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Orthodoxy  &  Suggestions


        Pasteurization is usually applied to low-acid foods to kill the vegetative
        cells of the common pathogenic bacteria and to lower the contamination
        level of spoilage organisms growing under refrigerated conditions of
        The reference parameters of pasteurization treatments suggested by the
        International Dairy Federation [IDF, 1994] are very effective in killing
        most of the vegetative pathogens, while some vegetative  pathogens of
        high heat resistance are not killed at all.
        On the basis of available knowledge about ecology and heat resistance of
        the set of organisms specific to non-acid, medium-acid or high-acid foods,
        four pasteurization treatments can be envisioned.

The heat processes devised to give different degrees of shelf life to food products are usually classified either as pasteurization or sterilization.
The former is a partial treatment, in that it destroys only the more labile
fraction of microbial population. The latter is a complete one, because the level of surviving organisms is lowered beyond any value detectable by usual analytical practices.
The two treatments differ greatly in the size of the lethal agent (heat)
Pasteurization is usually done at temperatures lower than 80-100°C
[176°- 212°F].
Sterilization is applied at temperatures ranging from 115°C to 145°C [239°-293°F].

Sterilized products are stable at room temperature.

Basically, the pasteurization is a process applied to milk:

“Pasteurization is a heat treatment process applied to a product
with the aim of avoiding public health hazard arising from patho-
genic microorganisms associated with milk.”[IDF, 1994].

In a more general way, a pasteurization process is normally applied to
give a limited shelf life to different kinds of food that must be stored
under refrigerated conditions.

    Heat treatments of intermediate size - between pasteurization and
sterilization - are applied to several products [usually fruit based products] to give them storage stability at room temperature.
    The storage stability at room Temperature is reached because such intermediate treatments are applied to products with physico-chemical characteristics adversely affecting the probability of microbial growth:
mostly pH lower than 3.9 (acid-foods),  aw lower than 0.92 (foods of
low-water activity), pH ranging from 3.9 to 4.7 (low-acid foods), or
intermediate values of both parameters.

All over the world the pasteurization parameters of choice are based on
heat treatments issued by national or regional regulations for treatment
of dairy products.
Well grounded reference parameters for the destruction of  pathogenic
and spoilage microorganisms by a pasteurization treatment have never
been scheduled.
Reference treatments analogous to either the minimum botulinum cook
of 3 min at 121.1°C (= 250°F ) or the Fo = 8 -15 [z = 10°C = 18°F] as
in the sterilization practice are not available for the pasteurization process.

It follows that the pasteurization treatments suffer from a certain degree of uncertainty, being devised on a non widely accepted codified basis.
A direct consequence of this situation is that a great deal of products of
very similar characteristics are subjected to pasteurization treatments of
quite different size [JOHNSON et al., 1990; LOPEZ, 1987].

The basis on which such heat treatments are founded are often buried under a layer of out-of-date rules and/or presumed practical evidence [statistically uncertain], really disregarding the some basic bit of modern thermobacteriology represented by the D and ‘z’ relationship.

Even the reference treatment of milk pasteurization at 161°F for 15 sec [IDF, 1994] is incomplete for the lack of an openly defined z value.

You can find here a design for a rationale of the pasteurization process, based on available knowledge of microbial ecology and heat resistance data, taking into account both sanitary and shelf life requirements of different products.


Pasteurized foods can be divided into two main categories:

   1 - products allowing growth of pathogenic organisms owing to the high pH [low acid foods, LAF] and high aw;
   2 - products not allowing growth of pathogenic organisms [Acid Foods, AF, having low pH , and/or Low aw].

    1 - Pasteurized products included in the first category [milk, milk based products, etc.] are are LOW ACID FOODS, LAF.
Usually LAF are stored under refrigerated conditions in the following range:

                0°C  <   5°C  <  10°C
               32°F  < 41°F  <  50°F

[reference temperature 5°C = 41°F].

Basically, the shelf life is expected to lie - following the size of the applied heat treatment - in the two sub-categories:

    1.1 - Short Storage Stability, SHS:  less than two weeks of safe storage [the pasteurization process would destroy only the vegetative pathogenic bacteria and viruses];
   1.2 - Long Storage Stability, LS:  up to 100 days of safe storage
[the pasteurization should destroy vegetative pathogens AND Cl. botulinum spores of the types B, E and F, able to grow under refrigerated conditions];

 2 - Products included in the second category are foods stored at room temperature:
                       10°C < 30°C <  50°C
                       50°F < 86°F < 122°F

having a shelf life limited only by the sensory properties of foods and the persistence of the hermetic condition of packages.

Two sub-categories can be envisioned, following the size of the applied heat treatment :

   AF: Acid foods     [pH lower than 3.8] or foods having aw < 0.92;

    MAF:  Medium acid foods     [ 3.8 < pH < 4.7].

Reference Pasteurization parameters

The heat treatments suggested by the International Dairy Federation [IDF, 1994] for the pasteurization of milk [15 seconds at 71.7°C = 161°F or 30 minutes at 62.7°C = 145°F] can be regarded as  “universal” reference treatments.
The IDF’s parameters are followed in several Countries: France, Canada, Australia, Italy, UK, and with limited modifications, by Germany and Switzerland [Staal, 1986]. The recommended IDF standard is matched by the EC directive 92/46/CEE too [1992].

