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FOOD STERILIZATION BY HEAT
A. Casolari





Introduction
PART  I
    Brief
PART  II
Details of the microbial destruction process
Microbial destruction and the sterilization process:
               A -  Theory of Microbial Heat Inactivation Kinetics.
               B -  Heat Process, FT , required   for preservation of  low-acid
                      foods.
               C -  Heat Process, FT , required to produce low-acid canned
                      foods SAFE from a Public Health standpoint.
               D - Mesophilic non-pathogenic sporeformers of the putrefactive
                     group.
               E - Comparison  between Sanitary and Preservation
                     requirements.
             F - The Thermophilic microorganisms and the Sterilization Process

REFERENCES



 

INTRODUCTION

STERILIZATION is a process employed to deprive microorganisms of their ability to multiply. The most reliable Sterilization process is obtained by application of Heat.

    Heat destruction of microorganisms is a gradual phenomenon: the longer is the treatment time at lethal temperatures, the larger is the number of killed microorganisms.
        As higher is treatment temperature:
                1 - as shorter is the time required to kill microorganisms;
                2 - as lower is heat induced damage to food products.

    Theoretically, absolute sterility does not exist.

    Only Low Acid Foods [LAF], having pH higher than 4.6, must be sterilized, because all microorganisms are able to grow in LAF.
    More acid products [pH equal/lower than 4.6] do not allow the growth of pathogenic sporeforming bacteria. Then Sterilization is not required.

    The purpose of the Sterilization Process [SP] is the destruction of all pathogenic organisms together with spoilage/non-pathogenic microorganisms, to  achieve the Safe Preservation at Room Temperature [SPRT] of treated products.
    Food products processed to obtain SPRT are usually  defined as ‘commercially sterile’.
 
 

PART  I

B R  I  E  F

LAF is a food having an equilibrium pH value higher than 4.7.

The Sterilization Treatment by Heat [STH] is regarded as suitable if of size equivalent to Fo=10 at least [Casolari, A., 1994, Food Microbiology 11, 75-84] or equal/higher than 8 [Stumbo et al., 1975, J. Food Science 40, 1316-1323].

Fo value is the number of minutes the food is actually exposed at the Reference Sterilization Temperature of  250°F = 121.11°C. Fo does not refer to total, ‘integrated heat’ applied/received by the entire mass of  product in the container or holding tube [that is the heat absorbed by the slowest-heating product at the center of the can or flowing at the center of  the holding tube, plus the heat absorbed by the fast-heating product located at the periphery of the can, or of the holding tube].
Fo refers to the  amount of heat received by product located at the coldest point in the container heated inside a retort or  hydrostatic sterilizer; or the fastest particle flowing in holding tube or plates of continuous sterilizers.
The total, ‘integrated Fo’ if of interest to structural/nutritional quality of sterilized products; not to the process of killing the microorganisms.
A STH of any chosen Fo can be applied at Temperatures lower or higher than 250°F. The following equation describes the relationship occurring between Fo and any treatment FT  of equivalent size obtainable at different temperature, T:

         FT  = Fo * 10 (Tr – T) / z                                          [1]

       FT  = Fo * 10 (250 – T) / 18          T = °F               [1.F]

      FT = Fo * 10 (121.11 – T) / 10         T = °C             [1.C]

According to equation [1.F & 1.C] the following EQUIVALENT STH [EFT]  can be obtained:
 

  °F   214   223   232   241   250   259   268   277   286   295
 EFT 1,000   316   100  31.6    10  3.16     1   0.32   0.1  0.032
  °C  101.1 106.1 111.1 116.1 121.1 126.1 131.1 136.1 141.1 146.1
The meaning of EFT  is that a STH of Fo = 10 can be applied by a treatment of 10 minutes at 250°F, or, what is the same, 100 minutes at 232°F, or 1 minute at 268°F, or 6 seconds at 286°F, obtaining the same lethality as regards the microbial destruction, what means destroying the same fraction of microorganisms.

STH is usually applied at the highest temperature allowed by the available equipment/product, since as higher is temperature as easier is to preserve prominent characteristics of foods [nutritional components, etc.]. The above effect does occur because the thermal degradation rate of organic matter increases less rapidly, increasing temperature, than destruction of microorganisms [‘z’ of organic matter is higher than ‘z’ of destruction of microorganisms; i.e.: its easier to destroy microorganisms than most organic molecules].
 
