52 MAINE AGRICULTURAL EXPERIMENT STATION. IQIO. 



three things : ( i ) The extent to which the growth of the fungus 

 in a medium changes the acidity of that medium; (2) the rela- 

 tive effect of different acids on growth; (3) the amount of a 

 given acid which it is necessary to add to prevent growth. 



Three inorganic acids, — hydrochloric, sulphuric and nitric, — 

 and 7 organic acids, — lactic, acetic, citric, tartaric, oxalic, malic, 

 and formic, — have been used. Normal solutions or solutions 

 of twice the normal strength in some cases have been used in 

 making up the medium to a given acidity. The acid was added 

 to the dextrose broth in most cases before sterilizing but in the 

 case of some of the volatile acids, the acid w^s added cold after 

 the tubes had been sterilized and the tubes were then incubated 

 for 3 days to prove sterility before inoculation. The cultures 

 were made in test tubes each containing 10 c. c. of the culture 

 medium. The acidity of the media is expressed in each case in 

 terms of Fuller's scale. 



To determine what change takes place in the acidity of cul- 

 ture media due to the growth of the fungus, a large number 

 of determinations were made. In a considerable number of 

 cases uninoculated tubes were kept as checks and titrated on the 

 same day as the inoculated ones. In determining the acidity, 

 the entire contents of the tube were poured into a porcelain dish 

 and the tube was rinsed with part of the 50 c. c. of distilled 

 water which was added. The dish was heated over a flame 

 until the contents boiled for about one minute and then the titra- 

 tion was made. 



It was found that in the case of 2 per cent, lactose broth or 

 of 2 per cent, mannite broth neither of which is a favorable 

 medium for the growth of the fungus, there was little or no 

 change in the acidity after 18 days. In 5 per cent glycerine 

 broth, the acidity increased from +12 to +18 at the end of 11 

 days. In 2 per cent dextrose broth, the increase in acidity 

 varied from one to two per cent as shown in the table given be- 

 low. In 5 per cent and in 10 per cent dextrose broth the acid- 

 ity increased from +20 to +53 in the first and from 

 + 15 to +52 in the second at the end of three weeks. The table 

 which follows shows that the acidity of the two per cent dex- 

 trose broth in which the fungus had grown for 2 or 3 weeks did 

 not change to any extent by allowing the fungus to continue its 

 growth for 2 weeks longer. 