OBJECTION.Neither the IDF [1994] nor the EC Directive [92/46/EEC, 1992],  provide a suitable “z” value over which to compute a series of equivalent treatments.  Such a situation does not take into consideration the actual Basis
of Thermobacteriology [Pflug, 1987], not even the suggestions already presented more than 40 years ago [Ball & Olson, 1957].

Notwithstanding, by joining both the IDF’s pasteurization parameters by a straight line it gives:

                      Log Pref  = 15.96 - 0.231 * °C              [1]
                                             z = 4.33 °C

              Log Pref  = 20.06 - .1283 * °F              [1.F]
                                 z = 7.79°F

The pasteurization parameters obtained following equations [1] and [1.F] are shown in Table 1.
        Table 1.  Equivalents Pasteurization parameters of milk, based on the equations [1] and
        [1F] obtained from the two parameters suggested by the IDF [1994]: 15 sec at 71.7°C
        and 30 min at 62.7 .
       °C      62       64       66     68       70       72       74
   °F    143    147.2    150.8    154.4     158     161.6     165.2
  minutes     43      15        5       1.8       0.6       0.2      0.07
      °C    70     71      72     73       74        75       76
     °F  158   159.8    161.6   163.4    165.2      167    168.8
  seconds    37    22     13     7.5      4.4      2.6      1.5
The IDF does not explicitly endorse the intermediate pasteurization treatments values thereby obtained. However the above reported parameters should be as reliable as those explicitly recommended, because they derive directly from the application of the universally accepted theory of thermobacteriology.

The EC Directive [1992] on the contrary does supply only one treatment [15 sec at 71.7°C], without an alternative treatment, so that no one can find the values of the tolerated “..any equivalent combination..” [the food-science incompetence of the EC lawmakers is already proverbial !].

The treatment times suggested by the IDF at 72°C and at 62.7°C are high enough to get more than 10D of nearly all the most heat labile bacterial pathogens.

OBJECTIONS:  15 seconds at 72°C are completely useless to kill the high heat resistant Listeria monocytogenes (D72 = 30 sec) [Schoeni et al., 1991] and the heat resistant S. aureus (D72° = 7.2 sec) [Evans et al., 1970].
Only about 4D of Borrelia [Lee et al., 1990] and of Salmonella enteritidis [Xavier & Ingram, 1993] , and only 2D of the L. monocytogenes of average heat resistance (D72° = 8 sec) [Boyle et al., 1990] can be expected after 15 seconds at 72°C.

On the contrary, the pasteurization time of  30 minutes at 62.7°C [the second parameter suggested by the IDF] would lead to about 4D of heat resistant S. aureus and about 7D of the heat resistant L. monocytogenes and more then 7D of S. senftenberg. More than 10D of all the less heat resistant pathogenic bacteria can be expected.

Such a disagreement between the bactericidal effectiveness of the pasteurization treatments suggested by the IDF at the higher and at the lower temperatures, derives directly from the disregarded [by IDF]  relevance of the ‘z’ value upon microbial heat resistance.
The FDA’s recommendations (1986, 1987) are exactly the same as those of IDF, as regards the pasteurization times at 72°C and 63°C, while the suggested treatments at temperatures higher than 72°C are very different (Tab.2).

                                Table 2: Minimum temperatures/times approved by FDA
                                (1987)  for the pasteurization of milk.

            °C        °F         seconds
          62.78      145         1800
          71.67      161             15
          88.33      191               1
          90      194               0.5
          93.89      201               0.1
          95.56      204               0.05
        100      212               0.01
Treatment times lower than 1 sec would be hardly of any interest in the

Anyway, the FDA parameters at the 5 highest temperatures (191-212 °F) are exactly equivalent to each other [ i.e. they are linearly related if
temperature is plotted vs. Log time, as is usual in termobacteriology]
on the basis of z = 10.44 °F = 5.8 °C:

                  Log sec = 18.28 - 0.0958 * °F       [2.F]
            Log sec = 15.21 - 0.1724 * °C   [2]

Objection. The treatment times devised at the two lower temperatures (145°F and 161°F) however, can not fit the above equation. They are more than 13 and 47 times lower, respectively, than times expected by equation [2 and 2.F].
The same situation does occur with the parameters devised by the FDA for the pasteurization of “Milk products of 10%fat or more...”[1986, 1987].  At the highest temperatures, the approved parameters match those expected by equation [2 and 2.F]: at lower temperatures the pasteurization treatments devised by the FDA are of 15 seconds at 74.4 °C and of 30 minutes at 65.6°C, instead of 15 sec at 72°C and 30 min at 63°C, respectively, as expected on the same z-based equivalent values [eq. 2, 2.F]. The reason(s) for the above discrepancies is not obvious.


Four types of pasteurization processes can be envisioned:

 - Chill-Pasteurization
 - Acid-Pasteurization
 - Path-Pasteurization
 - Hard-Pasteurization

1 - Chill-pasteurization (Chill-Past).

The Chill-Past treatment is designed to ensure the destruction of all pathogenic vegetative bacteria.

The Chill-Past treatment could be properly applied to all non-acid products, LAF, including milk and milk based products, having pH higher than 4.7 and water activity higher than 0.98.