 

PART  II

Details of the microbial destruction process

Microbial destruction and the sterilization process:

    A - Theory of Microbial Heat Inactivation Kinetics.
    B -  Heat Process, FT , required   for preservation of  low-acid
            foods.
    C -  Heat Process, FT , required to produce low-acid canned
            foods SAFE from a Public Health standpoint.
    D - Mesophilic non-pathogenic sporeformers of the putrefactive
            group.
    E - Comparison  between Sanitary and Preservation
            requirements.

A - 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 * ek*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 * 10k'*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.
DT is the time in minutes:
    - required for the inactivation curve to transverse one Log
      cycle,
    - required for destruction of 90% of the population,
    - required 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]

The value of D at the temperature T - usually written DT - is specific
to each type of microorganism, species, strain, physiological condition, environment,  etc.  [  specimen of heat resistance from Microbiofood ]

At the same temperature T a heat sensitive microorganisms may have DT = 2 minutes, for instance, while a heat resistant microorganisms may have DT > 2, that is 4 minutes, or 18 minutes, or 145 minutes, and so on.
Examples of different heat resistance among vegetative cells:
Listeria monocytogenes has a D80°C = 5.1 minutes [Schoeni et al., 1991]; Staphylococcus aureus has D80°C = 0.2 minutes [Evans et al., 1970].
Sporulated bacteria have usually a heat resistance much higher than vegetative cells: spores of Clostridium pasteurianum have D80°C =  100 minutes [Casolari & Giannone, 1966].

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 * D T - z  =    DT    = 10 * D T + z

Really eq. [8] can be written as:

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

                  z = [Tr - T] / [Log DT - Log DTr ]         [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.

For example: DT = 24 and then

                Log 24 - Log 0.1*24 = 1.38 - 0.38 = 1

so that if T = 80°C = 176°F and Tr = 86°C = 186.8 then:

            z = 86-80 = 6°C
            z = 186.8 - 176 = 10.8°F

as a matter of fact:
                            z(°C) = z(°F)*5/9
and
                           z(°F) = z(°C)*9/5

The value of ‘z’ is characteristic to each microorganisms in a defined environmental condition and is a parameter regarded as constant in the narrow range of temperature usually of relevance for the sterilization purposes.
D and ‘z’ are the two basic parameters defining completely the heat resistance characteristics of single microorganisms. [Analogous parameters are used to characterize the heat resistance of vitamins and other nutritional components of foods].

B - Heat Process , FT  , required  for preservation of low-acid foods.

The heat treatments required for preservation of canned foods are calculated on the basis of equations [5, 8].
According to eq. [5], the time t required to decrease the starting level of microbial contamination from No to the final level Nt is obtained from the following relationship:

                  t  =  D * [Log No - Log Nt]            [11]

As an illustration, let the DT of the most heat resistant [HR] Listeria monocytogenes be  D80°C  = D176°F = 5.1 minutes [Schoeni et al.,  1991] and the contamination level of the lot of sliced cabbage to be sanified [for instance] is expected to be No = 107  = 10,000,000 / kg;  the treatment time t required to reduce the HR Listeria to the level Nt = 1/ kg, can be calculated from [11], being Log 107 = 7 and Log 1 = 0 :

                        t = 5.1 * [7 - 0]
                        t = 35.7 minutes
The reduction of the contamination level from 107 to 1/kg can be
expressed equally well as a 7 decimal reductions [D*7, or simply 7D] of the contamination.
The nD concept [that is the number of decimal reductions obtained] is very practical because it can be applied to any difference between Log No and Log Nt equaling n.
Really, 7D of the contamination [as in the above example] is obtained after the same treatment time, whether

                Log No = 7 and Log Nt = 0

because  t = D*[7-0] = D*7

or
                Log No = 8 and Log Nt = 1

because  t = D*[8-1] = D*7

or
                Log No = 9 and Log Nt = 2

because  t = D*[9-2] = D*7

or
                Log No = 6 and Log Nt = -1

because  t = D*[6-(-1)] = D*[6+1] = D*7

or
                Log No = 5 and Log Nt = -2

because  t = D*[5-(-2)] = D*[5+2] = D*7

that is the difference between Log No and Log Nt is always equal to 7.
It follows that the number n of decimal reductions, nD, obtained after a treatment time t equals the difference in the contamination level before treatment and after the treatment t at T:

                    nD = Log No - Log Nt              [11.1]

The knowledge of nD is very interesting in practice because it is a parameter that can be applied easy to any starting/final level of contamination, and so allowing the immediate evaluation of the suitability of the treatment for the purpose.

