22 MICRO-ORGANISMS AND FERMENTATION. 



If the liquid is particularly sensitive to high temperatures, 

 it is necessary to fall back on the method of discontinuous 

 treatment, whereby the liquid is heated to a moderate degree 

 several times, with a suitable interval between each heating. 

 Frequently the alteration in taste produced by pasteurisation 

 can be partially removed by subjecting the liquid for a time 

 to a low temperature. A special difficulty met with, par- 

 ticularly in the case of beer, is that during storage or transport, 

 particularly at low temperatures, the pasteurised liquid 

 develops a turbidity, or forms a deposit, consisting usually 

 of albuminoid substances separating in the form of small 

 granules, or, in difficult cases, in flakes and skin formation. 

 It has usually proved necessary to control the preparation of 

 the malt if such a calamity is to be avoided. Care must be 

 taken that a slow and sufficiently advanced development of the 

 grain has taken place, accompanied by a full transformation 

 of its contents. Further, it is obvious that the fermentation 

 should have been vigorously carried out, and in this con- 

 nection it is particularly necessary to adopt pure ferments. 

 By cooling the beer to a low temperature before filtration and 

 pasteurisation it is possible to avoid the subsequent separa- 

 tion, as part of the material in question is separated in the 

 cooling process. 



In dealing with milk, heat is applied in the same way. In 

 this case the greatest possible difficulties are met with owing 

 to the great range of micro-organisms present in milk (lactic 

 acid bacteria, putrefactive bacteria, hay and potato bacilli, 

 etc.), many of which are only killed at a high temperature, 

 owing to their power of forming spores. Heating the milk 

 further results in separating or modifying components, which 

 may be of extreme value in nutrition (e.g., the enzymes), 

 even at comparatively low temperatures. Pasteurisation at 

 temperatures considerably below the boiling point may result 

 in the milk bacteria being killed, whilst the putrefactive 

 bacteria remain alive. As a consequence, the latter, freed 

 from competition, multiply rapidly, and form putrefying 

 matter in the milk, and this may occur to a considerable 

 extent if the milk is not stored at a very low temperature. 

 Actual sterilisation can only be secured if the milk is heated 