The Chill-Pasteurized products must be stored under refrigeration temperature (RT). The shelf life at RT ~ 10°C = 50°F will be shorter
than 2 weeks.
At higher storage temperatures and/or during longer storage time the Chill-Past products will spoil and may become toxic (Cl. botulinum).
All frozen products too (meat and/or vegetables, prepared foods, ice-cream, etc.) should be Chill-Pasteurized before freezing.

The Chill-Past treatment can be applied safely at temperatures ranging from 60 to 90°C [140 - 194°F], according to the time/Temperature parameters described by the two following equations:

[time in seconds]:

                   Log PChill,sec = 10.94 - 0.115 * °C      [4s]

               Log PChill,sec  = 13 - .064 * °F             [4s.F]

        [time in minutes]:

              Log PChill,min = 9.16 - 0.115 * °C            [4m]

            Log PChill,min = 11.22 - .064 * °F            [4m.F]

A sample of the Chill-Past parameters obtained from [4m] and [4s] is
shown in the following Tables 4a, 4b:

            4a:Chill-Past parameters.

    °C  60    62    64    66    68   70    72    74    76
  minutes 183   108    64     38     22   13      8      5      3
    °F 140 143.6 147.2 150.8 154.4 158 161.6 165.2 168.8

            4b:Chill-Past parameters

     °C    76     78     80     82     84   86     88     90
  minutes      3    1.6    0.9    0.5    0.3   0.2   0.11   0.06
    seconds    180     96     55     32     19    11      6     4
     °F 168.8  165.2  172.4  179.6  186.8  194  201.2  208.4
The PChill  parameters provided by [4m, 4s, 4mF, 4sF] will yield a number of decimal reductions (nD) of the most heat resistant S. aureus and Listeria monocytogenes greater than nD = 7 in the entire range of the pasteurization process carried out at temperature T in the range:  60°C < T < 90°C (140°F < T < 194°F) .

EFFICIENCY of the Chill-Past. The Chill-Past treatments would destroy less heat resistant pathogenic vegetative bacteria like E. coli O157:H7, Campylobacter, Yersinia, S. enteritidis, S. senftenberg, V. vulnificus, together with heat labile strains of S. aureus and Listeria,  with a very high probability, as shown in Table 5. Nevertheless, the suggested time / Temperature parameters are the lowest ones required by the destruction [7D] of the most heat resistant reference vegetative pathogens. Thus the suggested parameters can be regarded as the minimum parameters truly able to assure the destruction of all pathogenic non-sporeforming bacteria.

                Table 5. Destruction rate, nD, of the most common non-sporeforming
                pathogenic organisms achieved by Chill-Past treatments.

                                      Temperature  60°C   70°C   80°C   90°C
    Chill-Past, minutes     183    13    0.9  0.06
       Organisms     nD    nD    nD   nD
  E. coli O157:H7 a      155    >>>    >>>    >>>
   Campylobacter b      855    >>>    >>>    >>>
   Yersinia c      394    >>>    >>>    >>>
   Salm. enteritidis d      150    130    100    >>>
   S. typhimurium e      565    >>>    >>>    >>>
   S. senftenberg 774W f        14    163    >>>    >>>
   Shigella disenteriae g      261    >>>    >>>    >>>
   C. dyphteriae g    1115    >>>    >>>    >>>
   Mycobact. tuberculosis g        57    325    >>>    >>>
   Vibrio vulnificus h      871    >>>    >>>    >>>
   Borrelia burgdorferi k       68     93    129    >>>
   S. aureus [Heat Labile] m     203    >>>    >>>    >>>
   List. monocytogenes 
   [Heat Labile] n
      76     65      45     35 
  S. aureus
  [reference strain] o
         7     47    305    >>>
  List. monocytogenes 
   [reference strain] p
       28     18       12       7
                    (a) Doyle & Schoeni, 1984; (b) Doyle & Roman, 1981; (c) Sorquist, 1989;
                (d) Xavier & Ingham, 1993; (e) Dega et al., 1972; (f) Read et al., 1968;
                (g) Evans et al., 1970; (h) Cook & Ruple, 1992; (k) Lee et al., 1990;
                (m) Firstenberg-Eden et al., 1977; (n) Boyle et al., 1990; (o) Evans et al., 1970;
                    (p) Schoeni et al., 1991.
According to Kantor & Potter [1975] Echovirus and poliovirus 1 have
D60 = 30 minutes (z not known). Sixty minutes at 60°C were shown
repeatedly to kill both Hepatitis A and B viruses in suspensions of
unknown titer [LARKIN, 1983].
The Chill-Past  of 183 minutes at 60°C can then be expected to give more than 6D of both group of viruses. Several Legionella have D70° = 1.8 min (z not known) [Stout et al., 1986].
The Chill-Past of 13 min at 70°C will yield more than 7D of the most heat resistant Legionella [Stout et al., 1986].
After 183 minutes at 60°C the expected number of D would be about 180 for Brucella, 37 for Plesiomonas and 70 for V. cholerae [ICMSF, 1996].

The Chill-Past parameters can therefore be considered fully satisfactory from the sanitary view point.

2 - Path-Pasteurization (Path-Past).