The following equation too can be obtained from eq. [11]  :

                 Log Nt  =  Log No -  t / D               [12]

after which one can find:

    1 - the level of contamination Nt surviving the treatment time t, knowing the value of D and No of the microorganisms of interest;

    2 - the number of decimal reductions, nD, achieved after a treatment time t, by knowing only the value of D.

According to the above reported example, the contamination level of different microorganisms having D = 12 minutes is expected to be, after the treatment time of 35.7 minutes [7D of HR Listeria]:

                Log Nt = Log No - 35.7/12

so that if Log (No/g) = 5, Log (Nt/g) will be 2.025; what means that about 100 surviving organisms/g can be expected after 35.7 minutes at 80°C.

After eq. [12] we can see that:

                    Log Nt - Log No = - t/D
so that
                    Log No - Log Nt =  t/D

and, according to [11.1]:
                      nD = t/D                            [12.1]

after which we see that knowing t and D values one can obtain immediately the value of nD and then the degree of destruction of the reference organisms having decimal reduction time of D minutes.
Accordingly, the number of decimal reductions, nD, of microorganisms having D80°C = D176°F =  3.57 minutes, for instance, can be obtained from eq. [12.1]:  nD = 35.7/3.57 = 10 .
And more generally, we may say that a treatment of - for instance - 15 seconds at the temperature T will give 15D of microorganisms having DT = 1sec, 2D of microorganisms having DT of 7.5 sec, 1D of those having DT of 15 sec, 0.5D of those having DT = 30 sec, and so on.

Coming back to equation [12], we can deepen our understanding of the meaning of the surviving concentration, Nt, of microorganisms.
Let t = 24 minutes, D = 6 minutes and No = 100,000 microorganisms / sample unit. According to eq. [12] the following number Nt of surviving organisms is obtained:
  Log Nt  = 5 - 24 / 6
       = 5 - 4
       = 1
Whose means that after 24 minutes treatment, the product is not sterilized because each sample unit [can, jar, etc.] is still contaminated by the Nt = 10 surviving organisms. During storage, the surviving organisms could grow and spoil the product. If spoiling organisms belong to a pathogenic group, the product  may become toxic too.
For any sterilization process a suitable selection of Nt value must be done.
Let Nt = 1 survivor /10,000 units, i.e. Nt  = 10-4 , and No = 105 , D = 3 for instance,  and the time t required to achieve the chosen Nt level will be, from eq. [11]:
  t  = 3 * [ 5 - (- 4) ] = 27
So, after a treatment time  FT  = 27 minutes at the lethal temperature T the initial contamination of No =  100,000 microorganisms / unit will be reduced to the expected level of one survivor per 10,000  treated units.
The survival level of 1/10,000 = 10-4  organisms/sample unit could be regarded as a low enough survival probability and thus a satisfactory degree of final sterility.

According to Pflug  [1987.a]: “..the probability of a Cl. botulinum spore surviving...the probability of a mesophilic spore surviving...will give a more realistic picture of the microbial status of the product than use of descriptive terms, such as ‘sterile’ or commercial sterility.” [Pflug, 1987.a].
Really, the exponential nature of the inactivation kinetics does not allow the achievement of ‘absolute sterility’, but only some well defined  level of sterility.
The sterility level is more usefully expressed in terms of Survival Probability [SP], where:

                        SP = Nt / No                         [13]

Following eq. [12-13]:

   Log SP = Log (Nt/No) = Log Nt - Log No = - t / D      [14]

after which
                   SP = 10-t/D                       [15]
and also
                   SP = 10-nD                       [16]

Both safety and preservation are the most important objectives taken into consideration in calculating the sterilization process to be applied to low-acid canned foods.


FT  is the treatment time at the temperature T applied for the sterilization purposes.


The safety is reached by applying  FT   values high enough to destroy all pathogenic organisms.


The preservation is obtained by applying  FT   treatments high enough to destroy both pathogenic and spoilage microorganisms.

In either case, the required FT can be calculated by knowing:
    (i) the DT  and ‘z’ of the most heat resistant pathogenic organism,
   (ii) the  DT  and ‘z’ of the most heat resistant spoilage organism of
        interest,
  (iii) the expected level of No for both  Pathogenic [P] and Spoilage
        [S] organisms, and
 (iv) the tolerated survival value Nt for both P and S organisms.

