APPENDIX A 127 



stirring, on the steam bath. If this fails to reduce the acidity suffi- 

 ciently, then the process may be repeated. An excess of alkali, 

 however slight, must be avoided, because it will seriously discolor 

 media made from plant extracts. 



4. Sugar-containing media. All organic chemists are aware of the 

 changes wrought by the action of heat upon sugars. Dry heat con- 

 verts part of the sugar into furfuraldehyde. The moist heat of the 

 autoclave converts sugars into humuslike bodies and other substances 

 unfavorable to the growth of microorganisms. Henderson has found 

 that at a temperature so low as that of 37.5 C. a glucose solution 

 will, in the course of several days, lose its optical activity. 



An excess of heat should always be avoided in sterilizing culture 

 media containing either monosaccharides or disaccharides. The media 

 should be intermittently sterilized in the Arnold sterilizer by heating 

 for twenty minutes the first day, fifteen minutes the second day, and 

 ten minutes the third day. 



5. Solid vegetables. In plant-pathological investigations much 

 use is made of sterilized pieces of potatoes, beets, carrots, turnips, 

 rice, corn meal, and other vegetable substances. Much difficulty 

 is encountered at times in sterilizing potatoes and beets, because 

 they carry spores of very resistant soil organisms. When preparing 

 tubers or roots for culture media it is well to scrub them thoroughly 

 with a stiff brush, then wash in a 1 : 1000 bichloride solution, and 

 finally rinse with distilled water before cutting plugs or slices. 



There is some possibility that the heat of sterilization, acting upon 

 these carbohydrate substances, may produce compounds toxic to 

 microorganisms, as has been pointed out by L. 11. Jones. It is there- 

 fore well to use as little heat as possible for sterilization, although 

 the autoclave must generally be used in order to successfully sterilize 

 potato plugs. 



6. Milk. Sterilization will be much more successful if fresh, clean 

 milk is obtained. In cities one should obtain certified milk. Where- 

 ever possible, the milk should be obtained directly from the dairy. 

 Market milk often contains a large number of organisms, and trouble 

 arises from the heat-resistant types. The cream should be removed 

 by centrifugingand the milk placed in tubes or flasks for sterilization. 



Here again an excess of heat brings about undesirable changes 

 in the medium. Prolonged heating coagulates the casein more or 

 less completely and may transform the sugars into compounds less 

 favorable for bacterial growth. 



