338 Chapter 19
salt). Raw meat redox potential is − 50 mV;
this value is reduced when the meat is
heated: in sausages, it decreases from +20
to − 100 mV, depending on grinding condi-
tions and additives (Zamudio 2006 ).- Oxygen tension is decisive for the prolif-
eration of certain strains. This is the case
with strict anaerobes such as Clostridium
spp. or microaerophiles such as lactic acid
bacteria, which are present in very low
numbers or even absent in the presence of
oxygen. However, canned meat is a system
totally void of oxygen. - Food composition and structure also deter-
mine microbial colonization. Whereas
meat cuts are mainly contaminated on the
surface, microorganisms fi nd their way to
the inner part of the muscle, following the
muscle structure, mainly through the peri-
mysial layers. Ground products or those
added with other food components allow
microorganisms to colonize the inner as
well as the surface food structures.
Carbohydrates, fats, and proteins act as
protectors against microbial destruction by
heat.
In the striated muscle, caloric fl ux is also
affected by the muscle fi ber orientation.
Conductivity, if this fl ux is perpendicular to
the muscle fi bers, is 1.72 kJ/h m ° C at 78%
relative humidity, 0 ° C; at the same conditions
parallel to the muscle fi bers, conductivity is
1.76 kJ/h m ° C (P é rez and Calvelo 1984 ).
Several additives act as antimicrobial agents:
several medium - chain fatty acids, essential
oils (cinnamon, clove, garlic, onion, and
oregano), or proteins such as conalbumin.
Toxins
The pathogenicity of several microorganisms
present in foods depends on their infection or
intoxication ability. Infection is due to micro-
bial colonization on the human organism;
intoxication is a condition caused by the
intake of a toxin, produced by the secondary“ hurdle ” theory (Leistner 1985 ). In the case
of canning, although various treatment sever-
ities can be applied depending on the expected
shelf life, alternative preservation methods in
addition to heat are often used. For instance,
canned sausages also include the previous
addition of curing salts or the inclusion of
antioxidants such as phenols in smoked sau-
sages. According to the hurdle theory, it is
not necessary to apply more hurdles than
necessary to obtain microbiologically safe
products with a considerable long shelf life.
The net effect of the applied hurdles is an
interaction of individual effects; therefore, if
the ecology of a given food requires it, a
more severe heat treatment must be applied
due to intrinsic and extrinsic characteristics,
such as high pH and low conductivity com-
ponents (fat, carbohydrates, etc.), among
others.
However, product susceptibility to dete-
rioration of complex systems, such as that of
canned meat products, depends on a wide
variety of extrinsic and intrinsic characteris-
tics. The most important are the following:
- pH of most canned meats is relatively high
(pH > 6.5); a more severe heat treatment is
necessary to destroy potentially present
pathogens. A pH of 4.5 is the growth limit
for Clostridium botulinum , a strict anaer-
obe widely present in nature and seldom
found in canned foods. At this pH, C. botu-
linum can grow and produce toxins and
heat - resistant spores; processing condi-
tions to destroy this microorganism is the
calculation basis for thermal processing of
many foods. - Water activity (a w ) is also a criterion for
processing calculations; the limit value for
Cl. botulinum is 0.97 for psychrotrophic
species, and 0.95 for mesophile species.
Unfortunately, a w in most canned meats is
above this value. - Redox potential: there is a correlation
between redox potential, heating, and the
presence of additives (nitrates, phosphates,