Knowing the above quantities, the FT  at the temperature T can be calculated by the following relationship:

             FT  =  DT * nD                       [17]

where FT  is a multiple of the highest DT  recorded among D values of relevant S and P organisms.

By changing the treatment temperature, the values of  FT  = f (T)  are obtained, according to eq. [8], by the following:

         FT  = FTr  * 10 [Tr - T] / z                    [18]

where  FTr  is the treatment time at the reference temperature Tr .
 

C -  Heat Process required to produce low-acid canned foods SAFE from a Public Health standpoint.

The usually accepted ‘safe’ nd  [snd] value results from the early work of Esty & Meyer [1922] providing the treatment time required at several temperatures to obtain no survivors when using suspensions of billions of Cl. botulinum spores. Really, the exact number of spores used by Esty & Meyer [1922] is not known [nor quoted authors were able to calculate D values], while it was assumed to be very close to
1012 . Thus, the reference ‘destruction times’ [absence of survivors despite  No = 1012]  reported by Esty & Meyer [1922] caused a number of  log cycles decrease, nd, of Cl. botulinum spores very close to 12. Accordingly, the reference value of  snd = 12  was universally accepted.
Since the level of heat resistant Cl. botulinum spores can be safely expected to “...vary from  10,000  per container of product, which may occur in some mushrooms products, to 0.1 , which may be the level for meat..” [PFLUG, I.J., 1987a], then the Survival Probability of Cl. botulinum spores [SPB] will vary - following eq. [13-16] - from 10-8  to 10-13/container, since:

               SPB  =  Nt / No  = 10-snd                         [19]

after which the number of survivors / container, Nt, is expected to be:

               Nt  =  No * 10-snd
being
              Nt / No = 10-snd
and then:
               Log Nt = Log No - snd

Let No = 104 / container and then:

   Log Nt =  4 - 12   =  - 8
          Nt =  10-8

Let No = 10-1  and then:

   Log Nt = -1 - 12  = - 13

so that:
           Nt = 10-13

The highest heat resistance of the most heat resistant Clostridium botulinum spores is regarded to be [Ball & Olson [1957; Stumbo, 1973; Stumbo et al., 1975; Pflug & Odlaug, 1978; Pflug, 1987] the 'destruction time' [the theory of  exponential inactivation was not still working] of 4 minutes at 120°C recorded by Esty & Meyer [1922] using a very concentrated suspension of spores.
Such destruction time resulted from the more extensive collection of heat resistance data of Cl. botulinum ever studied, i.e.: a blend of spores from 109 strains obtained from different sources [Esty & Meyer, 1922].

The destruction times reported by Esty & Meyer [1922] were corrected by Townsend et al. [1938] for lags in heating and cooling, leading to D121.1°C  = D250°F =  2.45 min,  after which D121.1°C  = D250°F  = 2.45 / snd = 2.45 / 12 = 0.2 min.

The ‘z’ value recalculated on the data of Esty and Meyer [1922], was z = 9.8 °C = 17.6°F [Townsend et al., 1938].

Some investigators reported higher D and z values [see Casolari, 1994]. Nonetheless, the value of D121.1°C  = D250°F  = 0.2 minutes and z = 10°C = 18°F are currently the most widely accepted Reference Decimal Reduction Time and z values of Cl. botulinum spores of maximal heat resistance.


The D121.1°C  = D250°F  =  2.45 min was rounded up in practice to D121.1°C  = D250°F  = 3 min and called
“minimum botulinum cook”
[mbc]. A mbc of 3 min at 121.1°C = 250°F is equivalent to a value of  nd = 3/0.2 = 15.

[It is usually said that the mbc will produce 12 D. As shown above, the mbc is expected to produce 15D.]

It is a widely accepted concept too, that

any process, FT ,  equivalent to
Fo = D121.1°C  = D250°F = 3 min  [z  =  10°C = 18°F]
is a realistic mbc  because it will produce canned foods ‘safe’
from a public health standpoint
[Stumbo, 1973; Stumbo et al., 1975; Pflug & Odlaug, 1978].


 

The relevance of the z value in thermobacteriology results from the general equation [18] of heat inactivation as a function of temperature:

              FT = FTr*10(Tr - T) / z                   [20]

after which:
              FT  =  FTr * 10 ( 121.1 - T )  / 10                  [20.C]
 equivalent to
           F =  FTr * 10 ( 250 - T )  / 18                     [20.F]

where FTr in this case is the reference Treatment time of the
mbc = 3 minutes.

