82 METHODS OF STERILISATION 



number of steam-disinfectors to a careful examination. The simple and 

 inexpensive form described above is sufficient for the purposes of the fermenta- 

 tion physiologist. 



In the fermentation industries the method of destroying germs by steam is 

 highly prized on account of its convenience and efficacy. In breweries, for 

 instance, all the piping is steamed out, as also the wort cooler, and so on. The 

 Enzinger filter, however, cannot be treated in this way, owing to the softening 

 action of moist heat on the filter paper. 



The duration of exposure requisite for the destruction of germs by moist heat 

 can be considerably shortened by employing supersaturated high-pressure steam. 

 If, for example, the steam be used at a temperature of 1 20 C. (corresponding 

 to an extra-pressure of one atmosphere), an exposure of twenty minutes suffices 

 for sterilising liquids up to 50 c.c. in volume with certainty. For larger 

 quantities a corresponding additional exposure at 120 C. (five to ten minutes) 

 is given. The use of supersaturated high-pressure steam is attended with much 

 smaller outlay, but requires a strongly built autoclave. Laboratories already 

 possessing such an apparatus which is required, for example, in the determina- 

 tion of starch in cereals, &c. can also employ it to advantage for sterilising. 

 In many instances, too, a method of this kind is advisable, not only on account 

 of the saving in fuel, but also by reason of the fact that the chemical com- 

 position and nutritive quality of the liquid to be sterilised are less impaired 

 by fifteen minutes' exposure to 120 than by three hours' exposure in the Koch 

 apparatus. 



On the other hand, there are liquids which are so readily decomposed that 

 neither of the above methods of treatment can be thought of. An example of 

 these is afforded by the medium so frequently employed in mycological labora- 

 tories under the name of nutrient gelatin ; a solution of bouillon, wort, &c., 

 containing 8-10 per cent, of gelatin. This mixture, which sets at the ordinary 

 temperature of a room to the consistency of soft glue, and liquefies at about 

 25 C., would lose its pi-operty of setting if exposed to such degrees of heat, and 

 would thereby become useless. In such cases another method of killing the 

 germs must be employed namely, that first proposed by Tyndall, and known 

 as 



78. Intermittent Sterilisation. 



The powerful methods hitherto described have been considered necessary, for 

 the sole reason that the sample to be sterilised had to be regarded as presumably 

 containing highly resistant bacterial spores. In the absence of such forms, the 

 object in view is attainable by much milder means, and the liquids, &c., to be 

 sterilised can be converted into this more favourable condition by causing the 

 spores (possibly present therein) to germinate. It then becomes a much easier 

 task to deal with the resulting vegetative forms, since these latter perish at 

 temperatures below 100 C., and therefore so much the more certainly in a 

 current of steam. For this reason then the sample to be sterilised which, as 

 before, is supposed to contain the most highly resistant types of bacterial spores, 

 in addition to the comparatively feeble vegetative forms is exposed at first to a 

 temperature of 100 C. in the Koch steriliser for a short time. The duration 

 of ' his first treatment depends on the volume of liquid in the individual samples. 

 For flasks containing a charge of 10-15 c c - each, fifteen minutes will suffice ; 

 larger quantities warm through more slowly, and must be left in the steamer for 

 a correspondingly longer time. In every case the liquid should remain at a 

 temperature of 100 0. for about fifteen minutes. By this treatment only the 

 vegetative forms and weaker spores are killed, and the next step is to ensure 

 that the still living spores germinate, which is generally effected by simply 



