112 DISPERSE SYSTEMS 



which exists in niilk as a sohible calcium compound. This 

 compound is broken by the action of acid, and protein separates 

 as a curd. 



(c) The carbohydrate of milk, lactose, is split by various micro- 

 organisms, forming lactic acid, thus souring the milk and causing 

 curdling. 



Butter is simply the fat of the milk more or less completely 

 separated from the other constituents and forming a water-in-oil 

 emidsion. Whole, unchanged milk shows no tendency to form 

 butter. To form butter the fat particles are concentrated at the 

 siu'face by centrifugal action (or merely by allowing the cream 

 to rise), and then by causing the cream to sour, the fat is freed 

 from its emulsion with the colloidal matter. Since the hydrated 

 colloids tend to collect in the surface layer between the fat globules 

 and the dispersant aqueous phase of the cream, churning is 

 performed to break these layers and hasten the coalescence of 

 the fat. " The combined efforts therefore bring about a pro- 

 gressive increase in the concentration of the oil with a decrease 

 in the concentration of the hydrated colloid until the instability 

 of the oil in hydrated colloid becomes so great as to ' break ' and 

 yield the hydrated colloid-in-fat emulsion >vhich we call butter " 

 (Fischer and Hooker). That milk and cream are oil-in-water 

 emulsions can l)e proved microscopically. They wet paper and 

 are not greasy to the touch. Butter is a water-in-oil emulsion, 

 feels greasy, oils paper, and microscopically appears as a finely 

 divided aqueous colloid phase in a continuous oil phase. 



(2) Flesh. Under this head is included, not only the muscles of 

 various animals, but such cellular organs as the liver, kidneys, 

 thymus, etc. The colloidal nature of such tissues has already 

 been dealt with (see effect of cooking, below). 



(3) Eggs. The white of eggs is practically an albumin hydrosol 

 containing some crystalloids, while the yolk is an emulsion of 

 lipins (lecithin, etc.), in a hydrosol of protein (ordinary proteins, 

 and vitellin, a phospho-protein). 



B. Vegetable Foods. 



In the food of man, vegetable foods play as important a part 

 as animal products. (Generally, their make up is that of a mixed 

 hydrogel of protein, higher carbohydrates (and in the case of 

 oatmeal, maize, nuts, certain legumes and vegetables), a fair 

 proportion of fat. This gel is enclosed in a capsule of cellulose — 

 a higher carbohydrate which is very resistant to the action of 

 the human digestive juices. The capsule nuist be destroyed by 

 previous treatment, e.g., milling, cooking, chewing, etc., before 