The Path-Past can be applied to non-acid (pH > 4.7) and high water activity [aw > 0.96] products.
The Path-Past treatment is designed to destroy all pathogenic organisms, including Cl. botulinum spores of the non-proteolytic types B, E and F.
The shelf-life is expected to be greater than 3 months at temperature below 10°C = 50°F.
The Path-Past can be safely applied at temperatures ranging from 90 to 120°C [194°- 248°F], according to the following equations:

[time in minutes]:

        Log Ppath, min  =  11.07 - 0.099 * °C         [6m]

           Log Ppath, min =  12.83 - .055 * °F       [6m.F]

        [time in secionds]:

          Log Ppath, sec =  12.85 - 0.099 * °C    [6s]

          Log Ppath, sec =  14.61 - .055 * °F    [6s.F]

Following equations [6] the Path-Past parameters shown in Table 7 are obtained.
            Table 7. Equivalent parameters of the Path-Past treatment.
    °C   90   95   100   105   110   115   120
 minutes 145  46.2  14.8    4.7    1.5    0.5   0.15
     °F 194  203  212  221  230  239  248
EFFICIENCY of the Path-Past. The Path-Past treated foods could be safely stored at chill temperatures (lower than 10°C = 50°F) for a very long time, because the Path-Past parameters are expected to kill at neutral pH the Cl. botulinum spores types able to grow at temperatures lower than 12°C.

3 - Acid Pasteurization (Acid-Past).

The Acid-Past treatment is designed to decrease to a suitably low level the survival probability of the most heat-resistant spoilage micro- organisms in high acid (pH below 3.8) foods and in Low water activity Foods.

Products treated by Acid-Past can be safely stored under NNRCS
(“..Normal, Non Refrigerated Conditions of Storage..” [Federal register, 1978]).
The shelf life will be limited only by the durability of the leakage-proof (microbe-proof) condition of the container (LPCC).
The Acid-Past teatments would be applied at temperatures ranging from 90 to 105°C [194°-221°F]. Foods having aw lower than 0.92 can be pasteurized safely by Acid-Past.
The time/Temperature parameters of the Acid-Past treatments can be
obtained from the following equations:

          Log Pacid, min =  19.54 - .1956 * °C   [8]

          Log Pacid, min =  23.02 - .1087 * °F   [8.F]

The Acid-Past  parameters obtained from equations [8, 8F] are shown in Table 9.

                      Table 9. Equivalent  parameters of the Acid-Past treatment.

      °C       90       95    100    105
  minutes       85        9      1     0.1
     °F     194      203    212    221
EFFICIENCY of the Acid-Past.
The heat resistance of the Byssochlamys nivea and B. fulva ascospores
is not clearly defined. However, taking into account that more than six
decimal reductions were observed in less than 88 minutes at 90°C
[Bayne & Michener , 1979] it is quite reasonable to believe that
Acid-Past parameters [8,8F] would offer enough guarantees (at least
about 6D) against the survival of ascospore of these two species.
All the pathogenic and non-pathogenic vegetative bacteria, yeasts, moulds and viruses are killed (more than 40 D of the most resistant Listeria) by Pacid treatments.

4  - Hard-Pasteurization (Hard-Past).

The Hard-Past treatments are designed to kill all the spoilage
microorganisms of acid products having pH lower than 4.7 and higher
than 3.8.
The shelf life under NNRCS of the Hard-Pasteurized products is limited
only by the LPCC. The Hard-Past could be applied at temperatures
ranging from about 90 to 120°C [194 - 248°F].

The Hard-Past parameters are obtained from the following equations:

          Log Phard, min = 15.66 - .14 * °C        [12]

          Log Phard, min = 18.2 - 0.0778 * °F    [12.F]

        The Hard-Past parameters are shown in Table 11a,b.

            Table 11. - Equivalent time/Temperature parameters of the Hard-Past

       °C      97.5    100    102.5    105    107.5    110
  minutes    102    51    20.4    10      4     2
       °F    207.5   212    216.5    221    225.5    230


         °C    110    112    114    116    118    120
       seconds    120      57     30     16      8.3     4.3
        °F   230   233.6   237.2   240.8   244.4    248
EFFICIENCY of the Hard-Past. The Hard-Past parameters will give 13 and 7D of Cl. pasteurianum spores and 7 and 20 to B. coagulans spores, at 100° and 120°C, respectively.
According to Vaughn et al., (1952) the heat resistance of B. macerans and B. polymyxa spores would be:
    B. macerans: D100° (pH 4.8) = 2.2 min
    B polymyxa: D100° (pH 4.8) = 1.5 minutes)
It follows that heat-treatments provided by Hard-Past can be considered of a size high enough to destroy (more than 20D) this kind of spores too.
The Hard-Past is definitely safe also for the most heat resistant fungal
ascospores (see Acid-Past, above): the parameters provided by  equations [12, 12.F] should give to ascospores a number of D ranging between 57 and 400 at treatment temperatures ranging from 85° to 100°C, respectively.
The Hard-Past treatment could be applied to acid products [pH<4] to destroy efficiently [more than 7D] Alicyclobacillus spores [Previdi et al., 1997] too.


The five pasteurization processes can be compared on the basis of the treatment (minutes) they require in the same temperature range [Table 12].