Equations [20.C and 20.F] are of paramount important in thermobacteriology, because
by using either above equations one is able to find the right time FT  required at any preferred sterilization temperature that is EQUIVALENT to a heat treatment taken as reference [FTr].

According to equations [20.C and 20.F] a reference treatment of  FTr = F121.1°C  = F250°C = 3, for instance,  is exactly equivalent to the following treatments, as regards the killing rate:

    1.2  min at 125°C = 257°F
    3.9  min at 120°C = 248°F
  12.2  min at 115°C = 239°F
  38.7  min at 110°C = 230°F
122.2  min at 105°C = 221°F
386.5  min at 100°C = 212°F
Really, according to the theory, the above six heat treatments
- as well as any treatment obtained on the basis of equations [20.C, 20.F] - are exactly equivalent one another, in that they are all expected to cause ‘exactly the same’ number of decimal reductions, whichever reference microorganisms is concerned, while having z = 10°C = 18°F.
[ specimen from HProcess ]

Equivalent heat treatment times on the basis of  'z' different from z = 10°C = 18°F can equally be obtained by using the more general equation  [20] instead of equations [20.C and 20.F].
Accordingly, wishing to use  z = 8°C = 14.4°F, for instance, the equivalent sterilization treatment at different temperatures can be obtained from equations:

                 FT  = FTr *10(Tr-T)/9
or
                 FT  = FTr *10(Tr-T)/14.4
 
 
 

D - Destruction of Mesophilic non-pathogenic sporeformers of the putrefactive group.

TRUE STERILIZATION TREATMENT TIME
COMMERCIAL STERILITY

Several non-pathogenic mesophilic  sporeformers having heat resistance higher than that of Cl. botulinum usually occur in low-acid foods. They belong to the group of anaerobes of the putrefactive type. Soil is the primary source of these spores. Some species are common in human and animal intestine. They can contaminate  all canned foods. The most heat resistant anaerobic spores of the putrefactive group are those produced by a reference strain of Cl. sporogenes usually called Putrefactive Anaerobe 3679 [PA 3679]. Heat resistance of PA 3679 spores may be as high as D121.1°C =D250°F = 3 minutes [Cameron et al., 1980]. More usually, the D121.1°C = D250°F ranges between 1 minutes and 0.3 minutes [Stumbo et al., 1975; Pflug , 1987b]. According to Pflug [1987b] “..D121.1°C = 0.5 min probably is more realistic of canned food production conditions.”
A low enough survival probability of spoilage bacteria of this type is identified with what is called ‘commercial sterility’ [CS] of low-acid canned food products. According to Stumbo [1973] and Stumbo et al. [1975], a satisfactory level of CS is reached at  a survival probability of PA 3679-like spores of 10 - 4 /container. Pflug [1987b] suggested a Nt value of 10 - 6.
The incidence of  mesophilic Clostridium spores in raw beef, chicken and pork  was found to be about 2.8 spores/g on the average [Greenberg et al., 1966]. It may be safely expected that not more than about 1% of those mesophilic Clostridium spores may have high heat resistance. Nevertheless, Pflug [1987.b] said:  “..the actual number of resistant  [0.3 < D121.1°C < 0.7 minutes] mesophilic spores per container will be more nearly of the order of 10 per container.”
The treatment time required to achieve the commercial sterility of low-acid canned foods can be obtained, according to equation [17] by:

                  FT = D * [Log No - Log Nt]                    [21]

and the reference treatment:

        F121.1°C =  D121.1°C * [Log No - Log Nt]      [22.C]
        F121.1°C =  D250°F   * [Log No - Log Nt]      [22.F]

Accepting the suggestion of Pflug [1987.b] of  an average No = 10 / container and Nt = 10 - 6  per container, the expected sterilization value F121.1°C = F250°F of low-acid foods  will be, according to different D121.1°C values of PA 3679 spores, and following eq. [22.C]:

F121.1°C  =  0.3 * [1 - ( - 6 )] = 2.1
                =  0.5 * [1 - ( - 6 )] = 3.5
                =  0.7 * [1 - ( - 6 )] = 4.9
                =    1 *  [1 - ( - 6 )] = 7
By accepting instead the suggestion of Stumbo et al. [1975] of  a satisfactory  Nt = 10 - 4 , the F121.1°C  will be:
F121.1°C   =  0.3 * [1 - ( - 4 )] = 1.5
                 =  0.5 * [1 - ( - 4 )] = 2.5
                 =  0.7 * [1 - ( - 4 )] = 3.5
                 =    1 *  [1 - ( - 4 )] = 5
the Reference Heat Sterilization Treatment at 121.1°C = 250°F is currently termed Fo.


