COMBINED METHOD OF STERILISATION. 119 



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 re- 

 stricted 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 cul- 

 ture 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 some- 

 what 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 (100.5- 103 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 incapable 

 of further development ; the wort is therefore practically sterile. 

 Sometimes but, as E. CH. HANSEN (III.) has shown, not always 

 the complete destruction of all the germs (absolute sterility) 

 is attained in this way. However, the residual living germs in 

 the wort do not develop therein, though they will do so if trans- 

 ferred to a more favourable medium e.g. meat-broth. In this 

 case we have to do with relative sterility. The rapidity of the 

 effect is chiefly attributable to the influence of the hops, which, 

 in turn, owe their germicidal powers to the possession of certain 

 resinous bodies, generally known under the collective name of 

 hop-resins. The chemical properties and biological effects of 

 these bodies have been investigated by M. HAYDUCK (I.), Avho 

 found three different resins in hops, all of which are soluble in 

 alcohol, ether, and chloroform. One of these, viz., the brittle, 

 tasteless y-resin, insoluble in petroleum spirit, does not interest 

 us in the present instance, the germicidal properties of the hop 

 not being due to its influence, but to that of the two (extremely 

 bitter) soft resins, the a-resin and /3-resin. These two act power- 

 fully on the lactic acid- and butyric acid bacteria, but are in- 

 nocuous towards acetic acid bacteria, sarcina, and higher fungi 

 (especially yeast). The latter organisms are, however, subject 

 to the influence of the boiling temperature, so that the wort is 

 delivered in a sterile condition to the cooler, where it is infected 

 anew. The attempts of all discerning brewing technicists to 



