376 BACTERIOLOGICAL ANALYSES 
30 grams lactose 
4 grams sodium chloride 
150 grams gelatin 
1000 ¢.c. distilled water. 
Acidity 0.1 per cent. 
Media for Yeasts and Molds 
+ grams beef extract 
10 grams peptone 
12 grams agar 
1000 grams whey 
Acidity 0.2 per cent. 
Add 1 cc. of sterile one per cent tartaric acid solution to 
each plate before pouring the medium over the dilution. 
Incubation.—Incubate agar, litmus agar and whey ayar 
plates at 35 degrees C. (95 degrees F.) for at least three days 
before making counts. Incubate gelatin plates at 21 degrees C. 
(70 degrees F.) for four to five days before making counts. 
Making Counts.—The colonies on the plates are counted 
most conveniently by placing the plates over a standard counting 
plate. In the absence of such a plate, place the petri dish upside 
down on a dark surface and draw, with a blue crayon, radial lines, 
dividing the field into segments. For plates containing not to 
exceed 100 colonies eight to sixteen segments are sufficient for 
easy, counting. 
Qualitative Determinations.—Numerical counts on the four 
kinds of media recommended above usually furnish a fair general 
idea of the types of bacteria present. 
For the detection of gas-producing species, nutrient bouillon 
containing three per cent lactose and three per cent sucrose, re- 
spectively, in fermentation tubes, or nutrient agar containing 
three per cent lactose and three per cent sucrose, respectively, 
in test tubes, are serviceable. 
Cans of sweetened condensed milk that show gaseous fermen- 
tation (swell heads) are usually due to certain species of yeast, 
which thrive best in media containing sucrose. 
Cans of evaporated milk that show gaseous fermentation 
(swell heads) are usually caused by anaerobic putrefactive bac- 
