I30 Transactions of the Canadian Institute. [Vol. VII. 



So far as my experiments are concerned, they are the only micro- 

 organisms that actually increase in cheese. Cheese No. 3 illustrates the 

 increase of yeasts that may take place in cheese, and some of the 

 analyses in Table i support this conclusion. 



Whilst I have applied the name yeasts to this class, most of them 

 are species of Toj-ula, as they form no spores, even under the con- 

 ditions prescribed by Hansen. They may be classified roughly here as 

 beneficial and injurious. The former either act like the lactic acid 

 bacteria in milk, produce acid, and give a firm curd, or they may make 

 but little acid and after considerable length of time digest the casein. 

 The injurious species have more diverse habits. Thus some are able to 

 ferment lactose, form gas, and give rise to bad flavours in the cheese. 

 A species of torula isolated in 1900, besides causing gas formation, pro- 

 duced a bitter flavour, which gave much trouble in as many as four 

 factories in the same district. 



Another species of torula caused no perceptible change in milk, but 

 when grown with a lactic acid bacterium it produced a mottled appear- 

 ance in the cheese. I was able to produce such mottles in milk con- 

 taining a little cheese colour, and seeded with a lactic acid bacillus, or a 

 drop of lactic acid and the torula. 



The great difficulty which a cheese maker experiences when working 

 with yeasts is the remarkable tolerance they shew to acidity, so that a 

 maker is unable to repress an undesirable yeast fermentation by the 

 addition of a vigorous lactic acid starter. Some of the torulae I have 

 isolated grew luxuriantly in peptone solutions containing 2.25 per cent, 

 of lactic acid. This fact undoubtedly explains their increase in cheese. 



Nothing can be said as to the place of origin of these yeasts. 



Other bacteria in Cheese. — During these investigations a number of 

 other forms, not falling into any of the classes I have mentioned, have 

 been isolated. Some of these produced undesirable flavours and others 

 were inert. Nothing need be said of these in this paper, on account of 

 their occasional occurrence, or unimportance to my subject. 



My experiments and results are perhaps rather few, and too little 

 chemical work has been done to justify much theorizing on what causes 

 the ripening of cheese, but from a review of the works of others and my 

 own results, I may perhaps be justified in making a few remarks. 



Three most important facts seem well supported by good evidence 

 and trustworthy experiments : 



