From Wikipedia,
the free encyclopedia
The term Dry Sterilisation
Process, DSP, denotes a
dry aseptic
sterilisation process. It is
used for instance in the beverage
industry during cold aseptic
filling of beverages (juices,
waters,
UHT-milk, ...) into plastic
bottles made from
PET or
HDPE, and also for some
applications in the pharmaceutical
industry.
In cold aseptic filling the
sterile or near-sterile product is
filled into a bottle which has to
be sterilised prior to bottling to
avoid product contamination.
Caused by the heat-sensitive
plastic material, the
sterilisation process must not
heat the bottles. Therefore
chemical sterilisation processes
are used for this purpose. The Dry
Sterilisation Process uses an
aqueous solution of
hydrogen peroxide (H2O2)
with a concentration of 30...35%
to achieve the germ-killing
effect.
At first the bottles are placed
into a sterilisation chamber. This
chamber is designed to be a vacuum
chamber and is
evacuated by
vacuum pumps down to the
low vacuum range. A certain
amount of aqueous solution of
hydrogen peroxide is now delivered
to an evaporator and abruptly
evaporated. Driven only by the
pressure difference between the
hydrogen peroxide
vapor inside the evaporator
and the evacuated sterilisation
chamber, the vapor flows through
an appropriate piping into the
sterilisation chamber. The vapor
is strongly expanding when it
enters the chamber, undercooled
thereby and instantaneously
condensing. The forming
condensate layer is covering all
surfaces inside the sterilisation
chamber, all inner and outer
bottle surfaces and all surfaces
of the chamber itself.
The
heat of vaporization, released
by the
phase change from gaseous to
liquid, heats the forming
condensate layer in such a way,
that most of the hydrogen peroxide
molecules are thermally
dissociated thereby. The
resulting
free radicals, particularly
the oxygen atoms, are immediately
killing all the germs adhered to
the surfaces already during the
condensation. In contrast to other
sterilisation processes the
killing of the germs occurs
instantaneously without any need
for residence time.
The condensate layer is removed
from the sterilisation chamber and
all bottle surfaces immediately
after the condensation. This is
performed only by means of
appropriate vacuum pumps which
reduce the pressure inside the
sterilisation chamber below 1 Torr.
The condensate is rapidly
re-evaporating when the decreasing
chamber pressure reaches the
condensates
vapor pressure and the forming
vapor is removed from the chamber
by the vacuum pumps. This
re-evaporation effects a total
drying of the bottles and the
surfaces inside of the
sterilisation chamber and
completely removes all hydrogen
peroxide.
Prior to deloading of the
bottles from the sterilisation
chamber, the chamber is vented to
ambient pressure with sterile air
to avoid recontamination of the
sterile bottles.
The complete process time
amounts to 6 seconds. Using the
common reference germs for
hydrogen peroxide sterilisation
processes,
endospores of different
strains of
bacillus subtilis and bacillus
stearothermophilus, the Dry
Sterilisation Process easily
achieves a germ reduction of 106...108
(log6...log8) in
count reduction tests and also
in
end point tests.
The sterilised items leave the
sterilisation chamber in a
completely dry state. Only the
surface temperature of the items
is slightly increased by a few
degrees (10°...15°) during the
sterilisation process. Therefore,
the process is particularly useful
for the sterilisation of heat
sensitive items like plastic
bottles. It is also useful for
applications which require a high
germ reduction and short process
times.
NB #1: Unfortunately it is
common diction to say "the kill
rate is log6" or "the germ
reduction is log6", which strictly
speaking is not only wrong but
nonsensical. By saying this one
means that the germ reduction is 6
orders of magnitude or the
survival probability of each
single germ is 10-6.
(This wrong diction originates
from a misunderstanding of the
mathematical expression log 106
= 6)
NB #2: Strictly speaking it
is also wrong to talk about single
germs or the like. It's correct to
use the item cfu or colony
forming unit. The
main problem is not inevitably the
presence of germs (bacteria,
spores, ...) but their ability of
fast fissiparous, which gives an
exponential increase of the number
of the germs with time. If one
tries to count "a number of germs"
one has to, simply spoken,
cultivate them on an agar plate,
let them grow for a few days and
count the macroscopic colonies
which have formed. Each of these
colonies is resulting from 1 cfu
(= 1 "augmentable germ").
Example #1: One item which
has to be sterilized carries a
contamination of 107
germs prior to sterilisation. The
germ reduction capability of the
sterilisation process is 6 orders
of magnitude (=106 or
"log 6"), which means the survival
probability of the germs is 10-6.
If such items are sterilised the
average number of
"surviving germs" or, correctly
spoken, cfu's which are found on
the items after sterilisation is:
107 / 106 =
10 or 107 * 10-6
= 10.
Example #2 (statistically
equivalent to #1): A lot of items
which have to be sterilized are
carrying a contamination of 10
germs each prior to sterilisation.
The germ reduction capability of
the sterilisation process is 6
orders of magnitude (=106
or "log 6"), which means the
survival probability of the germs
is 10-6. If one
sterilises a statistically
significant number of these
items the average number of
cfu's which is found on the items
after sterilisation is: 10 / 106
= 10-5 or 10 * 10-6
= 10-5. This means
that, in average 1 cfu is
found per 105 = 100.000
items.