            Table 12. - Comparison of the Pasteurization parameters [minutes].
       °C     60     70     80     90   100   110 
               °F   140   158   176    194   212  230
   IDF [1994], milk    127    0.62    0.003   <<<    <<<   <<<
   Chill-Past   183    13     1   0.06   <<<  <<<
   Acid-Past   >>>   >>>  7709    85    1   0.01
   Path-Past   >>>   >>>   >>>   145   15   1.5
   Hard-Past   >>>   >>>   >>>  1269   51   2
As can be seen, the sanitization power of the five treatments is very different.
However, each process achieves a characteristic effect, based either on a satisfactory high probability of destruction of :
    - all vegetative pathogenic bacteria and viruses (Chill-Past);

    - all vegetative pathogenic bacteria together with Clostridium
      botulinum spores of the non-proteolytic types B, E and F

    - all organisms affecting the shelf stability of high-acid  or low aw
      products stored at ambient temperature (Acid-Past);

   - or all spoilage microorganisms jeopardizing shelf stability of acid
     products under NNRCS (Hard-Past) .

    The liquid milk Pasteurized according to IDF parameters
    might still carry pathogenic organisms.

    Liquid milk should be more safely treated by the Chill-Past
    process before storage under refrigeration.


The required temperature/time parameters of the Path-Pasteurization are safely obtained from the eat resistance data of the more heat resistant spores of pathogenic bacteria able to grow at chill temperatures, that is from the highest heat resistance values of Cl. botulinum spores of the non-proteolytic types B, F and E, as shown in Table 6.

        Table 6. Heat resistance [DT, minutes] of the most heat resistant Clostridium botulinum
        spores of the non-proteolytic types B, E, F.
      90°C     100°C    110°C    120°C   z °C   z °F
 type Ba     18.7      0.89     0.043   0.002   7.6   13.7
 type Ea    11.8     0.74     0.046     <<<   8.3  14.9
 type Fb   0.085    0.0025     <<<     <<<  6.54  11.8
  194 °F   212 °F   230 °F   248 °F
           (a)  Peck et al., 1993; (b) Lynt et al., 1979
                    <<<  =  very low.
The Cl. botulinum spores of the non-proteolytic type B are more heat resistant than spores of the non-proteolytic type E and type F.  The following equations describe heat resistance of the non-proteolytic type B spores:

                                    Log DT = 13.11 - 0.132 * °C                          [5]
                                                         z = 7.6 °C

                                    Log DT = 15.45 - 0.073 * °F                       [5.F]
                                                     z = 13.68°F

The most heat resistant spoilage microorganisms of the medium-acid products belong to the
two species Bacillus coagulans and Clostridium pasteurianum. The B. coagulans spores are
more heat resistant than Cl. pasteurianum spores in the lower range of temperatures.
Nevertheless, the first is a thermophilic bacterium. It follows that the latter might be of more interest.  The Hard-Past is envisioned so as to reduce the survival probability of the most heat resistant Cl. pasteurianum and B. coagulans spores to a level low enough to guarantee the
shelf stability of acid products under NNRCS. The heat resistance of spores of Cl. pasteurianum and B. coagulans at pH 4.5  can be safely described by the following equations:

Cl. pasteurianum  (a) :

                                                        Log DT (min) = 10 - 0.1*°C                          [9]
                                                                        z = 10°C
B. coagulans (b)2 :

                                                       Log DT (min) = 16.3 - 0.156* °C                [10]
                                                                               z = 6.4 °C

At 100 °C the spores of B. coagulans are more heat resistant than spores of Cl. pasteurianum. However the z of Cl. pasteurianum  (z = 10) is greater than the z of B. coagulans (z = 6.4).
So, the inactivation curves of these two leading bacteria are not parallel. At temperatures below 112.5°C the spores of B. coagulans have a heat resistance higher than  spores of Cl. pasteurianum; whereas at temperatures above 112.5°C, Cl. pasteurianum spores have a heat resistance higher than B. coagulans spores [Tab. 10].

    Table 10. Heat resistance [DT,minutes] of Bacillus coagulans and Clostridium pasteurianum

Species 90°C 95°C 100°C 105°C 110°C 115°C 120°C
B. coagulans 183 30 5 0.8 0.14 0.023 0.004
Cl. pasteurianum 10 3.2 1 0.32 0.1 0.032 0.01
194°F 203°F 212°F 221°F 230°F 239°F 248°F

The relationship fitting the highest heat resistance values of both microorganisms, namely
D90°C = 183.2 (B. coagulans) and D120°C = 0.01 (Cl. pasteurianum) could therefore be adopted because it covers all intermediate situations:

                                       Log DT  (min) = 14.81 - .14 * °C     [11]
                                                         z=7.14 °C

                                        Log DT  (min) = 17.35 - 0.0778 * °F [11.F]
                                                             z = 12.85 °F


The safest way to prevent the spreading of foodborne intoxications by a pasteurization
treatment is to apply time/Temperature Parameters of such a size as to reach a
satisfactory killing rate of all pathogenic microorganisms.

The inhibitory conditions (hurdles like pH, water activity, nitrate, storage temperature, etc.) affecting microbial growth [see predictive modeling] are of no interest, because the most
important single condition is that  all pathogenic organisms must be absent from the pasteurized product.

The highest effectiveness of the pasteurization effects upon the target FRE-Pathogens Condition
is achieved by the application of heat treatments to products pre-packaged into hermetically
sealed containers; on some minor degree, by the practice of aseptic processing and packaging.

