4. Aseptic Filling
    16. 1.8.5. Aseptic Blow Filling
    17. 1.9. Advantages and disadvantages of containers for beverages
    18. 1.9.1. Glass
    19. 1.9.2. Polylaminate carton
    20. 1.9.3. PET
    21. 1.9.4. HDPE
    22. 1.9.5. Cans
    23. 1.9.6. Pouches
    24. 1.10. Caps, closures, fitments
  3. 2.The right direction of sustainability
    1. 2.1. Material
    2. 2.2. Energy
    3. 2.3. Space
    4. 2.4. Time
  4. 3.Thermal treatment for product
    1. 3.1. Heat Exchangers for Liquid Products
    2. 3.1.1. Plate Heat Exchanger
    3. 3.1.2. Single Tube Heat Exchanger
    4. 3.1.3. Multi Tube Heat Exchanger
    5. 3.1.4. Triple Tube Heat Exchanger
    6. 3.1.5. Spiral Tube Heat Exchangers
    7. 3.1.6. Scraped Surface Heat Exchangers
    8. 3.2. Indirect and Direct Heating
    9. 3.3. Direct Heating UHT and ESL Designs
    10. 3.3.1. Direct Injection
    11. 3.3.2. Direct infusion
    12. 3.4. The best heat exchanger for your application
    13. 3.4.1. Heat Damage to food
    14. 3.4.2. System Selection Criteria
    15. 3.5. Conclusions
  5. 4.Understanding aseptic filling technology
    1. 4.1. Aseptic technology: an integrated system, not a series of connected machines.
    2. 4.2. Structure of an aseptic filling line
    3. 4.2.1. Sterilization
    4. 4.2.2. Container sterilization
    5. 4.3. Treatment of containers
    6. 4.3.1. Peroxyacetic Acid (POAA or PAA)
    7. 4.3.2. H2O2
    8. 4.4. PAA WET container sterilization
    9. 4.5. PAA vapour container sterilization
    10. 4.6. H2O2 CHP container sterilization
    11. 4.7. H2O2 VHP container sterilization
    12. 4.8. Preform sterilization technology
    13. 4.8.1. CHP sterilization
    14. 4.8.2. VHP sterilization
    15. 4.9. Cap sterilization technology
    16. 4.9.1. PAA spray sterilization
    17. 4.10. PAA immersion sterilization
    18. 4.10.1. CHP sterilization
    19. 4.10.2. VHP sterilization
    20. 4.10.3. Pre-sterilized caps handling
    21. 4.11. Energy-based sterilization without chemicals
    22. 4.11.1. UV light sterilization
    23. 4.11.2. Pulsed light sterilization
    24. 4.11.3. Ionizing radiation Sterilization
    25. 4.11.4. Electron beam sterilization
    26. 4.12. Aseptic Filling
    27. 4.12.1. Volumetric electronic filling
    28. 4.12.2. Weight filling
    29. 4.12.3. Other filling technologies
    30. 4.13. Capping
    31. 4.14. Bottle handling
    32. 4.15. Ancillary process equipment
    33. 4.15.1. Sterilizing solution production
    34. 4.16. Sterile water production
    35. 4.16.1. Utilities and fluids handling
    36. 4.16.2. CIP, SIP, COP, SOP
    37. 4.16.3. Integration of ancillary process units
    38. 4.16.4. Piping
    39. 4.16.5. Simplification of line handling
    40. 4.16.6. Radiation-based fluids sterilization
    41. 4.17. Line automation
  6. 5.Your new Aseptic Line
    1. 5.1. Preliminary Checklist
    2. 5.1.1. Volumes
    3. 5.1.2. Products
    4. 5.1.3. Design
    5. 5.1.4. Costs
    6. 5.1.5. Centralising production
    7. 5.2. Evaluation of the investment
    8. 5.2.1. Choose according to your own needs: the value curve
    9. 5.2.2. How to measure the performances of an aseptic line
  7. 6.Good maintenance: the best way to preserve the value of the investment
    1. 6.1. Service Culture
    2. 6.2. TPM
  8. 7.Improved safety: for the product, for operators and for the environment
    1. 7.1. Microbic Contamination
    2. 7.2. Contamination Control
    3. 7.3. Microbiological Isolator
    4. 7.4. Air Filtration
    5. 7.5. Differential Pressures
  9. 8.Aseptic filling and FDA
    1. 8.1. FDA Validation
    2. 8.2. Electronic Validation
    3. 8.2.1. GAMP 4 Module
    4. 8.3. Paper Recording vs Electronic Recording
  10. 9.Sell Aseptic to sell "more" and sell "better"
  11. 10.The Future of Aseptic
  12. Conclusions
  13. Addendum
    1. 1. Thermal treatment for products
Reference: Schlünder,E.U.:Dissertation Techn.Hochschule Darmstadt D 17, 1962.