CULTURE MEDIA 



General Properties 



Agar 



Gelatin 



Silica gel 



Source 



Chemical nature 



Reaction 



Melting point — usual concentra- 

 tion 



Solidifies, usual concentration . . . . 



Tryptic digestion 



Water of condensation 



Usual concentration 



Plant 



Carbohydrate 

 Faintly acid 



96°C. 

 40°C. 



Not affected 

 Present 

 1 to IH per 

 cent 



Animal 

 Protein 

 Acid 



25°C. 

 25°C. 



Liquefied 

 None 

 10 to 12 

 per cent 



Inorganic 

 Silicic acid 

 Acid 



None 



5 to 6 per 

 cent 



Agar-agar is prepared by extracting certain seaweeds (largely 

 Gelidium corneum) with hot water. It consists almost entirely of 

 polysaccharides (largely galactans with some pentosan^) and a 

 small admixture of protein and mineral matter. 



Approximate Chemical Composition of Agar and Gelatin 



Agar, 2 Gelatin, 



Per Cent Per Cent 



Moisture 16.0 14 to 15 



Ash, dry basis 4.4 0.6 



Calcium oxide 1.15 0.0 



Magnesium oxide . 77 • 0.0 



Nitrogen 0.40 18.3 



Pure agar has practically no buffer effect within the range com- 

 monly employed. 



Washed Agar. — This is prepared as follows: Take 1,000 grams 

 of agar shreds, place in an enamel pail or glass vessel with 10 liters 

 of distilled water and allow to stand for 24 hours at room tem- 

 perature. Place a piece of cheesecloth over the top of the vessel, 

 pour off the water and once more add fresh distilled water and 

 allow to soak another 24 hours. Now pour off the water, and 

 allow the agar to air dry in thin layers. 



Agar treated in this way is much lower in calcium and mag- 

 nesium, as well as in soluble organic matter. In the preparation 



1 KoNiG and Bettels, Ztschr. Unters. Nahr. u. Genussm., 10: 487, 1905. 



2 Fellers, C. R., /. Ind. Eng. Chem., 8: 1128, 1916. 