1. A - The great majority of food-borne outbreaks are due to non-
sporeforming microorganisms, like Campylobacter, Salmonella, E.coli O157:H7, Listeria,
Vibrio, Hepatitis A, Cyclospora, etc. [Altekruse et al., 1997; Tauxe, 1997].
The main purpose of the Pasteurization process required to give a shelf life of two weeks at refrigeration temperature, SHS, would be to lower the degree of microbial contamination of
this kind of target organisms to a  low enough level as to assure the complete safety of
products during storage, with the highest probability.

1. B - The pasteurization treatments required to give the expected shelf life of up to about 100 days at refrigeration temperatures, LSS, must be strong enough to destroy both non-
sporeforming pathogenic organisms and also the spores of the non-proteolytic Cl. botulinum
types B, E and F, this kind of cell and spores bein able to grow at refrigeration temperatures.

2 - The Pasteurization treatments leading to the indefinite storage stability at ambient temperature can be applied only to acid and/or low aw  products LAF and medium acid foods MAF.
The effectiveness of the required pasteurization treatment is linked to the destruction of the two groups of target organisms able to spoil the above kind of products [see below].

Heat treatments.
The usual residual contamination of pasteurized products is hardly lower than one spore/g
and less than 1000 vegetative organisms/g. Hopefully, the degree of killing of target microorganisms reached by the pasteurization process must be analogous to that reached
by the sterilization process.
The residual contamination level of sterilized products receiving a standard  Fo <= 10
[meaning a heat treatment equivalent to 10 minuts at 121.1°C = 250°F] is expected to
be less than 10-7 (1) spores per container (100g-1,000g).
Accordingly, the degree of killing reached by the heat treatments of the pasteurization process
must be based on the same principle of the sterilization process:  the level of survival probability
by the most heat resistant target groups of microorganisms must be lower than 10-7/container.

The meaning of 10-7 survivors.
A product can be expected to carry a contamination of at most one vegetative pathogenic
cell or spoilage spore provided with the highest heat resistance for each kg or liter. At the production rate of about 105 kg/day for about 102 days / year, the heat treatment must
kill at least 105 *102  = 107  undesirable microorganisms. A treatment of such a size
would free from pathogenic and spoilage organisms  1,000 tanks of 10,000 liters each,
containing one microorganism of maximum heat resistance per liter. Such a 10-7 [ten to
minus seven] survival level can then be regarded as very safe from both the sanitary and
storage stability view point.

Methods for designing the pasteurization heat treatments.
The selection of the pasteurization parameters must be based on the highest known heat
resistance values of microorganisms affecting the spoilage probability and/or the sanitary
quality of the four sub-categories of foods [because pasteurization parameters able to kill
the most heat resistant organisms are expected to kill all target organisms].

According to the classical rules of the Thermobacteriology, the heat resistance parameters
taken into account for comparison of heat resistance of microorganisms must be the decimal reduction time at temperature T [DT , the time required to kill 90% of the microorganism
at the temperature T] and z [the number of degrees change required for a DT change of
ten times].
Owing to the effect of ‘z’ on the D value at different temperatures,
the  heat resistance of different sets of target  microorganisms was compared over the whole
range of temperatures that could be used for the pasteurization treatment.

On these grounds, the highest D value at the lowest and at the highest temperature values most suitable for the pasteurization treatments were selected among those showed by all  the microorganisms involved in each sub-category.
The equation of the straight line joining  the highest D value at the lowest pasteurization
temperature (HD.LT)  to the highest D at the highest temperature (HD.HT) was calculated.
The time / temperature parameters obtained from the equation joining the above maxima is expected to define the killing rate expected to be the safest one with the highest probability, because they include the highest heat resistance data actually known, in the entire range
of the pasteurization temperatures and of the target microorganisms.

In this way, four equations were obtained, one for each sub-category of products.
This first type of equation represents, however,  the destruction times of only 90% of the microorganisms having the highest heat resistance in the selected range of temperature.
Nevertheless, a safe pasteurization treatment must meet the same criterion applied to
the food sterilization practice, that is the number of surviving organisms lower than 10-7
per container. On the basis of the first type of equation describing the changing of the
highest D as a function of  temperature, a second set of equations giving the highest
7D times as a function of temperature was obtained by only a mathematical treatment.
The four sets of time/temperature values obtained by the latter type of equations are
the treatment parameters of the envisioned four types of pasteurization.

The 7D-pasteurization parameters should be regarded as very safe reference treatments.
The most suitable range of temperature to be used for the pasteurization of each sub-category
of products was selected taking into account (i) the maximum heat resistance of each set
of target microorganisms and (ii) the available types of pasteurization equipment, from the
still retort [operating at lower temperatures for longer times] to High Temperature Short
Time [HTST] processing equipment [treatment times of few seconds].


In comparing the heat resistance data reported by literature for different  target pathogenic vegetative bacteria, those showing both decimal reduction times (DT ) at more than a single temperature - and/or the regression equation linking Log D vs. temperature, as is usual in thermobacteriology - were preferred. Otherwise, the D and/or z values were computed  from data sets showing survival or D values at three/four temperatures at least. Single heat resistance data were taken into account only in case of exceptionally high values.

The two most heat resistant pathogenic vegetative bacteria in the pasteurization range of
60-90°C (140 - 194°F), can be regarded to be the S. aureus [Evans et al., 1970] and
the Listeria monocytogenes [Schoeni et. al., 1991]. The heat resistance of the former
pathogen is higher in the lower range of the pasteurization temperature and the heat
resistance of the latter in the higher one [the z values of the two bacteria are different].

