176 COLLOIDS 7N BIOLOGY AND MEDICINE 



added to the fat in order, we are accustomed to assume, to give it 

 the taste of butter. It is yet to be determined whether it is not just 

 this addition, which gives the fat the dispersion characteristic of 

 butter, and thereby its greater digestibility. The darkening of 

 margarine in heating is undoubtedly to be attributed to the added 

 skim-milk. 



By changing the colloidally dissolved albuminous substances of 

 milk into the gel form, we obtain cheese. Coagulation can be brought 

 about by means of rennet (sweet milk cheese) or through acidification 

 (sour milk cheese). In cheese we have an emulsion of fat in a protein 

 gel; whereas in skim or sour milk cheese (kummel, Harz and hand- 

 kase), the amount of fat is only the small amount afforded by skim- 

 milk; it is quite high in the fatty cheeses (cream, Swiss, Camembert 

 and Roquefort). 



A process of great interest, as yet uninvestigated from the colloid- 

 chemical standpoint is the ripening of cheese. Through the action 

 of bacteria there occur changes in the structure of the cheese which 

 are specific for every variety and which cause the spotted appear- 

 ance found in the various kinds of cheese. 



For cheese, chemical tests are limited to the determination of the 

 water, fat, albumin and salt content and the possible adulterants. 

 The most important, namely, the swelling capacity in the presence 

 of the digestive ferments, is nowadays entirely ignored, although 

 this would furnish the simplest method of deciding the important 

 question of the digestibility of cheese. 



Honey should in the main be composed of sugar; it is neverthe- 

 less frequently adulterated with glucose and dextrin. My opinion 

 is that tests of the surface tension of dilute solutions would lead to 

 the detection of such colloidal adulterants. 



Flour, Dough and Baking Products. 



The examination of flour, in addition to the microscopic histologic 

 study, extends to its doughing and baking properties. These two 

 questions belong entirely to the province of colloid chemistry. 



Art is in advance of science in this matter. There exist the most 

 diverse methods for discovering the presence of foreign substances 

 in flour and for distinguishing its various varieties by determining 

 the temperature at which it becomes pasty. The baking capacity 

 in particular, which is intimately bound up with the swelling capacity 

 of the gluten, is applied colloid chemistry. The more glutenous the 

 flour, the more water it binds (38 to 60 per cent) and the greater is 

 its capacity to be kneaded. 



