14 BULLETIN 1025, U. S. DEPARTMENT- OF AGRICULTURE. 



present begins to affect the color of the juices, and even after such 

 prolonged use the earth may give considerable further service in 

 treating heavity colored juices. 



DIFFICULTIES ENCOUNTERED IN FILTERING TREATED JUICES. 



In the. earier work various treatments were employed in the attempt 

 to devise satisfactory methods of filtration after treatment with the 

 earth which would be usable on a small scale. Some difficulty in ob- 

 taining a perfectly transparent filtrate arises from the fact that dia- 

 tomaceous earth contains a considerable quantity of excessively fine 

 fragments of broken f rustules which pass through ordinary filters and 

 later partially settle out upon the walls of the containers. Lenher (17) 

 has shown that when silica or orthoclase is ground until most of the 

 particles are less than 0.004 mm. in diameter, shaken up in water, and 

 allowed to stand for several weeks the larger portion of the suspended 

 material settles quickly, but the finer particles form strata in the 

 liquid which persist for 10 days. At the end of 15 clays the solution 

 is stable, but a portion of the material is in colloidal solution and 

 shows Brownian movement and Tyndall effect. It is clear from re- 

 sults obtained in the laboratory that analogous conditions result 

 when diatomaceous earth containing minute fragments of broken 

 frustules is added to a fruit juice, as a progressive settling extending 

 over some days or weeks occurs when a sample of treated juice which 

 has been filtered through paper is allowed to stand undisturbed. 



As the centrifuge is successfully used in a wide variety of otherwise 

 difficult separations and has repeatedly been suggested as a substitute 

 for filtration in the clearing of fruit juices (11), its possibilities as a 

 means for clarifying ciders treated with diatomaceous earth were 

 thoroughly tested. After being thoroughly mixed with the earth, the 

 juices were repeatedly passed through a gear-driven laboratory super- 

 centrifuge having a maximum speed of 25,000 revolutions per minute. 

 A single passage through the machine was practically as effective as 

 three or more, since only a very small trace of material was recovered 

 by any treatment after the first. While the appearance of treated juices 

 was greatly improved, a considerable amount of colloidal material 

 remained in suspension even after repeated stirring with earth fol- 

 lowed by passage through the centrifuge. Juices so treated and sub- 

 sequently pasteurized and stored remained for 18 months as hazy 

 colloidal solutions. Attempts further to clarify such centrifuged 

 juices by ordinary filtration were unsuccessful, as any filter having 

 sufficiently close texture to hold back the suspended particles quickly 

 became choked by slime. 



For these reasons the complete removal of the added earth from 

 the juices by any of the more generally available methods of filtra- 

 tion appeared somewhat difficult at the outset. The best types of 



