156 Condensed Milk and Milk Powder 



concerned. It is generally understood, though not experimentally 

 proven, that species of micro-organisms which do not contain the 

 enzyme lactase have no gas-producing action on lactose. 



The great majority of cases of gaseous fermentation of sweet- 

 ened condensed milk are the result of the action of micro-organisms 

 on the sucrose, especially those which contain the enzyme "inver- 

 tase." The majority of yeasts secrete invertase and ferment sucrose, 

 producing alcohol and carbon dioxide to the same extent as in the 

 case of glucose fermentations. The process is considerably slower, 

 however, especially at the start, owing to the fact that inversion of 

 the sucrose must precede fermentation. For this reason gaseous 

 fermentations of sweetened condensed milk do not become notice- 

 able until the product is one or several weeks old. 



Contamination With Yeast on the Farm. — In most cases of 

 yeast fermentations of sweetened condensed milk, the source of 

 contamination lies in the factory. While such contamination may 

 and often does occur on the farm, the yeast cells, though they may 

 be spore-bearing, are destroyed by the heat to which the fresh milk 

 is subjected in the forewarmers and before it reaches the vacuum 

 pan. The thermal death point of all forms of yeast which have 

 come to the attention of the writer in connection with a vast num- 

 ber of investigations of fermented condensed milk was below 180 

 degrees F. If all the milk is properly heated in the forewarmers to 

 190 degrees F. or over, there is, therefore, little danger of fermented 

 miilk, caused by contamination of the fresh milk on the farm with 

 yeast. If, however, the heating is incomplete, or if some of the milk 

 passes into the vacuum pan without having been properly heated, 

 there is danger of milk, contaminated with these yeasts, to result in 

 fermented condensed milk. 



Contamination with Yeast in the Factory. — As previously 

 stated, yeast fermentation of condensed milk can almost invariably 

 be traced back to contamination in the factory. After the milk 

 leaves the forewarmers, or hot wells, it is never again heated to 

 temperatures high enough to destroy these destructive yeast cells. 

 The channels through which yeast contamination may occur in the 

 factory are many. 



Contaminated Sugar. — The sucrose itself may be contaminated 

 with yeast. This is frequently the case and especially so if the sugar 