Fo represents the number of minutes that are applied at 121.1°C = 250°F for the sterilization purpose, i.e. for obtaining the 'commercial sterility'.

THE COMMERCIAL STERILITY
is the condition achieved by a Sterilization Process
applied for the preservation of low-acid foods
obtained by applying heat treatments equivalents to Fo ranging from 5 to 7 minutes .


Some observations suggest that it would be safer to apply Fo values equal/higher than 10 minutes [Casolari, 1994] from both sanitary and preservation view point.
 

E - Comparison between Sanitary and Preservation requirements.

From the sanitary viewpoint, a heat treatment equal or higher than the minimum botulinum cook [mbc = 3 min at 121.1°C]  will give the very satisfactory low level of survival probability of  SPB  = 10-15/ container [see Section B].
As shown in Section C, the protection of canned foods against spoilage during storage at ambient temperature, can be achieved by heat treatments equivalent to 5-7 minutes at 121.1°C, that is 5 <  Fo < 7  [z = 10°C].
By applying treatments of the above size, the survival  probability of food spoilage microorganisms of the putrefactive group is expected to be SP = 10 - 4  -  10 - 6  per container, respectively [see Section C].
The survival probability of the most heat resistant spores of the most heat resistant  Cl. botulinum strain is expected to be, according to eq. [15]:
                SPB = 10 - Fo / Db                               [23]
Being Db = 0.2  the reference decimal reduction time of Cl. botulinum spores at 121.1°C, it follows that after heat treatments of Fo = 5 the SPB will be 10 - 25 .
After heat treatments of Fo = 7 the value of SPB will be 10 - 35 .
Both SPB values are of an exceedingly low level, much lower than SP.
Accordingly, any heat treatment FT  ‘equivalent’ [on the basis of the eq. [18]]  to 5 <  Fo <  7  can be regarded as exceedingly safe from the sanitary viewpoint.


F - The Thermophilic microorganisms and the Sterilization Process

A group of bacteria called thermophils are able to growth at temperature higher than about 45°C = 113°F up to about 75°C = 167°F.
The thermophils seldom grow [some eurithermophil strains of Bacillus stearothermophilus] at temperatures lower than 45°C down to 30°C = 86°F. Thermophilic bacteria may be facultative-aerobic or anaerobic and not pathogenic species had ever been found in the group. Aerobic thermophils belong to the species B. stearothermophilus, B. coagulans, B. circulans. Anaerobic thermophils usually found in foods belong to the species Clostridium thermosaccharolyticum and Desulfovibrio desulfuricans.
Usually, the spoilage of Low-acid Foods by thermophilic bacteria does not matter at warehousing and distribution in the mesophilic temperature range. Low-acid foods stored and/or distributed  under temperatures in the thermophilic range [higher than 45°C = 113°F] could perhaps be spoiled by thermophilic Bacillus of the flat-sour type and mostly by Bacillus stearothermophilus. Spoilage by thermophilic anaerobes is less likely, given the usual low contamination level of food products by this type of organisms.

[The spores of the anaerobic thermophilic bacteria are often so high heat resistant at temperature close to 121°C that it's impossible to destroy with usual Fo applied at temperatures close to - or lower than 121°C. In case of foods contaminated by such thermophilic anaerobes it is necessary to change raw material or to change the Sterilization Process toward the use of higher temperatures - up to 140 - 150°C (284 - 302°F). At such high temperatures the heat resistance of the thermophilic anaerobic spores is lower than that of the mesophilic spores. As a matter of fact, the z value of such high heat resistant spores is lower than that of mesophilic spores (usually close to 10°C = 18°F) and close to 5°C = 41°F. It follows that the same Fo is unable to destroy the anaerobic thermophilic spores at temperature close to - or lower than - 121.1°C = 250°F; while the same Fo applied at much higher temperatures, will do.]

The normal growth temperature  of  B. stearothermophilus-type  organisms ranges between 50 and 70°C [122 < °F < 158]. High spore crops of some strains may grow at lower/higher temperatures.

