92 METHODS OF STERILISATION 



general, and to that of the pathogenic and fermentative microbes in particular. 

 It is also important, as we shall soon see, for the technique of sterilisation. 



The destruction of germs by heat in certain nutrient solutions and food-stuffs 

 is often a very difficult task, because it necessitates temperatures that damage the 

 sample both as regards nutritive value and palatability. Success may, however, be 

 attained by combining the influence of heat with that of poison, although the iso- 

 lated action of either is incapable of killing the germs. This is the leading idea on 

 which is based the process of mixed or combined sterilisation, wherein the death 

 of the micro-organism is caused by the simultaneous application of two factors ; 

 one of which (the poison) is without influence on the chemical composition of the 

 sample, whilst the other (heat) is too low to set up any injurious decomposition. 



At first sight it may seem that the presence of poison restricts the application 

 of the process to such cases as the sterilisation, pure and simple, of a liquid, and 

 precludes its use when such liquid is intended for the cultivation of micro- 

 organisms or for human consumption. On more mature deliberation, however, 

 a contrary conviction will be formed. 



Many of the substances named in the preceding paragraph are in themselves 

 innocuous to the health of man, provided the quantity present is not too large ; 

 this is particularly the case with alcohol and the organic acids, and it is precisely 

 these acids that are generally employed for the preservation of numerous food- 

 stuffs. A fuller account of this subject will be given in a future chapter, so 

 we will simply refer to it here and pass on to the consideration of the second 

 question : Is the combined method also suitable for sterilising nutrient media 

 intended for mycological work ? 



Let us recall the observation that has been frequently made in previous 

 paragraphs with reference to the behaviour of micro-organisms under the 

 influence of physical and chemical forces. Just as a certain degree of heat is 

 fatal to one species, simply retards the development of a second, is favourable to 

 a third, and insufficient to allow the cells of a fourth species to grow at all so 

 given amounts of poison may be fatal to one species of organism, inert towards 

 a second, and even stimulating to a third. In other words, the constants of 

 influence of a given poison vary with different organisms. 



We are indebted to TH. SCHWANN (II.) for the first observations on the varia- 

 tions in behaviour thus exhibited, but to PASTEUR (I.) for the first practical appli- 

 cation thereof. Attention has already been directed to the susceptibility of the 

 putrefactive bacteria to the influence of acids, a property of which Pasteur availed 

 himself to protect his cultures of 1'erments (in the restricted sense of the term) 

 against injury on the part of such interlopers. For example, in order to study 

 acetic fermentation, he first acidified the artificial medium with acetic acid. By 

 means of a skilful combination of various anti-bacterial forces, properly adapted to 

 each particular case, a given nutrient medium can be freed from germs without 

 diminishing its suitability for the culture in view. One factor of this combined 

 method of sterilisation is usually heat. Many examples of this will be given in 

 the course of subsequent paragraphs, so that we will now simply refer to that 

 afforded by the boiling of beer-wort. 



At the moment when the still unhopped wort runs from the mash-tun into 

 the copper, it contains innumerable bacteria, chiefly derived from the malt. Not 

 only do these survive the mashing process uninjured, but their increase is such 

 that 0.07-0.12 per cent, of lactic acid is produced. The acidity of the wort is 

 somewhat further increased by the addition of the hops placed in the copper 

 before boiling is commenced. But, as a consequence of the conjoint influence 

 of the boiling temperature (ioo.5-io3 C.), the lactic acid and the hops, 

 the germs in the wort are as found by G. H. MORRIS (I.) at the end of 

 fifteen minutes' boiling, partly killed and partly so far weakened that they are 



