128 THERMOGENIC BACTERIA 



104. The Spontaneous Heating- of Hops 



is well known to every brewer from wide personal experience. For a more 

 intimate knowledge of the subject we are indebted to J. BEHEEXS (I.), who has 

 not only discovered a cause for the phenomenon, but has also traced a con- 

 nection between the latter and the well-known presence of trimethylamine in 

 hops, first shown by GRIESSMAYER (I.). Behrens found, in hops that had 

 become warm, a fission fungus, in a condition of almost pure culture, which he 

 named Bacillus lupuliperda, and explained to be nearly allied to the Bacillus 

 fluorescens putidus described by FLUGGE (I.). The cells of this newly-discovered 

 motile microbe are about 0.7 p in breath, and vary in length, according to the 

 conditions of cultivation, from 0.7 to 2.5 /*. It liquefies gelatin. Peptone 

 alone is insufficient for its support, a second substance from which it can derive 

 carbon being required ; consequently it belongs to the group of peptone-carbon 

 bacteria established by Beyerinck. It thrives freely in hop extract and quickly 

 renders sugar-free media alkaline by excreting copious quantities of ammonia 

 bases, especially trimethylamine. In presence of sugar the reaction of the 

 medium at first becomes acid, butyric acid being formed, although in Behrens' 

 experiments only to the extent of o.i per cent, at most. This microbe, which 

 gives rise to spontaneous heating in hops, and causes them to give off an odour 

 of trimethylamine, was found by Behrens in all the samples of hops examined 

 by him. It appears to be chiefly domiciled in the soil, arid passes thence to the 

 hop cones, which, being fairly hygroscopic, attract moisture when bagged, and 

 thus enable the bacillus to develop, the hops becoming ; ' warm " and commencing 

 to decompose, whereby they are reduced in value. Timely prevention may be 

 ensured by removing the bagging in which the hops are packed, and thus 

 admitting cool and dry air into the interior of the contents, in consequence 

 of which the activity of the microbe is lessened. Closer studies both on the 

 spontaneous heating of hops, and especially into the transformation products of 

 Bacillus lupuliperda and their relation to the production of rancidity in hops, 

 constitute a productive field for future research. 



The above remarks on the spontaneous heating of stored vegetable substances 

 may now be supplemented by a few observations concerning 



105. The Fermentation of Tobacco, 



which cannot well be included in subsequent chapters. The tobacco leaves, 

 when gathered, are allowed to become somewhat withered, and are then arranged 

 in moderate-sized heaps, where they undergo a so-called " sweating." The rise 

 of temperature occurring during this process is, as determined by Miiller- 

 Thurgau, a consequence of the activity of the leaf-cells, which transpire their 

 store of carbohydrates and convert their albuminoid matters into amides, the 

 heat thereby liberated effecting the gradual drying of the leaves. The water 

 vapour evolved condenses into the matting employed to cover the heaps, which 

 are then said to " sweat." The alteration of the nitrogenous bodies in the leaves 

 can also be effected by " shed drying." On this point more will be said in a later 

 chapter dealing with Botrytis cinerea in the second volume. 



As soon as the tobacco leaves have finished sweating and become "shed 

 ripe," they are made to undergo fermentation, for which purpose they are tied 

 in bundles and arranged in great heaps, containing as much as fifty tons of 

 tobacco. Hereupon active decomposition quickly ensues and the temperature 

 rises. NESSLER (I ) found this to be as much as 54 C. even on the second day, 

 but, as a rule, the heaps are not allowed to become warmer than 50 C., further 



