92 MKTIlons or STF.KII.ISATION 



nil, and to that of the pathogenic ami fermentati\ microbes in particular. 

 It i> al>o important, as we shall soon see, for the technique of sterilisation. 



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

 v difficult ta>k. because it neceit at cs t em pcrat ures t hat damage the 



.simple both a> rei:ards nutritive valueand 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 i> basul the process of mixed or combined sterilisation, wherein the death 

 of the mien -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 lir>t sight it may seem that the presence of poison restricts the application 



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



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



:ni>m> or for human consumption. On more mature deliberation, however, 

 a con t r.-i IT 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 ^ 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- 

 stufls. 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 

 p: i i-a graphs 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. 



\Ve are indebted to Tn. 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 lias 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 ferments (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 

 a >kilful combination of various ant i-hacterial forces, properly adapted to 

 each particul given nutrient medium can be freed from germs without 



dimini-l inability for the culture in view. One factor of this combined 



method of >tei ili> ;( t ion is usually heat. Many examples of this will bu given in 

 the - I subsequent p:nagiaphs. so that we will now simply icfer to that 



afforded by the boiling of beer-wort. 



At the moment when the .still unhopped wort runs fiom the mash-tun info 

 the co|'[er. it contain- innumerable bacteria, chiefly derived from the malt. Not 

 only do the>e surxive the mashing process uninjured, hut their increase is such 



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



uh.it f'ui t her increased by the addition of the hops placed in the copper 

 iKjfore boiling is commenced. But, as a con> quence of the conjoint influence 

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

 the germ> in the wort are as found by (I. H. Momus (T.) at the end of 

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



