VIABILITY or FREEZEfDRŒD ORGANISMS
.
99
so, by paying attention to the medium in which the organisms are suspended for drying,
great improvement in the survival rate can often be obtained. This subject has been dealt
within detail in previous papers in these courses.
But if survival on drying is poor, an attempt should be made to discover at what point
in the process death is occurring. So far we have found several organisms which are frost
sensitive which, if prefrozen in a suitable way, can be freeze-dried with reasonable success.
We have also found one organism, Vibrio cholerae, which, though frost resistant, is very
sensitive to drying; as yet we have found no method for protecting it well against drying ’
injury.
.
,
Preliminary experiments in the freeze—drying of the yeast Saccharomyces cerevisiae
were made by drying at controlled temperatures on nylon gauze in an apparatus similar
to that described by Meryman (1959). Drying at 0 °C the survival was 20 %. This survival
rose to 50 % when the drying temperature was lowered to—— 13 °C. Lowering the drying
temperature produced far worse results and the survival rate practically exactly followed
the curve of survival on fast freezing and slow thawing. This, therefore, seemed to be an
ideal organism for our experiments.
_
.
,
APPARATUS
The freeze—drying apparatus for these experiments requires to be able to prefreeze at a
constant rate which can be varied. It should also be able to dry at a constant temperature
which should be capable of variation down to at least— 35 °C.
The apparatus nally evolved consisted of a two-stage ‘Frigistor’ refrigerator mounted
on a water cooled base plate. A “bell jar” formed the top of the desiccator. The desiccator
chamber was connected to a vacuum pump in a phosphorous_ pentoxide trap. Figures 3,
4 and 5.
'
'
_
The rate of freezing is controlled by varying the current passing through the Frigistor.
At maximum current the minimum temperature obtained is between— 55 and—— 60 °C.
Temperatures are measured by thermoCouples connected to a recording galvanometer
which also has electric contacts which canbe’ used to control the drying temperature by
switching the current passing throug‘hthe Frigistor.
'
“
-
When drying is complete the current through the Frigistor can be reversed and the am- ‘
poules rapidly heated to ambient temperature.
'
-
'
RESULTS
,
°
*
Preliminary experiments showed that if S. cerevisiae was frozen to — _35 °C at ,1 °C/min.
even in distilled water the survival on rapid thawing was of the order of 70 %; With a
sugar added as a protective agent,,the
to nearly 100 %.
‘
Table I shows the percentage survival
drying this organism in various
single substances which
protective agents.every case prefreezm
\
was at 1 °Ç/min. Three drying temperatures were chosen —-12,5°,»—— 25° and— 35
'
È
%]
%
!é’s/
,
,'
,
,
,
*
.
99
so, by paying attention to the medium in which the organisms are suspended for drying,
great improvement in the survival rate can often be obtained. This subject has been dealt
within detail in previous papers in these courses.
But if survival on drying is poor, an attempt should be made to discover at what point
in the process death is occurring. So far we have found several organisms which are frost
sensitive which, if prefrozen in a suitable way, can be freeze-dried with reasonable success.
We have also found one organism, Vibrio cholerae, which, though frost resistant, is very
sensitive to drying; as yet we have found no method for protecting it well against drying ’
injury.
.
,
Preliminary experiments in the freeze—drying of the yeast Saccharomyces cerevisiae
were made by drying at controlled temperatures on nylon gauze in an apparatus similar
to that described by Meryman (1959). Drying at 0 °C the survival was 20 %. This survival
rose to 50 % when the drying temperature was lowered to—— 13 °C. Lowering the drying
temperature produced far worse results and the survival rate practically exactly followed
the curve of survival on fast freezing and slow thawing. This, therefore, seemed to be an
ideal organism for our experiments.
_
.
,
APPARATUS
The freeze—drying apparatus for these experiments requires to be able to prefreeze at a
constant rate which can be varied. It should also be able to dry at a constant temperature
which should be capable of variation down to at least— 35 °C.
The apparatus nally evolved consisted of a two-stage ‘Frigistor’ refrigerator mounted
on a water cooled base plate. A “bell jar” formed the top of the desiccator. The desiccator
chamber was connected to a vacuum pump in a phosphorous_ pentoxide trap. Figures 3,
4 and 5.
'
'
_
The rate of freezing is controlled by varying the current passing through the Frigistor.
At maximum current the minimum temperature obtained is between— 55 and—— 60 °C.
Temperatures are measured by thermoCouples connected to a recording galvanometer
which also has electric contacts which canbe’ used to control the drying temperature by
switching the current passing throug‘hthe Frigistor.
'
“
-
When drying is complete the current through the Frigistor can be reversed and the am- ‘
poules rapidly heated to ambient temperature.
'
-
'
RESULTS
,
°
*
Preliminary experiments showed that if S. cerevisiae was frozen to — _35 °C at ,1 °C/min.
even in distilled water the survival on rapid thawing was of the order of 70 %; With a
sugar added as a protective agent,,the
to nearly 100 %.
‘
Table I shows the percentage survival
drying this organism in various
single substances which
protective agents.every case prefreezm
\
was at 1 °Ç/min. Three drying temperatures were chosen —-12,5°,»—— 25° and— 35
'
È
%]
%
!é’s/
,
,'
,
,
,
*