Bacterial spores are usually far more heat resistant than vegetative
cells; thermophiles produce the most heat resistant spores while those of
psychrotrophs and psychrophiles are most heat sensitive. Since spore
inactivation is the principal concern in producing appertized foods,
much higher temperatures are used in appertization processes and in
the measurement of spore D values.
Yeast ascospores and the asexual spores of moulds are only slightly
more heat resistant than the vegetative cells and will normally be killed
by temperatures at or below 100 1 C,e.g.in the baking of bread. Asco-
spores of the mouldByssochlamys fulva, and a few other ascomycetes do
show a more marked heat resistance and can be an occasional cause of
problems in canned fruits which receive a relatively mild heat process (see
Section 5.5.4).
The heat resistance exhibited by the bacterial endospore is due
mainly to its ability to maintain a very low water content in the central
DNA-containing protoplast; spores with a higher water content have a
lower heat resistance. The relative dehydration of the protoplast is
maintained by the spore cortex, a surrounding layer of electronegative
peptidoglycan which is also responsible for the spore’s refractile nature.
The exact mechanism by which it does this is not known, although it
may be some combination of physical compression of the protoplast by
the cortex and osmotic extraction of the water. As the cortex is
dissolved during germination and the protoplast rehydrates, so the
spore’s heat resistance declines. Suspension of a germinated spore
population in a strong solution of a non-permeant solute such as
sucrose will reverse this process of rehydration and restore the spore’s
Table 4.3 Microbial heat resistance
Vegetative organisms (zB 51 C) D (mins)
Salmonellasp. D 65 0.02–0.25
SalmonellaSenftenberg D 65 0.8–1.0
Staphylococcus aureus D 65 0.2–2.0
Escherichia coli D 65 0.1
Yeasts and moulds D 65 0.5–3.0
Listeria monocytogenes D 60 5.0–8.3
Campylobacter jejuni D 55 1.1
Bacterial Endospores D 121
(zB 101 C)
B. stearothermophilus 4–5
C. thermosaccharolyticum 3–4
Desulfotomaculum nigrificans 2–3
B. coagulans 0.1
C. botulinumtypes A & B 0.1–0.2
C. sporogenes 0.1–1.5
C. botulinumtype E D 80 0.1–3.0
D 110 o1 second
Chapter 4 69