Thermal Sterilisation

Contents

  1. Introduction
  2. Thermal death rate kinetics
  3. Sterilisation of Foods
  4. Effect of temperature on D-value
  5. Sterilisation time - the general method
  6. HTST Processes
  7. Fo values for selected foods
  8. Problems

 

Introduction

Sterilisation is an important activity in the Food Industries. Sterilisation may be carried out by means of a number of processes including irradiation, sonication, ultrafiltration, ultra high pressure, heating and chemical methods. Of these, the last two are the most important commercially. Chemical methods are most commonly used either to sterilise surfaces for hygiene purposes or to maintain sterility after it has been achieved by other means (ie as preservatives)

There are two major processes, pasteurisation and sterilisation. These are essentially the same, differing only in their severity of treatment. Pasteurisation is used to kill vegetative organisms and rarely uses temperatures above 100° C. Sterilisation aims to inactivate bacterial spores and usually uses temperatures above 100° C.

The ideal is to kill or inactivate all viable organisms. However, this is not achievable in practice so the aim is to reduce risk of contamination to an acceptable level. Factors which determine the degree of sterilisation include safety, cost and effect on product. In the food canning, for example, industry, the dangers of permitting the survival of clostridium botulinum spores are such that a probability of contamination of 10-12 is aimed for. This is termed "commercial sterility"


Thermal death rate kinetics

Bacterial death normally follows first order kinetics i.e. the rate of inactivation is proportional to the number of spores present.

5.1

Where N is the number of viable cells present, t is time and kd(T) is the death rate constant expressed as a function of temperature. This equation may be integrated for constant Temperature to give

5.2

Where No is the initial cell count and Nt is the number of cells after time t. The quantity ln No/Nt is sometimes referred to as the "Del" factor (Ñ ) and is a measure of the desired degree of reduction aimed for.

The death rate constant, kd varies with temperature according to the Arrhenius equation.

5.3

Where ED is the activation energy for thermal cell death, AD is the Arrhenius constant for cell death, T is absolute temperature and R is the universal gas constant.

The time to reduce the number of cells by the desired amount is given by

5.4

If the temperature, T is constant, the integration of the RHS of equation 5.4 gives equation 5.2

5.2


Sterilisation of Foods

Determining the sterilisation time for foods is based on the same principles as described until now, but there are differences in the way in which the principles are applied. In particular, a property called the decimal reduction time or D-value is defined for an organism and sterilisation times are based on this.

Changing the base of the logarithms in equation 5.2 gives

5.21

The D-value is defined as the time to reduce the number of organisms by 1 log cycle i.e. N0/Nt = 10; i.e.

(log 10 = 1.0)

5.22

Since D = t, then

5.23

The D value is specific to a temperature. Therefore when quoting D-values for an organism, the temperature must also be quoted, usually as a subscript to the "D".

e.g. For clostridium botulinum, D121 = 0.2 min.

The definition of D-value can also be represented graphically (Fig. 5.21)


Fig 5.21 D-value for sterilising food.
(Note that D = 1/slope)

Examples of D-values for various organisms are given in table 5.21 below.

Table 5.21. Decimal reduction times (D-values) for various bacteria.

Organism

Temp, T /0C

D-value, DT

Campylobacter jejuni

55

1 min

Salmonella spp

60

0.98 min

Listeria monocytogenes

71.7

3.3 sec

Escherichia coli

71.7

1 sec

Staphylococcus aureus

71.7

4.1 sec

Clostridium perfringens

90

145 min

Clostridium botulinum

121.1

12 sec

Bacillus stearothermophillus

121.1

5.0 min

The sterilisation is usually specified in terms of a decimal reduction. This is the number of powers of 10 that the number of organisms is to be reduced by. (i.e. log N0/N)

The most usual decimal reduction is a "12D" reduction i.e. N0/N = 1012 and log N0/N =12. The sterilisation time is often referred to as the "F number". F is the product of the D-value and the decimal reduction. Thus, for a 12D reduction, F = 12 DT. Because C. botulinum is the basis of most food sterilisations, the F value is determined on the basis of a reference temperature of 121° C. In such cases, F is usually designated F0.

e.g. For a 12 D reduction of clostridium botulinum, F0 = 12 x 0.2 = 2.4 min.

The F0 value calculated this way represents a minimum. In practice, a higher degree of sterilisation and hence of F0 is usually achieved. There are various reasons for this


Effect of temperature on D-value

A consequence of the Arrhenius equation for the temperature dependence of kd is that log D is proportional to temperature. (Fig 5.22)


Fig. 5.22 Temperature dependence of D-value

The relationship between D-value and temperature is defined in terms of the z-value. This is defined as the temperature change needed to bring about a one log cycle change in D-value. In effect z is -1/slope of a graph of D vs. temperature;. i.e.

5.24


Sterilisation time - the general method.

The general method is so called because it is of general applicability to commercial sterilisation and pasteurisation processes, though it is most usually applied to sterilisation of canned foods. It requires the definition of one further parameter, called the lethal rate, Lv and a knowledge of the time-temperature curve for the food being processed.

The value of F0 for a 12D reduction of clostridium botulinum was quoted earlier as 2.4 min. This is based on a constant sterilisation temperature of 121.1° C. In practice, however, the processing is not all at 121.10C. There is a finite heating and cooling time so that some of the processing is inevitably below 121.10C. In addition, the heating steam temperature may vary and the final processing temperature may therefore vary.

To account for this, a parameter called the lethal rate, Lv is defined. This is a measure of the effect of temperature on the actual reduction achieved at any time. Lethal rate is defined from.

Rearranging equation 5.24 for temperatures, Tref and T

5.25

The ratio, Dref/D is defined as the lethal rate, Lv. Hence we can find Lv by taking antilogs of equation 5.25

5.26

Lv is the fraction of the decimal reduction value achieved at temperature, T compared with Tref. If we have a temperature-time profile, we can determine a lethal rate-time profile and hence the F0 value actually achieved for a given sterilisation can be found from

5.27

As with medium sterilisation, this is best determined by graphical or numerical integration based on the Lv-temperature profile.

The F0 value is the process F0 value and is the F0 value actually achieved as opposed to the F0 value desired. Process F0 values are almost invariably greater than the desired F0 value. The reasons for this are given at the beginning of this section.


HTST Processes

There has been a trend towards high temperature processes for sterilisation in recent years. There are a number of reasons for this

Because of the rapid heating and cooling times, continuous sterilisation lends itself to the use of higher temperatures. In many cases, a 10° C increase in temperature can reduce sterilisation times from minutes to seconds. For example, by raising the temperature for milk pasteurisation from 65° C to 75° C, the time can be reduced from 30 min to 15 s. Similarly, the UHT process for milk involves heating the milk to 135° C for 3.5 s , compared with sterilisation for 30 min at 115° C. In both cases, there is much less alteration to the flavour of the milk and, in the case of UHT, the loss of vitamins is considerably reduced.

These processes are generically known as High Temperature, Short Time processes, commonly abbreviated to HTST processes.


Fo Values for Selected Foods

Problems

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Produced by Geoff Walker
Last modified September 2000