The Principles of Canning for Fishery Products: Bacterial Spores Heat Resistance

It is desirable that the test organism in comparative thermal death time determinations be one which grows readily on the ordinary culture medium, with the production of an abundant yield of spores in a minimum time.

It should, furthermore, be an organism which has a characteristic type of growth, or should possess some readily determinable which will serve to differentiate it from contaminants likely to be encountered.

The heat resistance of bacterial spores is specified by the time required to kill 90 per cent of the population at constant temperature; this enables a comparison of heat resistance of spores of many different bacteria. For most spores of importance in canned food spoilage their heat resistance is measured at 121.1 ºC (250 ºF), a common retorting temperature, and is expressed as the D value. A typical plot of the number of survivors against heating time is shown in Figure 1. It can be seen that the time to reduce the population from 1 000 000 to 100 000 is the same as that required to reduce it from 100 to 10. That is, the D value is constant for specific bacterial spores when they are subjected to heat at constant temperature. In Table 1 are summarised the D values of bacterial spores important in canned foods.

Destruction of all spores of Clostridium botulinum is the minimum safety requirement , when thermally processing low-acid canned foods. Canners aim to reduce the probability of one spore surviving the thermal process to such a low level that, for all practical purposes, the contents of the container pose no health risk due to survival of Clostridium botulinum (spores). Experience has shown that a process equivalent in sterilising effect to twelve decimal reductions of the population of Clostridium botulinum is sufficient to protect consumer safety. Such a process is referred to as a "12 D" process and it is equivalent to holding the contents of the container at 121.1 ºC for 2.8 min (12 D= 12 x 0.23 = 2.8 min). A process as severe as this will satisfy requirements (under conditions of good manufacturing practice); however, it will be insufficient to reduce to a commercially acceptable level, the probability of survival for the extremely heat resistant spores (with D values of 2.0 to 5.0 min) of non-pathogenic bacteria. This is why canned fish manufacturers select a thermal process which goes beyond the safety requirements of destruction of Clostridium botulinum.

Fig.1. Survivor curve for bacterial destruction at constant temperature

Although the probability of survival for spores of non-pathogenic heat resistant bacteria may be several thousand times that for Clostridium botulinum spores, their presence is of no great concern to canners for two reasons:

1. Should they lead to spoilage, there is no associated health risk
2. They only grow at temperatures above 40 ºC (i.e., they are thermophilic) and their optimum growth temperature is around 55 ºC, which is above that in most warehouses ) and retail outlets

Table 1. Decimal reduction times (D-values) of bacteria

important in low acid canned foods

Organism	D value (min. at 121.1 ºC)
B. stearothermophilus	4.0 - 5.0
C. thermosaccharlyticum	3.0 - 4.0
D. nigrificans *	2.0 - 3.0
C. botulinum (A & B)	0.1 - 0.23
C. sporogenes (P.A. 3679)	0.1 - 1.5
B. coagulans	0.01 - 0.07

* Formerly C. nigrificans

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