The heat resistance data reported by Schoeni et al.(1991), obtained by treating a mixture
of 5 strains of L. monocytogenes, are higher than those reported by other authors, although
of quite similar order of magnitude.  On the contrary, the selected heat resistance data of
S. aureus as reported by Evans et al. [1970] are higher than usual for this type of organism.
The D value at 60°C of different strains of S. aureus - and in different environments - as
reported by most authors ranges from 0.9 to 6 minutes. Nevertheless, higher D60 values
can be found in the literature, though as single D data, not coming from a set of companion
data obtained at different temperatures. So,  D60  values of 12-15 minutes [El-Banna & Hurst, 1983] and D60  of 25 minutes [Bean & Roberts, 1975] are reported. Such values are more
than 5 times higher than usual and then they can not be disregarded without the risk of underestimating the true heat resistance of the pathogen, with unpredictable consequences
in the practice. Evans et al. [1970] tested the heat resistance of 9 strains of S. aureus in milk treated at  six temperatures (68.3 - 82.2°C) in a commercial plate heat exchanger, and they obtained a D value at 82.2°C that leads - assuming z = 5.11, as reported in the literature -
to a D at 60°C very close to the above maximum values of 12 <  D60° <25 minutes.
Then, choosing the Evans et al. [1970] data, both usual and exceptional heat resistance
data of S. aureus are included.
The expected heat resistance values of the two selected organisms are shown in Table 3.

Table 3 . Heat resistance [DT, seconds] of the most heat resistant non-sporeforming pathogenic bacteria.
z °C 60 °C 70 °C 72 °C 80 °C 90 °C z °F
S. aureus 5.1 1615 17.8 7.2 0.2 0.002 9.2
L. monocytogenes 10.0 427 46.7 30 5.1 0.56 18.0
140 °F 158 °F 161.1 °F 176 °F 185 °F
(a)  Evans et al., 1970. Computed equation: Log Dsec =14.95-T/ .11
(b)  Schoeni et al., 1991. Computed equation: Log Dsec = 8.4-T /10.4
As can be seen, S. aureus has heat resistance at 60°C (D 60°C ? 27 min) about four times
higher than Listeria (D 60°C = 7 min). Nevertheless, owing to the rapid change of D with temperature (z = 5,1), the D90°C  of S. aureus (only 0.0022 sec) is about 250 times lower
than that of Listeria. On the contrary, Listeria monocytogenes has the maximum (among the pathogens) resistance at 90°C, since D 90°C = 0.56 sec. The z value of L. monocytogenes
being twice that of S. aureus it prevents the former from having D60  higher than that of the
latter. S. senftenberg has D of intermediate values in the temperature range considered.
It can be safely expected that a straight line fitting the highest two DT values at the extremes
of the pasteurization temperatures, the one of Listeria at 90°C (D90°C = 0.56 sec) and that
of S. aureus at 60°C (D  60°C = 1615 sec) will necessarily include the heat resistance values
of all the vegetative pathogenic bacteria of intermediate resistance. The following relationship
can be obtained by joining the above DT:
                               Log DT,sec = 10.1 - 0.115 * °C   [3]
                                                          z = 8.7 °C

                            Log DT,sec = 12.16 - 0.064 * °F   [3.F]
                                                          z = 15.66 °F

and Low Water Activity Foods
Few microbial groups can spoil products with pH below 3.8 and/or water activity lower than
Sporeforming bacteria do not belong to these groups [the relevance of Alicyclobacillus is still uncertain][Previdi et al., 1997].
Some mold species can produce sexual spores called ascospores, of the highest heat resistance among micro-organisms  (moulds, yeasts, lactobacilli, etc.) usually involved in spoilage of such
acid and/or low aw products. The most heat-resistant ascospores are produced by strains belonging to the genus Neosartoria and Talaromyces.
Two different micro-organisms show the maximum heat resistance in the range considered:
N. fischeri in the lower temperature range and T. flavus in the higher one [Table 8].

       Table 8. The most heat resistant ascospores [DT, minutes].

       Moulds 85°C 87.5°C 90°C 92.5°C 95°C 97.5°C z°C z°F
Neosartoria fischeria 4.4 1.6 0.6 0.2 0.07 0.025 5.6 10.1
         "             " b 112 25 5.6 1.3 0.3 0.06 3.6 6.5
Talaromyces flavusc 26.5 8.8 2.9 1 0.3 0.1 5.2 9.4
         "            " a 70.8 25 8.9 3.2 1.1 0.4 5.4 9.7
         "           " b 20.4 7.8 3 1.1 0.44 0.17 6 10.8
185°F 189.5°F 194°F 198.5°F 203°F 207.5°F
         (a) Scott & Bernard, 1987; (b) Beuchat, 1986; (c) Quintavalla & Spotti, 1993;

The following relationship fits both extreme values in the temperature range of interest (D85°C = 112 min; D97.5°C = 0.4 minutes):

                                  Log D = 18.69 - .196 °C                           [7]
                                                z = 5.1 °C

                                   Log DT = 22.17 - .1087 * °F                  [7.F]
                                                        z = 9.2 °F



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Basics of  Thermobacteriology
Theory of Microbial Heat Inactivation Kinetics.

