304 



Journal of Agricultural Research 



Vol. Ill, No. 4 



tent and some with casein added to raise the protein content. One ounce 

 of salt to I pound of butter was used in some samples; no salt in others. 

 Slices of butter of each kind were put into Petri dishes and inoculated 

 with a series of molds obtained from butter. Among these were Oidium 

 lactis, Mucor sp., Aliernaria sp., and several species of Penicillium. The 

 dishes were then allowed to stand in an incubator at the temperature 

 and relative humidity of the laboratory for several days. Absolutely 

 no surface growth of mold was obtained. Part of these Petri dishes 

 were then placed in moist chambers and it was found that mold colonies 

 developed upon every sample so placed. These growths included not 

 only the species inoculated into the butter but other forms whose spores 

 were present in the butter as made. At the low relative humidities pre- 

 vailing in the laboratories of the Dairy Division from February to April^ 

 1914, no mold colonies were able to develop in butter representing a 

 range in water content greater than the usual range of percentage in 

 market butter. 



The addition of 2 to 3 per cent of water to butter containing but 14 or 

 15 per cent does not make the quantity of water present sufficient to 

 support mold growth aside from conditions of high humidity. 



REL.^TION OF HUMIDITY TO MOLD GROWTH 



In moist-chamber culture comparison between samples containing 

 normal and low protein with samples containing excess or added pro- 

 tein showed that mold growth was more rapid and extensive when pro- 

 tein was added. The failure of molds to grow in these same cultures 

 under the ordinary humidity conditions of the laboratory proved that 

 the essential factor in molding was not protein, but water. 



To define these humidity relations more closely, three desiccators 

 were prepared in which definite relative humidities could be maintained. 

 For this purpose the bases of the desiccators were filled with sulphuric 

 acid standardized to the specific gravities — from Hastings's (1909) table — 

 required to maintain, respectively, 90 per cent, 79.6 per cent, and 69.6 

 per cent relative humidity. Three samples were used: One sample of 

 butter was made with low-salt content (0.55 per cent); one at normal 

 salting (2.43 per cent) ; and one sample of butter fat, free from water, with 

 skim-milk powder added. 



The composition of these three samples is given in Table II. 



Table II. — Composition of samples of btillcr used in mold-growth tests 



