METABOLIC PRODUCTS 463 



Willstatter and 8chudel\s alkaline iodine nietliud. If 

 more than one of these methods is employed any dis- 

 crepancy between them may afford valuable information 

 as to the products formed. Thus a low value for glucose 

 by the optical method as compared with the reduction 

 method would indicate the formation of a laevo -rotatory 

 substance among the products, whilst a high value 

 would suggest the presence of a dextro-rotatory product. 



The fermentation mixture will consist of two parts, 

 insoluble and soluble. The insoluble constituents are first 

 removed by centrifugalisation or filtration. Difficult 

 filtration may often be improved by shaking the meta- 

 bolism solution with kieselguhr and filtering through a 

 thin layer of kieselguhr on filter paper prepared by 

 pouring a suspension of kieselguhr on to a wet paper in a 

 large Buchner funnel (about 1 g. is adequate for a funnel 

 15 to 20 cm. in diameter). Filtration is usually effective 

 for fungi, which form a more or less continuous felt of 

 mycelium, less effective for yeasts and usually ineffective 

 for bacteria for which centrifugalisation is usually 

 employed. 



The insoluble residue will contain the organism, such 

 products as may be insoluble in water (usually only 

 occurring in mould fermentations) and insoluble calcium 

 salts and excess of calcium carbonate, if the latter has 

 been employed. The organism is, of course, usually 

 known but should be examined in order to detect any 

 possible contamination with unwanted organisms. The 

 metabolic products of bacteria and yeasts are almost 

 invariably soluble in water unless the normal course of 

 fermentation has been interfered with by addition of 

 calcium carbonate or fixative agents such as sulphite, 

 dimedon, or /3-naphthylamine. In the latter cases the 

 residue will naturally be examined for the presence of 

 the expected products. If the fermentation has been 

 carried out in the presence of calcium carbonate the 

 production of calcium oxalate is possible, especially in 



