MICROFLORA OF BUTTER 123 



are the most active in hydrolysing fat, they do not spoil the-butter 

 to the same extent as the above-mentioned bacteria. Not only do 

 the moulds hydrolyse the fat, but they also consume part of the 

 free fatty acids, especially the lower members of the series ; the 

 acids produced by the bacteria, however, are allowed to accumulate 

 to such an extent that the fat hydrolysing bacteria, which, curiously 

 enough, are fairly sensitive to acid, are destroyed. This explains 

 the fact that very rancid butter may occasionally be sterile some 

 distance from the surface. The formation of the esters which are 

 so characteristic of rancid butter is due to Penicillium glaucum 

 and especially to Cladosporium butyri. The former, however, 

 only forms esters in symbiosis with Oidium lactis, which mould 

 also promotes ester formation by Cladosporium butyri. Certain 

 mycodermse which do not themselves hydrolyse fat may also 

 promote the formation of esters. According to the investigations 

 oiH.C. Jacobsen 1 , exactly the same microorganisms are responsible 

 for rancidity in margarine. 



All the above-mentioned organisms grow better and quicker 

 in unsalted butter than in salted butter. The water bacteria 

 are particularly sensitive to salt, and by packing the butter 

 in large casks the moulds, which require air, will obtain the 

 smallest possible surface for attack ; Danish butter and similarly 

 prepared butters therefore contain, as a rule, only lactic acid 

 bacteria and yeast. Among the yeasts, however, there are 

 certain torulae which are by no means innocuous and which 

 may, especially in symbiosis with lactic acid bacteria, hydrolyse 

 butter-fat to varying extents. The reason why the lactic 

 acid bacteria promote hydrolysis in such cases is that the 

 yeasts in question thrive best in presence of a little lactic acid 2 . 

 While the lactic acid bacteria, generally speaking, promote the 

 growth of yeasts and moulds, they inhibit the action of the fat 

 hydrolysing bacteria to a considerable extent, so that it is difficult 

 on the whole to say whether the souring of cream will as a general 

 rule contribute towards the preservation of butter. According to 

 the observations of Rogers and Gray 3 , butter from pasteurised 

 sweet cream keeps better than that from pasteurised sour cream, 

 and the addition of a little lactic acid to the pasteurised cream 

 has the same undesirable effect as souring with a starter. In all 

 circumstances it is essential that the starter used should contain 

 only pure cultures of good lactic acid bacteria, and no yeasts or 

 moulds. The safest method of preventing yeasts and moulds as 

 well as other harmful organisms from developing to any extent in 



1 " Folia Microbiologica," 1918, V. 2. 



2 Orla Jensen, " Maelkeritidende," 1910, p. 965. 



3 Experimental Station Kecord, 1909, No. 5. 



