The limits for the germination of Penicillium, as given by W. J.
Sykes,[E] are 2° to 43° C. (35° to 110° F.), and the most favorable
temperature 22° to 26° C. (72° to 79° F.). This author states also that
according to Pasteur the dry spores retained their vitality at 108°
C. (226° F.), but that they were soon killed when immersed in boiling
water. Klöcker[F] quotes Pasteur as saying that the conidia are killed
if exposed to a temperature of 127° to 132° C. for half an hour, but
that they retain life at 119° to 121° C.
[E] Principles and Practice of Brewing, 1907, p. 284.
[F] Ibid., p. 281.
A series of tests was made to determine the thermal death point of
the moist and dry conidia of the Penicillium used in the experiments,
a young, vigorous development on ketchup being used. The flasks were
kept under observation for a month after the tests were made, as in
many cases a development does not occur in the usual time. The high
temperatures applied for longer periods of time were tried first, but
both temperature and time were reduced as results from the series were
obtained. Only the conditions obtaining in the final tests are given in
the table. It was found that the Penicillium used did not have the high
resistance supposed.
The tests were made in small flat-bottomed 10-cc flasks, tomato
bouillon being used for the tests on moist conidia. The bouillon was
used so as to have the conidia in a nutritive medium after the test was
made, without transferring. The time for those at 100° C. was estimated
from the time of ebullition. At the end of the specified time, the
flasks were cooled promptly under running water. As the flat bottoms
gave comparatively large surface, the heating and the cooling could be
effected in a short time. For the tests below 100° C. a vessel of water
was heated to the desired temperature, and the flasks were immersed in
it and shaken constantly. The dry conidia were placed in test tubes
which were immersed in boiling water for the desired time and cooled
under running water, after which 10 cc of sterilized tomato bouillon
was added. After determining the death point in this manner and finding
it to be much lower than had been supposed, it was decided to make the
test again, but using ketchup as the medium. Ten grams of ketchup were
sterilized, then inoculated from a vigorous growth of mold, and tested
with a set in which the tomato bouillon was used. For those below 100°
C. the two flasks which were to receive the same temperature were held
in the vessel of water at the same time, so that as nearly as possible
the treatment would be identical. The following results were obtained:
_Thermal death point of moist and dry conidia of Penicillium._
PENICILLIUM IN 10 CC OF TOMATO BOUILLON.
-----------+------------+----------+------------+---------------------
No. of |Temperature.| Time of |Time before |Period of observation
experiment.| | heating. |germination.| and developments.
-----------+------------+----------+------------+---------------------
| _°C._ |_Minutes._| _Days._ |
1 | 85 | ½ | 3 |Dark strings from
| | | |spores; 9 days;
| | | |no development.
2 | 80 | 1 | |
3 | 75 | 1 | |
4 | 70 | 5 | |
5 | 65 | 5 | 3 |Dark strings running
| | | |from spores; 9 days;
| | | |growth normal, spots
| | | |on surface.
6 | 60 | 5 | 3 | Do.
7 | 55 | 5 | 3 | Do.
-----------+------------+----------+------------+---------------------
PENICILLIUM IN 10 CC OF KETCHUP.
-----------+------------+----------+------------+---------------------
1 | 100 | 3 | |
2 | 100 | 2 | |
3 | 100 | 1 | |
4 | 100 | ½ | |
5 | 100 | Instant.| |
6 | 85 | ½ | 2 |Colonies on sides;
| | | |8 days; surface
| | | |covered, green.
7 | 80 | 1 | 2 | Do.
8 | 75 | 1 | 2 | Do.
9 | 70 | 5 | 8 |Colony on surface.
10 | 65 | 5 | 9 | Do.
11 | 60 | 5 | 3 |Colonies on sides;
| | | |8 days; surface
| | | |covered, green.
12 | 55 | 5 | 4 | Do.
13 | | | 2 |Ring around sides;
| | | |3 days; surface
| | | |nearly covered.
-----------+------------+----------+------------+---------------------
DRY CONIDIA.
-----------+------------+----------+------------+---------------------
1 | 100 | 10 | 4 |Rough appearance,
| | | |like that in ketchup.
2 | 100 | 15 | 4 | Do.
3 | 100 | 20 | 7 |Slight growth.
4 | 100 | 25 | 10 |Growth barely
| | | |perceptible.
5 | 100 | 30 | 10 | Do.
6 | 100 | 35 | |Conidia stained
| | | |readily, showing they
| | | |were dead.
-----------+------------+----------+------------+---------------------
YEAST.
-----------+------------+----------+------------+---------------------
1 | 55 | 5 | 2 |Wrinkled film; liquid
| | | | turbid.
2 | 60 | 5 | 2 | Do.
3 | 65 | 5 | 2 |Thin, smooth film;
| | | |liquid clear.
4 | 70 | 5 | |
5 | 75 | 5 | |
6 | 100 | Instant.| |
-----------+------------+----------+------------+---------------------
The moist heat was very effective in destroying the vitality of the
conidia of Penicillium, the death point being 27° C. higher than the
maximum temperature for germination as given by Sykes. The heating was
more effective in destroying germs when applied to bouillon than to
ketchup, no development taking place for any temperature above 65° C.,
even when applied for a short time.
In the ketchup the lower temperatures for the longer periods of time
were more effective in checking the development, even though they did
not destroy the vitality. In the ketchup, with the exception of Nos. 9
and 10, the colonies started invariably along the sides of the flasks.
The greater access of air to those on the sides would account for
this. The conidia on the sides of flasks Nos. 9 and 10 must have been
destroyed, as no development took place in either case except in the
center of the surface.
The dry conidia were destroyed at 100° C. when heated for thirty-five
minutes; they did not reach a normal development in any case, even
when heated for only ten minutes, many of the conidia being destroyed
by this treatment. Where development failed to take place, the conidia
were stained with a water solution of eosin, so as to be sure that the
effect was death, and not an arrested development.
The results of the tests do not agree with those obtained in factory
practice, where the ketchup is cooked at 100° C. for at least forty
minutes and sometimes for fifty or fifty-five minutes, depending on the
consistency of the pulp.