The death of  microbial populations exposed to lethal temperatures follows the kinetics of  first-order reactions [Esty & Meyer, 1922; Ball & Olson, 1957; Stumbo, 1973; Pflug, 1987.b; Casolari, 1988].
According to the equation of exponential decay:

                   Nt = No * e-k*t                                           [1]

where Nt is the number of organisms surviving the treatment time ‘t’, No is the initial number of microorganisms and  ‘k’ is the death rate. Equation [1] is often rewritten as:

                   Nt = No * 10-k'*t                                               [2]

and more commonly in the logarithmic form:

                 Log Nt = Log No - k' * t                                    [3]

The death rate k’ of microbial populations can be obtained from eq. [3]:

                      k' = [Log No - Log Nt] / t                               [4]

after which the single most important parameter [D] describing heat resistance of microorganisms exposed to the lethal temperature T can be obtained:

                  DT = [1/k']
                       =  t / [Log No - Log Nt]                               [5]

DT  is called “Decimal Reduction Time ” at the temperature T.
D is the time in minutes required for the inactivation curve to transverse one Log cycle, that is the time required for destruction of 90% of the population or, what is the same, the time to reduce of 10-times the number of  living microorganisms. According to eq. [5], in the form:

                     t / D = Log No - Log Nt

the eq. [2] can be rewritten as:

                   Nt = No * 10-t/D                                              [6]

after which:

                Log Nt = Log No - t / D                                    [6.1]

The value of D is specific to each type of microorganism, species, strain, specific physiological condition, environment,  and mostly temperature.
At the Temperature T, as heat resistance is higher as D value is higher:
the organism having  DT = 5 is less resistant than an organisme having  DT = 12.
Accordingly, the Staphylococcus aureus having D72°C = D161.6°F = 7.2 seconds is less heat resistant than the Listeria monocytogenes having D72°C = D161.6°F= 30 seconds.

The Meaning of exponential inactivation.
The law of exponential decay is a general law that is applied to the destruction of any kind of organisms, whatever the lethal agent employed, as well as to the radioactive decay.
The exponential inactivation imply that treatments times of equal size will destroy the same percentage of organisms [not the same quantity].
Accordingly, if a treatment time of 20 minutes, for instance, at the temperature of 100°C = 212°F will destroy 90% of a species of bacterium, the 'decay' of the concentration of organisms following some 20 minutes treatments will be the described one:

       Nt / g      t, minutes Log Nt  % surviving N destroyed % destroyed
    100,000           0        5       100        0           0
     10,000         20       4         10     90,000         90
       1,000         40       3           1       9,000         99
          100         60       2          0.1          900        99.9
            10         80        1          0.01            90       99.99
              1        100       0          0.001             9      99.999
             0.1        120     - 1           0.0001          (0.9)       99.9999
            0.01      140     - 2          0.00001          (0.09)   99.99999

As can be seen, each tratment time of 20 minutes cause the destructio of the same
fraction of organisms [90% in this case] surviving the preceeding treatment, even if the absolute Number of organisms destroyed by each treatment decreases as the number of survivors decreases too.
In this case, since after each treatmnent of 20 minutes the fraction of killed organisms equals 90%, it follows that  DT = 20 minutes.
The equation describing the results of the above Table is the following, according to [6.1]:

                                        Log Nt = 5 - t / 20

The Meaning of Negative Log Nt.
Survival values of Nt = 0.1 or lower does not mean that only a fraction of a single organisms is surviving, and a fraction destroyed [what would have no meaning]. Values of Nt lower than 1 [ i.e. negative values of Log Nt] are interpreted on the probability basis; that is Nt = 0.1/g  is expected to mean 1 survivor each 10 g substrate [i.2. Nt = 0.1 = 1survivor/10g], Nt = 0.01/g = 1 survivor/100g, Nt = 0.001/g = 1 surv. / 1,000 g , and so on.

Heat Resistance and Temperature.
As for usual chemical reactions, the death rate of microorganisms increases with the increase in temperature, according to the Arrehnius law:

                          k = A * e-Ea/R*T                                       [7]

Thermobacteriologists prefer to link the effect of temperature upon the death rate of microorganisms to the parameter D [D = 2.3/k = 1/k’] of the equations [5-6]:

                      DT  = DTr  * 10[Tr-T]/z                             [8]

where DT  and DTr  are the D value at the temperature T and at the ‘reference’ temperature Tr, respectively;  ‘z’ is the reciprocal of the rate of change of death rate with temperature, i.e., ‘z’ is the number of degrees [°C, °F] required to achieve a tenfold change of DT  values:

                           0.1 * DT-z  =   D T  = 10 * DT+z

Really eq. [8] can be written as:

                  Log DT  = Log DTr  + [Tr - T] / z                          [9]

after which:

                  z  =   [ Tr - T ]  / [Log DT  - Log D Tr ]              [10]

so that when DT   and DTr  differ by 10-times, the difference of their logarithms
is one:
                Log DT  - Log ( 0.1 * DT ) = 1

and thus z = Tr - T.
The value of ‘z’ is characteristic to each microorganisms in a defined environmental condition and is regarded as constant in a narrow range of temperature.
D and ‘z’ are the two basic parameters required to define satisfactorily the
heat resistance characteristics of single microorganisms.
[Analogous parameters are used to characterize the heat resistance of vitamins
and other nutritional components of foods].