THE RELATION OF MICROORGANISMS TO MILK 447 



surrounded by a hazy zone, and always occur below the surface of the 

 medium. In lactose-agar stab cultures growth occurs along the entire 

 line of inoculation, but there is no surface growth. No liquefaction of 

 gelatin occurs. In bouillon the medium is uniformly turbid or it re- 

 mains clear with a slight sediment. On potato, growth is slight or is 

 absent. Milk is usually curdled within twenty-four hours at the opti- 

 mum temperature by members of the group, although some fail to cur- 

 dle the milk, since the maximum amount of acid produced is not suffi- 

 cient to cause this phenomenon. Still others cause curdling in the pres- 

 ence of small amounts of acids, in which case a rennet-like enzyme may 

 be present. No gas is produced in the fermentation of lactose, hence 

 the curd formed in milk is perfectly homogeneous; it shows but little 

 tendency to shrink and to express whey. In litmus milk the color is 

 discharged from the entire mass of medium before curdling occurs, due 

 to the reduction of the litmus to the colorless leuco-compound. Through 

 the action of the oxygen of the air the litmus is slowly reoxidized and 

 the pink layer, which immediately after curdling is but a few millimeters 

 in depth, is slowly extended until the entire mass of curd has a uniform 

 pink color. Saccharose, dextrose, maltose, and mannit are fermented. 

 The maximum amount of acid produced by organisms that are most 

 typical of the group is determined by the composition of the medium. 

 It is often said that the organisms causing the normal souring of milk 

 represent a group that can grow in a strongly acid medium. This is 

 true as far as acid salts are concerned, but free acid totally inhibits 

 growth. In a culture medium, which contains no substance that can 

 combine with the acid formed and thus remove it from the sphere of 

 action, no growth, or but very slight growth occurs. In sugar bouillon 

 and in milk, the amount of acid formed is determined by the amount of 

 substances in these liquids that can combine with the acid. In milk 

 such compounds are the casein and some of the ash constituents, 

 especially the phosphates. In normal milk, the maximum acidity 

 attained ranges from 0.9 to 1.25 per cent calculated as lactic acid. If 

 the content of neutralizing compounds per unit volume is varied by 

 concentration, dilution, or by the addition of such substances as cal- 

 cium phosphate, the maximum amount of acid produced by typical 

 cultures will be changed. In sugar bouillon the maximum acidity 

 produced rarely exceeds 0.25 per cent. 