According to Pflug [1987_b]: “ ..B. stearothermophilus spores in nature will have a weighed, effective D121.1°C  value of the order of 1.5 min, but it may be as high as 3.0 min or as low as 1.0 min.” .
Accordingly, after heat treatments of   5 <  Fo < 7   it may be expected a range of  1.7 - 2.3 <  nD  < 5 - 7 at the extreme values of heat resistance, and 3.3 <  nD  < 4.7 on the basis of  the average D = 1.5 min.

The contamination level of vegetables by this type of flat sour spores usually ranges between  0.1 and 1,000/g .
It follows that survival probability of flat-sour spores [SPFS] will ranges between

100.3 or 10-0.3  < Nt / kg <   104.3 or  103.7
as a function of the starting level of contamination [2 < Log No / kg  < 6] and on the basis of an average heat resistance of D = 1.5 min:
                10-1.3 or 102.7  < Nt / kg <   10-2.7 or  101.3

Survival probability of  10-0.3 - 10-1.3 means 0.5 - 0.05 surviving spores/kg, that is 5 spores every 10 one kg units and 5 spores each 100 one kg units, respectively.
Both survival probability are quite higher than the 10-4- 10-6  values accepted for the survival of heat resistant PA3679 spores.
Nevertheless, some survival [0.01 - 0.05 spores / container] of thermophilic microorganisms of the flat-sour type in low-acid foods is widely accepted because:
    1 - the high heat resistance of this kind of spores;
    2 - the storage temperature  in the temperate climate is low enough to prevent germination and growth of thermophilic spores;
    3 - thermophilic bacteria are not pathogenic.

The survival of thermophilic bacteria of the flat-sour type justify and strengthen the concept of Commercial Sterility, implying that both absolute sterility can not be achieved owing to the exponential behavior of microbial inactivation, and that some non-pathogenic/thermophilic bacteria have such a high heat resistance that it is impractical to destroy without damaging the treated foods.
 


R E F E R E N C E S

Ball, C. O. and Olson, F. C. W., 1957, “Sterilization in Food Technology”, McGraw-Hill, London

Bigelow, W. D., 1921. Logarithmic nature of thermal death time curves. Journal of Infectious Diseases 29, 528.

Cameron, M. S., Leonard, S. J. and Barrett, E. L., 1980. Effect of moderately acidic pH on heat resistance of Clostridium sporogenes spores in phosphate buffer and in buffered pea puree. Appl. Environ. Microbiol., 39, 043-949

Casolari, A., 1988. ‘Microbial Death’, in  Physiological Models in Microbiology, Vol. II, p. 1-44;  M.J. Basin and J.I. Prosser, Ed., CRC Press Inc., Boca Raton FL.

Casolari, A., 1994. About basic parameters of food sterilization technology. Food Microbiology 11, 75-84

Esty, J. R. & Meyer, K. F., 1922. The heat resistance of the spores of B. Botulinus and allied anaerobes. XI. J. Infestious Diseases 31, 650-663

Federal Register, 1979. Part 113. Thermally processed low-acid foods packaged in hermetically sealed containers. Federal Register 44, No. 53, 16215-16238. U.S. Gov. Printing Office

Greenberg, R. A., Tompkin, R. B., Bladel, B. O., Kittaka, R. S. and Anellis, A., 1966. Incidence of mesophilic spores in raw pork, beef and chicken in processing plant in the United States and Canada. Appl. Microbiol., 14, 789-793

Pflug, I. J., 1987a.Factors important in determining the heat process value, FT , for low-acid canned foods. Journal Food Protection 50(6), 528-533

Pflug, I. J., 1987b. Calculating FT-values for heat preservation of shelf-stable, low-acid canned foods using the straight-line semilogarithmic model. J. Food Protection 50(7), 608-620

Pflug, I. J. & Odlaug, T. E., 1978. A review of z and F  values used to ensure the safety of low-acid canned foods. Food Technology 32, 63-70

Stumbo, C. R., 1973, “Thermobacteriology in Food Processing”, Academic Press, London

Stumbo, C. R., Purohit, K. S. and Ramakrishnan, T. V., 1975. Thermal process lethality guide for low-acid foods in metal containers. J. Food Science, 40, 1316-1323

Townsend, C. T., Esty, J. R. & Baselt, F. C., 1938. Heat resistance studies on spores of putrefactive anaerobes in relation to determination of safe processes for canned foods. Food Research 3, 323-346
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