Biomedical Engineering Reference
In-Depth Information
sterilization criterion should be defined so that it is possible to design a process that
is safe but occurs within a finite time and which is economically and practically
feasible (Simpson et al.
2012
). According to Stumbo (
1973
) the commercial steril-
ization criterion was established arbitrarily. The commercial sterilization criterion
states that the minimum thermal process should reduce initial micro-organism con-
centration by 10
12
. This is well known as 12
D
concept or “botulinum cook”. Teixeira
(
2002
) discussed the origins of the 12
D
concept and outlined the original work of
Esty and Meyer (
1922
). They concluded that Stumbo (
1948
) was the first to recog-
nize the logarithmic nature of time temperature curve. The commercial sterilization
was used for several reasons (1) safety margins, (2) cooking requirements, and (3)
to prevent the growth of thermophilic spoilage microorganisms. The probability
argument says, in 12
D
treatment there will be one spore in 10
12
cans (Singh and
Heldman
2009
). Considering 100 million cans consumed per day, an estimate over
a 100-year period worldwide consumption will be 3.65 × 10
12
cans and the 12
D
cri-
terion would predict three to four outbreaks every 100 years (Simpson et al.
2012
).
The processing time to achieve 12
D
can be estimated when
D
T
value of target
micro-organism at the temperature
T
is known. However, the processing time varies
with the size of a can. Considering
D
121.1
= 0.21 min for
C. botulinum
, one spore per
g and can size of 0.1 L (density: 1 g/cm
3
), the processing time (
t
) to achieve 12
D
is
2.94 min from Eq. (
2.1
) (Simpson et al.
2012
). Similarly if the can size is 5 L, the
processing time is 3.29 min. Since thermal process could not be performed at a
specific temperature instead a varied temperature range during heating and cooling
time, thus thermal death time,
F
r
is defined for processing as (Holdsworth and
Simpson
2008
):
t
TT
z
−
r
=
∫
F
10
dt
(2.2)
r
0
where
F
r
is the processing time at a reference temperature (s),
T
r
is the reference
temperature (°C) and
z
is the thermal constant or
z
-value. If reference temperature
T
r
is 121.1 °C, then
F
r
is termed as
F
o
as:
t
T
−
121 1
.
=
∫
F
10
z
dt
(2.3)
o
0
Currently, a common commercial sterilization treatment for
C. botulinum F
o
is in
the range of 6-8 min, although some companies use
F
o
of 10 min or higher.
Considering this processing time, 5 L can size and the minimum time requirement
(6 min) at 121.1 °C, indicated 1.335 × 10
−25
spores per package. Applying the prob-
ability concept, we should expect one outbreak in several billion years (to be pre-
cise, one hundred thousand billion years). In fact, at least in the past 50 years no
outbreak has been directly related to the sterilization criterion (Simpson et al.
2012
).
However, there is no theoretical method available for the prediction of
F
o
, if other
hurdles are used in combination.
